Neutron diffraction measurement of weld residual stresses in an UOE linepipe subjected to mechanical expansion

Weld residual stresses in double submerged arc welded (DSAW) UOE linepipes can have significant implications on the integrity and in-service performance of the pipeline from which it is normally constructed, often for deep and ultra-deep water applications. Therefore they have to be considered as a crucial data input for accurate Engineering Critical Assessments (ECAs) of offshore pipelines. This has so far been based on the assumption that the residual stresses in the UOE pipe seam weld are equal to the yield strength of the parent material: an assumption that is likely to cause over-conservatism in conventional pipeline design. In the current study neutron diffraction technique was used to quantify the weld residual stresses of an UOE linepipe (18" OD × 25.4mm WT, grade X65) in post-expansion condition. The UOE linepipe manufacturing process is briefly introduced first, which involves forming steel plate into pipe, followed by DSAW seam welding and then pipe mechanical expansion. The sample preparation for neutron diffraction measurement of lattice parameters is described next in detail. Neutron diffraction was used to measure the residual elastic strain within the pipe through the precise characterisation of the interplanar crystal lattice spacing and the atomic lattice itself was used as a strain gauge. The measurements were mainly taken in and near the seam weld area of the pipe along the pre-defined lines in through-thickness and cross-weld orientations. The weld residual stresses in three primary directions, namely axial, hoop and radial, were calculated from the measured lattice parameters in as-supplied (welded and expanded) and stress-free conditions using Hooke's law of linear elasticity, and are systematically presented in the paper. It was found that weld residual stresses of the UOE linepipe in post-expansion condition were significantly lower than the yield strength of the parent material, and in most cases these were less than 50% of the actual yield strength of the parent material. The mechanism of weld residual stress formation and the influence of post-weld expansion on the reduction and through-thickness normalisation of weld residual stresses were studied through finite element (FE) modelling analysis. Finally an example ECA analysis was carried out for the current linepipe under an assumed service loading condition. The degree of over-conservatism that could result by assuming weld residual stresses to be equal to the yield strength of the parent material was demonstrated, as compared to the situation where the values of the required residual stresses were taken from the neutron diffraction measurement. It was therefore shown by the current study that the combined approach of neutron diffraction measurement and FE modelling could greatly enhance the understanding of performance of seam-welded UOE pipes.