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Motion Compensating Systems

The motion compensator isolates the drillstring from wave-induced motions

of the floating drilling vessel so drilling operations may proceed as if being carried

out from a fixed platform. This means all operations are performed more easily—

landing equipment, hole opening, drilling, coring, testing, and logging—and the

whole drillstring is subjected to less wear and shock. A reliable motion

compensator provides more safety to the drilling operation, making every hour of

drilling more productive. A typical motion compensator system consists of four

basic assemblies: the motion compensator unit, located between the hook and

traveling block; a hydraulic-pneumatic operating system; an air supply system; and

a drillers control panel.

The motion compensator unit is comprised of an upper yoke assembly to

which the traveling block is attached, a lower yoke assembly to which the hook is

attached, single or dual hydraulic cylinder assemblies connecting the upper and

lower yokes (fig. 6.24), a hydraulically actuated locking pin to lock the unit

mechanically in its retracted position, a hydraulic lock/deceleration valve assembly

and accumulator, and an extensometer to indicate point of cylinder stroke. In

addition, a rotary hose assembly is supplied to act as a conduit for all electrical and

hydraulic/pneumatic power to and from the unit.

The single cylinder unit utilizes a large central hydraulic cylinder mounted

between two halves of a split traveling block. Variable sheave reaving permits

stable and dynamic balance of the unit when operating with 8, 10, or 12 lines. In

the design shown, the split block and cylinder are an integral part of the unit, so no

other traveling block is required. The hydraulic cylinder is removed easily for

repair or replacement on a noninterfering basis, allowing normal drilling operations

to continue while the cylinder is absent.

The dual cylinder unit uses an upper and lower yoke assembly that is

attached between the traveling block and hook. The yoke assemblies are

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