Finite element and theoretical analyses of bisymmetric collapses in flexible risers for deepwaters developments

Unbonded flexible risers are manufactured with a stainless steel carcass, however the carcass layer may fail by two bisymmetric type collapses (ovality and eight modes). Therefore, this work presents analytical and finite elements modeling strategies to study both types of collapse risers fabricated with “S” carcass profile in order to predict collapse failure. The developed methodologies were applied to a 50.8 mm (2 in.) unbonded flexible riser. The results showed the importance to performs 3D finite element modeling for a proper carcass design and collapse assessment, calibration of the length of the finite element models and boundary conditions definition in order to obtain reliable results and computer time optimization. It was also found a difference in collapse loads for each type of collapse: for ovality type collapse, the development of a finite element of the carcass only is sufficient, but for the eight type of collapse it is necessary to develop a finite element model including carcass, internal polymer sheath and pressure armour. Furthermore, it was found that the analytical formulations developed to the date, can only evaluate the collapse properties of the carcass, but they are limited to be used for the design of the carcass for flexible risers.