Seabed characterisation and interactions with ORE: two major challenges
The seabed surface is shaped by the combined action of waves and currents. Depending on the area, this surface is composed of more or less coarse sediments. Sometimes the bedrock is exposed at the surface. Deeper down, the seabed is the record of the geological history of the area under study. It may be composed of loose sediments, over several tens or even hundreds of metres deep, and source rock. The characterisation of the seabed as well as the understanding of its interactions with the ORE structures are two aspects on which important R&D work is carried out by France Energies Marines, its members and partners.
Bathymetry and sediment characterisation
To analyse the surface part of an area, it is first necessary to know its bathymetry and characterise the sediments that make it up (nature, size, shape, porosity) at the scale of the grain or of a sedimentary unit. Bathymetry is carried out using sounders that provide a depth measurement over the entire area. High-frequency campaigns are necessary in order to make comparisons over given time intervals. This is the preferred approach in the framework of an ongoing project studying the underwater dunes off the coast of Dunkerque (DUNES project). Seabed characterisation is carried out in particular by in-situ sampling with a grab to study the first few centimetres. The collected samples are then analysed in the laboratory, for example to study their particle size distribution.
Study of hydrosedimentary processes
To better understand the dynamics of the shallow seabed, it is necessary to study the hydrosedimentary processes describing the movement of sediments. These processes are dependent on the hydrodynamic conditions present, which in turn depend on the morphology of the seabed. These conditions are evaluated using specific instruments such as the ADCP-type current profilers used in various completed or ongoing projects (HYD2M, THYMOTE, PHYSIC, DUNES and ELEMENT projects). These data can be coupled with optical and acoustic measurements of suspended and bottom-borne matter. This allows the estimation of sediment fluxes over the area (PHYSIC project).
The acquisition of bathymetry data, oceanographic data and a better understanding of the hydrosedimentary processes of an area also allows the configuration and improvement of hydrosedimentary models (DUNES project). These can be used, for example, to study the seabed dynamics under certain conditions or to estimate trends in seabed evolution, useful in a ORE context to assist in decision-making.
Geophysics and geotechnics
Deep characterisation is carried out using geotechnical and geophysical methods. The latter are non-intrusive and include reflection seismic and electromagnetic imaging. They provide access to the variation in the structure of the soil profile and identify its different layers. Geophysical surveys as a whole have the advantage of being less expensive and give a global view of the site studied. Geotechnical methods involve a phase of soil extraction by coring. Samples are obtained in order to classify the soil and determine its physical and mechanical properties. These methods are expensive and their results concern specific points of the site. An R&D project aims to correlate the data obtained by geophysical and geotechnical methods in order to define the mechanical and physical properties of the bottom of the entire area analysed (GEOSISMEM project).
Interactions between seabed and ORE systems
Seabed morphology can have a strong impact on the routing, installation and stability of subsea cables. Geometry and slopes can change rapidly. For the future Dunkerque wind farm, which will be located in a field of mobile underwater dunes, ensuring that the cables are submerged is a challenge. If the cables were to end up out of the sediment, they would be subject to current-induced constraints and risk breaking. Understanding these displacements in order to better define the route of the subsea cables is therefore essential (DUNES project).
The characteristics of the surface sediments are also an important parameter to be taken into account in the design of ORE structures. An R&D project now completed has enabled the study of the dynamics of coarse sediments (gravel, pebbles) in the Alderney Race to prevent damage to future tidal turbines (PHYSIC project).
Obtaining the mechanical characteristics of the soil at depth is also important for the design of the foundations. The future offshore wind farms in Saint-Nazaire and Fécamp will be built on limestone or chalk. This material, whose physical and mechanical properties change with time and the loads to which it is subjected, is very difficult to characterise. A project currently in progress is dedicated to the characterisation of its mechanical properties (SOLCYP+ project). It will allow to better design the foundations of future offshore wind turbines that will be installed in these particular rocks.
Photo credit: France Energies Marines
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Published on 02/11/2020