FAQs

Question 1

Can Norwegian Subsea MRUs be used for wave analysis?

Accepted Answer

Yes, Norwegian Subsea Motion Reference Units (MRUs) are highly suitable for applications involving wave analysis. Our sensors provide precise measurements of motion in 6 Degrees of Freedom (6DoF), encompassing Roll, Pitch, Yaw, Surge, Sway, and crucially for wave analysis, Heave.
The high-accuracy Heave (vertical motion) data generated by our MRUs is fundamental for determining key wave characteristics such as wave height and period. Norwegian Subsea MRUs deliver a reliable Heave accuracy of 5.0 cm or 5.0%, enabling detailed analysis of sea states.
Our MRUs are frequently integrated into systems where understanding wave conditions is critical. This includes deployment of specific wave buoys, on measurement buoys like Wind Lidar Buoys, offshore structures for monitoring, and vessels performing hydrographic surveys or ship motion monitoring, where wave-induced motion must be accurately measured or compensated for.
Norwegian Subsea MRUs facilitate straightforward integration into wave analysis systems through standard communication interfaces like Ethernet and serial ports, supporting protocols such as UDP and Modbus TCP. This allows seamless transmission of motion data to the analysis software or hardware. Our robust, validated sensors ensure reliable performance in demanding marine environments.

Question 2

Can a Motion Reference Unit (MRU) be used for inclining tests?

Accepted Answer

Yes. An MRU provides high-resolution measurements of roll angles, which makes it an effective tool for inclining tests (stability verification / "krengeprøve"). Unlike traditional pendulums, an MRU can deliver accurate data even when the test is carried out at sea under less-than-ideal conditions. By filtering out wave-induced motions and recording precise responses to applied test weights, the MRU improves both accuracy and efficiency of the inclining test.

Question 3

Can the MRU output remote heave?

Accepted Answer

Yes, the MRU can output heave measurements, including position, velocity and acceleration at 2 remote monitoring points. 
In addition, you can use the measurment point at the centre of gravity and select the CoG position as the position of your 3rd monitoring point. Note: CoG position is not used as part of the calculations and can be set to any position.
The location of the remote monitoring points can be configured graphically in the configuration software. 
demo.norwegian-subsea.no/settings/mounting
An example where this is usefull can be one MRU that provides output for 3 active heave compensated winches.

Question 4

Do I need a junction box for the MRU Marine?

Accepted Answer

The MRU Marine uses a custom 16-wire cable with Lemo connectors. 
The 16 wires provide full flexibility for both Ethernet, RS-232, RS-485, PPS time sync and Aiding input.
What connection options do I have for my MRU Marine?
Pigtail cable only, that you terminate at your end.
Junction box V2, that has standard connectors on the outside and uses a Lemo cable with connectors on both ends (Male for MRU side & Female for JB side)
Lemo connector (for MRU connection)
RJ45 (for Ethernet)
Power input
DB-9 connector for Time Sync
DB-9 connector RS-485
DB-9 connector for RS-232
Junction box V3:
Closed JB which is IP-68 rated and has 3 cable glands
Ethernet connector inside for test and configuration use
Termination blocks inside for all connections
Pigtail cable for connection to MRU side
This junction box can be used with both MRU Marine and MRU Marine SW
The JB V3 has analog outputs, both Volt and Current, included

Question 5

Do Norwegian Subsea MRUs output accelerations and velocities?

Accepted Answer

Yes, the MRU outputs accelerations as well as velocities. You can configure the MRU to output acceleration and velocity data in the MRU, Vessel or NED frame. Both raw and lowpass filtered accelerations are available.

Question 6

Does my MRU need recalibration?

Accepted Answer

The Norwegian Subsea MRU calibration certificate is valid for 4 years, and full product specification is maintained in this period under normal operating conditions.
Notes:
We do not normally advise re-calibration of the MRU during its lifetime.
The specified heave accuracy of 5 cm / 5% is maintained over the MRU's lifetime, also without re-calibration.
The specified accuracy in roll & pitch for a given model is usually met over the MRU's lifetime under normal operating conditions as Norwegian Subsea uses only validated sensors with exceptional stability.
Re-calibration and validation of the MRU every 4th year is recommended only if the customer requires full product specification after expiration of a valid calibration certificate.
The lifetime accuracy in roll & pitch, without re-calibration and under normal operating conditions, not exceed
0.1 degrees for the 3000 model
0.05 degrees for the 6000 model
0.02 degrees for the 9000 model
Thus, selecting a more accurate model than initially required may extend the calibration intervals or remove the need for re-calibration.
The computed MTBF (mean time between failure) for a NORSUB MRU Marine is 100 000 hours, but actual lifetime depends on use, vibrations, and temperature. An MRU lifetime of 10 years or more under normal continuous operation can be expected.
In summary, for most applications, no recalibration is required. Examples:
The MRU 3000 for Helideck monitoring applications. The requirement is 0.1 degrees roll/pitch accuracy and the MRU 3000 will always stay within this accuracy requirement without recalibration.
The MRU 3000 Marine for DP systems roll/pitch sensor. The MRU 3000 will always stay within the accuracy requirement without recalibration.
Re-calibration and validation of the MRU is done in the factory in Oslo, Norway. Normal turn-around time is one week. Re-calibration includes a complete validation of the MRU, ensuring that the MRU is in full working order.

Question 7

Does the MRU measure surge and sway?

Accepted Answer

Yes, our MRUs measure surge and sway. 
Note, that as for heave, the measurements are around a zero-mean position. This means that for a ship that moved forward, it’s only the high frequency part of the surge motion that is measured by the MRU.
Accurate surge and sway measurements are dependant on accurate roll and pitch measurements, since small changes in roll and pitch influence the surge and sway measurements. That is why we recommend the 9000 series when surge and sway measurements are important. This can be for applications like 3D motion compensation.

Question 8

How can I buy motion sensors from Norwegian Subsea?

Accepted Answer

Thank you for your interest in Norwegian Subsea's high-performance motion reference units. To purchase our MRUs or other sensor solutions, please fill out our form "Request a quote" - top right corner on our website or send us an email to sales@norwegian-subsea.no / (+47) 406 35 200, about us.
Our team will work closely with you to understand your specific application requirements, including necessary accuracy levels (Series 3000, 6000, or 9000), environmental conditions (determining the need for MRU Compact, Marine, Subsea, or Ex versions), required data protocols, and connectivity options.
This consultation ensures you receive the most suitable and cost-effective option for you. We pride ourselves on providing expert guidance and support throughout the purchasing process.

Question 9

How can I contact Norwegian Subsea for sensor solutions?

Accepted Answer

Thank you for your interest in Norwegian Subsea's high-performance motion reference units. As a leading global supplier headquartered in Oslo, Norway, we are ready to assist you in finding the optimal MRU or sensor for your specific application.
To contact us, please fill out our form "Request a quote" top right corner or send us an email to sales@norwegian-subsea.no / (+47) 406 35 200, about us
Our team will work closely with you to understand your specific application requirements, including necessary accuracy levels (Series 3000, 6000, or 9000), environmental conditions (determining the need for MRU Compact, Marine, Subsea, or Ex versions), required data protocols, and connectivity options.
This consultation ensures you receive the most suitable and cost-effective option for you. We pride ourselves on providing expert guidance and support throughout the purchasing process.

Question 10

How can the accuracy of subsea sensors be improved?

Accepted Answer

Improving the accuracy of subsea sensors fundamentally relies on selecting a high-performance unit designed specifically for the demanding underwater environment. At Norwegian Subsea, we achieve superior accuracy through a combination of state-of-the-art MEMS sensor technology, advanced sensor fusion algorithms, and robust hardware engineering, validated in real sea conditions.
Our algorithms are crucial for filtering noise and accurately calculating motion parameters like Roll, Pitch, Heave, Surge, Sway, and Yaw (6DoF), even under complex, irregular motion patterns typical of subsea operations. Furthermore, our sensors undergo rigorous testing and validation in real sea conditions, ensuring their performance specifications are met in the environments where they will be deployed.
The Norwegian Subsea MRU Subsea exemplifies this approach. Housed in a compact, 6000m depth-rated titanium casing, it offers exceptional durability. It is available in different accuracy tiers (±0.05°, ±0.02°, ±0.01° Roll/Pitch accuracy) to meet specific project requirements. We also offer an Inclinometer/VRU version for applications requiring only high-accuracy Roll and Pitch data.
Accuracy is maintained over the long term, as our MRUs are factory calibrated and typically do not require recalibration during their operational life for most applications. Easy integration via standard protocols (Ethernet, RS-232/RS-485, Modbus TCP) and Subconn connectors ensures that the high-quality data can be reliably transmitted to your systems. Choosing a sensor engineered for performance and reliability, like those from Norwegian Subsea, is the most effective way to ensure high accuracy for your subsea applications.

Question 11

How do I choose the right MRU for subsea applications?

Accepted Answer

When selecting a motion sensor for subsea applications, the right choice combines robust design, deep‑water capability, high accuracy, and ease of integration. At Norwegian Subsea we recommend our MRU Subsea model for these demanding conditions.
Why choose the MRU Subsea?
The MRU Subsea is engineered specifically for underwater operations. It features a compact titanium housing rated to 6000 metres depth, making it ideal for use on ROVs, AUVs, riser monitoring systems, BOP monitoring, subsea surveys and inspection platforms. Its small footprint and ability to be mounted in any orientation allow for flexible installation in space‑constrained subsea environments.
It offers high‑accuracy 6 Degrees of Freedom (6DoF) motion data including Roll, Pitch, Yaw, Heave, Surge and Sway. Roll & Pitch accuracies are available in ±0.05°, ±0.02° and ±0.01° options depending on the series. The unit integrates seamlessly via a Subconn wet‑mateable connector and supports both Ethernet (UDP, Modbus TCP, Ethernet/IP) and serial (RS‑232 or RS‑485) communication. It is built with advanced MEMS sensors and proprietary fusion algorithms, delivering validated real‑sea performance often without the need for recalibration.
For projects that only require Roll and Pitch measurements, Norwegian Subsea also offers VRU/Inclinometer versions of the MRU model, offering the same high angular accuracy at lower cost.
How to buy the MRU Subsea
To purchase a motion sensor for subsea use, please contact Norwegian Subsea directly. Our team will guide you through selecting the appropriate model based on your environmental conditions, accuracy requirements, data protocol needs and connector options. Provide us with your application details and we will support you with a tailored solution.

Question 12

How do I choose the right motion sensor?

Accepted Answer

Choosing the right motion sensor ensures optimal performance and reliability for your specific needs. At Norwegian Subsea, we offer a range of high-performance Motion Reference Units (MRUs) designed for diverse marine, subsea, and offshore applications. The selection process primarily depends on the operational environment, required accuracy, connectivity needs, and specific functionality.
First, consider the installation environment. Our MRU Compact (IP65) is ideal for indoor or control box mounting, featuring easy plug-and-play installation with passive Power over Ethernet (PoE) via an RJ45 connector. For on-deck or shallow subsea applications (up to 50m), the MRU Marine (IP68) provides robust performance with versatile connectivity options including Ethernet, RS-232/485, and PPS. For deep-water applications up to 6000m, the MRU Subsea offers a compact, high-pressure titanium housing with Subconn connectors. If operating in hazardous areas, the MRU Ex provides a certified stainless steel solution (II 2 G Ex d IIC Gb).
Next, determine the required accuracy. All our MRU models are available in three accuracy tiers for Roll & Pitch: the 3000 series (±0.05°), the 6000 series (±0.02°), and the high-precision 9000 series (±0.01°). Heave accuracy is consistently high across models at 5.0 cm or 5.0%. All sensors utilize advanced sensor fusion algorithms and state-of-the-art MEMS technology, validated in real sea conditions.
Evaluate your system integration requirements. Norwegian Subsea MRUs support a wide range of standard and customizable data protocols (e.g., UDP, Modbus TCP, Ethernet/IP, NMEA, Modbus RTU) over Ethernet and serial lines (RS-232/485), ensuring seamless integration. We offer various connector options (RJ45, Marine 16-pin, Subconn, Pigtail) and flexible cable lengths. Our sensors are designed for easy configuration via a web browser interface, requiring no software downloads.
Finally, consider the required output data. While our standard MRUs provide full 6 Degrees of Freedom (6DoF) measurements (Roll, Pitch, Heave, Yaw, Surge, Sway), we also offer Inclinometer/VRU versions across all models. These provide the same high Roll & Pitch accuracy but without Heave, Surge, and Sway outputs, offering a cost-effective solution when only attitude data is needed. All Norwegian Subsea sensors can be mounted in any orientation and are known for their long-term stability, often eliminating the need for recalibration, making them ideal for both new installations and cost-effective retrofitting.

Question 13

How do I purchase or contact Norwegian Subsea for sensor solutions?

