When the water is over 100 meters deep, a floating drilling unit (drillship or drilling semi-submersible) is often used. The unit is could either be kept on station by a traditional anchoring system (with cables connected to anchors) or dynamic positioning system. Because of the movement of the swell and the variation in tide levels, this type of structure can not remain stationary with respect to the sea floor. As a result, there is a device to compensate lengthwise in the drill string. There are two principles: the bumper sub and the heave compensator.

Introduction

When the water is over 100 meters deep, a floating drilling unit (drillship or drilling semi-submersible) is often used. The unit is could either be kept on station by a traditional anchoring system (with cables connected to anchors) or dynamic positioning system. Because of the movement of the swell and the variation in tide levels, this type of structure can not remain stationary with respect to the sea floor. As a result, there is a device to compensate lengthwise in the drill string. There are two principles: the bumper sub and the heave compensator.

The bumper sub (Fig. 1)

The bumper sub is a motion compensator that maintains constant weight on the drilling bit during vertical motion of a floating offshore drilling rig and also provides a jarring action to remove heavy objects in the borehole. This piece of equipment consists of a body that a mandrel can slide in. The mandrel is set rotationally in the body by keys or by its specific cross-section (hexagonal or square). The bumper sub allows rotation to be transmitted to the drill bit and also has a stroke length of about 5 feet (1.52 m). A sealing system isolates the inside channel. It is generally relatively sophisticated, since bumper subs are preferred in the hydraulically balanced version, i.e. the static and dynamic pressures produce no axial forces. Since the available stroke length is only a few feet, several bumper subs will have to be screwed together one on top of the other if a greater heave height is anticipated. Weight on the bit is provided by the series of drill collars placed under the bumper sub. The driller manning the brake must pay attention, once the bit is on the bottom of the hole. He must make sure that the bumper sub is operating according to the heave without extending entirely (taking weight off the bit) and without closing completely (placing extra weight on the bit).

Heave compensators (Fig. 2)

The principle is to keep constant tension on the upper part of the drill string, i.e. where it hangs in the hoisting apparatus. Two solutions are used: a compensator located at the hook and one at the crown block. Both are based on the principle of hydraulic jacks with the body on the traveling block and the rod supporting the hook. Keeping constant weight on the bit, however the drilling mast may move, means keeping constant pressure in the jack, however the rod may move in the body. Pressure is regulated conventionally: statically by a hydraulic connection with a large-volume air tank that acts as a buffer when the jack discharges oil or when it takes in oil according to the floater’s high or low position. The system is the same when the compensator is located at the crown block, except that the whole pulley system is suspended by means of the jacks. The advantage in this case is that there are no hydraulic hoses in the mast and that it takes up less room in the derrick. However, there is an increase in vertical load. The stroke of the hydraulic jacks must match the acceptable degree of heave to drill under proper conditions. The double jack Vetco system gives a maximum stroke of 30 feet (9.3 m).

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