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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. 2.16), 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 interconnected by two cylinders that
straddle the traveling block. Generally speaking, the dual type is
easier to install on an existing rig because the crown and traveling
blocks can be utilized and few, if any, derrick modifications are
required. However, both types have identical operating
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