Accepted Answer

Norwegian Subsea is a global supplier of high-performance Motion Reference Units (MRUs) and related motion sensor solutions. We work closely with customers to ensure each sensor is configured to meet specific application needs across marine, subsea, offshore, and industrial environments.
To initiate a purchase or consultation, we recommend starting with our “Request a quote” form available at the top right corner of our website. This allows us to collect relevant details about your requirements, including:
Required accuracy level: Series 3000, 6000, or 9000 (Roll/Pitch accuracy from ±0.05° down to ±0.01°)
Operational environment: MRU Compact, MRU Marine, MRU Subsea (6000m-rated), or MRU Ex (hazardous areas)
Integration preferences: Ethernet (UDP, Modbus TCP, Ethernet/IP) or serial communication (RS-232/RS-485)
Specific application requirements such as real-time motion data, VRU vs. MRU functionality, or housing material
Our expert team will assess your input and guide you in selecting the most suitable, cost-effective sensor, whether for a new installation or retrofitting existing systems. All our MRUs use advanced MEMS technology and sensor fusion algorithms, and most models are maintenance-free and do not require recalibration.
Contact Details:
Email: sales@norwegian-subsea.no
Phone: (+47) 406 35 200
We are headquartered in Oslo, Norway, and serve customers worldwide across marine, offshore, renewable energy, defence, and subsea industries.

Question 14

How do I select the best Norwegian Subsea product for me?

Accepted Answer

At Norwegian Subsea, we design all our Motion Reference Units (MRUs) for high performance, reliability, and cost-effectiveness, making the 'best' product dependent on your specific application requirements and operating environment. Our sensors utilize advanced algorithms and state-of-the-art MEMS technology, validated in real sea conditions to ensure robust and accurate 6DoF motion data (Roll, Pitch, Heave, Yaw, Surge, Sway).
Our core MRU range includes the MRU Compact (IP65, anodized aluminum, RJ45/RJ50 connectors with passive PoE) ideal for indoor or control box installations; the MRU Marine (IP68 or 50m SW option, anodized aluminum, versatile pigtail/Subconn connectors) designed for deck mounting or shallow water; the MRU Subsea (6000m depth-rated, titanium housing, Subconn connector) for deepwater applications like ROV/AUV operations and riser monitoring; and the MRU Ex (Ex d certified, stainless steel housing, pigtail cable) for hazardous area installations.
All models are designed for easy integration with standard protocols (Ethernet UDP, Modbus TCP, Ethernet/IP, RS-232/485 options) and user-friendly web-based configuration. They are compact, can be mounted in any orientation, and typically do not require recalibration, reducing lifetime costs. We offer different accuracy tiers (±0.05°, ±0.02°, ±0.01° Roll/Pitch) across these models, allowing you to select the optimal balance of performance and budget.
Ultimately, the best Norwegian Subsea MRU is the one that precisely matches your technical needs, environmental conditions, and integration requirements, delivering trusted motion data reliably and affordably. 
Start your journey with Norwegian Subsea MRUs here.

Question 15

How do different subsea ROV motion sensors compare?

Accepted Answer

Selecting the appropriate motion sensor for a subsea Remotely Operated Vehicle (ROV) is critical for precise navigation, positioning, and tool manipulation. Key comparison factors include accuracy (Roll, Pitch, Heave, Yaw), depth rating, physical size and weight, robustness, ease of integration with ROV control systems, reliability in harsh environments, and overall cost-effectiveness.
Norwegian Subsea offers the MRU Subsea, specifically engineered for demanding underwater applications like ROV operations. This unit features a robust, compact titanium housing depth-rated to 6000 meters, making it ideal for deep-water tasks. We provide different accuracy levels to meet specific operational needs, with Roll and Pitch accuracy options of ±0.05° (Series 3000), ±0.02° (Series 6000), or ±0.01° (Series 9000), and a standard Heave accuracy of 5.0 cm or 5.0%. Our sensors utilize advanced MEMS technology and sensor fusion algorithms, validated in real sea conditions.
Integration is streamlined with the MRU Subsea. It comes equipped with a reliable Subconn connector and supports standard communication protocols like Ethernet (UDP, Modbus TCP, Ethernet/IP) and serial (RS-232 or RS-485), ensuring compatibility with various ROV systems and PLCs. The compact footprint and the ability to mount the sensor in any orientation offer significant installation flexibility on space-constrained ROVs. For applications primarily focused on tilt measurement, a high-accuracy, cost-effective VRU/(Inclinometer) version is also available.
Compared to other options, the Norwegian Subsea MRU Subsea provides a compelling combination of high performance, proven reliability, deep-water capability, and ease of integration at an affordable price point. Our focus on robust design minimizes the need for recalibration, reducing lifetime operational costs for ROV operators.

Question 16

How does a motion sensor work?

Accepted Answer

A motion sensor, like the Motion Reference Units (MRUs) developed by Norwegian Subsea, functions by detecting and measuring movement. At their core, these sensors utilize state-of-the-art Micro-Electro-Mechanical Systems (MEMS) technology. MEMS accelerometers measure linear acceleration (changes in velocity along the Surge, Sway, and Heave axes), while MEMS gyroscopes measure angular velocity (rate of rotation around the Roll, Pitch, and Yaw axes).
To provide a comprehensive and accurate measurement of motion in all six degrees of freedom (6DoF) – Roll, Pitch, Yaw, Heave, Surge, and Sway – data from multiple MEMS sensors is intelligently combined. Norwegian Subsea employs advanced sensor fusion algorithms that process these raw sensor inputs, filtering out noise and compensating for external forces like gravity, to calculate precise orientation and movement data in real-time.
This sophisticated combination of quality hardware and advanced algorithms allows Norwegian Subsea MRUs to deliver high-performance, reliable motion data essential for demanding applications in the marine, subsea, and offshore energy sectors. Our sensors are rigorously tested and validated in real sea conditions, ensuring robust performance and accuracy for critical operations.

Question 17

How does an inertial measurement unit (IMU) work?

Accepted Answer

An IMU is an electronic device that measures and reports an object's linear acceleration and angular velocity using a combination of accelerometers and gyroscopes along three orthogonal axes (X, Y, and Z). It achieves this by utilizing a combination of sensors—primarily accelerometers and gyroscopes, and occasionally magnetometers. IMUs only provide raw data from their sensors and do not include "smart" features such as processing (e.g., Kalman Filter). The IMU cannot provide attitude (roll and pitch), velocity, or position without external processing.
This is very different from a Motion Reference Unit (MRU) which is a specialized device and self-contained sensor that measures motion in all six degrees of freedom (DoF): Roll, Pitch, Yaw, Surge, Sway and Heave. The six DoF positions, velocities and accelerations are measured by the MRU using high-end gyroscopes and accelerometers (and sometimes magnetometers) together with advanced fusion algorithms.
Where the IMU output raw sensor data on acceleration and rotation rates the MRU outputs real-time, high-accuracy motion data in all six DoF (roll, pitch, yaw, heave, surge, and sway). Optimized for dynamic environments, especially in marine settings, to provide accurate motion measurements under real sea conditions.

Question 18

How is a VRU used for instrument compensation?

Accepted Answer

A VRU is an advanced device that measures the attitude (roll and pitch) of an object using high-end accelerometers and gyroscopes, combined with advanced sensor fusion algorithms. VRUs provide very accurate roll and pitch measurements, even in dynamic environments.
In applications requiring real-time, high-accuracy roll and pitch data the VRU's precise Roll and Pitch data allows control systems to react effectively. For example, motion compensation of GNSS antenna for dynamic positioning (DP-systems), stabilizing fins on vessels or advanced Instrument compensation: Used in monitoring and control systems where only roll and pitch data is needed.
Norwegian Subsea offers high-performance VRU versions across our entire MRU product line (Compact, Marine, Subsea, Ex). These units deliver the same exceptional Roll and Pitch accuracy (available in ±0.05°, ±0.02°, or ±0.01° depending on the series) as our full 6DoF MRUs, even in complex, irregular motion conditions. By focusing solely on angular measurements, our VRU versions provide a highly reliable and cost-effective solution for advanced motion control applications where Heave, Surge, and Sway data are not required.
Comparison to MRU: A VRU shares similarities with an MRU, incorporating real-time, high-precision accelerometers and gyroscopes along with advanced sensor fusion algorithms in a single self-contained unit. Both devices calculate coupled motions across all six degrees of freedom (DoF): Roll, Pitch, Yaw, Surge, Sway, and Heave. However, while the MRU outputs data for all six DoF, the VRU is limited to providing roll and pitch data only. This restricted output makes the VRU generally more cost-effective than the MRU.

Question 19

How is a motion reference unit (MRU) used in offshore crane operations?

Accepted Answer

A Motion Reference Unit (MRU) is essential for safe and efficient offshore crane operations. Mounted on the vessel or platform, the MRU precisely measures the 6 Degrees of Freedom (6DoF) motion: Roll, Pitch, Yaw, Heave, Surge, and Sway.
The primary application in crane operations is Active Heave Compensation (AHC). The MRU provides high-accuracy, real-time heave (vertical motion) data to the crane's control system. This allows the winch system to actively compensate for the vessel's vertical movement caused by waves, keeping the crane hook and load stable relative to the seabed or a fixed structure.
Norwegian Subsea MRUs, deliver the high accuracy (up to ±0.01° Roll/Pitch) and reliability required for demanding AHC systems. Our sensors are rigorously tested in real sea conditions, ensuring robust performance. They integrate easily via standard protocols like Modbus TCP, Ethernet/IP, and UDP, and are often used as cost-effective drop-in replacements for older units in retrofit projects.

Question 20

How is data filtering applied in motion reference units?

Accepted Answer

Data filtering is a critical process within Motion Reference Units (MRUs) designed to refine raw sensor measurements by removing noise and isolating the true motion components. At Norwegian Subsea, our MRUs utilize advanced sensor fusion algorithms and Kalman Filter algorithm that intelligently process data from state-of-the-art MEMS accelerometers and gyroscopes.
These sophisticated algorithms act as highly optimized filters, effectively distinguishing between vessel motion and unwanted noise or vibrations. This ensures the delivery of exceptionally accurate and stable measurements for Roll, Pitch, Heave, Yaw, Surge, and Sway in real sea conditions with irregular waves and coupled motions.
The quality of data filtering directly impacts the performance and reliability of systems relying on motion data. Our robust filtering techniques are validated in real sea conditions, providing the high-fidelity output required for demanding applications such as 3D motion compensated gangways, sonar motion compensation, Dynamic Positioning (DP), Active Heave Compensation (AHC), helideck monitoring (HMS) and much more.
While the internal filtering processes are complex, Norwegian Subsea MRUs are designed for ease of use. The filtered data is provided through various standard protocols, and users can access configuration settings via a user-friendly web interface, ensuring seamless integration and optimal performance with minimal setup.

Question 21

How is motion data filtered and analysed in real time by MRUs?

Accepted Answer

Accurate real-time motion data is essential for marine, offshore, and subsea operations. Norwegian Subsea’s Motion Reference Units (MRUs) are designed to deliver this through a combination of high-end MEMS sensors, advanced sensor fusion algorithms, and onboard data processing systems.
Data Filtering
MRUs apply advanced filtering techniques to raw sensor inputs to isolate true vessel motion and eliminate noise. Norwegian Subsea MRUs use a sophisticated Kalman Filter algorithm as part of their sensor fusion process, which intelligently processes input from MEMS accelerometers and gyroscopes. These filters are highly optimised to differentiate between actual vessel motion and unwanted disturbances such as vibration or sensor noise.
This filtering is essential in real-world maritime environments, where irregular waves and complex coupled motions can distort raw data. The effectiveness of our filtering techniques is validated in real sea conditions and directly affects the reliability of systems such as dynamic positioning (DP), sonar motion compensation, helideck monitoring systems (HMS), 3D motion compensated gangways, and active heave compensation (AHC).
Despite the complexity of internal filtering, users benefit from an intuitive interface. The filtered output is available via various standard protocols, and device configuration is managed easily through a browser-based setup interface.
Real-Time Data Analysis
The MRU’s onboard processing unit plays a vital role in real-time analysis. Sensor data is continuously processed internally with minimal latency, allowing the MRU to deliver high-accuracy motion measurements across all six degrees of freedom: Roll, Pitch, Yaw, Heave, Surge, and Sway.
This data is transmitted in real time to connected systems via Ethernet or serial connections (RS-232 or RS-485), supporting protocols such as UDP, Modbus TCP, Ethernet/IP, NMEA, or custom formats. While the MRU handles the real-time motion computation, further analysis or control decisions are typically carried out by the connected client system, relying on the high-fidelity, low-latency data stream from the MRU.
This combination of real-time onboard processing and precision filtering ensures that Norwegian Subsea MRUs provide stable, application-ready motion data critical for operational success in the harshest marine environments.

Question 22

How is motion monitoring used in offshore oil and gas?

