GEOMAR Technologie: Heave Compensation


 
Even if the sea is not that rough and the sea state is moderate, often it is quite difficult to keep a payload at constant level above the sea floor. Stable positioning the equipment within the deep sea waters is not only disturbed by heave motions at the ocean surface but also by resulting forced vibrations of the cable. Thus, any compensation of both effects, the heave itself and the elasticities of the cable, is required to keep the payload several thousand metres below the ocean surface and some meters above the sea floor in an invariable vertical position, the one-and-only position.

 

Active Heave Compensation for Winches
 

Normally, for a compensation system a combination of storage winch and length compensation is usual. The storage winch has the function to compensate long-term alterations and the length compensation comprises the actual compensation of the heave effects. As a constructive solution for the length compensation is not critical with respect to the task of an active heave compensation system. It may be realized by two reels, around which the cable is conducted and which achieve a compensation with a modification of the distance of the reels with the help of a hydraulic cylinder. The system would also work with a lever compensator.

The heave can be calculated for different ships individually. For this simulation the heave randomly equals the natural heave and is statistically calculated, so the heave is different during each simulation start. The control algorithm is a fuzzy controller system. It has the advantage towards conventional controllers that many input values can easily be connected with one another.

The input values for the control are similar to the values which can be expected in a real, natural environment, because the simulation is carried out independently of the controlling system. Input values available to the controller are four variables:

From these input values, output values for the storage winch and the length compensation system are calculated:

A survey concerning the realisation of an active heave compensation showed, that conventional heave compensations are not able to position a platform which is located in a depth of several thousand metres precisely stable. The reason for remaining motions is that a cable of such a length has a rather high own weight, which causes reactions of the cable like a spring mass system. The computer simulation demonstrates, that this system can be improved by the new active heave compensation control concept.

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Sorry, partially only in German:
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