The Development of Suction Buckets for Offshore Wind Foundations

According to the Carbon Trust’s Offshore Wind Accelerator (OWA) program, the design, fabrication and installation of turbine foundations in an offshore wind farm makes up approximately 30% of its total capital cost. At a time when the focus in the industry is firmly on cost reduction, foundations are considered one of the key areas where cost savings can be implemented.

While it may be the driving force, cost is not the only issue concerning foundations. With the progression of the UK’s Round 3 of development and with the move to deeper water across the industry in general, the traditional monopile foundation will become less practical. Larger turbines (up to 10MW or more) are crucial to the future of the industry in terms of improving the cost of energy, and these turbines will need suitable and cost-effective foundations. There are several new types of foundation in development and testing, and suction buckets are one of the most promising options, with various prototypes already installed. The technology is far from new, having been used in the oil and gas industries for several decades, so it makes sense that as wind farms move further from shore, they move away from the traditional monopile foundation and progress to a more suitable foundation for the prevailing conditions.

Suction Buckets in the Oil and Gas Industries

The oil and gas industries are many years ahead of the wind industry in terms of expertise and experience at sea. The established technology at use in those industries can be adapted and developed for wind farms.

When oil recovery began in the 1970’s the early foundations were steel-framed jackets, which were fixed to the seafloor using open-end steel pipe piles that were driven through the jacket legs and then welded to the jacket. When oil was discovered in the North Sea, the subsoil was found to be stiff enough that shallow foundations could be used, and gravity platforms became the common design. To minimize potential problems with scour, the sides of the foundations were built with side walls called skirts which extended down into the subsoil. As the platforms were sunk, the skirts would penetrate until equilibrium was established and then any remaining water trapped between the platform and the seafloor was pumped out to create a pressure differential and force the platform down.

The concept of using skirts with smaller steel tubes and installing them with suction was first introduced in Norway in the mid 1990’s with the Norwegian Geotechnical Institute (NGI) playing a major role in their development. It was then that the term ‘Suction Bucket’ was first coined.

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The oil industry experienced a similar situation to the one facing the wind industry now, when installations were moved into increasingly deeper waters in the Gulf of Mexico. It became apparent that jacket foundations were not economical due to the vast amounts of steel required, and the difficulty of supporting the huge jacket structure was also becoming a technical challenge. Evolution continued from skirted gravity structures to bucket foundations and the new foundations became known as ‘Suction Caissons’.

A key stage in the development of suction buckets again involved NGI as they developed suction foundations for the Snorre A oilfield in 1992. The oil platform, owned by former North Sea operator Saga Petroleum AS, was a tension leg platform of a type that in other parts of the world would have been installed with 90 metre long pile foundations. However, due to the presence of large pebbles at 60 metres depth under the surface, this wasn’t possible and NGI developed suction anchors which only penetrated to a 12 metre depth. By 1999 the foundations were becoming more common and a world record was achieved in the Gulf of Mexico for the size and depth of the anchors used for the Diana Platform. The buckets measure 30 metres in height and 6.5 metres in diameter, installed at a depth of 1500 metres. That record has since been surpassed but it was a technological breakthrough at the time and by 2002 figures estimated that 485 suction caissons had been installed in 50 locations worldwide at depths of up to 2000 metres.

Trials with Wind Energy

The wind industry faces the same challenges of moving to deeper water and developing more cost–effective foundations. Suction buckets are a viable option.

The Carbon Trust Offshore Wind Accelerator was established to support and speed up the development of technologies that will reduce the cost of offshore wind, and in 2012 provided funding for the development of new foundation types that could meet the challenges that will be encountered in the UK Round 3 of development. Four projects were awarded funding, including Universal Foundations Suction Bucket Monopile, and SPT Offshores tribucket foundation.

SPT Offshore

The Tribucket foundations developed by SPT Offshore boast a range of potential advantages over traditional pile foundations and could offer a long-term solution for wind farms in deeper waters.

The design consists of a three-bucket configuration which is self-installing. The entire foundation and structure can be assembled in port, then transported and installed without piling or drilling. This quick and easy installation is more cost-effective, creates less noise and therefore has no effect on marine life, and the buckets can be removed and decommissioned in the future so there is no long-term effect on the underwater environment.

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