| China's rapid economic development, especially in the transportation and energy sectors, stimulates the soaring demand for oil and gas. It is urgent for China's oil and gas industry to solve the conflict between the soaring demand for oil & gas and the insufficient supply. In waters under China's jurisdiction, the deepwater areas are unexploited places for the oil & gas industry with tremendous potential as the main production areas.The water depth of Liwan 3-1 gas field which is considered as China's largest-ever offshore natural gas discovery is approximately 1500m. The extreme water depth and also the harsh environmental conditions of South China Sea pose tremendous challenges for the deepwater facilities.This paper is concerned with the investigation and comparison on hydrodynamics and motion performance of a typical Truss Spar and a preliminary designed Semi-submersible FPU (Semi-submersible) for the unique Liwan 3-1 environment through the numerical simulation. Firstly, the hydrodynamic information of the two platforms is acquired based on the potential fluid theory. Then, the 1st order Response amplitude operators (RAO) for surge, heave and pitch of the two platforms are calculated and compared in frequency domain. The viscous damping effects on the platform's response are especially emphasized in this section through an enhanced frequency domain method. Secondly, the calculational motions of Truss Spar based on the hull and mooring lines coupled and uncoupled models are compared in time-domain and the coupled effects on the platform's response are detailed studied for 3 extreme environment conditions for Liwan 3-1 area. Finally, the global motions of the Semi-submersible under the 3 extreme environment conditions are also computed based on the coupled model and used to be compared with the results of the Truss Spar platform.
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