Stability of Deepwater Drilling Semi Submersibles

Most stability standards for the design of semisubmersibles are based on quasi-static approaches. These methods do not take into account the dynamic wind and wave loading and the resulting platform response. Several model tests in recent years revealed that stability criteria of drilling semi-submersibles are influenced by the existence of nonlinear wave loads. These wave loads appeared to be induced by shallow water wave effects on top of the pontoons. This paper reviews these effects from model tests, from numerical calculations and from field experience concluding that for increasing natural periods of roll, the low frequency response increases. These motions reduce the availability of the unit for drilling operations. The authors share this experience in hope that designers of new semi-submersibles will take these dynamics into considerations.