The Search On The LRFD Design Method For Deep Sea Semi-submersible Platform

In the wake of developments in economics of China, the needs of crude oil are increasing. Inrecent years, China has actively promoted the deep-sea oil resources exploitation. Thesemi-submersible platform as the deep-sea oil and gas exploration equipment, was given highpriority by the relevant departments. In this paper, the load and resistance factor design methods(LRFD) of semi-submersible platform were studied.The researches on reliability theory of marine engineering are a little in China especially onLRFD design method. At present, the marine engineering LRFD design rule in China waspublished by the Chinese National Offshore Oil Corporation on August12,1996. It is equivalentto API RP2A-LRFD published by American Petroleum Institute in1993. This LRFD designmethod is based on the Gulf of Mexico marine environmental conditions. It can not fit well onthe design of semi-submersible platform operated in South China Sea, so it is necessary to takeaccount of the South China Sea’s special sea conditions, and develop the LRFD design methodfor the semi-submersible platform.In this paper, the studies on LRFD design method for overall structure and local structure ofHYSY981semi-submersible platform were done. The main conclusions are as follows:(1) The ultimate strength and wave load probabilistic model of HYSY981platform were set upand its reliability index was calculated. For the overall structure of HYSY981platform, itscapacity performs in line with the DNV rules. If the platform only operated in South China Sea,it was recommended that the load factor may be appropriate to reduce and the resistance factorincreases slightly.(2) For the local stiffened panels of HYSY981platform, it was divided into three levelsaccording to the degree of risk, then their load and resistance factors ware calibrated respectively.Compared with the implicit reliability index of the CCS specification based on the workingstress methods (WSD), it was found that the reliability index of the structures in normal area andsecond dangerous area were near to the implicit reliability index of the CCS specification, thereliability index in most dangerous area were larger than the implicit index of CCS specification.(3) For the connecting structure of pontoons of the platform, the reliability index was calculated.Comparing with the implicit index of CCS specification based on the WSD methods, the authorfound the implicit index of CCS specification is too small. The load and resistance factors werecalibrated according to the new index recommended by the author.