Jacking and suction installation of bucket foundation for offshore wind turbines

Research output: Book/ReportPh.D. thesisResearch

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Abstract

The production of renewable energy on the global scale experiences a large growth due to the well-known reasons. Offshore wind power is one of the most promising sources with a high development range. The costs of offshore energy are being rapidly reduced; however, there is still much to be improved. Lowering the costs of offshore wind farms is not only the key to lower energy prices, but primarily is the great contribution to the climate goals for the future.

The installation of foundations is often named as one of the main issues that influences the total costs of the offshore energy. The majority of offshore wind turbines are supported by monopile foundations. However, the demand for increasing size of offshore wind turbines is the reason why a better solution is desired. Therefore, more and more effort is put on the development of a suction bucket foundation that seems to be more cost-effective and environmentally friendly due to the suction installation manner. The concept is commonly used in the oil and gas industry, but as loading conditions for offshore wind turbines are very different, a further research and new design methods are required. The concept is already proven to be feasible, but the suction installation process is still not fully understood and can be optimized.

This thesis focuses on the bucket installation by analyzing the soil-structure response during the suction and the jacking installation. Medium-scale tests of the installation have been performed at Aalborg University laboratory in fine grained sand. The tests prove that the suction installation can be performed and easily controlled even in very dense sand. The suction applied during the installation can be much higher that the proposed suction limits and no failure is observed. Moreover, the tests results indicate a huge difference between the soil resistance against two different installations, as the seepage flow, induced by the applied suction, reduces the soil stresses inside the bucket and below the skirt tip. The cone penetration tests performed before and after each test confirm that the soil trapped inside the bucket is significantly loosened up.

The thesis covers also the numerical analysis of seepage around the skirt for different boundary conditions and with applied changes in the soil permeability of the inside soil plug due to the mentioned loosening. The numerical part is a basis for the analysis of the critical suction which is later on evaluated by laboratory tests results.

Finally, the thesis includes results of the test campaign where two different bucket models are used and compared. An increase in bucket foundation diameter requires an increase in skirt thickness at the same time. Otherwise, too thin structure will lead to a buckling failure during the installation. Obviously, the total cost of steel material increases significantly. However, a modular bucket with the internal stiffeners used for tests has a much lower skirt thickness. The changed shape gives much higher buckling resistance and savings in material costs at the same time. Jacking tests show that the soil resistance for the modular bucket is significantly higher than for the round bucket with a similar diameter, but the suction installation tests show that the reduced soil resistance is almost the same in both cases. These results are very promising, showing that large-diameter mono-buckets with modular shape can be feasible for suction installation.
Original languageEnglish
PublisherAalborg Universitetsforlag
Number of pages261
ISBN (Electronic)978-87-7210-536-9
Publication statusPublished - 2019
SeriesPh.d.-serien for Det Ingeniør- og Naturvidenskabelige Fakultet, Aalborg Universitet
ISSN2446-1636

Bibliographical note

PhD supervisor:
Prof. Lars Bo Ibsen, Aalborg University

Assistant PhD supervisor:
Assoc.Prof. Johan Christian Steffensen Clausen, Aarhus University (former Aalborg University)

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