Increasing confidence in the prediction of axial friction factors for offshore pipelines and cables

Seabed resistance to the axial movement of pipelines and cables – defined through use of an ‘axial friction factor’ – is key to predicting axial expansion, lateral buckling, pull-in forces and hydrodynamic stability. Simple pipeline- and seabed zone-specific friction factors are often used in pipeline analyses, despite the complexity of estimating the total restraint. This paper introduces various testing approaches and penetrometer tools – including pipes, toroids, rings and plates – used to characterise this interface behaviour. Centrifuge modelling using field samples of fine and course-grained soils was carried out alongside more conventional direct shear interface and tilt table testing. The results highlight some of the complexity that arises because resistance is influenced by interface roughness, stress level, pipeline settlement, and ‘wedging’. The results show the importance of measuring soil-specific interface properties rather than using generic estimates and demonstrate novel techniques to measure these properties.