Abstract
Nonbonded flexible risers serve in high-pressure environments, and the external hydrostatic pressure is mainly resisted by the carcass layer. If the external sheath layer of the flexible riser is damaged, seawater enters the annular, putting the carcass layer at risk of collapse. Therefore, accurate prediction of the critical collapse pressure of the carcass during the design phase is essential to prevent collapse and ensure the safety of the flexible riser. When the external sheath is damaged, the pressure armor does not directly resist external pressure; however, it can indirectly increase the critical collapse pressure by restricting the outward displacement of the carcass layer after collapse. In the present work, finite element models are first established to investigate the collapse behavior of the carcass with and without pressure armor. Based on the numerical results, the pressure armor layer is equivalently modeled as radial springs supporting the carcass, and an analytical method based on the principle of minimum potential energy is developed to predict the critical collapse pressure. The critical collapse pressure of the carcass with and without the pressure armor is calculated separately and it is verified that the presence of the pressure armor layer increases the critical collapse pressure. The critical collapse pressures obtained from the analytical method are then compared with the corresponding finite element results. Furthermore, the proposed analytical approach is validated against model test results. The comparison between the analytical and finite element results shows that the proposed method performs well for carcass layers without pressure armor. However, for cases including pressure armor, the predicted critical collapse pressure is underestimated compared with both experimental and finite element results, although the prediction is more accurate than that obtained for models without pressure armor. This underestimation can be attributed to the fact that, during collapse, the contact between the carcass layer and the pressure armor occurs over a finite region rather than at a discrete point. As a result, the equivalent representation of the pressure armor provides insufficient constraint, leading to a lower predicted critical collapse pressure.