Nowadays most wind turbines are positioned on large monopiles. Monopiles can be as long as 80 meters and weigh up to 1,300 tonnes. It is expected that the size of the piles will considerably increase in the near future as larger wind turbines of over 10 MW are already in development.
Owners of the turbine are obliged to remove the wind turbine at the end of its lifetime.
To be ready for this, the offshore wind sector needs to develop new techniques to remove wind farms efficiently, including the foundations. The application of smart techniques for full pile removal will avoid underwater cutting of piles, which is a risky operation, and allows for recycling the recovered steel for second use. In this way, offshore wind energy generation would be brought one step further towards circularity.
The technique that we investigate in this project is based on hydraulic extraction. This means that the pile is sealed and its void volume is pressurised using a fluid, e.g. seawater. The pressure will drive the pile upward, liberating the pile out of the soil.
Before this extraction technique can be applied safely and efficiently at full scale, we have to better understand the interaction between the pile and the soil before and during the process. We also need to minimize possible leakage of pressurised fluids and have to understand how much force is required while extracting the pile. Scale tests are therefore a prerequisite for the application of this hydraulic pile extraction at full scale.
In this project led by innogy we are going to gather data by testing a set of downscaled piles. The tests consist of a basin filled with water and soil at the bottom. We drive the piles into the soil and we will wait to allow them to settle in the soil. Next, we seal each pile and connect it to a pump or multiple pumps via a hose. The pumps deliver the pressure and we are able to adjust the flow rate during the extraction process. We will test various soil compositions to mimic the actual soil conditions at sea.
The project will deliver a description of the scale tests and a full set of data of the extraction process, which will be made available to the project partners. We evaluate the tests outcomes and provide insights and recommendations for next steps to develop the hydraulic extraction technique towards a commercial level.
Industry joins forces to study the sustainable decommissioning of offshore wind turbine foundations (news item on the website of Deltares 2019)
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