Submarine-pipeline developments are moving into deeper waters. As a result, thicker pipe walls are needed and hence the quotient of the pipe diameter and its wall thickness (i.e., D/t) reduces. On the other hand, there is a push to optimise properties and requirements for moderate water depths of several hundred metres. Design criteria in the prevailing submarine-pipeline design standard DNVGL-ST-F101 (2017) were developed for straight pipes with 15 ≤ D/t ≤ 60 but applicability is limited to D/t ≤ 45. There can be a desire to go beyond this limit when designing for moderate depths.

When pipe walls are becoming thinner, this makes the pipeline more susceptible to collapse. It has been found that, in some cases, the DNVGL-ST-F101 formulations that aim at predicting the collapse pressure of relatively thin-walled pipelines can lead to nonconservative results. This is a concern when the line-pipe manufacturing technique employs cold forming—such as the cross-section expansion step in the UOE and JCOE methods—and the adverse contribution of the Bauschinger effect is not mitigated in a suitable manner.

INTRODUCTION

The UOE and JCOE methods are used for manufacturing largediameter seam-welded line pipe. Both methods include a coldexpansion step, which can promote the Bauschinger effect. The pipe's compressive stress–strain curve can be affected in two different ways: (1) the plasticity plateau can be lowered (i.e., a reduction of the yield stress), and (2) the curve can become more rounded near its transition from linear-elastic to plastic behaviour. Current design formulations were derived after assuming bilinear (elastic-perfectly plastic) relations between stress and strain, and deal with only the first effect, via the fabrication factor. The second effect can cause significant narrowing of the range in which the material behaves linearly and is particularly pronounced for steels that are not subjected to (light) heat treatment after cold forming.

The collapse mechanism is a typical design driver for pipelines in deep and ultra-deep water. However, it can also be important for pipelines in intermediate water depths. Collapse is the failure mode where the pipe cross section becomes unstable due to excessive external pressure acting on the pipe wall. It is typically viewed as buckling or bifurcation behaviour because the cross section will deform significantly only when the acting pressure is very close to the limit capacity. In this range of pressure, a very small pressure increase will result in a relatively large growth of deformations. When the cross section cannot sustain any more pressure increase, it will ovalize excessively and ultimately flatten: this is called collapse.

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