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

Abstract

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.

A review on predicting critical collapse pressure of flexible risers for ultra-deep oil and gas production

“…16): one is the roll bending stage where the metallic wire experiences a sequence of bending and twisting events; another is the interlocking stage, where the profiled wire is wound onto a bobbin [83]. Owing to the practical difficulties, there is no available post-deformation stress relief operation that could be conducted to the flexible riser products [84]. Those products, therefore, contain unknown magnitude of residual stress in the cross-section of their armor wires.…”

“…By contrast, non-destructive approaches are gaining popularity among researchers for their advantages of determining the stress state in-situ on the manufactured risers. Fernando et al[84] first used the x-ray diffraction method to measure the distribution of residual stress in pressure armor. This method evaluated the magnitude of the residual stress by measuring the changes in the spacing of the lattice planes…”

Prediction of the critical collapse pressure of ultra-deep water flexible risers-a: Literature review

“…Residual stress is an imperfection also generated from the manufacturing process. Owing to the practical difficulties, the residual stresses contained in the carcass strip cannot be relieved after it has been wrapped onto a bobbin [42]. As the interlocked layers are embedded within tensile armors, it is extremely difficult to measure the residual stress of interlocked wires in-situ.…”

“…Therefore, some nondestructive methods, e.g. X-ray diffraction [42] or neutron diffraction methods [43], are employed by researchers to establish the preliminary studies of residual stress measurement.…”