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

Eur. Phys. J. Spec. Top.

Hopperstad

2016

Abstract. Anchors or trawl gear occasionally impact offshore pipelines, resulting in large local and global deformations. Impact velocities are typically less than 5 m/s, but local strain rates may be very high. In this study strain rate effects in an X65 offshore material was characterised by split Hopkinson bar tests, while the cross-section homogeneity and possible anisotropic behaviour were determined by quasi-static material tests. Further, dynamic impact tests at prescribed velocities were carried out on simply supported full scale X65 steel pipes. Next, deformation-controlled quasi-static tests with the same boundary conditions were conducted. The level of deformation in the quasi-static tests were set to be equal to what was attained in the dynamic tests. Finally, an assessment of the differences between the dynamically and quasi-statically loaded pipes was made in terms of force-displacement response, energy absorbed, and fracture. An optical light microscope and a scanning electron microscope were used to investigate fracture surfaces arising from the various tests.

Section: Introductionmentioning

confidence: 99%

Dynamic versus quasi-static loading of X65 offshore steel pipes

Eur. Phys. J. Spec. Top.

Hopperstad

2016

“…Kinematic hardening is not included as the global response differs only marginally in similar cases [18]. The von Mises equivalent stress σ eq is given as a function of the deviatoric part σ σ σ dev of the Cauchy stress tensor σ σ σ ,…”

Section: Constitutive Relationmentioning

confidence: 99%

Transverse Deformation of Pressurised Pipes With Different Axial Loads

Volume 3B: Structures, Safety and Reliability

Ilstad

et al. 2017

Self Cite

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.

“…This load cycle causes a complex stress and strain history which in turn may lead to fracture [5]. The load sequence 5 of denting followed by stretching was applied quasi-statically to strips of an X65 pipeline material by Manes et al [6], without producing any significant cracks. Further, Kristoffersen et al [7] carried out dynamic impact tests on simply supported pipes followed by quasi-static stretching of the dented pipes to emulate the rebound of a pipeline after release from a hooking event.…”

Section: Introductionmentioning

confidence: 99%

“…anchors or trawl gear [2,3], which is treated in the DNV GL guidelines [4]. This load cycle causes a complex stress and strain history which in turn may lead to fracture [5]. The load sequence 5 of denting followed by stretching was applied quasi-statically to strips of an X65 pipeline material by Manes et al [6], without producing any significant cracks.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Combined three-point bending and axial tension of pressurised and unpressurised X65 offshore steel pipes – Experiments and simulations

2018

Marine Structures

Self Cite

Subsea pipelines are occasionally struck and hooked by objects such as anchors or trawl gear. The initial denting, followed by potential hooking and displacement of the pipeline, give rise to a complex load and deformation history. Transverse displacements cause a simultaneous increase in tensile axial forces, further complicating the load sequence. This study examines the effect of applying one of three different axial loads (zero, constant, and linearly increasing) to a pipe while simultaneously deforming it transversely. The three tests were repeated with an internal pressure of 10 MPa (100 bar), and all tests were recreated numerically in finite element simulations using both iterative (implicit) and non-iterative (explicit) approaches. As expected, adding an axial load increased the pipe's resistance to bending in terms of force-displacement, and the same can be said of including internal pressure. However, a more localised dent was observed in the pressurised pipes, which in turn could affect the onset of failure. The experimental results were well captured by the finite element simulations.