Petroleum Dude: Heave compensation

Heave compensation: control of offshore cranes and drilling equipment.

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Compensating heave motion requires a control system, a number of sensor inputs and proper machinery to control the motion of the load. A typical system is indicated below:

Obtaining good heave compensation requires that we solve several problems that directly affect the performance of the system:

Measuring, calculating and estimating the movements of the vessel, or more precisely: the wave induced movements of the tip of the crane. A rule of thumb is that compensation performance is limited by the quality of the motion sensor system.

The heave machinery has a dynamic behavior of its own as well as a number of nonlinearities due to friction, valve characteristics etc, that complicates control design. A thorough understanding of these phenomena is vital to achieving good overall system performance.

A load hanging at the end of a long elastic wire, an umbilical or a drill string behaves like a spring-mass system. In unfortunate cases this system has resonant behavior, ie. the load's motion may be amplified and become larger than that of the vessel.

Eventually, the controller makes all the above subsystems work together in an optimal way. It extracts the essence of the sensor inputs, makes use of vital model parameters and calculates the control signals to the machinery. Depending on the system at hand, a number of different techniques may be applied: position feedback, velocity feed forward, force control by pressure and/or tension feedback, self-tuning control, repetitive control.

The ultimate goal is to adjust the compensator so that the movements of the vessel-crane assembly is perfectly compensated for at the load.