Wet Collapse of Carcass in Nonbonded Flexible Riser

Abstract The nonbonded flexible risers are critical transportation equipment in offshore oil and gas engineering, and their structural integrity is essential for the safe and reliable operation of oil and gas projects. The outer sheath layer, which is the component directly exposed to seawater, is relatively more susceptible to damage. Once the outer sheath layer is damaged, seawater can penetrate into the annular space, causing the external pressure to act directly on the structural layers. Under such circumstances, the flexible riser becomes more vulnerable to collapse. Although the pressure armor does not directly resist external pressure in wet collapse failure, it serves as a constraining structure against ovalization of the carcass, thereby indirectly increasing the critical collapse pressure. This paper analyzes the influence of the pressure armor on the wet collapse of flexible risers. Then a simplified finite element model was proposed by simulating the pressure armor as an elastic foundation surrounding the carcass. A full finite element model including the actual pressure armor was established in ABAQUS for validating the simplified model. The results show that the simplified model provides conservative predictions of the critical collapse pressure and confirm that the pressure armor significantly elevates the wet-collapse capacity by restraining carcass ovalization after instability. This approach therefore is suitable for preliminary design and large-scale parametric assessments due to its efficiency.