Subsea pipeline networks with components such as Wye-pieces and Pipeline End Manifolds (PLEMs) require protection from mechanical impacts. Pipeline scour is well understood and scour at solid caissons/gravity base foundations and piled foundations has been studied (Whitehouse, 1998; Whitehouse, et al., 2011; Sumer and Fredsøe, 2002). The scouring at small subsea gravity protection structures has received less attention than those with piles (Ottesen Hansen, 1997; Fog and Krogh, 2001) and hence there is uncertainty about predicting scour at such structures (Whitehouse, et al., 2010).
The design of subsea structures requires a composite assessment of scour and stability for the protected asset and the protection structure itself. A closed structure may lead to the formation of a scour footprint that is quite similar to a solid caisson or gravity base. For an open structure, the contribution to scour interaction from the different components is more difficult to assess.
Large-scale mobile bed laboratory experiments evaluated scour for a surface laid pipeline, with a spur and valves, and a generic subsea protection structure with mudmats and a perimeter skirt. Bathymetry mapping was achieved with a high precision underwater laser scanner. The experiments evaluated the contributions to scour arising from the different subsea components.
Sediment mobility was a key factor, with the perimeter skirt able to resist scour over part of its length when the mobility was low/moderate, whereas in a high mobility environment the skirt was completely undermined. The foundation beams were underscoured quite quickly and the eroded sediment was flushed out through the perforated mudmats, highlighting the importance of mudmat design. The scour pattern and rate was similar with or without the pipeline in place. The underscouring of the beams caused settlement and tilting of the structure, highlighting the importance of assessing scour and scour countermeasures for these types of structure. The scour assessment needs to be integrated into the structural, geotechnical and functional performance study for any structure and the new results illustrate the importance of understanding effects related to sediment mobility, the prevailing direction of currents, the design of mudmats and skirts, and the need for additional scour countermeasures.