Wave-seabed observations and a combined approach to determine required burial depth of offshore power cables under large storm waves and currents

The interaction of wave and currents with the seafloor and seabed can result in the erosion, liquefaction and failure of near surface sediments. The recent exposure of power cables for an offshore wind project in the United States has highlighted the need for more robust assessments of seafloor and seabed instability. Pipelines and cables can then become exposed or experience unacceptable and damaging deformations. This paper examines these processes. Experimental results have identified two different types of liquefaction. The first type results from the cumulative rise in pore pressures caused by repeated cyclic loading from waves. This type of lique-faction is most likely to occur in freshly deposited sediments that remain undrained or partially drained during wave loading events. The second type of liquefaction occurs from the combination of low initial stresses in the near seafloor sediments and the changing effective stresses within the seabed from wave action. This liquefaction will be more temporary and will depend on the time during each wave cycle when principal stresses are near zero or slightly negative. In addition to the experimental results two types of advanced numerical approaches are presented. These approaches greatly enhance the ability to make accurate predictions of seafloor and seabed instability. The first approach uses finite elements to examine stress changes within the seabed. Validation of the approach is presented in the paper by comparison with a previous analytical solution. A Computational Fluid Dynamics approach was used to determine the fluid velocities from waves and currents within the bottom boundary layer which can then be used to estimate the bottom seafloor shear stresses. These shear stresses can cause erosion with the potential to expose pipelines and cables. In addition to erosional and liquefiable zones, the paper also identifies a zone where the soil may yield and produce unacceptable deformations for pipelines and cables. The main recommendation from this study is that scour and erosion need to be evaluated by considering both fluid and soil mechanics aspects of the problem.

Automated summary

The interaction between waves, currents, and the seafloor can lead to erosion, liquefaction, and failure of near surface sediments. The recent exposure of power cables in a US offshore wind project has highlighted the importance of assessing seafloor instability. This paper examines these processes.