Earthquake Response of a Multi-Articulated Offshore Tower in Random sea

The present study deals with the seismic compliance of a multi-articulated offshore tower to three real (El Centro 1940, Taft 1952 and Northridge 1994) earthquakes in a random sea environment. The random waves are generated using Monte-Carlo simulation with the Pierson-Moskowitz (P-M) spectrum. The shafts of the tower are idealized as a rigid cylindrical column with 2 rotational degrees of freedom. Seismic forces are evaluated by dividing the tower into finite elements with masses lumped at the nodes. The earthquake response is carried out by random vibration analysis, in which, seismic excitations are assumed as a broadband stationary process. The dynamic equation of motion is solved in time domain. Nonlinearities due to variable submergence and buoyancy, added mass along with the geometrical nonlinearities associated with the system are taken into account. The results are expressed in the form of time-histories of the response quantities. Power spectral densities under different seismic and random sea environments are plotted.