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Improving the lateral stability of monopile foundations

Lookup NU author(s): Jamie Irvine, Peter Allan, Professor Barry Clarke, Jing-Rui Peng


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The UK, along with several other nations, has pledged to generate at least 10% of its electricity from sustainable sources by 2010 (Rowley et al, 2002). From the current range of renewable technologies, offshore wind has the potential to deliver the greatest quantities of energy, meaning that offshore windfarms are likely to provide a significant contribution to meet non-fossil fuel obligations. Locating windfarms offshore is appealing because wind velocities are generally higher over open water and there is less turbulence. The greater wind speeds mean more energy is produced. The offshore turbines at Tuno Knob, West Coast of Denmark, produce 38% more energy than an equivalent onshore windfarm at Fjaldene, inland Denmark, (Pedersen, 1998). The two windfarms are almost identical in terms of size, technology, tower height, age and ownership. Furthermore developers are finding it increasingly difficult to find suitable onshore sites for windfarms because of planning objections due to concerns over the visual and aural impact on the environment. Any offshore construction is expensive compared to that of the equivalent onshore construction. This is due to the difficulties of access, depth of water, delays due to bad weather and the need to ‘marinise’ the components to protect them from the corrosive offshore environment. Despite the fact that an offshore windfarm can generate more energy than an onshore windfarm, the electricity they generate is still slightly more expensive than that from onshore windfarms. A major component of the initial cost of an offshore wind turbine is the foundation. Currently the preferred foundation option in the UK is the monopile. The monopile foundation, illustrated in Figure 1, is simply a large diameter pipe, which is hammered or drilled into the seabed. The monopile effectively extends the turbine tower under the water and into the seabed. They are relatively quick to install, require minimal seabed preparation, are inexpensive and simple to produce and once installed are largely scour resistant. Importantly they also have a proven track record in the hostile, UK North Sea having been used successfully in the UK’s first development at Blyth and in the world’s largest offshore windfarm at Horns Rev, West of Denmark. At this stage it is likely that monopiles will be used in at least fourteen of the proposed windfarm developments in the UK. Alternative foundation types include multipile, gravity base and suction caisson foundations (Figure 1). A multiple foundation consists of a steel frame emanating from the base of the tower; the steel frame holds three or more small piles, which are driven into the seabed. Gravity base foundations rely on their mass and gravity to keep the structure in place. A large steel or concrete structure is positioned on the seabed and filled with heavier ballast material to keep it in place. A suction caisson is basically an upturned steel bucket. The caisson is lowered to the seabed and then water is pumped out creating negative pressure inside. This suction pulls the caisson into the seabed and helps hold it in place. Figure 1: Wind Turbine Foundation Types The loading on offshore wind turbine foundations is comprised of a relatively low vertical load due to the weight of the turbine, blades, gearing and mast, and a large lateral overturning moment due to wind, wave and current forces. However monopiles are the least stiff of the foundation types available for offshore turbines. This has led to concerns about lateral movement at the pilehead causing soil degradation and ‘potholing’ around the pile. This paper describes how the addition of vertical fins to a monopile enhances the lateral load capacity and increases stiffness therefore reducing the horizontal movement at the pilehead. The following sections describe the theoretical benefits of a finned pile, the computer simulations carried out and the results of small-scale laboratory tests. The results are then analysed, conclusions drawn and plans for future research outlined.

Publication metadata

Author(s): Irvine JH; Clarke BG; Peng JR; Allan PG

Editor(s): Newson, T.A.

Publication type: Conference Proceedings (inc. Abstract)

Publication status: Published

Conference Name: BGA International Conference on Foundations "Innovations, Observations, Design and Practice"

Year of Conference: 2003

Pages: 371-380

Publisher: Thomas Telford

Library holdings: Search Newcastle University Library for this item

ISBN: 9780727732446