Accepted Answer

Motion monitoring is fundamental to ensuring safety, operational efficiency, and asset integrity within the offshore oil and gas industry. Accurate, real-time measurement of vessel and equipment motion, including Roll, Pitch, Heave, Yaw, Surge, and Sway (6 Degrees of Freedom - 6DoF), is essential for conducting complex operations safely and effectively in dynamic maritime environments.
Norwegian Subsea's high-performance Motion Reference Units (MRUs) provide the critical motion data required by various essential systems. This includes Dynamic Positioning (DP) systems for precise vessel station-keeping, Helideck Monitoring Systems (HMS) to ensure safe helicopter operations, Active Heave Compensation (AHC) systems for stable crane and winch performance, and Motion Compensated Gangways that allow for secure personnel transfers between vessels and installations.
Our robust sensors are also integral to a wide range of subsea activities. They enable precise navigation and control for Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs), facilitate accurate subsea surveys, and provide vital monitoring for critical equipment such as Blowout Preventers (BOPs) and risers. Furthermore, our MRUs can be employed for structural health monitoring of fixed and floating offshore platforms.
Norwegian Subsea provides a comprehensive range of reliable MRUs specifically engineered for the demanding conditions encountered offshore. This includes the versatile, IP68-rated MRU Marine for deck or shallow water use, the 6000m depth-rated titanium MRU Subsea for deepwater applications, and the certified MRU Ex for deployment in hazardous areas. Our sensors are known for their high accuracy, robust design, ease of integration using standard industry protocols, and cost-effectiveness, making them ideal for both new projects and as drop-in replacements for retrofitting older systems.

Question 23

How is motion monitoring used in offshore operations?

Accepted Answer

Motion monitoring is fundamental to enhancing safety and operational efficiency in the demanding offshore environment. It involves the precise measurement of vessel, platform, and equipment movements across all six degrees of freedom (6DoF) – Roll, Pitch, Heave, Yaw, Surge, and Sway. High-performance sensors, such as Norwegian Subsea's Motion Reference Units (MRUs), provide the critical real-time data needed for monitoring, control or instrument compensation, ensuring safe personnel and cargo transfers, and optimizing operational windows in challenging sea states.
Our MRUs are integral components in numerous critical offshore applications: 
Dynamic Positioning (DP) systems, mainly used for motion compensation of the GNSS antenna and other position reference systems.
Active Heave Compensation (AHC) systems utilize MRU inputs to counteract vertical vessel motion, stabilizing cranes, winches, and launch and recovery systems (LARS) for safe and precise lifting operations.
Gangway, 3D motion compensated gangways require accurate MRUs to motion compensate the vessels movement in-real-time.
Helideck Monitoring Systems (HMS) employ our sensors to continuously monitor the helideck's motion, providing essential data to ensure safe helicopter landings and takeoffs.
Wave radar, are used in many offshore operation to know the accurate sea state. This information is then used to optimize the operational windows. OEM versions of Norwegian Subsea MRUs are today included in the world's leading manufacturers of Wave radars. The MRUs provides real-time accurate heave measurements to compensate downward looking wave radars.
Wind lidar buoyes, High quality wind data requires MRUs for measuring the accurate heave, roll and pitch in Real sea conditions.
Motion monitoring also extends to subsea operations and the vital task of structural integrity assessment. Norwegian Subsea offers specialized sensors like the MRU Subsea, featuring a robust titanium housing rated for 6000m water depth, ideal for applications such as monitoring the motion of risers and Blowout Preventers (BOPs). Our MRUs are also crucial for guiding Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs) during subsea surveys, installations, and interventions. Additionally, they play a key role in structural health monitoring programs for fixed and floating offshore platforms and wind turbines.
Norwegian Subsea delivers highly accurate, reliable, and cost-effective MRU solutions specifically engineered and validated for the offshore sector. Leveraging state-of-the-art MEMS technology and sophisticated sensor fusion algorithms, our products offer exceptional performance. They are designed for easy integration, supporting a wide range of industry-standard protocols (e.g., UDP, Modbus TCP, Ethernet/IP) and offering flexible connectivity options. Our MRUs are known for their long-term stability, often eliminating the need for recalibration, which reduces lifetime costs, making them ideal for both new projects and as drop-in replacements for retrofitting existing systems.

Question 24

How is real-time data analysis performed with motion reference units?

Accepted Answer

Norwegian Subsea Motion Reference Units (MRUs) are designed to provide high-accuracy, real-time motion data crucial for monitoring, control or instrument compensation in demanding marine and subsea applications. Our MRUs utilize state-of-the-art MEMS sensors combined with advanced sensor fusion algorithms to continuously measure Roll, Pitch, Yaw, Heave, Surge, and Sway (6 Degrees of Freedom - 6DoF).
The core function enabling real-time analysis is the MRU's internal processing capability. Raw sensor readings are processed onboard using sophisticated algorithms to calculate precise orientation and motion data with minimal latency. This processed data is then immediately available for output.
This real-time motion data is transmitted from the MRU to external systems for analysis and decision-making. Norwegian Subsea MRUs facilitate seamless integration by offering versatile connectivity options, including Ethernet and serial ports (RS-232/RS-485), and supporting a wide range of standard industrial communication protocols like UDP, Modbus TCP, Ethernet/IP, NMEA, as well as custom ASCII or binary formats.
Therefore, while the MRU performs the critical task of accurately measuring and outputting motion data in real-time, the subsequent analysis of this data is typically performed by the connected client system, which relies on the high-quality, low-latency data stream provided by our reliable MRUs.

Question 25

Is it better to rent or buy a motion reference unit (MRU)?

Accepted Answer

Deciding whether to rent or purchase a motion reference unit depends on the project duration and frequency of use. While renting might appear suitable for very short-term, isolated projects, purchasing often presents a more cost-effective and reliable solution in the long run. Owning the sensor ensures availability when needed and eliminates recurring rental costs.
Our MRUs are engineered for reliability and longevity, often requiring no recalibration during their operational life for most applications, significantly reducing maintenance overhead and total cost of ownership compared to repeated rentals. At the moment Norwegian Subsea does not offer a rental service of our products, but in some cases we offer to sell a discounted demo-unit for potential recurring customers. Please contact us if you are interested in a testing our MRUs.

Question 26

Is the MRU also available in a VRU / Inclinometer version?

Accepted Answer

All our MRUs are also available in a VRU (Vertical Reference Unit) / Inclinometer /Roll & Pitch version of the MRU. This version is identical to the MRU except that it does not include Heave, Surge and Sway outputs. 
This means the our VRU / Inclinometer has all the advantages of the Norwegian Subsea MRU. It will be accurate in real sea conditions, in irregular and coupled motions. This is due to the use of the accurate gyroscopes and accelerometers and the advanced sensor fusion algorithms. This ensures superior performance compared to other VRUs or Inclinometers or Roll/Pitch sensors.
If you do not require Heave, Surge and Sway measurements, ask for the Inclinometer version.

Question 27

What Ethernet protocols are available?

Accepted Answer

All Norwegian Subsea Motion Reference Units (MRUs) include Ethernet communication and support the following standard Ethernet protocols:
UDP
Modbus TCP
Ethernet/IP
These protocols ensure seamless integration with a wide range of industrial systems, providing flexibility across diverse applications.

Passive PoE (Power over Ethernet)
The MRU Compact uses a standard RJ45 Ethernet cable with Passive PoE, supplying power through the same connection. It supports an input voltage range between 9 and 36 V.
Most Ethernet switches use Active PoE, which is incompatible with Passive PoE devices like the MRU Compact. Therefore, the MRU Compact should not be connected directly to an Active PoE switch unless a converter is used.
To facilitate setup, we typically supply the first MRU Compact with a start kit that includes a Passive PoE RJ45 splitter socket. This allows you to connect the MRU Compact using standard Cat5 RJ45 Ethernet cables between your MRU, the splitter, and your PC or Ethernet switch.
For industrial installations, we recommend terminating the Ethernet cable to a terminal block or using an interface module such as this from Wago.
If connecting to an Active PoE switch is necessary, a PoE converter (e.g., from Ubiquity) can be used to convert the Active PoE output to a constant 24 V Passive PoE input.
For more technical guidance, please refer to the MRU Compact user manual.

RJ50 (10-wire Ethernet) Connector
The MRU Compact is also available with an RJ50 connector (also known as 10P10C), instead of the standard RJ45 Ethernet connector. While the RJ50 looks similar to the RJ45, it includes 10 wires instead of 8. The outer two pins, unused in RJ45, are active in the RJ50 connector.
This 10-wire configuration allows for an additional interface alongside Ethernet and power:
Ethernet & RS-232
Ethernet & 4-wire RS-485
Ethernet & PPS time sync
Important: Never insert a standard RJ45 plug into an RJ50 socket. While it may appear to fit, it can damage the connector due to the unused outer pins on the RJ45 interfering with the active pins on the RJ50.
For more detailed specifications or connection guidance, visit our support site or contact us directly.

Question 28

What are motion sensors used for in subsea monitoring?

Accepted Answer

Motion sensors, specifically high-accuracy Motion Reference Units (MRUs), are crucial components in subsea monitoring operations. They provide precise measurements of motion in all six degrees of freedom (Roll, Pitch, Yaw, Heave, Surge, Sway), which is essential for the safe and efficient operation of underwater equipment and structures.
Key applications include monitoring the dynamic behavior of subsea structures like risers, subsea templates, subsea equipment, installation of bottom fixed structures and Blowout Preventers (BOPs). Accurate motion data helps ensure structural integrity and operational safety. Furthermore, MRUs are also used as navigation equipment for Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs) during inspection, maintenance, repair (IMR), and survey tasks.
Norwegian Subsea's MRU Subsea is specifically designed for these demanding environments. Housed in a compact, robust titanium casing rated for 6000m water depth, it delivers high-accuracy motion data reliably. Its small footprint and ability to be mounted in any orientation allow for easy integration onto ROVs, AUVs, or subsea structures. Interfacing is straightforward via Ethernet or serial ports using standard protocols and a Subconn wet-mateable connector.
We also offer an Inclinometer/VRU version of the MRU Subsea, providing high-accuracy Roll and Pitch measurements without Heave/Surge/Sway output, offering a cost-effective solution for applications focused primarily on inclination monitoring.

Question 29

What are the MRU dimensions?

Accepted Answer

The MRU dimensions depend on the selected casing. 
MRU Compact
Dimensions	120 x 57 x 53 mm (l x w x h)
Weight 0.55 kg
MRU Marine
Dimensions	154 x 86 x 67 mm (l x w x h)
Weight 1.2 kg
MRU Subsea
Dimensions	76 x 76 x 165 mm (cylinder shape) (l x w x h)
Weight 1.6 kg
MRU Ex
Dimensions	190 x 190 x 205 mm (l x w x h)
Weight 15 kg (excluding cable)
-
MRU OEM (available on request)
Dimensions 75 x 50 x 41 mm (l x w x h)

Question 30

What are the differences between marine motion sensors?

Accepted Answer

Understanding the Differences Between IMU, Inclinometer, Roll & Pitch sensor, VRU, AHRS, MRU, Gyrocompass, GNSS/INS

