Assessing subsea wellhead settlement risk in ultra-deepwater weak shallow sediments: A stage–condition coupled vertical support framework

In ultra-deepwater areas with weak shallow sediments, stage-dependent wellhead loads can exceed the support available from disturbed-soil recovery and cementation, triggering local shear failure and progressive settlement. Existing approaches typically assess conductor capacity or final-state stability without jointly considering stagewise load transfer, support-path activation, weakest-link control, and post-failure rebalancing. This study develops a stage–condition coupled framework for evaluating subsea wellhead vertical support and progressive-settlement risk. The framework distinguishes component installation, load transfer, and support-path activation, and limits composite support by the weakest link in the load-transfer chain. Three linked criteria assess peak capacity, local shear failure, and residual rebalancing. Benchmarking against four deepwater-well cases was restricted to the conductor-capacity submodule and yielded a mean relative capacity error of 7.35%, with classifications consistent with available field observations. This comparison does not constitute validation of the full stage–condition coupled framework. A representative South China Sea case shows that the first six construction stages satisfy the peak design threshold without triggering failure. The governing condition occurs under the riser-tension workload after blowout preventer/lower marine riser package (BOP/LMRP) installation, when the vertical load reaches 4774.1 kN. At 24 h of waiting-on-cement (WOC), the peak and residual safety factors are 0.974 and 0.499, respectively, while maximum local soil-shear utilisation reaches 1.660. Residual capacity is 51.3% of peak capacity and remains insufficient for post-failure rebalancing. Over the tested parameter ranges, conductor embedment and site strength produce the largest maximum relative safety-factor changes, at 214.1% and 103.8%, respectively. In the fixed-state WOC sweep, capacity reaches a soil-controlled plateau by the 6 h sampling point, with no further gain through 48 h. Increasing embedment to 80 m at baseline site strength raises the peak safety factor to 2.049 without triggering failure. These results support coordinated control of stage loads and effective support paths, with global capacity and local shear limits assessed together. The framework provides a quantitative basis for identifying governing construction conditions and evaluating settlement-mitigation measures.