Largest ever testing of wind turbine piles completed

Monday, 7 September, 2015 - 09:45

To find new design methods for offshore wind farms, Dong Energy and ESG have tested 28 wind turbine piles at two different offshore locations with typical North Sea conditions. First results indicate large opportunities for cost reductions in the offshore wind sector.

The joint industry project PISA (pile soil analysis) supervised the tests of 28 offshore wind turbine piles for Dong Energy and the British testing services company ESG to validate and assess a new design method designed by the academic group.

The testing took place in Cowden, England and in Dunkirk, France. The clay till site in Cowden and the dense sand site in Dunkirk represent typical surface soil conditions found in much of the North Sea. Since both sites had already been used for pile testing –mainly by the oil and gas industry–, researchers already had access to a huge amount of data about the soil. Piles with three different diameters were tested; the largest had a diameter of 2 metres and are some of the largest ever tested. The 28 piles have been supplied by the companies Bladt and Dansteel.

Challenging testing

The tests conducted were very extensive and used various precision instruments. The main focus was on the static monotonic loading, but also the response of the piles under cyclic lateral loading.

Steve Turner, Project Director from ESG, who undertook the testing, recalls the special requirements of these tests: “The PISA project has provided some of the most challenging testing we’ve ever undertaken. With the largest test, we were simultaneously monitoring more than 250 different precision instruments, whilst applying a load greater than the weight of 37 London double decker busses.”

Saving potential

But it seems that overcoming those challenges was worth it for the results. As Alastair Muir Wood, Lead Geotechnical Engineer Dong Energy and Technical Manager for the PISA Project, explains: “We’re very pleased with the test results, which confirm that traditional design methods in these soils are very conservative. The results indicate that in these site conditions there may be opportunities for savings identified by reducing the quantity of steel in the foundation. In other words, there’s a savings potential that will contribute to reducing the cost of electricity.”

Jesper Skov Gretlund, R&D Project Manager, said: “The PISA Project is a great example of inter-industry collaboration to solve a common problem. If the thickness or length of the steel piles can be reduced by even a small fraction, the saving in cost is quite considerable since smaller construction vessels can be used and larger turbines constructed. The next challenge is to analyse all of the data collected in order to refine our methods and apply these findings to our foundation designs.”

New results support new designs

Over the next six months, PISA will analyse the data from all tests and use it to confirm the new foundation design methods. If this goes according to schedule, their final report will be delivered to the project partners in January 2016.

The PISA academic working group is led by Oxford University and includes the Imperial College London and the University College Dublin. The academic working group supervised the testing on site as each of the 28 piles was pulled sideways into the soil until failure. The research project PISA is being carried out by an industry working group headed by Dong Energy and involves EDF, RWE, Statoil, Statkraft, SSE, Scottish Power, Vattenfall, Alstom and Van Oord. PISA is being run under the framework of the Carbon Trust Offshore Wind Accelerator (OWA).

Tanja Peschel / Dong Energy

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