The Stress Analysis and Residual Stress Evaluation of Pressure Armour Layers in Flexible Pipes Using 3D Finite Element Models

A pressure armour layer is an essential feature of un-bonded flexible pipes. The layer is made of an inter-locked helically wound metal wire of profiled section, whose primary use is to provide the circumferential strength of the pipe to resist internal pressure. The general design philosophy of the layer is defined in API 17J 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. During pressure armour layer manufacturing (a cold forming process), the armour wire is however subjected to a sequence of cyclic bending and twisting deformations which take it beyond its material elastic limit. This paper presents FE structure models for investigating the detailed local and residual stress variation during the forming process, and the subsequent stress relaxation as a result of the factory acceptance test (FAT). A study case is presented for illustrating the typical stress and strain behaviour after FAT pressurization. The paper also introduces X-ray diffraction technology as a method for residual stress measurement on full scale samples.