Value of Heave Compensators to Floating Drilling

Abstract

This examination of the motion compensator concludes that it is one of the biggest advances made in offshore drilling in recent years. Analysis of the operation of rigs equipped with motion compensators indicates a saving of about 16 days per rig-year. Optimum bit loads allow maximum penetration rates and increased bit life. Another advantage is safety in penetration rates and increased bit life. Another advantage is safety in well killing and BOP-stack handling. Introduction Before the design and introduction of the first heave or drillstring compensators, variations in the distance between the bit and the drilling level owing to wave action were accounted for with the aid of bumper subs. As offshore exploration moved into deeper and rougher water and failures achieved new dimensions in terms of cost, an alternate and more flexible method was sought. The result was the motion compensator first produced in the late 1960's when Vetco Offshore, Inc., installed a twin-cylinder model on the Wodeco IV, a drill barge working in the Santa Barbara channel. This equipment operated successfully within its design limits, and the concept of hydraulic or pneumatic heave compensation was proved viable. At least seven companies are now engaged in the design and manufacture of compensators. Each machine has a mode of operation peculiar to itself and possesses its own series of peculiar to itself and possesses its own series of problems. Shell Expro has been able to analyze the problems. Shell Expro has been able to analyze the performance of both the Vetco and the RUCKER Shaffer performance of both the Vetco and the RUCKER Shaffer models since they were installed on Shell-contracted rigs in the North Sea in Jan. 1974. Operating problems of other models such as the Western Gear Single Cylinder and the Ocean Science and Engineering, Inc., Crown Compensator have been assessed wherever possible. Savings in terms of weather down-time, bit life, and personnel and equipment safety have been observed during the latter part of the winter and throughout the summer of 1973-74 and are discussed in some detail. Now that drilling has moved further north, almost to latitude 62 degrees, the weather has produced a greater impact on rig operations. West of the Shetland Islands, where extremely long swells have been observed with correspondingly high heaves, one rig would certainly have been shut down without a heave compensator. Drilling has continued successfully in weather conditions producing heaves in excess of 12 ft and blowout-preventer producing heaves in excess of 12 ft and blowout-preventer (BOP) stacks have been landed with similar movement. Principles of Operations Principles of Operations Motion compensators may be located in the crown block or between the traveling block and the hook; they may operate either pneumatically or hydraulically, or by a combination of the two. During rig heave conditions, the operating medium is forced back and forth by means of hoses between the compensator's piston/cylinder arrangement and a number of pressure bottles or accumulators (Fig. 1). In the traveling-block units, hydraulic hoses hang from their respective standpipes in the derrick, whereas the compensated crown eliminates these - all hoses are fitted to the outside of the derrick. The mode of operation may be considered active or passive, or a hybrid of the two. In the purely passive passive, or a hybrid of the two. In the purely passive system (Fig. 2), the activating fluid results in air being compressed and expanded in and out of a bank of accumulator bottles; use is made of the adiabatic gas law: p1V1k = p2V2k V1 k p = p2 − p1 = p1 [(--------) −1],.......(1) V1−AX JPT P. 938