Scour at cable protection rock berms - Model test observations

Rock berms or concrete mattresses have traditionally been applied for protection of shallow buried, surface laid or crossing pipelines and cables. In most cases the installed protection has worked satisfactorily, but under certain adverse conditions, field evidence has shown examples of severe scour both at mattresses and rock berms. The paper presents and discusses physical model tests of rock berm scour in combined waves and current. Both single and double berm configurations were studied. The conclusions from the tests were two-fold addressing: 1) Scour development caused by rock berm(s), and 2) Structural robustness of the rock berm when subject to wave action and seabed model scale effects. The model tests support field evidence that scour is prone to develop in strong current environments and when the protection is aligned oblique or perpendicular to the prevailing currents. The tests also demonstrate larger structural integrity of wide crested berms as opposed to sharp crested berms.