Håvar Ilstad

The behaviour of an offshore steel pipeline material subjected to bending and stretching

Guidelines have been worked out on how to design sub-sea pipelines in fishing-rich areas subjected to the possible interference by trawl gear or ship anchors. One topic of special interest for the offshore industry is pipelines first subjected to impact from an anchor before being dragged along the seabed. After removal of the load, the pipe will be straightened due to rebound and present axial forces. The material in the deformed impact zone will experience a complex stress and strain history, which subsequently can cause cracking, leading to leakage or full failure. To study these topics, full-scale testing is not straightforward and thus a simplified approach is chosen as a first step in the present study. Motivated by the observed local curvature in impacted pipelines, three-point bending tests of plate strips cut from a typical offshore pipeline have been carried out and the strips subsequently stretched to a straight position. One objective of these tests was to investigate whether cracking in the plate strip could occur after such a loading sequence. Material tests with specimens taken in different directions and at different locations in the actual pipe were carried out to calibrate an appropriate constitutive relation (taking anisotropy and kinematic hardening into account) and a simplified fracture criterion. Numerical simulations of the complete loading sequence were finally carried out and the predicted response was validated against the experimental data.

Damage and Failure in an X65 Steel Pipeline Caused by Trawl Gear Impact

Offshore pipelines subjected to accidental impact loads from trawl gear or anchors may experience large global deformations and large local strains, creating a complex stress and strain history. In this study experiments and numerical simulations have been carried out to investigate the impact of a pipeline which is subsequently hooked and released. Material and component tests have been performed to investigate the behaviour during impact, and to observe if/when fracture occurs. The pipes were first impacted in a pendulum accelerator at varying velocities before they were pulled straight in a tension machine. Fracture was found in the impacted area of all the pipes during straightening. Material tests were done to determine the characteristics of the X65 grade steel. Numerical simulations showed excellent compliance with the impact phase, while the load level in the stretching phase was a bit overestimated.Copyright

Transverse Deformation of Pressurised Pipes With Different Axial Loads

Pipelines residing on the seabed are exposed to various hazards, one of them being denting, hooking and release of the pipeline by e.g. anchors or trawl gear. As a pipeline is displaced transversely in a hooking event, an axial tensile load resisting the displacement builds up in the pipeline. This study examines the effect of applying three different axial loads (zero, constant, and linearly increasing) to a pipe while simultaneously deforming it transversely. A fairly sharp indenter conforming to the prevailing design codes was used to deform the pipes. These three tests were repeated with an internal pressure of about 100 bar for comparison. Adding an axial load appeared to increase the pipe’s stiffness in terms of the force-displacement curve arising from deforming the pipe transversely. The internal pressure also increased the stiffness, and produced a more local dent in the pipe compared with the unpressurised pipes. All tests were recreated numerically in finite element simulations. Generally, the results of the simulations were in good agreement with the experiments. INTRODUCTION Pipelines are an integral part of the offshore industry and will continue to be so for the foreseeable future. Multiple hazards are present in the waters [1], and close to the coast pipelines ∗Corresponding author may suffer impact and hooking by e.g. anchors or trawl gear [2]. An initial impact typically causes a dent in the pipe, and if the impacting object hooks the pipeline it may displace it significantly, during which membrane forces arise in the pipeline. When the pipeline is released, it recoils back towards its initial position, thereby creating a complex load history. The open literature provides studies on impact against tubular structures of various character, ranging from rectangular cross-sections [3] to the more complicated T-joints [4]. Circular cross-sections are the most common, and are studied experimentally [5], theoretically [6] and numerically [7]. Inclusion of pressure in pipes during impact has also been investigated [8, 9]. Manes et al. [10] attempted to recreate the loading sequence of impact, hooking and subsequent release of an X65 pipline by subjecting strips taken from an actual offshore pipeline to quasistatic three point bending tests. The strips were then pulled straight and checked for fracture, which was present only as minor surface cracks without exerting any influence on the forcedisplacement curves. Later, simply supported X65 steel pipes were subjected to a dynamic impact before being pulled straight in quasi-static tension to emulate the release after hooking [11]. Here, fracture was a dominant part of the problem. When investigating fracture, dynamic effects from springback can be important [8, 12]. During an impact and hooking event the pipeline will deform locally and a dent will form under the impactor, and large Proceedings of the ASME 2017 36th International Conference on Ocean, Offshore and Arctic Engineering OMAE2017 June 25-30, 2017, Trondheim, Norway