A stress-based safety assessment and scheme determination framework for trenching and lowering-in process of submarine pipelines

During long-term service, ocean-current-induced seabed scour may reduce the burial depth of submarine pipelines, leading to shallow-buried segments or free spans. Trenching and lowering-in is commonly adopted to restore pipeline burial, but selecting appropriate trench length and depth remains challenging because the pipeline stress response evolves nonlinearly during progressive excavation, settlement and trench-bottom contact. In this study, a nonlinear finite-element model is developed to evaluate the stress–deformation response of pipelines during trenching and lowering-in, considering pipe–soil interaction, large deformation and trench-bottom contact. A large-diameter X60 submarine oil pipeline that has operated for more than 20 years is used as a case study. Parametric analyses are conducted to quantify the effects of wall thickness, trench length, trench depth and operating pressure on suspended-span development and maximum von Mises stress. The results show that four characteristic stress peaks are generated by trench-shoulder support and trench-bottom contact, with the dominant peak located near the trench shoulder. Increasing trench length produces a two-stage stress response, characterized by rapid stress growth before contact and stabilization after a stable contact region forms. This behavior supports the definition of a critical trench length associated with peak bending demand. Trench depth markedly amplifies settlement and peak stress, whereas operating pressure has a secondary influence. For large target burial depths, multi-step trenching is recommended. Finally, a stress-based screening framework is proposed to identify admissible trench length–depth combinations under construction-stage assumptions.