Response Behavior of Triangular Tension Leg Platforms under Regular Waves Using Stokes Nonlinear Wave Theory

Tension leg platforms (TLPs) are compliant-type offshore structures generally used for deepwater oil/gas exploration. Dynamic analysis of three triangular TLP models under regular waves is presented, considering the coupling between various degrees of freedom. The analysis considers various nonlinearities developed due to change in tether tension, change in buoyancy, and hydrodynamic drag force. The hydrodynamic characteristics of the wave loading on the structure are computed using Airy’s wave theory and Stokes’ fifth-order theory. The hydrodynamic coefficients $C_d$ and $C_m$ vary along the water depth. The low frequency drift response and the high frequency springing response are not considered in the present study. The equation of motion has been solved in time domain using Newmark’s integration scheme. Numerical studies are conducted to compare the coupled response of triangular TLPs under regular waves using Airy’s wave theory evaluated with Chakrabarti’s modification and that obtained by using Stokes’ fifth-order nonlinear wave theory. Results show that the coupled response in surge and pitch degree of freedom obtained using Stokes’ theory is lesser than that obtained using the Airy’s theory. Additionally, the presence of current in the wave field increases the responses in all degrees of freedom in all the cases taken for the study.