Monopiles are the most used foundations for offshore wind turbines. Because the turbines are getting bigger, the monopiles will also get 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 as there are already 15 MW wind turbines on the market. Depending on the water depth, a 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 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.
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 previous GROW project on this technique HyPE-ST, we have already successfully demonstrated the hydraulic extraction method for different soil configurations on a small scale. We concluded that with this technology it is possible to dismantle the entire monopile almost silently. However, before we can confidently use this extraction technique on a full scale, we still need to investigate and prove some aspects and demonstrate it on a medium scale.
In this project, we aim to prove the hydraulic extraction concept on an intermediate scale corresponding to the technology readiness level (TRL) of 6, i.e. to demonstrate the technology in a relevant environment. Through our activities, we will enhance our understanding of pile-soil interaction and demonstrate a proof of concept in representative soil conditions. We will further develop and validate the analytical extraction model - developed and used in the original HyPE-ST project - to predict the required breakout pressures for different configurations.
The type of work in this project includes preparatory works and experimental design, laboratory-scale testing, intermediate-scale testing, model validation as well as data analysis and reporting. The tests are performed at a scale of approximately 1:5, which translates into a pile diameter of ca. 1.5 m on a monopile with a diameter of 8 m.
The result of this project will be a proof of concept for hydraulic extraction of medium-scale monopiles. In the future we expect that the technology can be applied for the removal of large scale monopiles. The work will also result in a validated analytical model predicting the required breakout pressure for different soil conditions in full-scale offshore application.
All recorded data will be available for the project partners, including the application tool for hydraulic pile extraction. This tool will be based on the developed analytical model. We will prepare a full final report containing all relevant project outcomes. We expect the monopile hydraulic extraction technique to be ready for commercial application by 2025.
This project is supported by the Netherlands Enterprise Agency and TKI Wind op Zee.