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:
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.
From these input values, output
values for the storage winch and the length compensation system
are calculated:
English version | Deutsche Version