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The Concept and its Potential

universalfoundationsAfter ten years of dedicated and innovative technology development 'Bucket Foundations' are now being 'rolled out' into the commercial market. The expectations are that this concept will reduce the cost of energy, especially when that energy is to be produced in deeper waters.

By Henrik L. Nielsen, Universal Foundation, Denmark

{access view=!registered}Only logged in users can view the full text of the article.{/access}{access view=registered}The Bucket Foundation concept combines the benefits and main proven assets of a gravity base foundation, with a monopile and a suction bucket. Importantly the design also includes a patented installation system, which controls the vertical alignment of the total foundation as it sucks itself into the seabed, reducing the overall installation time significantly.

In brief, these new foundations consist of an up-ended bucket, which, via a closing lid is connected to a shaft that, in turn, connects to the wind turbine tower, met mast or other offshore surface installation. Once installed the ‘bucket’ encloses a large volume of sediment, which helps provide a high load-bearing capacity (see Figure1).

The development of this new concept began back in 2001 at Aalborg University, Denmark, with analytical studies, numerical modelling and laboratory tests that eventually led to pit tests of 2 x 2 metre and 4 x 4 metre buckets. Installation methods and techniques were developed from these prototypes using suction and skirt-tip injection. The installation of Bucket Foundations can be divided into two phases: (1) self-penetration of the bucket and (2) penetration of the bucket by means of suction. In phase 1 the skirt penetrates into the seabed by gravity. In phase 2 the penetration is increased by applying suction to the inside of the ‘bucket’. The suction creates an upward flow in the sediments within the ‘bucket’, reducing the effective stresses in the sediments beneath the skirt edge, and results in a net downward force on the top of the bucket, as illustrated in Figure 2.

The reduction in effective stresses greatly reduces the penetration resistance, allowing the skirt to penetrate the sediments further. The suction applied is limited by the gradient that causes piping channels in the soil within the bucket (i.e. the critical gradient at which the effective stress equals 0). Once piping channels are created the suction can no longer be sustained. If the suction is kept at a minimum, and does not cause piping in the soil, the soil will regain its strength when pumping is ceased. The critical gradient , where gw is the unit weight of water and g' is the submerged unit weight. The exit hydraulic gradient i can also be expressed in terms of the applied suction p and the seepage length s as   . The critical suction resulting in formation of local piping channels is therefore  . Combining these formulae with empirical experience the critical suction can be expressed as  , where D is the diameter of the bucket, and h is the penetrated length of the skirt. Thus, if h is known, the critical suction can be controlled.

In 2002 the first full-scale Bucket Foundation was installed using the suction technique. A Vestas V90, 3MW turbine was erected on top of a full-scale prototype of the Bucket Foundation, with a diameter, D, of 14 metres, and a skirt height, d, of 6 metres. The turbine is still in production, and now, after nearly ten years of performance data has been acquired, it is possible to say that 'Bucket Foundations' are one of the most proven foundation technologies available.

In 2009 a Bucket Foundation was installed at Horns Rev 2 as support for a met mast. Here the suction installation technology was reversed as a tilt of approximately 1 degree out of vertical was reduced to less than 0.1 of a degree. The ‘bucket’ was simply lifted 1 metre and reinstalled within the given tolerances (see Figure 3).

In conjunction with the installation requirements given above, geotechnical and structural design will cover all the limit states of Bucket Foundations under combined loads from wind, waves and currents. From these considerations a design approach can be developed covering the basic design, followed by a conceptual design, and finally a detailed design. This design procedure is verified by DNV, to fulfil the requirements given in the Offshore Standard DNV-OS- J101, 'Design of Offshore Wind Turbine Structures'.

The flexibility of these foundations lies in the variety of possible designs, and this has a direct bearing on the ‘universal’ prospects of the concept. Depending on the actual ground, meteorological and ocean conditions and the load regime, the bucket skirt height (d), the diameter (D), and shaft dimensions can be varied to give the optimal material usage and still provide an adequate bearing capacity and stiffness as required by the integrated turbine foundation system.

Recent participation in the Carbon Trust foundation competition – Offshore Wind Accelerator – yielded a promising result: 'Bucket Foundations' were chosen as one of the winning concepts out of a total of 104 entries. 'Bucket Foundations' show an estimated cost reduction for a final installed foundation of 15–30% over other competitor systems.

With a strong industry player involved as the major shareholder, the potential now exists for introducing 'Bucket Foundations' as the solution for large-scale wind farms. 'Bucket Foundations' are aimed directly at the offshore wind energy sector, enhancing technical performance while also reducing the significant costs for offshore foundation installations. With the acquisition of Universal Foundation, Fred. Olsen related companies are now positioned to provide a full, packaged solution for offshore wind farm foundations – from feasibility study/design to the finished installation of an integrated system of foundation and turbine.

About Universal Foundation
First Olsen, the engineering arm of Scandinavian shipping firm Fred. Olsen, acquired 60% of the Danish company Universal Foundation A/S (formerly known as MBD Offshore Power A/S) in August 2011. The remaining interests in Universal Foundation are held by the Danish utility company DONG Energy Power Holding A/S, Novasion ApS and Aalborg University, with whom the concept foundations were developed and tested.{/access}

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