A brief review of the characteristics of simple passive heave compensators is presented in this paper before proceeding to demonstrate the extension of performance which is obtainable from a servo controlled compensator. Dealt with in some detail are the principles of operation, performance characteristics and mechanical and operational design features. The potential of the system for wider application is discussed. Documentation of operational experience gained by Global Marine Inc. onboard their drilling vessel GLOMAR CHALLENGER with respect to an active heave compensator is presented.
One of the problems in drilling the ocean floor from a floating platform is the isolation of the drill string from the vertical motion or heave, of the platform. Accommodation of this motion is generally achieved either by permitting relative axial movement at some point in the drill string, or by incorporating a heave compensator into the suspension gear for the drill string.
The passive or "simple spring" type of heave compensator has a major disadvantage in that, when a large nominal load is to be supported and the allowable variation in lifting effort is small, the pneumatic spring required is so large and heavy that the cost of the system becomes prohibitive. A passive heave compensator system, therefore, is less effective in deep sea drilling and marine coring operations, such as the Deep Sea Drilling Project of the Scripps Institution of Oceanography, than an active system which, by the incorporation of a power injection system, can reduce air storage requirements by 60% in comparison with a passive system, and allow a load variation of only ± 0.5%. The D/V Glomar Challenger, which is employed by the Scripps Institution for the Deep Sea Drilling Project is operated by Global Marine Inc. A study was therefore undertaken by Global Marine to evaluate and select a heave compensator system, to be installed onboard the GLOMAR CHALLENGER, which would meet the following objectives:-
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Provide a more effective means of eliminating vertical motion of the drill string during vessel operations.
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Permit drilling under conditions which will allow a maximum penetration rate at optimum bit load.
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Permit recovery of cores of improved quality, and with minimum disturbance.
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Provide for an improvement in bit life with attendant economic savings, and the capability of achieving deeper penetration with fewer bit changes.
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Permit the more effective use of tools such as the re-entry coring system, the logging unit and other down hole tools.
To meet these objectives the following specification was drawn up:-
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The resultant system was of the servo-controlled active type, designed and patented by Brown Brothers and Company Limited, in collaboration with Global Marine Inc. (1), (2), and manufactured and set to work by the former Company between January and November 1973. This unit was the first of a range of heave compensators which is now produced by Brown Brothers.
The present paper reviews briefly the principles of the simple passive heave compensator and proceeds to the description of a servo-controlled active compensator which extends the performance range of the passive type system, with particular reference to the system onboard the GLOMAR CHALLENGER.