When navigating the complexities of motion sensing and navigation systems, it is essential to understand the distinctions between devices and systems like Inertial Measurement Units (IMUs), Inclinometers, Roll & Pitch sensors, Vertical Reference Units (VRU), Attitude and Heading Reference Systems (AHRS), Motion Reference Units (MRUs), Gyrocompasses, and GNSS-Aided Inertial Navigation Systems (GNSS/INS). Each serves specific purposes and offers different levels of functionality, accuracy, and application scope.
Inertial Measurement Unit (IMU)
Definition: An IMU is an electronic device that measures and reports an object's linear acceleration and angular velocity using a combination of accelerometers and gyroscopes along three orthogonal axes (X, Y, and Z). It achieves this by utilizing a combination of sensors—primarily accelerometers and gyroscopes, and occasionally magnetometers. IMUs only provide raw data from their sensors and do not include "smart" features such as processing (e.g., Kalman Filter). The IMU cannot provide attitude (roll and pitch), velocity, or position without external processing.
Key Features:
Sensors Included: Accelerometers (measure linear acceleration) and gyroscopes (measure angular rate).
Output: Raw sensor data on acceleration and rotation rates.
Functionality: Provides fundamental motion data without further processing to determine attitude, velocity, or position.
Applications: Used in systems where raw inertial data is sufficient or will be processed further by an external system.
Example Uses:
Basic navigation systems: Where external processing is available.
Robotics: For control algorithms requiring raw motion data.
Stabilization of cameras and drones.
Inclinometer
Definition: An Inclinometer, also known as a tilt or level sensor, measures the angle of inclination or tilt of an object with respect to gravity. It determines the angle between the sensitive axis of the device and Earth's vertical direction (gravity vector). It is important to note that an Inclinometer only measures static angles, and therefore cannot provide accurate data in dynamic conditions involving movements or vibrations. Inclinometers are generally less expensive due to simpler components.
Key Features:
Sensors Included: Primarily uses accelerometers to detect gravity's direction.
Output: Measures static angles accurately when the device is stationary.
Functionality: Determines roll and pitch in static conditions, but lacks accuracy in dynamic settings.
Applications: Used in systems that are stationary or moving very slowly and do not experience vibrations.
Example Uses:
Simple Leveling: Leveling equipment that is not in motion.
Monitoring Tilt: For structures, bridges, and buildings.
Measuring Slope: Useful for landslide-prone areas.
Roll & Pitch Sensor
Definition: A Roll & Pitch sensor, also known as a dynamic inclinometer, is a device designed to measure an object's attitude (roll and pitch). It incorporates gyroscopes to enhance its dynamic measurement capabilities. Compared to an inclinometer, a Roll & Pitch sensor is more advanced, featuring an onboard processing unit that performs basic sensor fusion by integrating gyroscope data. While it offers improved performance over inclinometers (compensating for errors caused by non-gravitational accelerations) it is less accurate than a VRU or MRU due to the use of lower-quality accelerometers, gyroscopes, and less sophisticated sensor fusion algorithms in its onboard processing unit.
Key Features:
Sensors Included: Accelerometers and gyroscopes.
Output: Measures rotation around two axes, roll and pitch, with enhanced dynamic measurement capabilities compared to an inclinometer
Functionality: It delivers improved attitude measurements (roll and pitch) in dynamic conditions compared to an inclinometer, but falls short of the accuracy achieved by a VRU or MRU.
Applications: Used in less critical systems that do not require highly accurate roll and pitch measurements in dynamic conditions. These systems can tolerate some movement and vibration, but their accuracy decreases as movement and vibration increase.
Example Uses:
Orientation Monitoring: For smaller vessels, barges, and buoys.
Control Systems Feedback: For stabilizer fins in smaller boats.
Stabilizing Platforms: Such as cameras and antennas.
Vertical Reference Unit (VRU)
Definition: A VRU is an advanced device that measures the attitude (roll and pitch) of an object using high-end accelerometers and gyroscopes, combined with advanced sensor fusion algorithms. VRUs provide very accurate roll and pitch measurements, even in dynamic environments.
Key Features:
Sensors Included: High-end accelerometers and gyroscopes.
Output: Real-time, high-accuracy roll and pitch data.
Functionality: Functionality: Designed for dynamic environments, particularly in marine settings, to deliver precise roll and pitch measurements under real sea conditions.
Applications: Delivers real-time, high-accuracy roll and pitch data for monitoring, control, or instrument compensations within the maritime, offshore, and subsea sector.
Example Uses:
Dynamic Positioning Systems
Advanced Instrument Compensation: Used in monitoring and control systems where only roll and pitch data is needed.

Comparison to MRU: A VRU shares similarities with an MRU, incorporating real-time, high-precision accelerometers and gyroscopes along with advanced sensor fusion algorithms in a single self-contained unit. Both devices calculate coupled motions across all six degrees of freedom (DoF): Roll, Pitch, Yaw, Surge, Sway, and Heave. However, while the MRU outputs data for all six DoF, the VRU is limited to providing roll and pitch data only. This restricted output makes the VRU generally more cost-effective than the MRU.
Summary: Inclinometer, Roll & Pitch Sensor, and VRU
Although an Inclinometer, Roll & Pitch Sensor, and VRU all measure attitude (roll and pitch), they vary significantly in accuracy, capabilities, and ideal applications:
Inclinometer: Best suited for basic static tilt measurements. It is ideal for stationary or slow-moving applications where simplicity and low cost are primary concerns
Roll & Pitch Sensor: Capable of measuring roll and pitch angles in conditions with some motion. It is suitable for applications requiring dynamic orientation data but not high accuracy. However, it is not ideal for environments with rough sea conditions or rapid, complex motions (e.g., coupled movements).
VRU: Delivers high-precision roll and pitch measurements in dynamic and challenging environments, such as rough sea conditions. It is indispensable for applications requiring accurate attitude data, even in the presence of vibrations and complex, coupled motions in real sea conditions.
Choosing the right device depends on factors like dynamic performance, complexity, cost, and the specific requirements of your application.
Attitude and Heading Reference System (AHRS)
Definition: An Attitude and Heading Reference System (AHRS) is an integrated system that builds on the capabilities of a Roll & Pitch sensor, offering complete orientation information—roll, pitch, and yaw (heading). It achieves this by processing data from an IMU combined with additional sensors, such as magnetometers or, in some cases, a north-seeking gyro (commonly referred to as a gyrocompass). An AHRS features an onboard processing unit that calculates attitude (roll and pitch) and heading using sensor fusion algorithms such as Kalman filters. It integrates data from accelerometers, gyroscopes, and magnetometers, applying these algorithms to correct sensor errors, biases, and drift, ensuring accurate and drift-free orientation. While combining sensors in an AHRS addresses many limitations, magnetic disturbances can still pose a challenge. Both internal and external disturbances may affect the magnetometer, potentially impacting the accuracy of the system's heading estimation.
Key Features:
Sensors Included: Accelerometers, gyroscopes, and magnetometers.
Output: Orientation data (roll, pitch, and heading).
Functionality: Incorporates an onboard processing unit that utilizes sensor fusion algorithms (e.g., Kalman filters) to accurately compute orientation data.
Applications: Provides real-time, accurate orientation data for navigation and control systems.
Example Uses:
Maritime navigation systems
Ship Stabilization Systems
Aviation and drones
Motion Reference Unit (MRU)
Definition: A motion Reference Unit (MRU) is a specialized device and self-contained sensor that measures motion in all six degrees of freedom (DoF): Roll, Pitch, Yaw, Surge, Sway and Heave. The six DoF positions, velocities and accelerations are measured by the MRU using high-end gyroscopes and accelerometers (and sometimes magnetometers) together with advanced fusion algorithms. An MRU has high accuracy Roll and Pitch measurements, and measures oscillatory Heave, Surge and Sway motions for wave periods at sea. Linear motions with very long periods, or steps, cannot be measured by an MRU as it assumes a 0 mean heave position.
Key Features:
Sensors Included: High-end accelerometers and gyroscopes (and sometimes magnetometers).
Output: Provides real-time, high-accuracy motion data in all six DoF (roll, pitch, yaw, heave, surge, and sway).
Functionality: Optimized for dynamic environments, especially in marine settings, to provide accurate motion measurements under real sea conditions.
Applications: Provides real-time, high-accuracy motion data in alle 6 DoF for monitoring, control, or instrument compensations within the maritime, offshore, and subsea sector.
Example Uses:
Monitoring systems: Ship motion monitoring, helideck monitoring, structural monitoring, and riser monitoring.
Control systems: Active heave compensation in cranes, winches, and gangways.
Instruments compensation: Hydrography, wave radar, dynamic position, and wind lidar.
Gyrocompass
Definition: A gyrocompass is a navigational instrument that determines true north by utilizing high-precision gyroscopes, such as fiber optic gyros (FOG), ring laser gyros (RLG), or traditional spinning-mass gyros. Unlike magnetic compasses, which are susceptible to magnetic anomalies, a gyrocompass identifies true north based on the Earth's rotation, aligning with the geographic North Pole. It provides constant and reliable heading information without dependence on external signals like GPS, which can be disrupted or unavailable. The incorporation of advanced, high-precision gyroscopes contributes to the high cost of gyrocompasses.
A gyrocompass has a settling time to stabilize and accurately align with true north after initialization, which can vary depending on the system's design and environmental factors. At high northern or southern latitudes, the gyrocompass faces challenges due to the diminishing horizontal component of Earth's rotation, making it harder to align to true north.
Key Features:
Sensors Included: High-precision gyroscopes and accelerometers
Output: True north heading information.
Functionality: Uses gyroscopic principles and Earth's rotation to maintain a true north orientation.
Applications: Provides stable and accurate heading information, essential for navigation in vessels and aircraft.
Example Uses:
Maritime Navigation systems: Ships, autonomous underwater vehicles (AUV) and working class Remotely Operated Vehicles (ROV).
Alignment of Subsea installations
Aircraft heading reference systems
GNSS-Aided Inertial Navigation System (GNSS/INS)
Definition: An Inertial Navigation System (INS) is a navigation device that utilizes motion sensors (accelerometers), rotation sensors (gyroscopes), and advanced sensor fusion algorithms to continuously calculate position, orientation, and velocity through dead reckoning, without relying on external references. When integrated with Global Navigation Satellite Systems (GNSS), forming an INS/GNSS system, it provides absolute position, orientation, and velocity in six degrees of freedom (6 DoF) by combining data from both systems. The GNSS calibrates the INS, enabling faster position and angle updates compared to GNSS alone. In other words, the INS fills in the gaps between GPS positions measurements. Moreover, if the GNSS signal is lost, the INS can maintain position and orientation calculations, although accuracy may drift over time, depending on the duration without GNSS signals.
Key Features:
Sensors Included: High-end accelerometers and gyroscopes, and GNSS receiver.
Output: Absolute position, orientation, and velocity (in 6 DoF).
Functionality: The INS determines absolute position, orientation, and velocity in six degrees of freedom (6 DoF), while the GNSS provides periodic position updates to correct any drift in the INS.
Applications: Used where continuous and accurate position and navigation data are required.
Example Uses:
Hydrography: Accurate motion compensation of Sonars, ensuring high-quality seafloor maps.
Gangways and 3D-motion compensated cranes: Where absolute position is needed.
Comparative Summary of All Sensors: | IMU | Inclinometer | Roll & Pitch Sensor | VRU | AHRS | MRU | Gyrocompass | GNSS/INS
Measured Parameters: Vary from raw linear acceleration (IMU) to comprehensive motion data (MRU and GNSS/INS).
Dynamic Performance: Differences in sensor sophistication determine accuracy levels in dynamic environments.
Applications: Range from simple leveling tasks (Inclinometer) to advanced marine (GNSS/INS).
Complexity and Cost: Higher complexity typically leads to increased cost, as seen in MRUs, Gyrocompasses, and GNSS/INS systems.
Note: Selecting the appropriate sensor depends on the required measurements, environmental conditions, and budget.
Use an IMU when you need raw acceleration and rotational data.
Use an Inclinometer for simple, static tilt measurements.
Use a Roll & Pitch Sensor for lower-level attitude measurements in a dynamic setting.
Use a VRU when precise roll and pitch measurements are needed in a dynamic environment.
Use an AHRS for roll, pitch, and heading data.
Use an MRU for comprehensive motion data in dynamic marine environments.
Use a Gyrocompass for accurate true north heading.
Use a GNSS/INS for continuous, absolute position, velocity, and orientation data.

Question 31

What are the main error sources in motion measurements at sea?

Accepted Answer

Measuring motion accurately in dynamic offshore environments presents several challenges. Error sources can generally be categorized into sensor-intrinsic factors, environmental influences, algorithmic processing limitations, and installation inaccuracies.
Intrinsic sensor errors include noise inherent in the MEMS accelerometers and gyroscopes, bias instability (drift over time), scale factor inaccuracies, and sensitivity to temperature variations. The performance of the sensor fusion algorithm, typically a Kalman filter, which combines data from multiple sensors to estimate orientation and motion (Roll, Pitch, Yaw, Heave, Surge, Sway), is also critical. Imperfections in the algorithm model or tuning can introduce errors, especially during complex or irregular motions.
External and installation-related factors significantly impact accuracy. Misalignment of the sensor unit relative to the vessel or structure's coordinate frame, insecure mounting leading to vibration amplification, and inaccurate lever arm compensation (calculating motion at a point of interest away from the sensor) are common sources of error. For systems aided by GNSS, errors in satellite positioning data (e.g., due to signal blockage or multipath) can also affect overall accuracy.
Norwegian Subsea addresses these challenges by utilizing state-of-the-art MEMS sensors and advanced, rigorously validated sensor fusion algorithms. Our MRUs undergo comprehensive calibration, ensuring high accuracy (with ±0.05°, ±0.02°, and ±0.01° Roll/Pitch options) and stability, often eliminating the need for recalibration during the product's lifecycle. Robust mechanical design and clear installation guidelines further minimize environmental and installation-related errors, delivering reliable 6DoF data for demanding marine and offshore applications.

Question 32

What are the specifications of Norwegian Subsea MRUs?

