Subsea cable trencher performance on sand dunes

Abstract

Numerous offshore wind farms have been constructed 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, it is common to use tracked remotely operated vehicles, equipped with two water jetting swords. These swords fluidise the seabed and generate a backward flow of water-sediment mixture, allowing the cable to sink into the seabed. The southern part of the North Sea has a highly variable seabed topography characterised by sandwaves and megaripples. These seabed features have a significant influence on the trenching process. Existing models do not allow for an accurate estimation of the influence of seabed slopes on the trenching process and are often not based on fundamental physical processes. Two separate numerical models are developed; a jet trenching model describing the cable burial process and a traction model describing the seabed trafficability.

The jet trenching model is divided into three parts; an erosion model describing the erosion of soil by the waterjets, a sedimentation model describing the re sedimentation process and resulting trench shape and a cable model describing the cable deflection. The erosion and sedimentation model combined describe the flow of water and sediment in the trench. The erosion model is based on a specific energy approach to determine the maximum allowable trencher velocity, limited by the eroding capacity of the jets. The sedimentation model describes the flow of water-sediment mixture through a rectangular channel, based on the shallow water equations. The channel width is able to evolve due to breaching and the bed elevation is controlled via erosion and sedimentation. The shallow water equations are solved on a staggered grid, following a one-dimensional finite volume scheme. A moving boundary is imposed on one side of the grid to simulate trencher movement. Seabed topography can be imported to model trencher performance on sand dunes.

Tractive performance of the vehicle is modelled by considering its driving state. A constant velocity is assumed, hereby balancing thrust and resistance forces. Resistances due to static sinkage, slip sinkage, seabed slopes, current and internal running gear friction are included. The driving thrust force is found by integration of shear stress over track-seabed contact area, including effects of slippage and constant seabed slopes.

A sensitivity study has been performed on the jet trenching model, where a strong influence on achieved depth of lowering was found to be caused by grain sizes and depth of the jetting sword below seabed. Influence of trencher velocity on depth of lowering was found to be associated with grain sizes. A higher trencher velocity has a positive effect on the achieved depth of lowering in coarse sand, whereas in fine sand the trencher velocity has a negligible influence on the depth of lowering. Validation of the model with field data shows reasonable agreement regarding average depth of lowering. When including sand dunes, results of the model show a similar depth of lowering trend as observed in field data. However, the amplitude of depth of lowering variation is underestimated by the model.

The sensitivity study performed on the traction model showed that resulting slip ratio and power demand have a strong dependency on track-seabed contact area and corresponding normal pressure distribution. Work remains to include the effect of variable seabed slopes, since the current model is based on constant seabed slopes.