Motion Reference Units (MRU)

How an MRU works

An MRU carries three gyros and three accelerometers on orthogonal axes. It fuses them to compute roll and pitch, using the accelerometers to find gravity over the long term and the gyros to follow fast motion. Heave comes from integrating vertical acceleration twice, with filtering that removes drift. Real-time heave works within a stated range of wave periods. Longer swells on large floaters push the limit. Some units also output a delayed heave signal that is more accurate, which suits survey but not a heave compensator.

MEMS sensors are common in standard MRUs. Higher grades use fibre-optic gyros and add heading, and some add full inertial navigation.

What uses the MRU

• The DP controller uses roll and pitch to remove motion from GNSS antenna and hydroacoustic transducer positions.
• Active heave compensation on drawworks, cranes and motion-compensated gangways uses heave and heave velocity.
• Helideck monitoring systems use roll, pitch and heave rate to set landing limits.

DP class 2 and 3 units normally carry three MRUs, also called vertical reference sensors, so the controller can vote out a faulty one.

Key specifications

Standards and what a used unit must show

No IMO performance standard covers MRUs. Acceptance comes from the class society's DP rules and type approval, the unit's FMEA and the annual DP trials. Environmental tests follow IEC 60945 or the class test specification. For helideck use, the helideck standard and its certifying body set what the motion data must show. For heave compensation, the drilling system's class notation covers the control loop. High-grade inertial units with fibre-optic gyros can fall under dual-use export control, so check with the OEM before moving one between countries.

Failure modes and inspection

• A drop or hard knock shifts sensor calibration while the unit still looks perfect.
• Calibration drifts with age, so OEMs set recalibration intervals by application.
• Connectors and cables corrode where the MRU sits in a damp void.
• A mounting bracket that has been moved or bent leaves the axes misaligned.

A bench check compares the unit's output with a second MRU or a reference on a level surface and a tilt table. On board, compare roll and pitch across the three DP MRUs and check alignment with the hull axes. The OEM recalibrates the sensors at its own facility.

Documentation

• Original factory calibration certificate and any OEM recalibration.
• Firmware version and configuration file, including lever arms.
• History of use and transport, including whether it moved in its original packing.
• Class type approval for DP use.

Integration with your unit

Mount the MRU near the centre of motion, rigidly, with its axes aligned to the hull. Enter the lever arms to each monitored point so the unit can output motion at the GNSS antenna, the helideck or the crane tip. Confirm the DP controller or heave compensator reads its protocol and rate. Older MRUs may need an OEM configuration tool that no longer runs on current computers.

Refurbishment, used or surplus

An MRU back from OEM recalibration is the safest used buy for DP or heave compensation. A heave error moves the drill string or a gangway tip. A used unit of the same model is a fast swap for a failed one because the consumer already reads its output. Unused surplus from cancelled projects is worth buying once recalibrated.

Related equipment

MRUs work alongside gyrocompasses and feed weather monitoring systems for helideck motion. Naventr can find out from the Seller how and when the unit was last calibrated.