Ten years ago the problem of attenuation of ship heave motion was receiving only scattered attention from the drilling industry, mostly because the offshore oil industry was mainly operating from jack-up rigs in 200 feet of water or less. However, the last five years have seen a rising interest in deep water drilling from floating vessels and the problems associated with heave motion have prompted a number of ideas to be patented for various types of motion compensators. These ideas can be generally divided into three approaches. One approach places the compensating device between the hook and the block. In another, the device is attached to either the fast line or dead line, alternately paying out or taking up line. The third approach moves the crown block. However, because simplified analytical models of such systems indicated difficulties would be encountered; or because of other reasons, most of these ideas remained unused in the patent files. However, a few small units that hung between the block and the hook were built for wire line work in 1965, and later a larger unit operating on the dead line was built by Rucker for the STAFLO. In February, 1970, a motion compensator designed for general drilling operations was invented by E. Larraldel and a small prototype was built by Vetco Offshore Industries. It was tested in June, 1970, in the Santa Barbara Channel. The initial success of the test led the company to make a detailed investigation of the dynamics of the machine. Additional tests were also conducted which led to the design of a larger machine capable of drilling to 20,000 feet. The results of this effort are the substance of this report.
The Vetco Motion Compensator is basically a hydraulic-pneumatic device which follows the heaving ship while transmitting very little of this motion to the hook. Basic models of the machine have either a 200 or 300 ton operating capacity and are offered with either a 15 foot or 20 foot stroke capability. Static load capacity is equal to the rated capacity of the derrick and blocks. The basic parts of the motion compensator are shown in Figure 1. Two cylinders are mounted in a frame connected to the traveling block and move up and down with heave motion. Within the cylinders are pistons connected by rods to a framework supporting the hook. The pistons remain relatively stationary with respect to the earth so that as the cylinders move upward with the ship, fluid is driven out, and as they move downward; fluid returns. This active fluid volume from the cylinders is received by piston type accumulators which have fluid on one side and pressurized gas on the other. Incoming fluid drives the accumulator piston against the gas, thus decreasing the gas volume and creating a pressure rise. Likewise, the outgoing fluid will create a pressure drop. The magnitude of the pressure changes is reduced by connecting the accumulators to additional gas vessels known as accumulator back up bottles.
