Aerodynamic behavior of offshore triangular tension Leg platforms

Abstract

Tension Leg Platforms have large stiffness in vertical plane whereas behave as moored structures in horizontal plane. TLP motions have time periods falling in the wind excitation frequency range due to its compliant nature. Dynamic response analysis of Triangular TLP to a low frequency wind forces with random waves is presented in this paper. Nonlinear effects due to variable submergence of structure, hydrodynamic forces, 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 pontoon structures 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. High frequency effects as springing & ringing have not been considered. 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 structural response are investigated. The effect of the offset of aerodynamic center (AC) and center of gravity (CG) of the platform on the coupled response of triangular TLP is also studied. Results indicate that the low frequency wind forces alter the response of TLP to a considerable extent 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. Copyright © 2004 by The International Society of Offshore and Polar Engineers.