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

During long-term service, seabed scour driven by ocean currents can reduce the burial depth of submarine pipelines, leading to shallow-buried segments and free spans. Trenching and lowering-in is widely used to restore burial, yet selecting an appropriate trench length and depth remains challenging because the pipeline stress response evolves nonlinearly during progressive trenching and settlement. In this study, a nonlinear finite-element model is developed to capture the stress–deformation behavior of pipelines during trenching and lowering-in, accounting for pipe–soil interaction, large deformation, and trench-bottom contact. A large-diameter X60 submarine oil pipeline that has operated for more than 20 years in the investigated sea area is used as a case study. Parametric analyses are performed to quantify the effects of wall thickness, trench length, trench depth, and operating internal pressure on the suspended span development and the maximum von Mises stress at critical sections. The results indicate that the stress response exhibits four characteristic peaks associated with trench-shoulder supports and trench-bottom contact, with the dominant peak occurring near the trench shoulder once contact is established. Increasing trench length produces a two-stage response featuring a rapid stress increase prior to contact and a stabilized response after a stable contact region forms, supporting the definition of a critical trench length associated with peak bending demand. Trench depth markedly amplifies settlement and peak stress, whereas operating internal pressure within the investigated range has a secondary influence. For large target burial depths, multi-step trenching with smaller depth increments is recommended to control peak stress while achieving the design depth. Based on the parametric database, a stress-based scheme-determination framework is further established to delineate admissible trench length–depth combinations for practical field design.