How Does Active Heave Compensation Work?

An AHC system consists of three core elements working in a closed feedback loop:

• Motion Reference Unit (MRU) — An inertial sensor package mounted on the vessel that measures heave displacement, velocity, and acceleration in real time.
• Control system — A computer that processes the MRU signal and calculates the required compensating motion, accounting for system dynamics, delays, and filtering.
• Hydraulic actuator — A cylinder driven by a hydraulic power unit (HPU) that moves the crane wire or load in the opposite direction to the measured heave.

The controller continuously adjusts the actuator’s position to keep the load stationary in an earth-fixed reference frame. This closed-loop approach can achieve compensation efficiencies of 90–98%, significantly outperforming passive systems in demanding conditions.

For a broader comparison of active and passive approaches, see active vs passive heave compensation.

The fundamental difference between AHC and passive heave compensation is that AHC requires continuous external power. The hydraulic power unit must be sized to deliver the peak flow and pressure demanded by the worst-case heave velocity and load combination.

Typical power requirements range from 100 kW for smaller systems to over 500 kW for heavy-lift applications. This power is not just consumed during peaks — the HPU must run continuously to maintain system pressure and responsiveness.

Energy management is a critical design consideration. In each heave cycle, the system alternates between motoring (lifting the load against gravity) and braking (lowering it). Advanced systems recover braking energy, but the overall power footprint remains substantial compared to passive alternatives. For more on the energy aspects, see our page on active heave compensation principles.

AHC performance depends critically on the quality of the heave measurement. Modern MRUs use accelerometers and gyroscopes to measure all six degrees of vessel motion, then extract the vertical heave component through signal processing.

Key challenges include:

• Phase lag — Any delay between measurement and actuation reduces efficiency. Advanced predictive algorithms compensate for system response time.
• Low-frequency drift — Integrating acceleration to get displacement introduces drift that must be filtered without removing valid heave signal.
• Noise rejection — The controller must distinguish genuine heave from vibration, crane slewing, and other disturbances.

The MRU is typically installed close to the crane pedestal to minimise the effect of roll and pitch on the heave measurement at the lifting point.

Active heave compensation is typically specified when:

• Very high positioning accuracy is required (e.g., subsea connector mating, J-tube pull-in).
• The load weight changes significantly during the operation.
• Sea states are severe enough that passive efficiency is insufficient.
• The operation demands controlled motion at specific speeds (e.g., constant lowering velocity).

Norwegian Dynamics is an inline AHC available in both topside and subsea-rated configurations, designed for integration with existing crane systems. For many operations, however, an adaptive passive system provides sufficient performance at a fraction of the cost — making the choice between active and passive a key part of compensator selection.