Reeling Analysis Including Strain Concentrations at Girth Welds

Abstract

Dimensional variations across a girth weld undergoing reeling leads to strain concentrations close to the weld that have to be taken into consideration in crack growth evaluations. The aim of the present paper is to outline an analysis procedure for simulating the complex load history, and to illustrate the effect of material and wall thickness variations between the two joints on each side of the girth weld. Verification analyses are also made towards full-scale tests simulating the cyclic loading during the reeling process. The computer software ANSYS was applied with contact and plasticity formulation, and the load history through reel, aligner and straightener was modelled. A combination of beam and shell elements was applied for the pipe, ending up in an efficient tool for design analysis. The nonlinear material model included kinematic hardening and was based upon a Ramberg-Osgood type of stress-strain curve. The effect of kinematic hardening through cyclic loading was studied in an effort to define the material properties of the as-installed pipe. The numerical model also came out with contact pressures between the reel and the pipe, being of special interest for thermally coated pipes. Ovalisation of the pipe was available, and goes into the bending moment capacity for expansion control. A full-scale bending test of clad pipe around a girth weld was performed, simulating the load history through the reeling process. Measured strains and forces were compared with numerical results as a verification of the analysis model. A separate ANSYS model was created for the test case, based on the material models in the global model. The location of strain concentrations in the vicinity of the girth weld was given special attention in the comparison between tests and numerical simulation. Material tests for backing steel and clad, respectively, around the circumference of the as-tested pipe illustrate the strain hardening effect as basis for evaluation of the kinematic hardening parameters implemented in the numerical models. Analyses were then used for a parameter study on strain concentrations with yield stress and wall thickness variations across the girth weld. Also, the amplifying effect of local buckling during reeling is evaluated, as basis for defining needed back-tension.