A motion compensator
is a device that decreases the undesirable effects of the relative motion between two connected objects. Motion compensators are usually placed between a floating object and a more stationary object, such as a vessel
or a structure fixed to the seabed
. The motion compensator does not prevent the motion, but tries to eliminate the negative effects of the movement. These negative effects include (1) the changes in force and stresses, and (2) the hysteresis
, ie, the rapid start-and-stop "jerking" of the objects. <!-- please fact-check this -->
A heave compensator
is a kind of motion compensator. 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.
The simplest motion compensator is the anchor
chain of a ship
. Not only does the anchor prevent the ship from drifting, but the chain itself dampens the movement of the ship due to undulating motion of the waves. Generally, motion compensators are implemented as spring
. For very large forces (dozens to hundreds of tonnes), the springs are implemented as gas springs: hydropneumatic
devices — a plunger cylinder buffered by a volume of gas.
Examples of heave compensators include: