The response of offshore structures to seismic sea environment in deep water conditions is a complex subject. In such a harsh environment, special offshore structures called compliant offshore structures are economically more attractive than conventional fixed offshore structures. Articulated towers are among the compliant offshore structures that freely oscillate along with the waves and current, as they are designed to have lower natural frequency than that of ocean waves. The present study deals with the comparative seismic compliance of a multi-hinged articulated tower with two different seismic sea environments. For this purpose, time histories of El Centro 1940 and Taft 1952 earthquakes are used and the results are compared with each other, as well as with mild random sea state only. The tower is modeled as an inverted pendulum connected to the sea bed through a universal joint with a concentrated mass at the top, having double rotational degree of freedom.

Many studies have been reported on the seismic analysis, but most of them relate to land-based structures; however, the information regarding the seismic analysis of offshore structures, especially articulated offshore towers, is relatively fewer. Most of the works on these towers are devoted to waves or wind responses (Bar-Avi and Benaroya, 1997; Will et al., 1999; and Kumar and Datta, 2008). Extremely little literature is available that discusses the seismic response of compliant towers. As such, in one of the studies by Brynjolfsson and Leonard (1988), the stochastic response of guyed offshore towers with MDOF subjected to earthquake loads in the presence of steady ocean current was investigated. The current and earthquake were assumed to be in the same direction, a conservative assumption. The nonlinearities associated with cable stiffness and fluid structure interaction were linearized. They concluded that the displacement statistics agree reasonably well with the results from the time simulation of an equivalent nonlinear SDOF system.

The increased damping of the structure with increasing current significantly reduces the statistics of the forces and moments on the tower. Kawanishi et al. (1994) studied the tsunami response due to an earthquake to analyze tension leg platform, using analytical model to calculate tension in tendons. It was concluded that tsunamic considerations are required for TLP design, as the tendon tension became 146% of the balanced initial tension. And, under unbalanced initial tension, it is necessary to adjust tendon length so that tendon tension is reduced. Ryu and Yun (1997) studied the combined effects of seismic and hydrodynamic loading on guyed offshore tower. They formulated the equation of motion for earthquake and wave/current loadings, incorporating the effect of hydrodynamic damping due to water. The hydrodynamic drag force was linearized. The maximum responses were found to be significantly less than those found by conventional spectral method based on stationary assumption. Recent studies on the seismic response of articulated offshore tower (Islam and Ahmad, 2003 and 2006; and Hasan et al., 2008) show the relative importance of the seismic response, in comparison to the response due to wave forces. It was found that the maximum response for an earthquake alone is 21% more than that due to the combined effect of the sea wave and earthquake.

Structural Engineering Journal, Multi-Hinged Articulated Offshore Tower, Seismic Sea Environment, Conventional Fixed Offshore Structures, Seismic Analysis, Hydrodynamic Damping, Conventional Spectral Method, Seismic Forces, Earthquake Excitations, Hydrodynamic Damping Matrix, Deck Displacement, Power Spectral Density Function.