ABSTRACT

Whether needed for transporting oil, gas, hydrogen, carbon dioxide or water, pipelines are being designed for and installed in increasingly deeper waters; water depths of more than 2,000 m are no longer an exception. The main driver for the design of such pipelines is their resistance against hydrostatic pressure, especially during installation. Selker et al. (2022, 2023) have developed an improved method for predicting the collapse pressure and compared their analytical results with the outcome of finite-element analyses. In this paper, the performance of the improved model and the conventional DNV model (DNV-ST-F101, 2021) is compared with the results of full-scale collapse testing. The predictions of the improved model are significantly closer to the actual test results of cold-formed pipe in as- fabricated condition compared with the conventional model. For a pipe subjected to (light) heat treatment, both methods’ estimates are relatively close to the test results. The Bauschinger effect and the associated change of the stress–strain curve's shape must be reflected in the model to achieve consistent and accurate results.

INTRODUCTION

Line pipe can be fabricated in various ways. The characteristics of the final pipe joints depend on the adopted manufacturing route. For example, seamless pipe and seam-welded pipe differ fundamentally in their dimensional and mechanical properties. Typical manufacturing methods for seam-welded, large-diameter offshore line pipe are the UOE (U-ing, O-ing and expansion) and JCOE (J-ing, C-ing, O-ing and expansion) techniques. In these methods, steel plates are cold formed into cylinders and the edges are seam welded. Next, the cross-section is cold expanded to improve its roundness. Geometric improvement enhances the pipeline's capacity to resist external pressure; however, cold forming has some unfavourable consequences as well. The expansion step will increase the material's tensile strength in the direction of applied expansion (the circumferential direction). Unfortunately, this diminishes the associated compressive strength because of the Bauschinger effect. Moreover, the shape of the material's stress–strain curve changes. These effects can be offset by a (light) heat treatment, during which the pipe joint is heated to and kept at 210–235°C for at least 3 minutes (Kerstens et al., 2014). This can be part of the anti-corrosion coating's application procedure or a separate fabrication step.

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