Active Heave Compensator for Heavy Hanging Body Suspended from Offshore Floating Unit

Abstract

When a heavy facility or equipment is set on or picked up from the sea bed in deep sea area, it seems to be difficult to keep safety anytime throughout the long operation period, because of the risk of encountering rough sea conditions.Although conventional heave compensator using air spring mechanism has been already applied in such an operation, the compensation performance of such a passive system would not be sufficient against large heave motions in rough sea conditions, and dynamic hook load would come to dangerous level, especially in case of resonance condition of the air spring due to long period heave motions.In this research, it has been studied to stabilize the position of suspended heavy mass in sea water and reduce the dynamic hook load by adding active control force to the passive air spring mechanism.Simplified model tests and corresponding theoretical simulations have been performed and the results are shown in this paper.Tested model consists of vessel part, suspended mass part and heave compensation mechanism part. The responses of those were measured in various wave conditions. Heave compensation mechanism was simplified using AC servo motor controlled so as to simulate the performance of the ideal air spring and also to simulate hybrid mode together with active control force. The command of the active control force is derived from the signals of vertical accelerometer sensing vessel heave motion, and pulse generator sensing relative motion between the vessel and the suspended mass.The following conclusions have been derived by the model tests and corresponding theoretical simulations.(1) Variable hook load in hybrid mode (with active control force) has been reduced to about 1/10 compared with the case of fixed mode, and about 1/3 compared with the case of passive mode (using air spring only).(2) Theoretical simulations have shown good agreement with the tested model results.