Worn pipes collapse strength: Experimental and numerical study

•We successfully collapsed full-scale intact and worn pipes similar to real cases.•A finite element model was used to make a parametric study, for both intact and worn pipes.•Analytical models were analyzed based on tests and FEM results.•Analytical models are representative of thin worn walled pipes.•Thicker worn pipes collapse are not well assessed by analytical models.

After drilling an oil or gas well the open well-bore is usually cased with steel pipes, which must be properly designed to support all predicted loads (pressures) along its service life. Such casing can be subject to material loss after deployed. One of the reasons for material loss comes from that the well-bore is drilled deeper with rotating drill pipes after casing installation. The interaction between the rotating drill-pipes and casing inner wall leads to the casing wear, which can significantly reduce the wall thickness at particular regions. Casing designers usually assume evenly distributed inner casing wear. Under this assumption the remaining wall is constant and the predictive burst and collapse strength equations presented by standards are applied, but resulting in much lower strength values than the real case.Few authors studied the pipe remaining strength under more realistic wear assumptions. Kuryama et al. presented an analytical formulation based on pipes with circular cross-section and an equivalent wall thickness eccentricity to simulate material loss over an angular section. Sakakibara et al. presented a model for collapse strength prediction of worn pipes with initial geometric imperfection (cross-section ovalization) and constant pipe wall loss within a given angular section. None of them combined real initial (ovalization and eccentricity) and produced (casing wear) geometric imperfections. This paper presents the full scale experimental set up and results for thin and thick walled intact and worn pipes under applied external hydrostatic pressure. The test procedure included pipes’ geometry mapping and wear production to match real conditions. The specimens were collapsed and numerical analysis based on finite element analysis and an analytical model were carried out to simulate physical conditions. The numerical results were then extended to a broad range of pipes with different geometries and steel grades representative of drilling well applications. As expected, one developed model developed predicts really well thin walled pipes, but not for thicker ones.