The preliminary development of the novel concept of using a large catamaran, known as Gas Cat, as a floating natural gas processing and offloading facility is outlined. The proposed system is based on two ship-shaped hulls joined by a spanning superstructure. For off-loading purposes a carrier may dock with or be tethered to the catamaran. A concept design has been developed based on two retrofitted VLCCs allowing for the processing and storage of 1 million bbls of condensate and approximately 240,000 m3 of LNG. A key aspect of the development of this concept is the accurate estimation of the motions of the catamaran in a variety of operational scenarios. Model experiments were conducted in the Model Test Basin of the Australian Maritime College using a 1:78 scale model of two full-form hulls converted into a catamaran configuration. Tests were conducted in head, beam and oblique seas for two hull spacings and a range of wave heights. The experimental results show that, for the range of wave conditions tested, good linearity of the motions can be expected with respect to wave height. An increase in demihull separation was found to significantly reduce the sway, heave and roll motions in beam seas. However a change in demihull separation had little influence on the motions in oblique seas. A change in heading angle from head seas to beam seas significantly increased the sway, heave and roll motions whilst reducing pitch motions. Bow quartering seas was seen to be the worst heading angle for yaw. The results from the experiments allowed the expected motions of the Gas Cat to be determined in extreme weather conditions.

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