A pipeline’s resistance to collapse is governed by geometric imperfections, material properties and residual stresses. The offshore pipeline design code DNV-OS-F101 provides a method for predicting collapse of pipelines with diameter to wall thickness (D/t) ratios between 15 and 45. This paper examined the various factors that could influence the collapse resistance of several pipe geometries, such as ovality, eccentricity, material stress-strain behavior and residual stresses in the hoop and longitudinal directions. A total of 132 cases were carried out, using 2D and 3D Nonlinear Finite Element Analysis, to predict the collapse pressure of several realistic pipe geometries. Results of this study suggest that the DNV-OS-F101 predictions are conservative and applicable for a wide range of D/t ratios. While there is close correlation between Finite Element prediction and DNV-OS-F101 prediction, there is a degree of conservatism at low D/t ratios using DNV-OS-F101 equations. Hence there would be scope for further optimization of pipe wall thickness design against the collapse limit state at low D/t ratios.

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This paper examines the magnitude and location of the maximum residual stress induced in pipes after the process of bending, reverse-bending and straightening. Dimensional analysis is used to establish generalized equations relating the maximum residual stress magnitude and location to the pipe geometry, maximum bending curvature and pipe material’s yield stress. 64 design cases based on an analytical solution assuming elastic-perfectly-plastic material behavior have been conducted. Regression analysis has revealed that the magnitude of the maximum residual stress can be conservatively approximated by a simplified quadratic equation involving the maximum axial bending strain, whereas the location of the maximum residual stress can be approximated by a linear function based on the same. Both equations are expected to be valid and conservative for X65 and X70 grade steel pipes under global maximum axial strain between 1% and 3%. Non-linear finite element analysis based on a realistic design example with isotropic hardening material is used to validate the prediction results based on the simplified equations.

It is commonly understood that residual stresses can have significant effects on structural integrity. The extent of such influence varies and is affected by material properties, manufacturing methods and thermal history. Welded components such as pipelines are subject to complex transient temperature fields and associated thermal stresses near the welded regions. These thermal stresses are often high in magnitude and could cause localized yielding around the deposited weld metal. Because of differential thermal expansion/contraction episodes, misfits are introduced into the welded regions which in turn generate residual stresses when the structure has cooled to ambient temperature. This paper is based on a recently completed Joint Industry Project (JIP) led by DNV GL. It briefly reviews published experimental and numerical studies on residual stresses and strength-mismatched girth welds in pipelines. Several Finite Element Analysis (FEA) models of a reeling simulation have been developed including mapping an initial axial residual stress (transverse to the weld) profile onto a seamless girth-welded pipe. The initial welding residual stress distribution used for mapping was measured along the circumference of the girth welds. The predicted residual stresses after reeling simulation was subsequently compared with experimental measurements.