Cyclic polypropylene pipeline coating interface strength with granular materials at low stress

Laying subsea pipelines on dynamic seabeds comprising non-cohesive soils remains a challenge to geotechnical and pipeline engineers. Smooth polymer coatings are applied to protect the steel pipeline, but these relatively soft surfaces are subject to abrasion when pipes experience buckling or walking. Repeated start-up and shutdown cycles leads to cyclic movement across seabeds and even excess pore water pressure generation, so the interface strength may evolve in the process. This research subjects polypropylene pipeline coating speci-mens to submerged cyclic interface shear tests using two granular soils. Strength is observed to reduce by between ~10% and ~25% depending on the soil type and applied normal stress over the course of cycling to a cumulative horizontal displacement of ~1200mm. The reduction in strength is thought to occur in part due to grains creating striations and then repeatedly sliding up and down the same striation creating a smoothening effect for individual grains. Post-cyclic monotonic interface tests on the same surface specimen show an enhanced interface strength relative to the initial cyclic strength. This has implications for pipelines which may cycle locally and then walk or buckle to another location onto fresh un-sheared seabed sediments. Findings from this research are expected to reduce epistemic uncertainty in design and improve value for money in offshore engineering projects.