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Title:
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Future XL monopile foundation design for a 10 MW turbine in deep water
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Author(s):
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Published by:
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Publication date:
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ECN
Wind Energy
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22-12-2016
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ECN report number:
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Document type:
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ECN-E--16-069
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ECN publication
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Number of pages:
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Full text:
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43
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Download PDF
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Abstract:
The monopile foundation is the dominating support structure type for offshore wind farms in construction today. It is anticipated that also in future wind farms the monopile will remain a preferred choice due to the manufacturing speed and installation experience. However, larger, heavier turbines are being developed and wind farms are being installed in deeper waters, testing the technical and economic limits of monopile foundations against other foundation types.
This report contains the results of an ECN internal knowledge project on future large monopile foundations. One of the main objectives was to create a preliminary design for a 10 MW support structure in 50 m water depth. This is a degree larger than the current designs in the industry and is aimed to put our current models to the test. The basic design features a foundation pile diameter of 9.3 m and a total steel mass above mudline of 1850 tons. The design has not been optimized but was subjected to an ultimate load, buckling and fatigue check. A global buckling check for a buckling failure mode twice the length of the support structure was not passed and the applicability and implications of this failure mode must be further investigated. The basic design passed all other ultimate load checks and fatigue check at the mudline. Largest diameter in the support structure is 9.3 m which is within manufacturing capabilities.
The second objective was to identify the limitations of current design methods and simulation tools. The soil-structure interaction can be modelled by lateral springs (PY curves) with the current tools available to ECN. However, for large diameter foundations other physical load bearing mechanisms are at play and the PY curves do not suffice for accurate soil-structure modelling. For a 9.3 m monopile, modelling the hydrodynamic loading by diffraction does not have a very large influence on the magnitude of the loads, hence Morison’s equation may still be used without being too conservative. Finally, there are assumptions made in evaluating the fatigue damage by using an SN curve and Miner’s rule.
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