How Does Passive Heave Compensation Work?

A passive heave compensator is essentially a pneumatic spring. It consists of a hydraulic cylinder connected to one or more gas accumulators charged with nitrogen. The load’s weight compresses the gas to a working pressure, and the piston settles at a mid-stroke equilibrium position.

When the vessel heaves upward, the crane tip rises but the gas expands, allowing the piston to extend and keep the load stationary. When the vessel drops, the gas compresses, and the piston retracts. The load, having significant inertia, barely moves.

The key engineering challenge is achieving near-zero stiffness at the working point. If the gas spring is too stiff, it transmits vessel motion to the load. If too soft, the system drifts to end-of-stroke. Engineers size the gas volume and pre-charge pressure so that the spring rate closely matches the load weight, creating an effective soft spring that isolates heave. For the detailed mathematics, see our page on passive heave compensation fundamentals.

A pure gas spring with no damping would oscillate uncontrollably near its natural frequency. Passive heave compensators include hydraulic damping valves — typically orifice-based — that resist oil flow between the cylinder and accumulators.

Damping serves two critical functions:

• Resonance control — If the wave period approaches the system’s natural period, the response would amplify without damping. Hydraulic resistance limits this amplification to safe levels.
• Landing control — During subsea landing, increased damping slows the load’s descent for a controlled touchdown.

The damping level is a trade-off: too little and resonance becomes dangerous; too much and the system becomes stiff, reducing compensation efficiency at typical wave periods.

A well-designed PHC system typically achieves 70–90% compensation efficiency, meaning the load’s residual motion is only 10–30% of the crane tip motion. Efficiency depends on several factors:

• Wave period — PHC works best when the wave period is short relative to the system’s natural period. At very long periods, the system tends to follow the vessel.
• Gas volume — Larger gas volumes yield softer springs and better efficiency, but increase system size and cost.
• Damping level — Higher damping improves resonance safety but reduces efficiency away from resonance.
• Load matching — The system performs best when the actual load matches the design load. If conditions change, a basic PHC cannot adapt.

This last limitation is what drives interest in adaptive passive heave compensation, where the gas spring is automatically adjusted to maintain optimal performance as conditions change.

Passive heave compensators are used across a wide range of offshore operations:

• Subsea lifts and installation
• Splash zone crossing
• Tensioning of risers, cables, and umbilicals
• Crane shock absorption for wind turbine installation

Norwegian Dynamics offers PHC solutions ranging from the cost-effective RIGEL for straightforward lifting tasks, to the ANTARES adaptive system for demanding operations requiring consistent high performance across varying conditions.