Local plastic deformation in pressure and tensile armour layers of flexible risers

Flexible pipes are commonly used for connecting seabed flow-lines to floating production facilities. The general riser design consists of an internal carcass for collapse resistance, a polymer fluid barrier, carbon steel interlocked circumferential pressure armour layer for resisting internal pressure loads, helically wound carbon steel tensile armour layers to resist axial loads and a watertight external sheath. Much of the analytical work that has been carried out on flexible composite pipe is based on the early stress analyses of wire ropes, and this is primarily because of the helical geometry of many of the metallic elements such as pressure armour and tensile armour layers of a flexible riser. The general design philosophy of the layer is defined in API 17J (American Petroleum Institute) in terms of the stress "utilisation" factor that specifies the maximum allowable average hoop stress in the layer, which is conventionally produced by the elastic stress analysis.

The layers are subjected to severe cyclic bending and twisting deformation during manufacturing of the pipe which makes the material to exceed the yield point. Consequently, residual stresses are developed in the pipe material and variable amounts of non zero stresses exist in the metallic layers of the newly manufactured pipes. An attempt has been made to model the amount of residual stress evolved during manufacturing stage for two different metallic layers, namely; pressure armour and tensile armour layers and its effects on overall pipe's performance while in service.

The strategy is to apply the finite element (FE) method by creating a3 dimensional wire model of the segment of pipe. Solid structural elements with plasticity were employed for the analyses. Kinematic hardening with Baushinger effect, contact and friction effects were also taken into account. Precise boundary conditions were applied to the model for elastic-plastic bending. The resulting residual stresses have been transferred to 3D axisymmetric model to investigate the internal and external pressure effects. This new procedure was carried out for both layers satisfying equilibrium and compatibility conditions for the pressure and tensile armours to find the common interface pressure and contact loading. FAT (Factory Acceptance Test) condition is simulated to investigate the level of residual stress variation. Experimental measurements of residual stress by X-Ray diffraction are in well agreement with FE results.