Investigation of Seabed-Anchor Interaction of Floating Offshore Wind Turbines

• Applicant: Professor Dr.-Ing. Jürgen Grabe
• Subject Area: Geotechnics, Hydraulic Engineering
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 526159939

In the future, floating offshore wind turbines (FOWTs) will increasingly offer the possibility of generating renewable energy even at locations with great ocean depths. An essential element is the safe and reliable anchoring of these platforms at the sea bed. In practice, a fixed bearing has been assumed up to now. It is well known that this assumption is not conservative. The underlying physical principles of the seabed-anchor interaction have yet not been completely understood. This interaction depends on the anchor type, its installation process, and the drainage conditions of the seabed. Other influences result from load direction and velocity. Yielding and failure results from the complex combinations of wave, current, wind, and operational loads, which pose a challenge to the safe and reliable operation of FOWTs. The deployment of FOWTs is still in its early stages. While there is a transfer of knowledge from the oil and gas industry in certain areas, the aforementioned influences on sedbed-anchor interaction and the interaction itself are largely unexplored. The research project is intended to contribute to a fundamental mechanical understanding of the soil-anchor interaction so that it can be modelled as a component of the design of anchored FOWT. Based on the fundamental research, a simplified drag embedment anchor model (anchor force as a function of anchor chain motion) will be developed to describe the soil-anchor interaction, taking into account anchor geometry, seabed, and load history. The anchor model describes the reaction force of the anchor for a given velocity and direction of the anchor load. To achieve the defined objectives, anchor models will be developed based on the following steps. In the beginning, the movement of the floating offshore platform of a reference installation and the resulting force at the anchor point due to wind, waves and currents will be calculated for different load scenarios. Preliminary numerical investigations follow this to calculate the anchor resistance and displacement considering the seabed-anchor interaction based on a continuum model for the saturated seabed, a rigid body motion for the anchor and possibly the part of the anchor chain on the seabed, and a contact model for the anchor/seabed and chain/seabed interface. Model tests in the centrifuge will validate the numerical model at prototype scale. In addition, the tests will serve to determine the anchor pull-out resistance. A parameter study will be used to investigate the soil-anchor interaction for the reference plant based on the validated FE model. In the final phase of the research project, an anchor model will be developed as a component of future mooring analyses.

• DFG Programme: Research Grants