Analysing Scour Interaction Between Submarine Pipelines, Valve Stations And Mechanical Protection Structures

摘要

Subsea pipeline networks with components such as Wye-pieces and Pipeline End Manifolds (PLEMs)rnrequire protection from mechanical impacts. Pipeline scour is well understood and scour at solidrncaissons/gravity base foundations and piled foundations has been studied (Whitehouse, 1998; Whitehouse,rnet al., 2011; Sumer and Fredsøe, 2002). The scouring at small subsea gravity protection structures hasrnreceived less attention than those with piles (Ottesen Hansen, 1997; Fog and Krogh, 2001) and hence therernis uncertainty about predicting scour at such structures (Whitehouse, et al., 2010).rnThe design of subsea structures requires a composite assessment of scour and stability for the protectedrnasset and the protection structure itself. A closed structure may lead to the formation of a scour footprintrnthat is quite similar to a solid caisson or gravity base. For an open structure, the contribution to scourrninteraction from the different components is more difficult to assess.rnLarge-scale mobile bed laboratory experiments evaluated scour for a surface laid pipeline, with a spurrnand valves, and a generic subsea protection structure with mudmats and a perimeter skirt. Bathymetryrnmapping was achieved with a high precision underwater laser scanner. The experiments evaluated therncontributions to scour arising from the different subsea components.rnSediment mobility was a key factor, with the perimeter skirt able to resist scour over part of its lengthrnwhen the mobility was low/moderate, whereas in a high mobility environment the skirt was completelyrnundermined. The foundation beams were underscoured quite quickly and the eroded sediment was flushedrnout through the perforated mudmats, highlighting the importance of mudmat design. The scour pattern andrnrate was similar with or without the pipeline in place. The underscouring of the beams caused settlementrnand tilting of the structure, highlighting the importance of assessing scour and scour countermeasures forrnthese types of structure. The scour assessment needs to be integrated into the structural, geotechnical andrnfunctional performance study for any structure and the new results illustrate the importance of understandingrneffects related to sediment mobility, the prevailing direction of currents, the design of mudmatsrnand skirts, and the need for additional scour countermeasures.