Motion compensator
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A motion compensator is a device that decreases the undesirable effects of the relative motion between two connected objects. In general a motion compensator is built in the connection between a floating object and the connection to the seabottom, or the connection to another vessel, or a fixed structure. The motion compensator does not compensate the motion, but tries to eliminate the negative influence of the movement (difference).
The simplest motion compensator is the anchor chain of a floating object. The shape of the chain makes that in case of moving of the ship, the forces on the anchor and the ancher chain remain acceptable. In general however a motion compensator is a "spring". For large forces this spring is in general a hydro pneumatic spring: a plunger cylinder connected to a gas volume.
A heave compensator is a kind of motion compensator, a device or mechanism which decreases the undesirable effects of the relative movement between two connected objects. Whereas most motion compensators will compensate for movement in all directions, the heave compensator will compensate for movement in only one direction, for instance, for vertical movement. In practice, the words motion compensator and heave compensator are used interchangeably. It is implemented as a spring, or most commonly as a gas spring.
In order to minimize changes in the force relative to the movement, one must increase the gas volume and decrease the hysteresis of the installation. Several patents exist on kinematicconstructions that give the same result as an increase in gas volume. To overcome the hysteresis as well as the limited gas volume, one can add active elements to the installation. The main spring function is given by the gas volume, the pressure difference and the hysteresis is compensated by this extra device. As normally there is a lot of energy related to heave compensators, it is almost impossible to have a full "active heave compensator": this energy also must be dissipated!
Examples of heave compensators include:
Fine examples of motion compensators are: