Motion Compensator

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 Vessel(s) or The Vessel may refer to: Biology *Blood vessel, a part of the circulatory system and function to transport blood throughout the body *Lymphatic vessel, a thin walled, valved structure that carries lymph *Vessel element, a narrow wat ...

or a structure fixed to the

seabed The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

. 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 Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...

, i.e., the rapid start-and-stop "jerking" of the objects. 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. Sensor technologies being used are inertial sensors and GNSS (example: iMAR Navigation) or image processing. The simplest motion compensator is the

anchor An anchor is a device, normally made of metal , used to secure a vessel to the bed of a body of water to prevent the craft from drifting due to wind or current. The word derives from Latin ''ancora'', which itself comes from the Greek ...

chain of a

ship A ship is a large watercraft that travels the world's oceans and other sufficiently deep waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research, and fishing. Ships are generally distinguishe ...

. 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 Spring(s) may refer to: Common uses * Spring (season), a season of the year * Spring (device), a mechanical device that stores energy * Spring (hydrology), a natural source of water * Spring (mathematics), a geometric surface in the shape of a h ...

s. For very large forces (dozens to hundreds of tonnes), the springs are implemented as gas springs:

hydropneumatic Hydropneumatic suspension is a type of motor vehicle suspension system, designed by Paul Magès, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers, notably Rolls-Royce ( Silver Shado ...

devices — a plunger cylinder buffered by a volume of gas. Examples of heave compensators include: *

Drill string compensator A drill string compensator decreases the influence of the heave of a drilling vessel on the drill bit. Drill string compensators are also known under the more general name: heave compensator. Drill string compensators are used onboard drill ships ...

s *

Riser tensioner Riser may refer to: Engineering * Stair riser, the vertical elements in a set of stairs * Riser, another name for a theatre platform * Riser, a length of vertically oriented piping used to deliver fluid, gas, or electrical signals or power upward * ...

s *Conductor tensioners *

Guideline tensioner A guideline tensioner is a hydropneumatic device used on an offshore drilling rig that keeps a positive pulling force on the guidelines from the platform to a template on the seabed. The guidelines act as a guidance for equipment and tools that mus ...

s More advanced heave compensated systems are often specified as systems with

Passive Heave Compensation Passive heave compensation is a technique used to reduce the influence of waves upon lifting and drilling operations. A simple passive heave compensator (PHC) is a soft spring which utilizes spring isolation to reduce transmissibility to less t ...

Active Heave Compensation Active heave compensation (AHC) is a technique used on lifting equipment to reduce the influence of waves upon offshore operations. AHC differs from Passive Heave Compensation by having a control system that actively tries to compensate for any m ...

or combinations of these. A new approach to advanced heave compensation systems is offered by

Balanced Heave Compensation Balanced heave compensation is a technology engaging the principle of a balanced-arm lamp for offshore motion compensation. Working principle The technical working principle can be summarized as converting the non-linear force of a gas spring ...

, increasing safety while lowering energy consumption and spring adjustment times. {{DEFAULTSORT:Motion Compensator Mechanisms (engineering)