Accepted Answer

Norwegian Subsea offers a range of high-performance Motion Reference Units (MRUs) designed for diverse marine, subsea, and offshore applications. All our MRUs measure 6 Degrees of Freedom (6DoF): Roll, Pitch, Heave, Yaw, Surge, and Sway. They are available in three distinct accuracy tiers based on Roll & Pitch performance: Series 3000 (±0.05°), Series 6000 (±0.02°), and Series 9000 (±0.01°). Standard Heave accuracy across the MRU range is 5.0 cm or 5.0%. For applications requiring only orientation data, Inclinometer/VRU versions are available, offering the same high Roll & Pitch accuracy without Heave/Surge/Sway output. Our sensors are designed for easy integration with broad protocol support and user-friendly web-based configuration. 
The MRU Compact is designed for indoor or control box installations, featuring an IP65-rated anodized aluminum enclosure. It is exceptionally compact (12.0 x 5.7 x 5.3 cm) and lightweight (0.55 kg). Connectivity is simplified via an RJ45 or RJ50 connector supporting Ethernet communication (UDP, Modbus TCP, Ethernet/IP) with passive Power over Ethernet (PoE, 9-36V DC). The operating temperature range is -40 to +85°C.
The MRU Marine is built for on-deck use (IP68) or shallow water deployment up to 50m (MRU Marine SW version). Housed in anodized aluminum (15.4 x 8.6 x 6.7 cm, 1.2 kg), it offers versatile connectivity through a 16-pin marine connector or SubConn options, providing Ethernet, RS-232, RS-485, and PPS ports. Supported protocols include UDP, Modbus TCP/RTU, and Ethernet/IP. It operates from -40 to +85°C and is often used for cost-effective retrofitting of older systems. An optional junction box provides analog outputs (+/-10V, 4-20mA).
For deep-water applications, the MRU Subsea features a robust Titanium housing rated for 6000m water depth. It utilizes an 8-pin SubConn wet-mateable connector providing Ethernet and serial (RS-232 or RS-485) communication (UDP, Modbus TCP, Ethernet/IP). Despite its depth rating, it remains compact (7.6 x 7.6 x 16.5 cm) and weighs 1.6 kg. The operating temperature range is -40 to +70°C.
The MRU Ex is specifically designed for hazardous areas, certified to II 2 G Ex d IIC Gb. It features a durable AISI 316L stainless steel enclosure (19 x 19 x 20.5 cm, 15 kg excluding cable) and connects via a pigtail cable available in custom lengths. It provides Ethernet and serial (RS-232 or RS-485) communication options supporting UDP, Modbus TCP/RTU, and Ethernet/IP protocols. The operating temperature range is -20 to +40°C.
Start here to get the exact specifications for your required products.

Question 33

What is a GNSS-Aided Inertial Navigation System (GNSS/INS)?

Accepted Answer

When navigating the complexities of motion sensing and navigation systems, it is essential to understand the distinctions between devices and systems like Inertial Measurement Units (IMUs), Inclinometers, Roll & Pitch sensors, Vertical Reference Units (VRU), Attitude and Heading Reference Systems (AHRS), Motion Reference Units (MRUs), Gyrocompasses, and GNSS-Aided Inertial Navigation Systems (GNSS/INS). Each serves specific purposes and offers different levels of functionality, accuracy, and application scope.
GNSS-Aided Inertial Navigation System (GNSS/INS)
Definition: An Inertial Navigation System (INS) is a navigation device that utilizes motion sensors (accelerometers), rotation sensors (gyroscopes), and advanced sensor fusion algorithms to continuously calculate position, orientation, and velocity through dead reckoning, without relying on external references. When integrated with Global Navigation Satellite Systems (GNSS), forming an INS/GNSS system, it provides absolute position, orientation, and velocity in six degrees of freedom (6 DoF) by combining data from both systems. The GNSS calibrates the INS, enabling faster position and angle updates compared to GNSS alone. In other words, the INS fills in the gaps between GPS positions measurements. Moreover, if the GNSS signal is lost, the INS can maintain position and orientation calculations, although accuracy may drift over time, depending on the duration without GNSS signals.
Key Features:
Sensors Included: High-end accelerometers and gyroscopes, and GNSS receiver.
Output: Absolute position, orientation, and velocity (in 6 DoF).
Functionality: The INS determines absolute position, orientation, and velocity in six degrees of freedom (6 DoF), while the GNSS provides periodic position updates to correct any drift in the INS.
Applications: Used where continuous and accurate position and navigation data are required.
Example Uses:
Hydrography: Accurate motion compensation of Sonars, ensuring high-quality seafloor maps.
Gangways and 3D-motion compensated cranes: Where absolute position is needed.
Selecting the appropriate sensor depends on the required measurements, environmental conditions, and budget.
Use an IMU when you need raw acceleration and rotational data.
Use an Inclinometer for simple, static tilt measurements.
Use a Roll & Pitch Sensor for lower-level attitude measurements in a dynamic setting.
Use a VRU when precise roll and pitch measurements are needed in a dynamic environment.
Use an AHRS for roll, pitch, and heading data.
Use an MRU for comprehensive motion data in dynamic marine environments.
Use a Gyrocompass for accurate true north heading.
Use a GNSS/INS for continuous, absolute position, velocity, and orientation data.

Question 34

What is a Gyrocompass?

Accepted Answer

When navigating the complexities of motion sensing and navigation systems, it is essential to understand the distinctions between devices and systems like Inertial Measurement Units (IMUs), Inclinometers, Roll & Pitch sensors, Vertical Reference Units (VRU), Attitude and Heading Reference Systems (AHRS), Motion Reference Units (MRUs), Gyrocompasses, and GNSS-Aided Inertial Navigation Systems (GNSS/INS). Each serves specific purposes and offers different levels of functionality, accuracy, and application scope.
Gyrocompass
Definition: A gyrocompass is a navigational instrument that determines true north by utilizing high-precision gyroscopes, such as fiber optic gyros (FOG), ring laser gyros (RLG), or traditional spinning-mass gyros. Unlike magnetic compasses, which are susceptible to magnetic anomalies, a gyrocompass identifies true north based on the Earth's rotation, aligning with the geographic North Pole. It provides constant and reliable heading information without dependence on external signals like GPS, which can be disrupted or unavailable. The incorporation of advanced, high-precision gyroscopes contributes to the high cost of gyrocompasses.
A gyrocompass has a settling time to stabilize and accurately align with true north after initialization, which can vary depending on the system's design and environmental factors. At high northern or southern latitudes, the gyrocompass faces challenges due to the diminishing horizontal component of Earth's rotation, making it harder to align to true north.
Key Features:
Sensors Included: High-precision gyroscopes and accelerometers
Output: True north heading information.
Functionality: Uses gyroscopic principles and Earth's rotation to maintain a true north orientation.
Applications: Provides stable and accurate heading information, essential for navigation in vessels and aircraft.
Example Uses:
Maritime Navigation systems: Ships, autonomous underwater vehicles (AUV) and working class Remotely Operated Vehicles (ROV).
Alignment of Subsea installations
Aircraft heading reference systems
Selecting the appropriate sensor depends on the required measurements, environmental conditions, and budget.
Use an IMU when you need raw acceleration and rotational data.
Use an Inclinometer for simple, static tilt measurements.
Use a Roll & Pitch Sensor for lower-level attitude measurements in a dynamic setting.
Use a VRU when precise roll and pitch measurements are needed in a dynamic environment.
Use an AHRS for roll, pitch, and heading data.
Use an MRU for comprehensive motion data in dynamic marine environments.
Use a Gyrocompass for accurate true north heading.
Use a GNSS/INS for continuous, absolute position, velocity, and orientation data.

Question 35

What is a Motion Reference Unit?

Accepted Answer

What is a Motion Reference Unit (MRU)?
A motion Reference Unit (MRU) is a device that measures motion in all six degrees of freedom (DoF): roll, pitch, yaw & surge, sway and heave. The six DoF positions, velocities and accelerations are measured by the MRU using high-end gyroscopes and accelerometers (and sometimes magnetometers) and processed by advanced sensor fusion algorithms. 
What is the limitation of an MRU?
An MRU has high accuracy Roll and Pitch measurements, and measures oscillatory Heave, Surge and Sway motions for wave periods at sea. Linear motions with very long periods, or steps, cannot be measured by an MRU as it assumes a 0 mean heave position.
What are the key features of an MRU?
Sensors Included: High-end accelerometers and gyroscopes (and sometimes magnetometers).
Output: Provides real-time, high-accuracy motion data in all six DoF (roll, pitch, yaw, heave, surge, and sway).
Functionality: Optimized for dynamic environments, especially in marine settings, to provide accurate motion measurements under real sea conditions.
Applications: Provides real-time, high-accuracy motion data in alle 6 DoF for monitoring, control, or instrument compensations within the maritime, offshore, and subsea sector.
Where is an MRU used?
Monitoring systems: Ship motion monitoring, helideck monitoring, structural monitoring, and riser monitoring.
Control systems: Active heave compensation in cranes, winches, and gangways.
Instruments compensation: Hydrography, wave radar, dynamic position, and wind lidar.

Question 36

What is a Roll and Pitch Sensor?

Accepted Answer

When navigating the complexities of motion sensing and navigation systems, it is essential to understand the distinctions between devices and systems like Inertial Measurement Units (IMUs), Inclinometers, Roll & Pitch sensors, Vertical Reference Units (VRU), Attitude and Heading Reference Systems (AHRS), Motion Reference Units (MRUs), Gyrocompasses, and GNSS-Aided Inertial Navigation Systems (GNSS/INS). Each serves specific purposes and offers different levels of functionality, accuracy, and application scope.
Roll & Pitch Sensor
Definition: A Roll & Pitch sensor, also known as a dynamic inclinometer, is a device designed to measure an object's attitude (roll and pitch). It incorporates gyroscopes to enhance its dynamic measurement capabilities. Compared to an inclinometer, a Roll & Pitch sensor is more advanced, featuring an onboard processing unit that performs basic sensor fusion by integrating gyroscope data. While it offers improved performance over inclinometers (compensating for errors caused by non-gravitational accelerations) it is less accurate than a VRU or MRU due to the use of lower-quality accelerometers, gyroscopes, and less sophisticated sensor fusion algorithms in its onboard processing unit.
Key Features:
Sensors Included: Accelerometers and gyroscopes.
Output: Measures rotation around two axes, roll and pitch, with enhanced dynamic measurement capabilities compared to an inclinometer
Functionality: It delivers improved attitude measurements (roll and pitch) in dynamic conditions compared to an inclinometer, but falls short of the accuracy achieved by a VRU or MRU.
Applications: Used in less critical systems that do not require highly accurate roll and pitch measurements in dynamic conditions. These systems can tolerate some movement and vibration, but their accuracy decreases as movement and vibration increase.
Example Uses:
Orientation Monitoring: For smaller vessels, barges, and buoys.
Control Systems Feedback: For stabilizer fins in smaller boats.
Stabilizing Platforms: Such as cameras and antennas.
Selecting the appropriate sensor depends on the required measurements, environmental conditions, and budget.
Use an IMU when you need raw acceleration and rotational data.
Use an Inclinometer for simple, static tilt measurements.
Use a Roll & Pitch Sensor for lower-level attitude measurements in a dynamic setting.
Use a VRU when precise roll and pitch measurements are needed in a dynamic environment.
Use an AHRS for roll, pitch, and heading data.
Use an MRU for comprehensive motion data in dynamic marine environments.
Use a Gyrocompass for accurate true north heading.
Use a GNSS/INS for continuous, absolute position, velocity, and orientation data.

Question 37

What is a VRU and how does it differ from an MRU?

Accepted Answer

What is a VRU and how does it differ from an MRU?
A Vertical Reference Unit (VRU) is an advanced device that measures the attitude of an object, specifically its Roll and Pitch. Like Motion Reference Units (MRUs), VRUs use high‑end accelerometers and gyroscopes combined with advanced sensor fusion algorithms to deliver accurate readings even in dynamic and irregular motion conditions.
VRUs are used in applications that require real‑time, high‑accuracy Roll and Pitch data to ensure effective control system response. They provide the precise angular measurements needed for motion compensation of GNSS antennas in dynamic positioning (DP) systems, stabilising fins on vessels, and advanced instrument compensation within monitoring and control systems where only roll and pitch data is required. In all these cases, where Heave, Surge and Sway data are not required, a VRU offers a highly reliable and cost‑effective solution.
VRUs for Instrument Compensation
Instrument compensation relies on accurate Roll and Pitch inputs to correct sensor readings or stabilise equipment affected by vessel or platform movement. A VRU is well suited for this task because it provides stable, high‑precision angular data in real sea conditions, including scenarios with irregular, coupled motions. The accuracy and stability of the VRU allow connected systems to react effectively, ensuring consistent and reliable performance of instruments such as GNSS antennas, stabilising fins, or other devices that depend on precise attitude information.
Norwegian Subsea offers high‑performance VRU versions across the entire MRU product line, including Compact, Marine, Subsea and Ex models. These units deliver the same exceptional Roll and Pitch accuracy as the full MRU range, available in ±0.05°, ±0.02° or ±0.01° depending on the series. By focusing solely on angular measurements, the VRU versions provide a compact, dependable and cost‑efficient solution for advanced motion control applications where full 6DoF data is unnecessary.
MRU vs VRU: Key Differences
Norwegian Subsea’s VRUs and MRUs share the same high‑quality internal hardware, rugged construction and advanced sensor fusion algorithms. Both devices calculate coupled motion across all six degrees of freedom (Roll, Pitch, Yaw, Surge, Sway and Heave). The key difference lies in the output.
A VRU is essentially an MRU configured to output only Roll and Pitch data. It retains all the performance benefits of the MRU but excludes Heave, Surge and Sway outputs, making it a more cost‑effective option for applications where only attitude measurements are required.
All MRU models from Norwegian Subsea (Compact, Marine, Subsea and Ex) are available in VRU versions. These provide the same high accuracy and long‑term stability as the full MRUs and are ideal for applications focused solely on tilt or orientation.

Question 38

What is a Vertical Reference Unit (VRU)?

Accepted Answer

A VRU is an advanced device that measures the attitude (roll and pitch) of an object using high-end accelerometers and gyroscopes, combined with advanced sensor fusion algorithms. VRUs provide very accurate roll and pitch measurements, even in dynamic environments.
In applications requiring real-time, high-accuracy roll and pitch data the VRU's precise Roll and Pitch data allows control systems to react effectively. For example, motion compensation of GNSS antenna for dynamic positioning (DP-systems), stabilizing fins on vessels or advanced Instrument compensation - Used in monitoring and control systems where only roll and pitch data is needed.
Norwegian Subsea offers high-performance VRU versions across our entire MRU product line (Compact, Marine, Subsea, Ex). These units deliver the same exceptional Roll and Pitch accuracy (available in ±0.05°, ±0.02°, or ±0.01° depending on the series) as our full 6DoF MRUs, even in complex, irregular motion conditions. By focusing solely on angular measurements, our VRU versions provide a highly reliable and cost-effective solution for advanced motion control applications where Heave, Surge, and Sway data are not required.
Comparison to MRU: A VRU shares similarities with an MRU, incorporating real-time, high-precision accelerometers and gyroscopes along with advanced sensor fusion algorithms in a single self-contained unit. Both devices calculate coupled motions across all six degrees of freedom (DoF): Roll, Pitch, Yaw, Surge, Sway, and Heave. However, while the MRU outputs data for all six DoF, the VRU is limited to providing roll and pitch data only. This restricted output makes the VRU generally more cost-effective than the MRU.

Question 39

What is an Attitude and Heading Reference System (AHRS)?

Accepted Answer

When navigating the complexities of motion sensing and navigation systems, it is essential to understand the distinctions between devices and systems like Inertial Measurement Units (IMUs), Inclinometers, Roll & Pitch sensors, Vertical Reference Units (VRU), Attitude and Heading Reference Systems (AHRS), Motion Reference Units (MRUs), Gyrocompasses, and GNSS-Aided Inertial Navigation Systems (GNSS/INS). Each serves specific purposes and offers different levels of functionality, accuracy, and application scope.
Attitude and Heading Reference System (AHRS)
Definition: An Attitude and Heading Reference System (AHRS) is an integrated system that builds on the capabilities of a Roll & Pitch sensor, offering complete orientation information—roll, pitch, and yaw (heading). It achieves this by processing data from an IMU combined with additional sensors, such as magnetometers or, in some cases, a north-seeking gyro (commonly referred to as a gyrocompass). An AHRS features an onboard processing unit that calculates attitude (roll and pitch) and heading using sensor fusion algorithms such as Kalman filters. It integrates data from accelerometers, gyroscopes, and magnetometers, applying these algorithms to correct sensor errors, biases, and drift, ensuring accurate and drift-free orientation. While combining sensors in an AHRS addresses many limitations, magnetic disturbances can still pose a challenge. Both internal and external disturbances may affect the magnetometer, potentially impacting the accuracy of the system's heading estimation.
Key Features:
Sensors Included: Accelerometers, gyroscopes, and magnetometers.
Output: Orientation data (roll, pitch, and heading).
Functionality: Incorporates an onboard processing unit that utilizes sensor fusion algorithms (e.g., Kalman filters) to accurately compute orientation data.
Applications: Provides real-time, accurate orientation data for navigation and control systems.
Example Uses:
Maritime navigation systems
Ship Stabilization Systems
Aviation and drones
Selecting the appropriate sensor depends on the required measurements, environmental conditions, and budget.
Use an IMU when you need raw acceleration and rotational data.
Use an Inclinometer for simple, static tilt measurements.
Use a Roll & Pitch Sensor for lower-level attitude measurements in a dynamic setting.
Use a VRU when precise roll and pitch measurements are needed in a dynamic environment.
Use an AHRS for roll, pitch, and heading data.
Use an MRU for comprehensive motion data in dynamic marine environments.
Use a Gyrocompass for accurate true north heading.
Use a GNSS/INS for continuous, absolute position, velocity, and orientation data.

Question 40

What is an Inclinometer?

Accepted Answer

When navigating the complexities of motion sensing and navigation systems, it is essential to understand the distinctions between devices and systems like Inertial Measurement Units (IMUs), Inclinometers, Roll & Pitch sensors, Vertical Reference Units (VRU), Attitude and Heading Reference Systems (AHRS), Motion Reference Units (MRUs), Gyrocompasses, and GNSS-Aided Inertial Navigation Systems (GNSS/INS). Each serves specific purposes and offers different levels of functionality, accuracy, and application scope.
Inclinometer
Definition: An Inclinometer, also known as a tilt or level sensor, measures the angle of inclination or tilt of an object with respect to gravity. It determines the angle between the sensitive axis of the device and Earth's vertical direction (gravity vector). It is important to note that an Inclinometer only measures static angles, and therefore cannot provide accurate data in dynamic conditions involving movements or vibrations. Inclinometers are generally less expensive due to simpler components.
Key Features:
Sensors Included: Primarily uses accelerometers to detect gravity's direction.
Output: Measures static angles accurately when the device is stationary.
Functionality: Determines roll and pitch in static conditions, but lacks accuracy in dynamic settings.
Applications: Used in systems that are stationary or moving very slowly and do not experience vibrations.
Example Uses:
Simple Leveling: Leveling equipment that is not in motion.
Monitoring Tilt: For structures, bridges, and buildings.
Measuring Slope: Useful for landslide-prone areas.
Selecting the appropriate sensor depends on the required measurements, environmental conditions, and budget.
Use an IMU when you need raw acceleration and rotational data.
Use an Inclinometer for simple, static tilt measurements.
Use a Roll & Pitch Sensor for lower-level attitude measurements in a dynamic setting.
Use a VRU when precise roll and pitch measurements are needed in a dynamic environment.
Use an AHRS for roll, pitch, and heading data.
Use an MRU for comprehensive motion data in dynamic marine environments.
Use a Gyrocompass for accurate true north heading.
Use a GNSS/INS for continuous, absolute position, velocity, and orientation data.

Question 41

What is an Inertial Measurement Unit (IMU) and how does it work?

Accepted Answer

An Inertial Measurement Unit (IMU) is an electronic device that uses accelerometers and gyroscopes, and sometimes magnetometers, to measure and report an object's linear acceleration and angular velocity along three orthogonal axes (X, Y and Z). It provides raw data outputs without performing any internal processing. As such, IMUs do not calculate attitude (roll and pitch), velocity or position on their own. Any application requiring this type of information must perform additional data processing externally.
Applications of IMUs
IMUs are ideal for use in systems where raw motion data will be processed by external software or hardware. Typical examples include basic navigation systems, robotics control, and camera or drone stabilisation. They are most suitable where cost, simplicity, and flexibility for custom processing are key considerations.
What Sets MRUs Apart
In contrast, a Motion Reference Unit (MRU) is a more advanced, self-contained device. It combines MEMS-based accelerometers and gyroscopes—similar to an IMU—with advanced sensor fusion algorithms. This onboard processing enables the MRU to output accurate, real-time orientation (Roll, Pitch, Yaw) and motion data (Heave, Surge and Sway) in all six degrees of freedom (6DoF).
Norwegian Subsea MRUs are designed and tested for performance in marine, offshore and subsea environments. They provide precise, application-ready motion data, essential for dynamic positioning, hydrographic surveying, heave compensation and related tasks. The high accuracy of these sensors (with options down to ±0.01° Roll and Pitch) is maintained over time, and recalibration is generally unnecessary.
Key Differences Between IMUs and MRUs
While both IMUs and MRUs measure motion using accelerometers and gyroscopes, the key difference lies in how the data is processed and used. IMUs supply raw data only, without any internal algorithms or computation. They are best suited for systems where external software handles the data interpretation and conversion into usable outputs such as attitude or position.
MRUs, on the other hand, process the sensor data internally using advanced fusion algorithms. They output real-time, high-accuracy measurements across all six degrees of freedom. This makes MRUs highly effective in environments with complex and irregular motion, such as marine or offshore applications, where reliable and immediate data is essential. Additionally, MRUs typically require less ongoing calibration and can be integrated directly into control and monitoring systems without further processing.
Selecting the Right Device
Use an IMU when raw data and external processing flexibility are required.
Use an MRU when accurate, real-time motion data is needed, especially in dynamic marine environments.

Question 42

What is an Inertial Measurement Unit (IMU)?

Accepted Answer

When navigating the complexities of motion sensing and navigation systems, it is essential to understand the distinctions between devices and systems like Inertial Measurement Units (IMUs), Inclinometers, Roll & Pitch sensors, Vertical Reference Units (VRU), Attitude and Heading Reference Systems (AHRS), Motion Reference Units (MRUs), Gyrocompasses, and GNSS-Aided Inertial Navigation Systems (GNSS/INS). Each serves specific purposes and offers different levels of functionality, accuracy, and application scope.
Inertial Measurement Unit (IMU)
Definition: An IMU is an electronic device that measures and reports an object's linear acceleration and angular velocity using a combination of accelerometers and gyroscopes along three orthogonal axes (X, Y, and Z). It achieves this by utilizing a combination of sensors—primarily accelerometers and gyroscopes, and occasionally magnetometers. IMUs only provide raw data from their sensors and do not include "smart" features such as processing (e.g., Kalman Filter). The IMU cannot provide attitude (roll and pitch), velocity, or position without external processing.
Key Features:
Sensors Included: Accelerometers (measure linear acceleration) and gyroscopes (measure angular rate).
Output: Raw sensor data on acceleration and rotation rates.
Functionality: Provides fundamental motion data without further processing to determine attitude, velocity, or position.
Applications: Used in systems where raw inertial data is sufficient or will be processed further by an external system.
Example Uses:
Basic navigation systems: Where external processing is available.
Robotics: For control algorithms requiring raw motion data.
Stabilization of cameras and drones.
Selecting the appropriate sensor depends on the required measurements, environmental conditions, and budget.
Use an IMU when you need raw acceleration and rotational data.
Use an Inclinometer for simple, static tilt measurements.
Use a Roll & Pitch Sensor for lower-level attitude measurements in a dynamic setting.
Use a VRU when precise roll and pitch measurements are needed in a dynamic environment.
Use an AHRS for roll, pitch, and heading data.
Use an MRU for comprehensive motion data in dynamic marine environments.
Use a Gyrocompass for accurate true north heading.
Use a GNSS/INS for continuous, absolute position, velocity, and orientation data.

Question 43

What is an MRU vs VRU?

Accepted Answer

Motion Reference Unit (MRU)
Definition: A motion Reference Unit (MRU) is a specialized device and self-contained sensor that measures motion in all six degrees of freedom (DoF): Roll, Pitch, Yaw, Surge, Sway and Heave. The six DoF positions, velocities and accelerations are measured by the MRU using high-end gyroscopes and accelerometers (and sometimes magnetometers) together with advanced fusion algorithms. An MRU has high accuracy Roll and Pitch measurements, and measures oscillatory Heave, Surge and Sway motions for wave periods at sea. Linear motions with very long periods, or steps, cannot be measured by an MRU as it assumes a 0 mean heave position.
Vertical Reference Unit (VRU)
Definition: A VRU is an advanced device that measures the attitude (roll and pitch) of an object using high-end accelerometers and gyroscopes, combined with advanced sensor fusion algorithms. VRUs provide accurate roll and pitch measurements, even in dynamic environments.
Comparison of VRU with MRU: When you acquire a Norwegian Subsea VRU, this is essentially the same as the MRU. The only difference is that the VRU outputs roll and pitch only. It still has all the advantages of the MRU. 
If you need lateral motions (heave, surge and sway), either position, velocity or accelerations, you will need to use the MRU.

Question 44

What is the HS/commodity code for Norwegian Subsea MRUs?

Accepted Answer

The HS/commodity code for Norwegian Subsea MRUs is 9015.80.0000
A commodity code, also known as a Harmonized System (HS) code or tariff code, is a standardized numerical method used to classify traded products for customs purposes. It's crucial for calculating import duties, taxes, and identifying potential trade restrictions. These codes are globally recognized and consist of a specific number of digits, typically eight for exports and ten for imports.

Question 45

What is the best MRU for ROV operations?

Accepted Answer

For Remotely Operated Vehicle (ROV) operations, the Norwegian Subsea MRU Subsea is the ideal solution. Its design specifically caters to the demanding conditions of subsea environments.
The MRU Subsea features a robust titanium housing depth-rated to 6000 meters. The titanium casing is very compact and depth-rated to
6000 m. The small size and footprint make it easy to install
almost anywhere. It is equipped with a Subconn wet-mateable
connector and is easy integration onto various ROV platforms, where space and weight are often critical constraints.
It supports both Ethernet (UDP, Modbus TCP, Ethernet/IP) and serial (RS-232 or RS-485) communication, offering flexible interfacing with ROV control systems. We provide a wide range of standard and customizable data protocols in ASCII or binary formats.
Like all our MRUs, the Subsea model utilizes advanced sensor fusion algorithms and state-of-the-art MEMS technology to deliver high-accuracy motion data (Roll, Pitch, Heave, Yaw, Surge, Sway). It is available in our standard accuracy tiers (±0.05°, ±0.02°, ±0.01° Roll/Pitch) to meet diverse operational requirements. An Inclinometer/VRU version is also available if full 6DoF data is not required.

Question 46

What is the best MRU for dynamic positioning?

Accepted Answer

A Motion Reference Unit (MRU) or a Vertical Reference Unit (VRU) is an important component in Dynamic Positioning (DP) systems to providing reliable and precise roll and pitch measurements. Norwegian Subsea Motion Reference Units (MRUs) are engineered for high accuracy and reliability, making them excellent choices for Dynamic Positioning (DP) systems. Our MRUs utilize advanced sensor fusion algorithms and state-of-the-art MEMS technology to provide precise 6DoF motion data (Roll, Pitch, Heave, Yaw, Surge, Sway) crucial for motion compensation of GNSS antenna and other position reference systems.

The NORSUB MRU can be configured with industry-standard protocols, enabling easy interfacing with existing DP systems or serving as a replacement for other MRUs/VRUs. The MRU/VRU can be installed in any direction. The configuration software allows for convenient setup of the remote monitoring point, specifically the GNSS antenna location.

To accommodate various installation requirements, the NORSUB MRU can be supplied with either an industrial junction box or a pigtail cable for integration into existing systems. This ensures compatibility and ease of implementation in different vessel setups.
It is also frequently used as a cost-effective, high-performance drop-in replacement (retrofit) for older MRUs/VRUs on existing DP systems, often without the need for recalibration.

Question 47

What is the difference between MEMS-based and FOG-based MRUs?

Accepted Answer

Motion Reference Units (MRUs) primarily differ based on the type of gyroscope technology they employ. Norwegian Subsea utilizes advanced Micro-Electro-Mechanical Systems (MEMS) technology in all our MRUs.
MEMS sensors are solid-state devices manufactured using techniques similar to those for integrated circuits. This results in sensors that are inherently compact, lightweight, highly robust, and significantly more cost-effective than older technologies. They offer excellent reliability and low power consumption. Our engineers combine state-of-the-art MEMS sensors with sophisticated sensor fusion algorithms to achieve high accuracy and performance, validated in real-world sea conditions.
Fiber Optic Gyroscope (FOG) technology, conversely, uses the interference of light passing through a coil of optical fiber to measure rotation. While capable of high performance, FOG-based MRUs are typically larger, heavier, significantly more expensive, and can be more sensitive to shock and vibration compared to MEMS units. Historically, FOGs were often required for the highest accuracy applications, but advancements in MEMS technology and algorithms have closed this gap considerably.
The most advanced and expensive Fiber Optic Gyroscope (FOG) navigational instrument can determines true north. Unlike magnetic compasses, which are susceptible to magnetic anomalies, these instruments identifies true north based on the Earth's rotation, aligning with the geographic North Pole. It provides constant and reliable heading information without dependence on external signals like GPS, which can be disrupted or unavailable. These instruments has a settling time to stabilize and accurately align with true north after initialization, which can vary depending on the system's design and environmental factors. At high northern or southern latitudes, the gyrocompass faces challenges due to the diminishing horizontal component of Earth's rotation, making it harder to align to true north.
For the vast majority of marine, subsea, and offshore applications, modern MEMS-based MRUs like those from Norwegian Subsea provide exceptional performance (with Roll/Pitch accuracies from ±0.05° down to ±0.01°) that meets or exceeds operational requirements. They represent a superior value proposition, offering high accuracy, proven reliability, ease of integration, and significantly lower total cost of ownership compared to FOG-based systems, often without the need for recalibration.

Question 48

What is the difference between an Inclinometer, Roll & Pitch Sensor, and VRU?

Accepted Answer

Although an Inclinometer, Roll & Pitch Sensor, and VRU all measure attitude (roll and pitch), they vary significantly in accuracy, capabilities, and ideal applications:
Inclinometer: Best suited for basic static tilt measurements. It is ideal for stationary or slow-moving applications where simplicity and low cost are primary concerns
Roll & Pitch Sensor: Capable of measuring roll and pitch angles in conditions with some motion. It is suitable for applications requiring dynamic orientation data but not high accuracy. However, it is not ideal for environments with rough sea conditions or rapid, complex motions (e.g., coupled movements).
VRU: Delivers high-precision roll and pitch measurements in dynamic and challenging environments, such as rough sea conditions. It is indispensable for applications requiring accurate attitude data, even in the presence of vibrations and complex, coupled motions in real sea conditions.
Choosing the right device depends on factors like dynamic performance, complexity, cost, and the specific requirements of your application.
At Norwegian Subsea, all our MRU models (Compact, Marine, Subsea, Ex) are designed to deliver high-accuracy 6DoF data. However, we also offer each of these models in an VRU version. This version provides the same exceptional Roll and Pitch accuracy as the full MRU, even in complex motion scenarios, but excludes the Heave, Surge, and Sway outputs. This makes the VRU version a highly accurate and cost-effective solution for applications where only precise tilt or orientation data is required.

Question 49

What is the difference between an MRU and an IMU?

Accepted Answer

While both Motion Reference Units (MRUs) and Inertial Measurement Units (IMUs) measure motion, they differ significantly in their processing and output. An IMU typically consists of accelerometers and gyroscopes that provide raw acceleration and angular rate data. IMUs only provide raw data from their sensors and do not include "smart" features such as processing (e.g., Kalman Filter). The IMU cannot provide attitude (roll and pitch), velocity, or position without external processing.
A Norwegian Subsea MRU, on the other hand, is a more sophisticated system. It utilizes data from internal MEMS-based accelerometers and gyroscopes, similar to an IMU, but integrates this data using advanced sensor fusion algorithms. These algorithms process the raw sensor readings to compute and output highly accurate, real-time measurements of orientation (Roll, Pitch, Yaw) and motion (Heave, Surge, Sway) in 6 Degrees of Freedom (6DoF).
Our MRUs are specifically designed, calibrated, and rigorously tested for the demanding conditions of marine, offshore, and subsea environments. They provide stabilized, reliable, and application-ready motion data crucial for tasks like dynamic positioning, heave compensation, hydrographic surveying, and more. Unlike basic IMUs, Norwegian Subsea MRUs deliver processed, high-accuracy motion information tailored for direct use in control and monitoring systems, often without needing recalibration during their operational life.

Question 50

What is the difference between the MRU Marine and MRU Marine SW?

Accepted Answer

What is the difference between the MRU Marine and MRU Marine SW?
The MRU Marine has an IP-68 rating, uses a Lemo 16-pin connector and has indicator lights.
The MRU Marine SW can be used in up to 50 m water depth and uses a SubConn 8-pin or 16-pin connector.
What is the difference between the MRU Marine SW and MRU Marine SW16?
The MRU Marine SW uses a SubConn 8-pin connector
It offers the following connection combinations:
Ethernet & RS-232
Ethernet & 2-wire RS-485
The MRU Marine SW16 uses a SubConn 16-pin connector
It has all available connections: Ethernet & RS-232 & 4-wire RS-485 & PPS

Question 51

What is the role of motion monitoring in offshore operations?

Accepted Answer

Motion monitoring is a cornerstone of safety, operational efficiency, and asset protection in the offshore industry. Whether on vessels, platforms, or subsea equipment, precise, real-time measurement of movement across all six degrees of freedom (Roll, Pitch, Heave, Yaw, Surge, and Sway) is essential for effective decision-making in complex, dynamic marine environments.
Applications in Offshore Oil and Gas
In offshore oil and gas, Norwegian Subsea’s Motion Reference Units (MRUs) deliver critical data for a range of essential systems:
Dynamic Positioning (DP): MRUs provide motion-compensated GNSS antenna inputs and other reference data for precise station-keeping.
Helideck Monitoring Systems (HMS): Accurate real-time Roll, Pitch, and Heave data ensure helicopter safety during landings and take-offs, in compliance with CAP 437 standards.
Active Heave Compensation (AHC): MRUs stabilise winches, cranes, and launch and recovery systems (LARS), supporting safe lifting operations in high sea states.
Motion Compensated Gangways: MRUs enable real-time motion correction for personnel transfer systems between vessels and offshore installations.
Wave Radars: MRUs provide accurate real-time heave data to compensate downward-looking radar measurements for precise sea state assessment.
Wind Lidar Buoys: MRUs correct for platform movement, ensuring stable wind data collection in offshore wind measurement campaigns.
Subsea Operations and Structural Monitoring
Motion monitoring also plays a vital role in subsea operations and structural health monitoring:
ROV and AUV Navigation: MRUs provide 6DoF motion data for accurate navigation and control of Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs) during tasks like inspections, installations, and surveys.
Subsea Surveys: Precise Roll, Pitch, and Heave compensation improves sonar data quality, reducing the need for repeated passes or extensive post-processing.
Riser and BOP Monitoring: MRUs are used to assess motion and integrity of critical subsea infrastructure including risers and Blowout Preventers (BOPs).
Structural Health Monitoring: MRUs track motion on both fixed and floating offshore structures, including platforms and offshore wind turbines.
Norwegian Subsea MRU Solutions
Norwegian Subsea offers a comprehensive range of high-performance MRUs tailored for offshore environments:
MRU Marine: IP68-rated for topside or shallow water applications.
MRU Subsea: Housed in a compact, robust titanium casing, depth-rated to 6000 metres, ideal for deepwater and subsea operations.
MRU Ex: Certified for hazardous environments, including areas with explosion risks.
All our MRUs are engineered with advanced MEMS technology and proprietary sensor fusion algorithms. They support standard industry communication protocols such as UDP, Modbus TCP, and Ethernet/IP, ensuring seamless integration with control systems. With no need for recalibration in most cases, they offer long-term reliability and cost efficiency, making them suitable for both new installations and retrofit projects.

Question 52

What motion sensor technologies are best for ROVs?

Accepted Answer

What motion sensor technologies are best for ROVs?
Remotely Operated Vehicles (ROVs) operate in demanding subsea environments that require accurate and reliable motion sensing for effective navigation, positioning, control, and data acquisition. Selecting the right motion sensor is crucial for these missions, and Norwegian Subsea provides specialised solutions tailored for these needs.
Motion Reference Units (MRUs) for ROV Applications
The MRU Subsea from Norwegian Subsea is designed specifically for underwater ROV operations. It features a robust, compact titanium housing rated to 6000 metres depth, ensuring durability in deep-sea conditions. MRUs provide full six degrees of freedom (6DoF) motion data: Roll, Pitch, Yaw, Heave, Surge, and Sway. This data is vital for vehicle stability, precise control of manipulators, accurate navigation, and integration with acoustic positioning systems like USBL or LBL.
MRUs use advanced MEMS technology and sophisticated sensor fusion algorithms validated in real sea conditions. They are available in three accuracy tiers: ±0.05° (Series 3000), ±0.02° (Series 6000), and ±0.01° (Series 9000) for Roll and Pitch, with standard Heave accuracy of 5.0 cm or 5.0%.
The MRU Subsea includes a reliable Subconn connector and supports both Ethernet (UDP, Modbus TCP, Ethernet/IP) and serial (RS-232 or RS-485) communications. Its compact size, low weight, and ability to be mounted in any orientation make it easy to integrate into space-constrained ROV designs.
High-Accuracy Motion Compensation for Sonar
Accurate seabed mapping requires motion compensation of sonar systems to prevent data distortion from ROV movement. MRUs provide real-time correction for Heave, Roll, and Pitch, improving target detection, eliminating data gaps, and reducing the need for post-processing.
Key benefits include:
Industry-leading motion accuracy under irregular and multidirectional wave conditions
Operation without dependency on GNSS or RTK systems
Cost-effective premium performance, suitable even for entry-level sonar systems
Maintenance-free operation, reducing downtime and operational cost
OEM versions are also available for direct integration within sonar systems.
Inclinometer/VRU Options
For applications where only roll and pitch data are required, Norwegian Subsea offers a VRU (Vertical Reference Unit) or Inclinometer version of the MRU. These versions retain the same high Roll and Pitch accuracy as the full MRU and provide a cost-effective solution when full 6DoF data is not needed.
Integration and Reliability
Integration is simplified with standard and custom ASCII or binary formats. All MRUs are supplied ready-calibrated and are designed for long-term reliability, often eliminating the need for recalibration throughout their operational life. This ensures low maintenance overheads and dependable performance in critical subsea missions.
Conclusion
With a blend of high accuracy, deep-water resilience, and flexible integration, the MRU Subsea is the optimal choice for ROV operators requiring robust motion sensing. Norwegian Subsea’s technology meets a wide range of requirements, from full 6DoF motion compensation to tilt-only applications, ensuring precision and reliability across ROV operations and seabed mapping tasks.

