Recent failures of multi-layer pressure sheath risers have shown that the carcass may fail in the top termination due to excessive axial loads. This is a new failure mode for flexible risers, recently presented by the authors in more general terms. The present paper explains details of the established load model and the validation against mid-scale tests, risers failed during operation, and operating risers close to failure by this new mode. The key driver in the model is the temperature contraction of pressure sheath layers. Also influenced by changes in polymer properties over the operational history, temperature and time is explained. Other contributing factors in the load model are gravity-component and bore pressure.
The prediction model for the carcass loads are developed during Statoils investigation in 2011–12. The model is regarded representative for 20% of the most exposed risers. Several of the input parameters are uncertain and a Monte Carlo simulation approach is selected to study the variability and predict the probability of failure, given that radial contact pressure is sufficiently low.
The approach adopted in the model may be applicable to other risers where polymers and steel components act together, and in such circumstances act as a guide for alternative model developments.