Pipe clamping mattresses (PCMs) are used to anchor subsea pipelines against axial movement caused by thermal stress. They comprise a hinged concrete structure that clamps onto a section of pipeline, with concrete ballast logs then installed in such a way to secure the clamping action and ensure that the entire submerged weight of the PCM contributes to enhanced axial friction. Until recently, PCMs had not been installed in locations with mobile seabed, and hence scour prediction and mitigation had not been investigated in detail. This paper reports on a series of experiments that were undertaken to enable scour prediction for a pipeline fitted with a PCM, and to investigate scour mitigation options. Initially, 2D experiments were performed in a narrow flume to establish how the onset of scour and maximum scour depth is enhanced in steady flow by the presence of the PCM, with results showing a similar trend to early research concerning pipeline spoilers. Next, 3D experiments were performed in the Large O-Tube (LOT) facility to investigate scour around a pipe with a single PCM in tidal flow conditions. The results show how the development of scour is influence by flow intensity and direction relative to pipeline alignment. Finally, scour mitigation testing was undertaken in the LOT to explore how rock berms placed adjacent to the PCM could limit scour development. Collectively, the experimental results enable designers to predict if scour may occur beneath PCMs and provide insight into how to mitigate scour. The findings of this work are also relevant to predicting scour depth when other objects are attached to the top of a pipeline or cable, including marine growth.

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