Modelling the waterjet cable trenching process on sand dunes

Numerous offshore wind farms have been installed recently in the southern part of the North Sea. Their infield and export cables are buried for protection against dropped or dragged objects. In sandy soils, burial is carried out by remotely operated tracked vehicles. Two swords with waterjets are used to fluidize the sand and generate a backward flow of the water-sediment mixture. The southern part of the North Sea has a highly variable seabed topography characterized by sand waves and megaripples. These seabed features can hinder significantly the trenching process. At the moment it is not possible to make an accurate estimate of the influence of sand dunes on the trenching process. The trench formation process is split into two parts; a front section where the seabed is eroded by waterjets (erosion model) and a rear section where the sand grains are settling in a backward flow (sedimentation model). The erosion model is made based on the assumption that the specific energy required to fluidise sand is equal to the specific energy required to cut sand with a blade. The blade is considered to have a small blade angle and to operate at zero meter waterdepth, following Miedema (2015). For a given jetting configuration and trench dimensions this results in a limiting trencher velocity. A volume balance between situ soil, waterjet flow and entrained flow gives the backwash flow rate and concentration. The last two are used as input for the sedimentation model. The sedimentation model relates water flow, sediment transport, bed evolution and trench width evolution, based on the shallow water equations. The governing equations represent horizontal momentum and mass conservation of the water-sediment mixture and horizontal mass conservation of the sediment. A numerical onedimensional finite volume model is proposed, which is solved on a staggered grid. An elastic cantilever beam model is used to determine the cable shape as it sinks in the trench. Subsequently the depth of lowering of the cable is determined by the intersection of the cable and trench shape. The combined fluidization, sedimentation and cable model is validated against full scale field data.