Local Stress Assessment of Flexible Unbonded Pipes Using FEA

Detailed stress analysis of unbonded flexible pipes has always been a challenging exercise which requires significant amount of computational resources and state of the art knowledge of flexible pipe modelling techniques. Lloyd’s Register EMEA and Brunel University have embarked on the development of a model which captures the response of the individual unbonded layers of flexible pipes using the ABAQUS finite element package employing 3D explicit and 3D implicit integration rules. The model accounts for nonlinear interaction between layers and provide the hysteresis response. The use of periodic boundary conditions to eliminate boundary effects at the pipe ends has also been investigated. The model predicts stress and strain conditions in metallic and plastic layers. These FEA predictions of the 3D non linear models are compared with the results of analytical methods for axisymmetric loads. This verification is carried out for three basic cases as follows: i) axial tension ii) internal/burst pressure iii) radial external pressure. Good agreement has been shown for stiffness and stress values predicted by the FEA and analytical formulations. Some initial results are presented for the bending behaviour in which the hysteresis obtained in the case of cyclic application of the design minimum bending radius is simulated.