This paper reviews the basic stability considerations applied in the design of semisubmersible drilling units. Rules and regulations governing the stability of mobile offshore drilling units (MODU's) were first introduced in the late 1960's. There has been considerable debate over the level of safety achieved in the present generation of MODU's based on existing rules. The assumptions implied in the early rules are discussed and the merits of a "yardstick" approach to the rules are presented. The paper summarizes the evolution of stability rules for mobile offshore drilling units and describes how the rules have changed due to public reaction to several major disasters.
Current state of the art for predicting downtime for offshore equipment is briefly reviewed and a new method for calculating downtime, using a Markov chain probability model to simulate all known operational and weather constraints, is described.The results of this method, applied to Santa Fe Engineering and Construction's Choctaw II pipelaying barge, are compared wi th actual downtime exper ienced dur ing one working season in the North Sea.
This paper describes how two model basins, testing the same semisubmersible design, produced different motion response results. Subsequent t?sting revealed the phenomena that the heave response of the modeTis extremely sensitive to small changes in wave period. Calculations show that oversimplification of the heave response can produce significant overestimates of actual heave values, and, hence an overestimate of weather downtime that a drilling vessel will experience. It can be concluded that detailed model tests are very important for the accurate determination of vessel performance.----------
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