Finite Element Analysis of Cold Field Bends for Sour Service Applications

Abstract

In onshore pipelines, cold field bends are regularly used for planned and in-situ pipeline route adjustments. The cold field bending operation consists of curving permanently a straight pipe until the desired change of alignment is achieved. The bends are achieved by locally bending the pipe against a die using a pipe bending machine. Bending against the die imposes a transversal plastic deformation over a short distance, many such local deformations are made to form the required cold field bend. Thus, cold field bends experience plastic strains from the formation process and residual stresses are left in the finished product. The onshore pipeline industry typically disregards residual stresses from cold bending as these are difficult to evaluate; mainly, because the main pipeline stress software packages used by the industry do not capture them. This empirical approach has provided a good record of pipeline service reliability for many years; although, international codes such as ASME B31.8 transfer the ultimate responsibility to determine whether such stresses should be evaluated to engineers (projects). For applications in sour service environments, where additional plasticity during operation generally needs to be avoided, the influence of these residual stresses may be significant, and the robustness of the current practice should be considered. In this context, this paper presents a comparative finite element analysis (FEA) using ABAQUS to evaluate the impact of the residual stresses from cold field bending on the overall stress state during operation. Analyses are performed using a representative single 14″ × 19.05mm × 80D × 14.7° cold field bend which is buried in a non-cohesive soil. To bound the range of local curvatures that the bend has to withstand during the formation process, models are run with a bend uniformly bent to 52D which relaxes to 80D, and a bend with 24 short sections bent to 17D (die radius) which relaxes to an average of 80D with a non-constant curvature along the bend length. To capture the impact of the type of element over the ovalisation and capture the influence of the residual longitudinal and hoop stresses, models are run with pipe, elbow and shell element models. Based on the results of these analyses, this paper recommends additional modelling and testing requirements for cold field bends for more sensitive applications such as sour service. These requirements intend to complement the approach currently adopted by the industry and ensure the fitness for service of cold field bends.