Recent incidents on the Norwegian shelf showed that the inner steel structure of a flexible riser, the carcass, can experience extensive axial loading. These events resulted in carcass overload followed by a spin-out and tear-off at the carcass end, eventually followed by shutdown of production. The carcass axial tension capacity has previously not been considered a critical design issue for flexible pipes. The incidents resulted in an extensive program, initiated by Statoil, to find the root cause of the problem. Both analytical and computational efforts validated through extensive testing of carcass axial capacity has been conducted. Advanced finite element analysis was used to establish both the carcass capacity and also the carcass load level as a function of pitch length. The numerical results are compared and validated towards experimental data. The results form a basis to suggest operational policies to mitigate the risk of new failures.

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