The Hydraulic Pile Extraction – Scale Tests (HyPE-ST) project aimed at both the fundamental understanding and the demonstration of the feasibility of hydraulically extracting monopiles for decommissioning. The project is a one-year R&D Joint Industry Project executed within the GROW research program.
Before this extraction technique can be applied safely and efficiently at full scale, better understanding of the interaction between the pile and the soil before and during the extraction process must be gained. Possible leakage of pressurised fluids must be minimized and understanding of how much force is required while extracting the pile must be acquired. For the application of this hydraulic pile extraction at full scale, scale tests are therefore a prerequisite. This is the main objective of this project.
In the second half of 2019 more than 30 extraction tests took place. Recently, the last test was in the Water and Soil Flume of Deltares in Delft, the Netherlands. The tests were performed at a scale of 1:20 and 1:30 of a monopile with a diameter of 8 m. Four different soil conditions were used:
- medium dense sand
- dense sand
- medium stiff clay
- layered soil
The piles were installed by impact driving. The tests have been instrumented and during the extraction process, several parameters have been monitored. Among which:
- extraction pressure
- pile displacement
- pore-water pressure
- plug displacement
The extraction technique involves sealing the monopile after removal of the top structure of the wind turbine and pressurizing a fluid (water) inside its void. This will push the pile upwards. The pressure exerted by the hydraulic pressure should be enough to overcome the frictional resistance forces along the shaft of the monopile. This breakout pressure depends on several factors such as soil type, size of the monopile, the permeability of the soil, its strength, among others.
Recent experiences from the limited number of decommissioning of offshore wind turbines, have demonstrated that the difficulties of cutting underwater and below the seabed tend to be underestimated. External cuts require customized equipment like clamped frames and guiderails, in order to support the cutting tool. As for internal cuts, off-the-shelf cutting tools from the oil and gas industry can be used only to a limited extent because the diameters of monopiles for wind turbines keep increasing. More importantly, any cut below the level of the local seabed requires an excavation in order to make the level of the cut accessible for the cutting tool.
A removal method that requires neither an excavation nor any cuts of the foundation structure could be substantially less expensive than a partial pile removal. This is due to narrower vessels and cranes and possibly less H&S groups as no diving operations are required. In addition, no part of the pile would be left behind on site and the steel can be reused. Hydraulic extraction is such a method. This technique is environmentally more sustainable than a partial removal of piles. Not only because of the recycle element, but it also disturbs of the soil less than cutting under the seabed. This is why hydraulic pile extraction contributes to a more circular economy and sustainable use of the seas.
Future development steps
The HyPE-ST project has demonstrated the feasibility of the hydraulic extraction method for a variety of soil configurations. The tests took place on piles that were 20 and 30 times smaller than regular monopiles. It is promising that the tests prove the concept at the lab scale. Now testing at larger or full scale is required. This is what the project partners are currently considering. Possibly other interested parties will join in.
HyPE-ST is coordinated by Innogy and includes 6 partners: Deltares, Delft Offshore Turbine B.V., IHC IQIP B.V., Jan de Nul, Sif Netherlands B.V. and TNO. The partners are research institutes, offshore contractors, an offshore wind project developer, a monopile manufacturer and a wind turbine developer.
The access to the partners for the HyPE-ST project was facilitated by TKI Wind op Zee. Ahmed Elkadi, Geotechnical engineer at Deltares: “This support, together with the financial support of TKI Wind op Zee and the Netherlands Enterprise Agency (RVO), was crucial for the HyPE-ST project.”
Find out more about HyPE-ST.