Measurements of Residual Stresses Using Mechanical Strain Relaxation Methods and Mapping in a Thin-Walled Girth Welded Pipe Mock-Up

Abstract

Girth welded pipes such as those located offshore on platforms in the North Sea are subjected to highly corrosive environment. The need to consider welding residual stresses in the assessment of the fitness for service and damages to these pipes when investigating local corrosion damages across a welded region is therefore important for the operators of the platforms and the manufacturers of the pipes. This paper presents a review of work carried out to ascertain the welding residual stresses present within a thin-walled girth welded pipe mock-up made from steel API 5LX Grade 52. The mock-up was manufactured to replicate typical pipes used to convey gas, oil and water through the platforms. The mock-up was of diameter 762mm and of thickness 19mm. The incremental deep hole drilling (iDHD) technique and the contour method were applied to characterize the residual stresses in the weld and heat affected zone of the specimen. The results of these measurements are presented and compared to highlight agreements and discrepancies in the measured residual stress distributions using these different techniques. Most residual stress measurement methods are limited in terms of their stress and spatial resolution, the number of measurable stress tensor components and their quantifiable measurement uncertainty. In contrast, finite element simulations of welding processes provide full field distributions of residual stresses, with results dependent on the quality of the input conditions available. As measurements and predictions are not often the same, the true residual stress state is therefore difficult to determine. In this paper, through-thickness residual stress measurements are made using the contour and iDHD methods and these residual stresses measured using the iDHD technique are then used as input to a residual stress mapping technique provided within a finite element analysis to reconstruct the residual stress field in the whole specimen. The technique is applied iteratively to converge to a balanced solution which is not necessarily unique. The solution can then be reused for further simulations and residual stress analyses, such as corrosion simulation. Results of the reconstruction are presented here.