Since the first Spar platform was installed in Gulf of Mexico(GOM) in 1996, Spar platform has been regarded as competitive floating structure for deepsea oil production. Recently truss-type Spar platforms, which are significantly modified from the previous one, are being deployed in GOM. The conventional Spar has a long circular cylindrical hull, meanwhile the truss Spar consists of upper circular tank, middle truss part with some horizontal plates and lower ballast tank at the keel. Since these two types of Spars are quite different in shape, their motion characteristics may also be quite different. In this paper, the vertical motion characteristics of the truss Spar in waves are examined analytically and numerically. The upper and lower ballast tanks are discretized by constant panels for potential calculations, while the generalized Morison's formula is used to estimate drag forces induced by the relative motion of the truss part. The equation of motion in waves is solved by the 4th-order Runge-Kutta integration scheme. The heave response of the truss spar in regular waves is examined, particularly at the resonance frequency. In addition, the pitch stability of the truss Spar is investigated based on the Mathieu equation. The result clearly indicates that the motion behavior of the truss Spar is better than the conventional one mainly due to increased viscous damping.
Recently Spar platforms have been used for deepwater oil production, because the motion characteristics are excellent even in severe sea states primarily due to the deep draft (Converse and Bridges,1996; Glanville ect.,1991). However, the conventional Spar platform undergoes large heave motions at resonance due to small damping, to say 8~10 times of incident waves (Haslum, 2000; Rho and Choi, 2002). Largely amplified heave motions may invoke the restoring arm to vary with time due to significant changes in GM with time.