Motion Reference Units (MRU)
What is an MRU?
A Motion Reference Unit (MRU) is an inertial sensor that measures a vessel’s motions in all six degrees of freedom: surge, sway, heave (translations) and roll, pitch, yaw (rotations). In offshore lifting, the MRU provides the real-time heave signal that active heave compensation (AHC) systems use to drive the compensating winch.
Modern MRUs combine accelerometers and gyroscopes — typically MEMS-based or fibre-optic — with digital signal processing to separate true heave motion from sensor noise and long-period drift. Key specifications include heave accuracy (typically ±5 cm or 5% of amplitude), update rate (10–100 Hz), and maximum measurable period (typically up to 30 seconds).
Common MRU manufacturers include Kongsberg (Seatex MRU series), iXblue, and SMC. The MRU is mounted at or near the crane pedestal to measure the motions at the point of interest.
MRU in Active vs Passive Systems
Active heave compensation (AHC): The MRU is essential — it provides the feedforward heave signal that the control system uses to drive the winch motor or hydraulic cylinder in anti-phase with the vessel motion. Without an accurate MRU, AHC cannot function. The MRU signal is processed through a Kalman filter or similar algorithm to predict the heave motion one or two samples ahead, giving the control system time to respond.
Passive heave compensation (PHC): A standard PHC does not require an MRU — it responds mechanically to load changes through gas spring dynamics. However, some advanced passive systems use MRU data for monitoring and performance verification. The ANTARES adaptive PHC uses sensor feedback to optimise its operating parameters in real time, achieving near-active performance without the full complexity of an AHC system.
Installation and Calibration
Proper MRU installation is critical for AHC performance. Key considerations:
- Location: Mount at or near the crane pedestal. Mounting far from the crane introduces lever-arm errors as roll/pitch motions create different heave at different locations on the vessel.
- Alignment: The MRU axes must be aligned with the vessel’s coordinate system. Misalignment causes cross-coupling errors where roll motion contaminates the heave signal.
- Calibration: Initial calibration establishes sensor offsets and scale factors. Some MRUs self-calibrate using GNSS aiding.
- Redundancy: Critical AHC systems often use dual-redundant MRUs to maintain operation if one sensor fails.
For compensator selection, understanding whether the crane system includes an MRU (and its specifications) helps determine whether an active, passive, or adaptive passive solution is the best fit.
