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Abstract

Deep draft multi-spar (DDMS), a novel deepwater platform, possesses the similar hydrodynamic and motion performances with the Spar platform. Therefore the DDMS may have instability of Mathieu’s type because the heave natural period is close to the half pitch natural period. The objective of this paper is to numerically study the damping effects on the Mathieu instability and the mechanism of this singular phenomena. In this simulation, the damping is determined through the free-decay tests based on a rigorous coupled hull and mooring model. The nonlinear motion equations of coupled heave and pitch considering the time-varying restoring forces are established and solved with six damping cases by using the 4th order Runge-Kutta method. Five regular wave cases of different wave heights and periods including heave natural period and half pitch natural period are conducted. The results indicate that the heave damping significantly influences the occurrence of pitch instability, meanwhile the damping contribution of heave plates and mooring lines also play an important role. Three types of pitch instability are concluded. Finally, four random wave conditions including two swell wave cases and two 100-y return period conditions for South China sea and Gulf of Mexico are applied. The calculated results show that the DDMS platform does not suffer from the Mathieu instability under these real ocean environments.

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