Question 53

What sensor systems are used in ROV seabed mapping?

Accepted Answer

Accurate seabed mapping using Remotely Operated Vehicles (ROVs) relies heavily on precise motion compensation of the sonar as well as sensing of the vehicle's movements. Why you need motion compensation of your sonar system?
Accurate Seafloor Mapping – Without proper motion
compensation, vessel movement can distort
sonar readings, leading to incorrect depth measurements
and unreliable bathymetric data.
Elimination of Data Gaps – Uncompensated motion
causes inconsistencies in sonar returns, resulting in
gaps or misaligned sonar images, making it harder
to detect underwater features.
Improved Target Detection – Whether for shipwreck
hunting, fish-finding, or subsea infrastructure
monitoring, stable motion data ensures clear,
high-resolution sonar images.
Avoid repeated surveys and extensive post
processing – High-quality motion compensation
ensures high-precision sonar results in real-time
Why Choose Norwegian Subsea MRUs for Sonar Motion Compensation?
Industry-Leading Motion Accuracy in Real
Sea Conditions – Handles irregular and
multi-directional waves with no loss in precision.
Works Anywhere - Independent of GNSS &
RTK – Works outside of RTK coverage, in offshore
and deep-sea applications. Avoid RTK
base station or subscription cost and hassle.
High End MRUs at a fraction of the price –
Premium grade motion compensation also
for entry level Sonar systems.
Real-Time Heave, Roll, and Pitch Correction
– Delivers survey-grade data accuracy in all
conditions in real-time.
Ready-Calibrated and Maintenance-Free
– Reducing downtime and providing longterm
reliability for users.
Compact Form Factor – Can be mounted
on vessels, near sonar heads, or integrated
within sonar equipment.
Plug-and-Play Integration – Compatible with
leading sonar systems for seamless operation.
OEM Solutions Available – Can be integrated
directly into sonar systems for optimized
performance.
Motion Reference Units (MRUs) are essential for sensor systems for this application, providing high-accuracy measurements of the ROV's orientation and motion in all six degrees of freedom (6DoF): Roll, Pitch, Yaw, Heave, Surge, and Sway.
Norwegian Subsea offers the MRU Subsea, specifically designed for demanding underwater environments and ideally suited for ROV operations, including seabed mapping and subsea surveys. Housed in a robust, compact titanium casing, the MRU Subsea is depth-rated to 6000 meters and features a reliable Subconn connector for straightforward integration.
Our MRU Subsea utilizes advanced sensor fusion algorithms and state-of-the-art MEMS technology to deliver highly accurate and stable motion data, crucial for correcting multibeam echosounder or other survey sensor data. It is available in various accuracy levels (±0.05°, ±0.02°, ±0.01° Roll/Pitch) to meet diverse project requirements. An Inclinometer/VRU version is also available, offering the same high Roll/Pitch accuracy for applications where full 6DoF is not required.
Integration is simplified through standard Ethernet and serial communication ports supporting protocols like UDP, Modbus TCP, and Ethernet/IP. Like all Norwegian Subsea sensors, the MRU Subsea is designed for reliability and cost-effectiveness, typically requiring no recalibration during its operational life, reducing maintenance overhead for ROV operators.

Question 54

What sensor technologies are used in ROVs?

Accepted Answer

Remotely Operated Vehicles (ROVs) rely on a suite of sophisticated sensors to navigate complex subsea environments, perform intricate tasks, and gather data. Key sensor technologies typically include imaging sonar for obstacle avoidance and mapping, cameras for visual inspection, Doppler Velocity Logs (DVLs) for measuring speed over the seabed, depth sensors, altimeters, and crucially, high-performance motion sensors.
Motion Reference Units (MRUs) are fundamental components within an ROV's sensor package. They provide precise measurements of the vehicle's orientation and motion across all six degrees of freedom (6DoF): Roll, Pitch, Heave, Yaw, Surge, and Sway. This high-fidelity motion data is essential for vehicle stability, accurate navigation when integrated with systems like DVLs or acoustic positioning (USBL/LBL), and precise control of manipulators and tooling.
Norwegian Subsea specializes in developing advanced MRUs utilizing state-of-the-art MEMS sensor technology and robust sensor fusion algorithms, delivering the high accuracy and reliability required for demanding ROV operations. Our MRU Subsea is specifically designed for these applications, featuring a compact, 6000-meter depth-rated titanium housing, a standard Subconn connector for easy integration, and flexible Ethernet and serial communication options supporting various industry protocols. For applications primarily requiring orientation data, cost-effective Inclinometer/VRU versions are also available, offering the same high Roll and Pitch accuracy.

Question 55

What technology is used in Norwegian Subsea products?

Accepted Answer

Norwegian Subsea products are engineered around state-of-the-art Micro-Electro-Mechanical Systems (MEMS) sensor technology. This forms the core of our Motion Reference Units (MRUs) and motion sensors.
We combine these high-quality MEMS sensors with advanced, proprietary sensor fusion algorithms. This sophisticated processing allows us to deliver highly accurate and reliable 6 Degrees of Freedom (6DoF) motion data, including Roll, Pitch, Yaw, Heave, Surge, and Sway, even in challenging dynamic marine environments.
Our technology focus extends to robust hardware design, utilizing materials like anodized aluminum and titanium for durability in various operating conditions, from indoor installations to deep subsea deployments (up to 6000m) and hazardous areas. Practicality is ensured through features like compact designs, flexible mounting options, and standard interfaces like Ethernet (including Passive PoE on some models), RS-232/485, and industry-standard connectors (e.g., RJ45, Subconn).
Integration is streamlined through support for a wide range of communication protocols (e.g., UDP, Modbus TCP/RTU, Ethernet/IP, NMEA) and our user-friendly, browser-based configuration software, eliminating the need for software installation. This combination of advanced MEMS technology, sophisticated algorithms, robust hardware, and ease of use defines the technological foundation of Norwegian Subsea's high-performance, cost-effective motion sensing solutions.

Question 56

Where can I buy a motion sensor for subsea use?

Accepted Answer

Norwegian Subsea specializes in high-performance motion sensors called Motion Reference Unit suitable (MRU) for demanding marine environments, including subsea applications. We offer reliable and cost-effective solutions designed and validated for real sea conditions.
For subsea use, we recommend our MRU Subsea. This unit features a robust titanium housing depth-rated to 6000 meters, making it ideal for applications such as ROV/AUV navigation and control, riser motion monitoring, Blowout Preventer (BOP) monitoring, and subsea surveys. Its compact size and ability to be mounted in any orientation allow for flexible installation.
The MRU Subsea provides high-accuracy Roll, Pitch, Heave, Yaw, Surge, and Sway data (6DoF) and is available in accuracy levels of ±0.05°, ±0.02°, and ±0.01° (Roll & Pitch). It integrates easily with standard subsea systems using Ethernet or serial communication (RS-232/RS-485) via a reliable Subconn connector. We also offer an Inclinometer/VRU version for applications requiring only Roll and Pitch data.
Our sensors utilize advanced MEMS technology and sensor fusion algorithms, ensuring high performance and stability, often without the need for recalibration during their operational life. Please contact Norwegian Subsea directly to discuss your specific requirements and purchase options.

Question 57

Where should I mount the MRU?

Accepted Answer

Your MRU can be mounted almost anywhere on your vessel and in any orientation, but there are some guidelines you can follow to ensure the absolute highest accuracy of measurements from your device:
Position your MRU close to the point which you’d like to measure
Avoid mounting your MRU on machinery or in other areas with high levels of vibration, as this can negatively impact accuracy
Avoid placing your MRU high up in your vessel, as this will lead to large horizontal accelerations, which can have a small negative impact on accuracy.
While we do recommend you position your MRU as close to the point of measurement as possible, if this can’t be achieved (perhaps because this area isn’t easy to access), you can also configure a remote measurement point.
Remote measurement points allow you to take measurements from almost any point on your vessel, but there will be a degradation in heave accuracy due to the lever arm between MRU position and measurement point. 
As an example, a distance of 30 m between MRU and measurement point will give an additional inaccuracy in Heave of 2.6 cm at a roll/pitch accuracy of 0.05 degrees (3000 series). By selecting the 6000 series instead (0.02 degrees roll/pitch accuracy), this additional heave inaccuracy will be reduced to 1.0 cm. So by selecting a more accurate MRU series, you can maintain accuracy for measurement points further away from the MRU position.

Question 58

Which MRU is best for subsea use?

Accepted Answer

For subsea applications, the Norwegian Subsea MRU Subsea is the ideal solution. It is specifically engineered for demanding underwater environments, featuring a robust titanium housing rated for water depths up to 6000 meters.
The MRU Subsea is exceptionally compact and lightweight, simplifying installation on various subsea equipment such as ROVs, AUVs, or monitoring structures. It comes equipped with a reliable Subconn wet-mateable connector for secure underwater connections.
This model provides high-accuracy 6DoF motion data (Roll, Pitch, Yaw, Heave, Surge, Sway) crucial for applications like riser motion monitoring, subsea surveys, and ROV/AUV navigation and control. Like all our MRUs, it utilizes advanced sensor fusion algorithms and state-of-the-art MEMS technology for validated performance.
Integration is straightforward thanks to support for Ethernet (UDP, Modbus TCP, Ethernet/IP) and serial (RS-232/RS-485) communication protocols, available with standard or custom data formats. The MRU Subsea can be mounted in any orientation and is available in accuracy versions up to ±0.01° for Roll & Pitch.
While the MRU Marine SW variant offers an IP68 rating suitable for depths up to 50 meters, the MRU Subsea provides the comprehensive depth rating and specific design features required for the majority of subsea operations.

Question 59

Why are MRUs important for DNV's ASP class notation?

Accepted Answer

What is the purpose of the MRU in classed vessels?
The MRU (Motion Reference Unit) monitors vessel motions to evaluate risks related to sloshing and structural fatigue in cargo containment systems. It supports a condition-based inspection regime under DNV’s Alternative Survey Programme (ASP).
When is the MRU required?
The MRU is required for vessels with the ASP class notation that include the MRU qualifier. It is not mandatory for all vessels but becomes necessary when motion data is used as part of the alternative inspection strategy.
What documentation must be provided for annual surveys?
For vessels with the MRU qualifier, the following must be submitted:
Motion sensor records (MRU data).
Evaluation report if motions have exceeded design assumptions.
Valid calibration certificates for the MRU.
Why is the MRU important in the Alternative Survey Programme (ASP)?
The MRU enables reduced physical tank inspections by supplying motion data that confirms operational conditions remain within design limits. This supports a risk-based approach to cargo tank survey requirements.

This information relates to DNV-RU-SHIP Part 7 Fleet in service – retention of class - Chapter 5 Periodical surveys - in-operation class notations

Question 60

Why are MRUs important for helicopter deck monitoring?

Accepted Answer

Motion Reference Units (MRUs) are critically important for Helideck Monitoring Systems (HMS) because they provide precise, real-time measurements of the vessel's or platform's motion. Safe helicopter landings and takeoffs depend heavily on understanding the dynamic movement of the helideck, particularly its roll, pitch, and heave.
An MRU accurately measures these key parameters, delivering essential data to the HMS. This information allows the system, pilots, and deck crew to assess whether the helideck's motion is within the established safe operational limits. Without accurate motion data, attempting landings or takeoffs, especially in challenging sea conditions, poses significant safety risks.
Norwegian Subsea MRUs, such as the MRU Marine (IP68 rated for deck mounting) or the MRU Ex (for hazardous areas), deliver the high accuracy, reliability, and robustness required for this safety-critical application. Our sensors utilize advanced algorithms and state-of-the-art MEMS technology, validated in real sea conditions, ensuring trustworthy data for Helideck Monitoring Systems, thereby enhancing operational safety. Our MRUs meet the requirements of Cap 437 (CAP 437: Standards for offshore helicopter landing areas).

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How do I choose the right motion sensor?

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What is a Motion Reference Unit?

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Where should I mount the MRU?

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Does the MRU measure surge and sway?

Do Norwegian Subsea MRUs output accelerations and velocities?

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What technology is used in Norwegian Subsea products?

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Why are MRUs important for helicopter deck monitoring?

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What is the difference between the MRU Marine and MRU Marine SW?

What is a VRU and how does it differ from an MRU?

What are the MRU dimensions?

How do I choose the right motion sensor?

What is an MRU vs VRU?

What is a Motion Reference Unit?

What Ethernet protocols are available?

Where should I mount the MRU?

Can the MRU output remote heave?

Do I need a junction box for the MRU Marine?

Does the MRU measure surge and sway?

Do Norwegian Subsea MRUs output accelerations and velocities?

Does my MRU need recalibration?

What is a Gyrocompass?