Multihinged Articulated Offshore Tower under Vertical Ground Excitation

Abstract

In this study, the response of a multihinged articulated offshore tower subjected to different seismic excitations in the presence of random waves was investigated. The study includes near-fault as well as far-fault earthquake ground motions. The contribution of the vertical component to the overall seismic behavior of the articulated tower is examined. The nonlinearities associated with the system owing to variable submergence, drag force, variable buoyancy, and added mass, along with the geometry, are also considered. The nonlinear dynamic equation of motion is formulated considering the Lagrangian approach, which is solved in time domain by the Newmark-beta integration scheme. The results are expressed in the form of time histories and spectral densities of the dynamic responses. Dynamic response quantities such as rotational angle, hinge shear, axial force at the articulation, and bending moment at peak ground acceleration in different seismic sea environment are discussed. The spectral responses under different seismic sea excitation have different contributions to the response evaluation of the tower. Of these, the occurrence of peaks nearer to the average loading frequencies is a clear indication of the influence of seismic excitation on the structural response. Moreover, with the inclusion of the vertical ground motions, the demand in the axial thrust at the base articulation increases, which in turn seems to have an effect on moment and shear demands at their respective critical sections. Therefore, the integrity of the articulating system may be at high risk if subjected to stresses exceeding the design limits.