Hydrodynamic loads on offshore wind turbines
Drag, inertia and cross-flow forces on offshore foundations including secondary structures
A comprehensive physical model test campaign in a towing tank facility coupled with advanced numerical simulations in OpenFOAM® to study the wave-structure interaction in various sea states.
Evaluating hydrodynamic loads on foundation structures
The high cost of offshore wind turbines compared to land-based wind turbines is a major barrier for the offshore wind industry. The cost difference is mainly due to the foundation principle as well as operation and maintenance. As such, a careful consideration and evaluation of hydrodynamic loads on foundation structures is very important. DONG Energy has built more than a third of the total offshore wind capacity in the world. As European Union (EU) countries strive to meet the EU’s 2020 renewable energy targets, DONG Energy wanted to:
In close collaboration with DONG Energy, we:
Simulation of acceleration and deviation of the flow around the tested WTG foundations in various wave regimes: jacket leg with secondary structures (left), close-up of the monopile with secondary structures (middle), and close-up of secondary structure on the monopile (right). © DHI
Physical model testing of wave loads on wind turbine generator (WTG) foundations
This project required the establishment of a robust, consistent, and validated experimental physical model setup. The process of establishing and validating the data basis and analyses methodologies was followed by a certifying company.
We achieved this by testing the physical models with plain cylinders and varying surface roughness. The results obtained from our physical model setup and procedure agreed well with existing datasets reported in literature.
We executed an extensive experimental campaign that comprised wave loads on the model cylinders with secondary structures for several foundation types including:
The experimental programme encompassed various environmental conditions that:
The focus of the experimental part was maintained on the in -line and cross-flow force responses on the structure by a typical Morison approach. The integrated time series of wave loads on the structure are expressed in terms of force coefficients representing drag and inertia terms.
Monopile foundation with the secondary boat landing structures in the lab (left) and monopile foundation with the secondary boat landing structures in the field (right). © DHI
Numerical simulations of wave loads on WTG foundations
For the second part of the project, we utilised the knowledge and results obtained from the experimental programme and put them into a numerical framework. A CFD model in OpenFOAM® was setup to accurately simulate the oscillating flow around the structural elements and consecutively produce the integrated time series of force responses on the WTG foundation structures.
In conjunction with the time series analysis obtained from the physical model testing, the force time series were expressed in terms of drag and inertia coefficients. The results from the numerical CFD simulation agreed well with the results from the physical model setup. This enabled us 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.
Example of velocity contour and streamlines for the monopile structure in oscillating flow. © DHI
In-house CFD model to simulate wave loads
The analyses of more than 300 tests detailing the flow around and the accompanying forces on WTG foundation structures were made available to Ørsted.
Reduction in chemical dosing
As a result of the implementation of biological phosphorus removal, almost complete abatement (99%) of chemical dosing of precipitation is expected.
We also conducted a CFD simulation short-course and provided model predictions of wave-structure interaction. This ensured that Ørsted secured in-house competences for setting up, executing, and analysing model simulations. The experimental tests, CFD simulations, and the shortcourse enabled Ørsted to investigate flow under well -defined conditions. This information helped them 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.
DONG Energy (now Ørsted) develops, constructs and operates offshore and onshore wind farms, solar farms, energy storage facilities, and bioenergy plants, and provides energy products to its customers. Ørsted ranks #1 in Corporate Knights' 2020 index of the Global 100 most sustainable corporations in the world and is recognised on the CDP Climate Change A List as a global leader on climate action.
