Damping

A passive heave compensator is fundamentally a spring-mass system. Like any spring-mass system, it has a natural frequency at which it will resonate if excited. Without damping, the response at resonance would grow without bound.

Damping serves three essential functions:

• Resonance control — Limits the amplification at and near the system’s natural frequency to safe levels.
• Transient response — Controls how the system responds to sudden changes in load or wave conditions, preventing excessive overshoot and oscillation.
• Landing and retrieval — Provides the ability to control the load’s descent speed during subsea landing and manage forces during retrieval.

In offshore heave compensators, damping is achieved hydraulically. As the piston moves, oil flows between the cylinder and the gas accumulators through orifices or control valves. The resistance to this flow generates a force proportional to the piston velocity, dissipating kinetic energy as heat in the hydraulic fluid.

The damping force is typically a function of velocity:

• Linear damping — Force proportional to velocity. Simple to model but rare in practice.
• Quadratic damping — Force proportional to velocity squared. Characteristic of orifice flow. More damping at high velocities, less at low velocities.
• Variable damping — Adjustable orifices or proportional valves that allow the damping level to be changed during operation.

Many modern compensators, including Norwegian Dynamics ANTARES, feature adjustable damping that can be optimised for different operational phases — low damping for maximum heave compensation efficiency during lowering, and high damping for controlled landing speed during the final approach.

Damping is always a compromise between competing objectives:

• Too little damping — The system resonates dangerously, with large amplification near the natural frequency. Transient events cause prolonged oscillation.
• Too much damping — The system becomes stiff, transmitting more vessel motion to the load. Compensation efficiency drops because the damper resists the relative motion that the spring is trying to create.

The optimal damping level depends on the operating conditions. For heave compensation during normal lowering, relatively low damping (5–15% of critical) gives the best efficiency whilst providing adequate resonance safety. For landing or splash zone transit, higher damping may be applied temporarily to control the load’s motion.

In real offshore operations, the ability to adjust damping gives operators flexibility to optimise performance for each phase of the lift. A typical sequence might involve:

• Lift-off — Moderate damping for stable load pick-up.
• Splash zone — Increased damping to manage rapidly changing hydrodynamic forces.
• Deepwater lowering — Reduced damping for maximum heave compensation efficiency.
• Landing — High damping to control descent speed for a gentle touchdown.

The Norwegian Dynamics RIGEL provides fixed or manually adjustable damping for straightforward operations, whilst the ANTARES system offers automated damping control integrated with its adaptive gas spring — ensuring optimal performance throughout each phase without manual intervention. For guidance on selecting the right system for your application, see the heave compensator selection guide.