Upul Shanthilal Fernando

An Investigation Into Fretting Behaviour in Pressure Armor Wires of Unbonded Flexible Pipes

The possibility of fretting damage in pressure armor wires of flexible pipes has been investigated. A novel experimental facility which is capable of simulating nub/valley contact conditions with dynamic slip, representative of actual pipe loading, has been developed. The test setup is equipped with a state of the art data acquisition system and a controller with transducers to measure and control the normal load, slip amplitude and friction force at the contact in addition to the hoop stress in the wire. Tests were performed with selected loading and the fretted regions were examined using an optical microscope. Results show that the magnitude of contact loading and the slip amplitude have a distinct effect on surface damage. Surface cracks originated from fretting scar were observed at very high contact loads in mixed slip sliding while surface damage predominantly due to wear was observed under gross slip. The position of surface cracks and the wear profile have been related to the contact pressure distribution. The evolution of friction force and surface damage under different slip and normal pressure conditions are presented. The effect of a general grease lubricant on friction behavior is also discussed.© 2007 ASME

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.Copyright