Search

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 Cd and Cm 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. [ABSTRACT FROM AUTHOR]

Oil and gas production from deep-water offshore fields represent a major structural engineering challenge for the industry. The tension leg platform (TLP) is a well-established concept for deep-water oil exploration. It is necessary to design an offshore TLP such that it can respond to moderate environmental loads without damage, and is capable of resisting severe environmental loads without seriously endangering the occupants. Seismic analysis of triangular TLP under moderate regular waves is investigated. The analysis considers nonlinearities due to the change in tether tension and nonlinear hydrodynamic drag forces. The coupled response of TLP under moderate regular sea waves due to change in initial pretension in the tethers caused by seismic forces (vertical direction) is then investigated. Seismic forces are imposed at the bottom of each tether as axial forces. The tether tension becomes unbalanced when the hull is under offset position. The vertical component of seismic force is an important item to take into consideration, because it is directly superposed to pretension of tethers. The change in initial pretension due to the vertical component of the earthquake affects the response of the triangular TLP in degrees-of-freedom experiencing such forces. The tether tension varies nonlinearly when the platform is subjected to seismic forces caused by the El Centro earthquake and artificially generated earthquake using Kanai–Tajimi's power spectrum. The response due to earthquakes varies with the intensity of the input ground motion. The seismic response of the triangular TLP exhibits nonlinear behavior in the presence of waves and it is non-proportionately influenced by the wave period and the wave height. [ABSTRACT FROM AUTHOR]