Experimental Data and Modelling of Cable and Pipe Axial Friction on a Rocky Seabed

As part of recent research into the on-bottom stability of subsea pipes and cables on rocky seabeds, a set of physical model tests have been performed investigating the effective lateral and axial friction factors. The lateral friction factor experience led to a new model for the effective friction which scales up the cable-rock interface friction coefficient allowing for the roughness of the rocky surface that the cable moves over. This paper presents the results of axial friction testing which showed a similar enhancement of friction linked to the roughness of the rock surface in a different way. These test results demonstrated that the axial resistance could also be many times greater than the interface friction force when the characteristic rock particle sizes are comparable to or greater than the cable diameter. This arises from a ‘wedging’ action that leads to inclined contact between the cable and the rock, which causes an increase in the sliding friction. Data and modelling of this behaviour is provided to assist application of these results to design.