Knowing the nature of the soil to choose the type of foundation
In order to exploit offshore renewable energy, the production system must be kept in place under extreme environmental conditions. It is necessary to hook to the seabed, either directly into the ground for fixed systems or via a line for floating systems. The foundation is therefore the component that holds the turbine subject to its own movements, waves, current and deformations due to the reactions of the ground. The two main parameters that condition the choice of the type of foundation to be used are the water depth and the nature of the soil. Different types of foundation have been developed to offer optimum resistance according to the soil resistance mechanisms. However, it is essential to carry out geotechnical studies at a very early stage to select the right technology from the outset.
Carbonate soils: a special case to study
Many offshore technologies have been anchored offshore for several decades in depths ranging from 30 m to 3,000 m, mainly in the oil and gas sector. There is therefore considerable experience for the relatively soft soils that contain hydrocarbon deposits. In the case of ORE, it is not the nature of the soil that primarily guides the choice of farm settlement areas. However, this can be a major technical and economic constraint when new conditions are encountered. A collaborative project aimed to study the behaviour of foundations in calcareous soils that are very present on the coasts of northern France (SOLCYP+ project). These types of soils are complex and present visco-elasto-plastic behaviour inducing non-linear behaviours, a source of uncertainty in long-term predictions. Their responses to monotonic and cyclic vertical and lateral stresses were studied.
Optimising foundations design
The engineering methods currently used for ORE come from the petroleum sector. However, new engineering challenges require adaptation. In the case of floating offshore wind turbines, for example, it is virtually impossible to have no vertical loading on the anchors due to shallow water depths. In addition, the positioning of ORE in the power generation sector requires optimised design to limit cost, while ensuring the robustness of the systems.
A collaborative project studied the limits and improvement of monopile design methods (SOLCYP+ project). The increase in the unity power of the bottom-fixed wind turbines induces the use of monopiles with very low slenderness. Geotechnical centrifuge tests are thus carried out to characterise their behaviour and create a database. This database makes it possible to calibrate a specific methodology developed within the framework of the project and illustrates the inadequacy of the “flexible pile” method used until now and based on the principle of a fixed base and a spreading of the lateral load by bending the monopile. In the field of floating offshore wind turbines, there are no specific recommendations or standards for the design of anchors and foundations. In addition, very little sand resistance data is available. A working group to which France Energies Marines contributes has therefore been created by the Comité Français de Mécanique des Sols et de Géotechnique (CFMS) to draft recommendations on these subjects.
Photo credit: Mike Mareen / AdobeStock
Cyclic loadings of offshore wind turbine monopiles
Expert in fluid-structure interactions
ORE Systems & Farms R&D Manager