Page 119 - 6634

changes but is displaced back and forth between the compensator and accumulator

by the compensator piston. The piston, then, carries the hook load by virtue of

hydraulic pressure, 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. 6.25).

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

120