This paper focuses on the variability in the maximum wave loads exerted on a caisson-type offshore platform in extreme multidirectional seas. In September 1998, the structure became the first production platform to be deployed in the Sea of Okhotsk offshore Sakhalin Island. Results from a program of physical model tests are presented and discussed. The discussion highlights the variability of maximum forces encountered with finite-length realizations of random wave activity. Simple statistical techniques to quantify this variability and define confidence bands are described. This paper illustrates the importance of conducting multiple tests with different realizations of a random sea state when determining maximum design loads for an offshore structure.
In September of 1998, the Molikpaq drilling platform was successfully deployed 16 km NE of the coast of Sakhalin Island at the Piltun-Astokhskoye (PA) field. The Molikpaq is the first production platform to be installed offshore Sakhalin Island in the Russian subarctic. The Molikpaq, an Arctic offshore drilling caisson, was originally developed by Gulf Canada Resources Ltd. for year-round exploratory and delineation drilling operations in the Arctic offshore (Hnatiuk & Felzien, 1985). The structure has a total width of approximately 90 m at the waterline and 111 m at its base. In deeper water, the caisson can be set down on a pre-dredged berm or some other suitable base structure. Ice conditions at the PA field are milder than in the Canadian Beaufort Sea, but the wave conditions are between 2 – 3 times more severe than what the Molikpaq had been exposed to in her previous deployments. The 100-year return period storm at the PA field features a significant wave height of 9.8 m in a water depth of approximately 31 m. This design sea state contains breaking waves and waves that are very steep and highly non-linear.