Transient response mechanism of the deepwater artificial seabed tower system under mooring tether failure conditions in internal solitary waves

A new-principle Deepwater Artificial Seabed (DAS) tower system is proposed, enabling the development of large deepwater fields with prevailing shallow-water technology and equipment. Nevertheless, its dynamic stability and structural safety are confronted with potential threats from the widespread Internal Solitary Waves (ISWs), especially in the worst-case scenario of the mooring tether fracture. Hence, this investigation is intended to explore the nonlinear transient response mechanism of the DAS tower coupling system under mooring tether fracture conditions in ISWs. A coupled transient analysis method is established and validated by the nonlinear response experiments carried out in a density-stratified internal wave tank. The DAS tower system's nonlinear responses under intact and damage conditions are analyzed contrastively from aspects of the temporal motions of the Artificial Seabed (AS), the overall stability of the subsurface wellhead, and the mechanical performance of the rigid riser and mooring systems. The resultant transient effects from the mooring tether fracture acting on the DAS tower system in ISWs are quantitatively characterized from a time-frequency viewpoint. The relative importance of key design parameters is investigated and determined. The research results confirm that the temporal response extremums of the DAS tower system along the surge, sway and yaw directions under mooring tether fracture conditions in ISWs result mainly from the ISW loads, while those of the relative heave motion, roll and pitch responses, and the mooring tension variation are dominated by the transient effects due to the mooring tether breakage. The von Mises stress on the rigid riser passes the allowable level due to the internal coupling effects of the AS, leading to subsequent failures of the DAS tower system. A new mooring pattern is recommended to improve the survivability of the overall system under one tether failure condition in ISWs.