Impact against X65 offshore pipelines

Abstract

This thesis presents both experimental and numerical investigations into some of the plethora of parameters influencing pipeline impact behaviour and the potential fracture arising thereof. The seamless pipes studied are made from an X65 offshore steel widely used by the industry. A succinct review of today’s valid design codes (and analytical or empirical methods) for problems similar to this one is presented. Impact tests against empty and water-filled pipes at different velocities were carried out to simulate a collision by trawl gear or an anchor. Subsequent straightening of the pipe representing a rebound after the impact revealed that fracture always presented itself given sufficient stretching (especially if the initial impact velocity had been at the high end of the spectrum). In some cases fracture (not necessarily visible on the surface) was present even after impact only, where in a metallurgical examination cleavage fracture surfaces were observed in the initially ductile material. Equivalent quasi-static three-point bending tests showed no such signs of fracture initiation, meaning that the problem being dynamic is an important factor and the cracking most likely initiates during the rebound after the impact. Through quasi-static uniaxial tensile tests the X65 steel was characterised as isotropic and homogeneous across the cross-section, with kinematic hardening being present in the material. Testing at elevated strain rates showed that viscous effects caused the flow stress to increase, while the fracture strain remained as for the quasi-static tests. As very high compressive strains were observed in the component tests prior to fracture, a material test using notched specimens was contrived to investigate this further. Specimens were compressed to various levels of plastic strain before being stretched to failure in tension. The tests showed that when the preceding compression increased, the strain to fracture in the following tensile step decreased. Metallurgical studies showed more shallow pores in the compressed specimens and cracked particles were prominent sights, both being indications of earlier onset of fracture. Cleavage fracture was observed in both the material and component tests where large compression preceded tension. Quasi-static stretch-bending experiments indicated that adding an axial tensile load to the pipe increased its resistance to bending. The same tests were repeated with the addition of internal pressure, which provided further resistance to bending while changing the local deformation significantly. Finite element simulations of the impact were in general very accurate, whereas in the stretch phase the force was typically overestimated. This was mainly caused by fracture being inadequately described by the numerical model, thereby offering more resistance to straightening compared with the experiments. Fully coupled fluid-structure interaction simulations of the impact against water-filled pipes were also completed with satisfying accuracy, employing a variety of different techniques. The effect of submerging the pipe in water was investigated numerically. Unit cell simulations with constant triaxiality were used to investigate the fracture mechanisms related to a load cycle of large compression before tension. Results indicated that increasing compression led to an accelerated void growth during tension, but the onset of coalescence appeared to be delayed, contrary to the experimental data. Very high local stresses after compression and load reversal indicate what might initiate cleavage fracture, so making use of a stress based fracture criterion is a natural progression from this thesis. In tension only, the unit cell simulations gave good predictions of the fracture strain given that the triaxiality remained fairly constant in the tests (i.e. notched tests). In summary the global response of the numerical simulations was very accurate, whereas a small scale phenomenon as initiation of fracture occurs on a scale much smaller than the element size in these global models and is as such not represented with sufficient accuracy and other approaches are needed. Experiments in general, and the advent of the technologies like scanning electron microscopy, are of paramount importance for understanding the physical processes at hand.