DHI Analyses Effects of Wave Loads on Offshore Wind Foundations

To help DONG Energy gain a better understanding of the effects of wave loads on offshore wind turbine foundations including secondary structures, DHI has performed physical model tests and conducted numerical simulations to analyse wave-structure interaction.

DONG Energy wanted to gain a better understanding of the hydrodynamic loading on wind turbine foundations with secondary structures attached and improve the present conservative design approaches and reduce the material consumption of steel in the foundations.

The project required the establishment of a robust, consistent, and validated experimental physical model setup. DHI executed an extensive experimental campaign that comprised wave loads on the model cylinders with secondary structures for several foundation types including:

• monopile with and without secondary boat landing structures
• jacket with and without boat landing structures

The experimental programme encompassed various environmental conditions that:

• matched the sea states experienced in practice
• took into consideration the orientation of the WTG foundation structures relative to the stroke of the wave motion and the main wave direction

Next, DHI utilised the knowledge and results obtained from the experimental programme and put them into a numerical framework. A Computational Fluid Dynamics (CFD) model in OpenFOAM® was set up to accurately simulate the oscillating flow around the structural elements and consecutively produce the integrated time series of force responses on the wind turbine generator (WTG) foundation structures.

The results from the numerical CFD simulation agreed well with the results from the physical model setup. This enabled DHI to validate the numerical model, which could be used to broaden the range of results from the parametric experimental programme and in the future design of offshore WTG foundations.

The experimental tests, CFD simulations, and a short-course DHI provided enabled DONG Energy to investigate flow under well-defined conditions. This information helped the company understand how present design tools should be adjusted or replaced in order to more accurately predict in-situ loads and support the evaluation of the fatigue design life of foundation structures.