Adaptive passive heave compensation

Adaptive passive heave compensation

Adaptive passive heave compensation is an improvement to passive heave compensation, that allows stiffness and damping to be adjusted during usage. It is particularly useful when performing a lifting operation that starts topside and moves through the splash zone, through potential resonance and finally landing at deep waters.

Lifting challenges and how they are solved

In the splash zone, the motion is often larger due to wave impacts on the lifted object. If the stiffness or damping of the heave compensator is too low, the full stroke of the compensator might be used up, and a dynamic shock could occur. Adaptive passive heave compensators can solve this challenge by reducing the gas volume and restricting flow between cylinders and accumulators during this phase.

Buoyancy is another challenge that is solved by the adaptive passive heave compensator. With a traditional passive heave compensator, buoyancy would cause retraction of the compensator stroke as the weight is lowered, potentially causing the stroke to go to zero and causing slack slings during the lowering. Adaptive passive heave compensators can measure that the equilibrium of the compensator stroke has changed and adjust the internal pressure to maintain correct equilibrium. This is usually done by venting gas to the atmosphere.

Temperature changes when lifting from topside into relatively colder seawater will cause the pressure of the heave compensator to drop. As a rule of thumb, this effect is about 1% per 3°C. This effect tends to extend the piston rod of the heave compensator. The adaptive passive heave compensator can have the ability to inject high-pressure gas to compensate for this effect.

Water pressure increases with about 1 bar for every 10 meters of depth. This tends to retract the piston rod of the heave compensator. The adaptive passive heave compensator can vent gas to the sea (if the seawater pressure is less than the internal pressure) or pump gas to the sea to reduce internal pressure and maintain correct piston rod equilibrium.

During the landing at the seabed, it is often required that the stiffness is very low to achieve good efficiency, this is achieved by connecting additional gas volume to the cylinder (by opening valves).

Negative aspects of adaptive passive heave compensation vs passive heave compensation

The main negative aspects are increased cost, increased risk of equipment failure, and increased weight. Risk of equipment failure is greater than for a passive heave compensation unit due to an increased number of components and the risk of electrical failure due to water ingression.

References

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