Dynamic versus quasi-static loading of X65 offshore steel pipes

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 was set to be equal to that 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.