Residual Stress Measurements in Tensile Armours of Flexible Pipes by the Contour and X-Ray Diffraction Methods

Excessive levels of residual stresses (RS) in metallic components of flexible pipes can lead to failure by fatigue or stress-corrosion cracking (SCC) mechanisms. Accurate measurements of residual stresses are essential in order to support comprehensive integrity assessments of these structures.

This paper aims at proposing a methodology for in-field evaluation of the residual stresses in the tensile armour of flexible pipes. This methodology is evaluated by comparing results from the contour method (CM), a portable (PXRD) and a benchtop (BXRD) X-ray diffractometer.

A set of samples with a controlled RS state was used to evaluate PXRD depth-profiles by comparison with BXRD measurements. CM was used to investigate stress values elsewhere in the cross-section of the samples. PXRD was then used to measure the total stress on a full-scale flexible riser.

Strong texture was seen in XRD measurements due to the preferential orientation in the samples, but a method to overcome this is proposed. RS measurements with PXRD were in close agreement with BXRD values in all samples. Total stress profiles obtained by PXRD on a tensile armour of flexible pipe are given. CM did not provide reliable results in the near-surface region of shot-peened samples, but reasonable agreement was found with BXRD after manual adjustment of a smoothing parameter when depths from 0.3 mm to 1.5 mm were analyzed.

A methodology has been established for non-destructive in-loco profiling of the total stress state in tensile armours of flexible pipes with PXRD; in a real application scenario it could provide valuable information that could help to understand damage initiation mechanisms.