Monopiles are the most commonly used foundations for offshore wind turbines. Due to the increased size of the turbines, the monopiles are also getting bigger and heavier. Currently installed monopiles can be 80 meters long and weigh up to 1,300 tons. We expect the size of the piles to increase significantly in the near future as there are already 15 MW wind turbines on the market. Depending on the water depth, future monopile can weigh more than 2,000 tons.
At the end of its life, turbine owners must completely dismantle the wind turbine structure. To be ready for this, the offshore wind sector is developing new techniques for efficiently removing wind turbines, including the foundations.
The project has explored a smart technique for removing the entire pile. This avoids a risky cutting operation under water and also allows the steel obtained to be recycled for reuse. In this way, offshore wind energy is brought one step closer to full circularity.
Hydraulic extraction is expected to be a near-silent method of pile removal. It will therefore cause very little disturbance to marine mammals. No excavation is required for hydraulic pile extraction. The layers of the seabed remain unaffected. The removal method is therefore gentle on benthos, the organisms that live on, in, or near the seabed.
In the HyPE-ST project, we have investigated a promising technique based on hydraulic extraction. This method involves sealing the pile after removal of the top structure, filling the void with water and pressurizing it, thus forcing the pile to move upward. Before we can safely apply this method offshore on a large scale, we need to better understand the interaction between the pile, fluid and different soil types.
We conducted an extensive hydraulic extraction test campaign and performed the tests on a 1:20 and 1:30 scale for a prototype monopile with a diameter of 8 meters. We tested four different soil conditions: medium dense sand, dense sand, medium-stiff clay and layered soil. We installed the piles through impact driving. Then, we sealed the piles and connected them to a pump via a hose. The pump provided the pressure and we had the option to adjust the flow during the extraction process. During the extraction process, we checked several parameters including pressure, flow, pile displacement and plug displacement.
The project successfully demonstrated the hydraulic extraction method for different soil configurations, identified the relevant conditions for the pile extraction and developed a model to predict the breakout pressure. On the tested scale, we saw that the breakout pressure was highly dependent on the soil type and soil configuration. In particular, the presence of a soil layer with a low permeability can have a major influence on the breakout pressure. We, therefore, concluded that additional research efforts are needed to ensure a safe operation offshore before this extraction technique can be applied in stratified soils on full scale.
Based on the results, we conclude that by using hydraulic extraction it is possible to dismantle and recycle the entire monopile almost silently and with very little impact on the seabed.
Before we can confidently use this extraction technique on full scale, we still need to investigate and prove a number of aspects. The technology is currently at the technology readiness level (TRL) of 4. In a follow-up study, we aim to reach TRL level 6. In a final project, we want to demonstrate the extraction technology offshore. We expect the monopile hydraulic extraction technique to be ready for commercial application by 2025.
Hydraulic extraction tests on a medium scale will allow us to better understand the effects of scaling. These onshore tests shall be conducted in natural soil that is representative for the North Sea. Accompanying lab tests on a very small scale shall give answers to specific phenomena of pile-soil interaction.
This project is supported by the Netherlands Enterprise Agency and TKI Wind op Zee.
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Afgedankte monopiles werken zichzelf met waterdruk uit de bodem (Article in Cobouw in Dutch 2020)
Sustainable monopile decommissioning (presentation at GROW side-event 2019)
Industry joins forces to study the sustainable decommissioning of offshore wind turbine foundations (news item on the website of Deltares 2019)