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DONG Energy (now Ørsted) wanted to gain a better understanding of the effects of wave loads on offshore wind turbine foundations. To help them do this, we performed physical model tests and used OpenFOAM® to conduct numerical simulations to analyse wave-structure interaction. DONG Energy will use the information to adjust their design tools for more accurate predictions.

Challenge

  • Determining (with improved accuracy) the wave loads on wind turbine foundations, including boat landing structures.
  • Reducing steel consumption to lower costs
  • Improving the design of the fatigue life of the foundation structure

Solution

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.

Results

  • Improved understanding of the wave-structure interaction in relation to the stroke of the wave motion and the propagating wave direction.
  • Established design diagrams for drag, inertia, and lift coefficients for cylinders with secondary structures
  • Improving the design of the fatigue life of the foundation structure
  • Provided tailor-made short-course to DONG Energy on the Computational Fluid Dynamics (CFD) code OpenFOAM® simulation, model prediction of wave-structure interaction, and the force response on the structures
  • DONG Energy gained in-house competences for setting up, executing and analysing model simulations to obtain in-line and cross-flow force coefficients for future design

‘Comprehensive physical and numerical modelling enabled detailed analyses, providing a solid basis for selecting the relevant wave loads for optimising material consumption in the foundation structures.’


Dr. Finn Rüdinger
Senior Load Engineer, Loads, Aerodynamics and Control
DONG Energy (now Ørsted)


The full story

Challenge

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:

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

In close collaboration with DONG Energy, we:

  • established design diagrams of force response on the foundations with secondary structures from physical model testing
  • built a Computational Fluid Dynamics (CFD) model using OpenFOAM® that was capable of simulating similar hydrodynamic forcing – we validated the numerical model simulations using results obtained in the experimental campaign
  • delivered the CFD model and provided a short course to the DONG Energy, enabling them to execute and analyse the CFD simulations in order to obtain inline and cross-flow force coefficients for future design applications

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


Solution

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:


  • 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

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


Results

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.


About the client

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.


Case story - Highlights flyer

Hydrodynamic loads on offshore wind turbines

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