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maintained by a counterbalancing force induced by air pressure in
the accumulator. The pressure is controlled from the driller's panel,
allowing rapid variation in effective bit weight.
A combination hydraulic lock/deceleration valve blocks the
flow of oil from the compensator cylinder, preventing extension of
the piston rods while making connections. This is an important
time-saving feature because it eliminates the need to cycle the
piston rods fully and double-set the slips. The hydraulic
lock/deceleration valve also allows immediate pickup of the drill
pipe with the cylinders in any extended position so the drill pipe is
held off bottom and motionless with respect to the vessel. In this
way, the BOP ram rubbers or bag preventer may be closed quickly
and safely around the drill pipe.
In operation, the weight of the drillstring is supported by
the hydraulic cylinder assembly positioned between the hook and
traveling block. As the drilling vessel heaves with the sea, the
piston rods extend or extract to isolate the vertical motion of the
vessel from the string. When starting to drill, the operator
determines the weight on the hook and simply adjusts the system
pressure to balance the hook load. Desired bit load is achieved by
reducing the system operating pressure by a sufficient amount for
the desired bit weight on bottom (fig. 2.17).
While running the drillstring, the compensator unit is
mechanically locked in its fully closed position. When the last
joint of drill pipe has been added to the string, the compensator is
unlocked and stroked out (to its fully extended position). The
traveling block is then lowered to land the bit on bottom. After
tagging bottom, the traveling block is lowered farther to compress
the compensator cylinders to the midpoint of their stroke.
A motion compensating system that utilizes a passive
operation must rely on a resisting force from the drillstring to
indicate that motion has occurred on the ship. This force normally
is provided by the weight of the drillstring, the vertical drag of the
drill pipe in the hole, and the inertia of the drillstring itself. During
wire-line operations (such as logging or testing) there usually is
not enough drag and inertial force to cause the compensator to
stroke. In these instances, the additional force is provided by a
sensing line. The sensing line is run over a sheave hung from the
hook and is connected to the drilling riser at one end, attached to
the derrick floor at the other end. Tension is then applied to the

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