Objectives  As oil and gas exploration enters the field of ultra-deep water, once the outer structure of the flexible riser is damaged, the inner carcass will carry huge external hydrostatic pressure. As one of the geometric imperfections, the gap between the carcass and pressure amour may cause a significant reduction in the collapse strength of flexible risers, which will lead to the so-called wet collapse.

  Methods  Aiming at this problem, this paper presents a spring-supported arch model for estimating the collapse strength of a subsea flexible riser with a layer gap, and the wet collapse process of the carcass is divided into pre-contact and post-contact phases. In the pre-contact phase, the instability behavior of the inner carcass is analyzed using the stability theory of single ring structure. When the interlayer gap is closed, the carcass begins to be supported by the pressure armor, and the structure enters the post-contact phase, here the outside pressure amour is considered as springs which support the detached portion of the inner carcass. At the same time, the finite element collapse model is employed to verify the reliability of this spring-supported arch model for a set of gap widths.

  Results  The predictions of the proposed models agree very well with the numerical results, and the collapse pressure is very sensitive to the stiffness of the pressure amour and layer gap width. When the initial gap width between layers increases from 0 to 0.5 mm, the collapse pressure of the flexible riser will decrease by about 18%。If the tightly fitted carcass is supported by a stiffer pressure amour, the collapse strength will be greatly improved.

  Conclusions  For a flexible riser with a harder pressure amour and a smaller layer gap, the proposed analytical model can effectively predict its hydrostatic crushing strength.