Time-domain dynamic response analysis of progressive mooring failure for deep-sea semi-submersible platforms

• First deepwater semi-submersible study on progressive mooring failures under 100-year extreme sea states (South China Sea). • Wave direction dictates platform dynamics: Aligned conditions maximize surge drift (46.5 % ↑), while 90° waves amplify sway 348 %. • Consecutive same-location failures cause catastrophic drift (205.8 m displacement, 310 % ↑ vs. single failure) and 315.5 % yaw increase, degrading mooring redundancy. • A comparative analysis of the platform's responses under intact and progressive mooring failure conditions was conducted through 51 scenarios, covering failures at different/same locations with varying sequences. The study, under combined wind-wave-current loads, incorporated first-order wave forces, second-order wave drift forces, and wave drift damping. • Failure sequences induce mirrored transient responses but identical final equilibria, proving mooring symmetry governs recovery. Deepwater semi-submersible platforms play a pivotal role in the exploration and exploitation of marine resources. The "Deep Sea No.1″ semi-submersible platform in the South China Sea serves as the reference case for numerical simulations. Progressive mooring failures may significantly increase the risk of subsequent damage to remaining mooring lines, making the platform's self-stabilization capacity after consecutive failures critical for operational safety. Numerical simulations investigate the response of a deepwater semi-submersible platform to progressive mooring failures under 100-year return period extreme sea states. Coupled dynamic analysis of the floater-mooring system was performed in the frequency and time domains in this numerical simulation. The analysis incorporates coupled wind-wave-current interactions to evaluate platform motions, fairlead tensions, and drift characteristics. The study examines 51 scenarios (including 3 intact cases) covering diverse sea conditions and mooring failure configurations, with comparative analysis between intact and failed conditions. Mooring lines are intentionally disconnected at specified time instants to simulate transient/steady-state platform responses and residual mooring tensions. Results demonstrate significant wave direction effects on surge/sway/yaw motions, with maximum fairlead tensions in head seas and strong drift direction-wave heading correlation. Consecutive same-location failures cause irreversible catastrophic drift with 315.5 % yaw angle increase versus single failures, degrading residual mooring capacity while increasing adjacent line tensions. Different failure sequences induce symmetrical transient drift and fairlead tension variations due to mooring system symmetry, with consistent final equilibrium positions despite divergent transient drift directions.