Aerodynamic Response of Offshore Spar Platforms

Offshore Spar platforms are the floating platforms and behave as moored structures in horizontal as well as vertical planes in deep oceans. Under the influence of wind, superstructure of the spar is subjected to aerodynamic drag forces in the windward direction and the underwater portion is exposed to random waves and currents. Spar motions have time periods falling in the wind excitation frequency range due to its compliant nature. Dynamic response analysis of spar platforms to a low frequency wind forces with random waves is presented in the paper. Nonlinear effects due to variable submergence of the structure, hydrodynamic forces and cable tension variation are also considered. The random sea state is characterized by Pierson-Moskowitz sea surface elevation spectrum. The wave forces on the elements of the structure are calculated using Morison's equation with Airy's linear wave theory ignoring diffraction effects. The fluctuating wind has been estimated using Emil Simiu's wind velocity spectrum for offshore structures. The nonlinear equation of motion is solved in the time domain by numerical integration procedure. The wind structure interactions along with the influence of various other parameters on the structure's response are investigated. Results indicate that the low frequency wind forces alter the response of the spar to a considerable extant which otherwise is not enhanced in the presence of random waves alone. The mean wind modifies the mean position of the surge response to the positive side, causing an offset. Apart from the prominent peak occurring at the natural frequency, other peaks also appear showing the super-harmonic responses.