In the North Sea, carcass failures have over the last years been the largest failure category for flexible pipes, causing multiple riser replacements . The carcass failures could be split into two groups: axial tear-off and collapse.
In order to determine the resistance to collapse, a 3D analysis model has been built using the explicit finite element (FE) program LS-Dyna.
In addition, the LS-Dyna results were compared with a simplified 2D FE approach, using the non-linear implicit solver MARC.
The 3D model allows for more complex sensitivities, such as curved pipe and carcass tension.
The FE analyses were based upon measured carcass profile geometries and material data from hardness measurements at several positions along the carcass profile, including effects of strain hardening during manufacturing.
Three different carcass profiles of three different risers were analysed. The study includes sensitivities on straight and curved pipe sections, axial preloaded carcass, carcass ovality, radial gap between carcass and pressure sheets and pressure increase velocity.
Collapse resistance of axial strained carcass and bent pipe is not well documented and this paper will highlight some of the observed effects.
The results from the FE analyses showed good correlation between the vendor data  both in shape of the buckling mode and capacity.