Impacts of Condensate Storage on the Hull Structure Design of Semi Submersible Platform

In order to develop the gas field in South China Sea, a deep-draft semi-submersible platform with 20000 m3 condensate storage tanks was selected. There is little industrial experience for semi-submersible platform with the condensate producing and storage tanks. This paper presents the impacts caused by the condensate producing and storage on the hull structure design in FEED phase. The design life is 30 years without returning to drydock for repair or maintenance. The main unique features are illustrated as follows. Firstly, the double-wall voids (cofferdams) of 1.8 meters width were designen to prevent condensate leakage in the column around the condensate tanks. The double-wall bulkheads were connected by flats with manholes for maintenance access. this is different from conventional semi-submersible platform. Secondly, the non-watertight bulkheads in the middle of the condensate tank were needed to meet the efficient strength requirements for structural design. The upper and lower 2m height gaps on the bulkhead are convenient for the flowing of bottom condensate and top gas in condensate tank respectively. It is proven that the optimized design of continuous non-watertight bulkhead for the condensate tank is feasible, safe and reliable based on the global strength and fatigue analysis. Thirdly, The additional corrosion margin of 2mm was added to the boundary of frequently ballast and de-ballast tanks in the nodes and columns, and to the bottom plate and the side shell of 2m height from the bottom of condensate tanks considering the SEMI design life of 30 years. Fourth, the pressure head for condensate tank scantling design was increased by 10 m per condensate tank vent head location on topside. Fifth, there are some indirect effects due to the condensate storage, such as the increase of displacement and the load on the SEMI. Therefore, the hull structure needs to be reinforced, especially on the key connections, such as the column and pontoon connection, the deck and hull connection. It is recommended to pay attention to these critical locations with detailed local analyses.