Foundations are crucial to offshore wind farms and are one of the major costs involved in establishing a site. They must be able to withstand the weight of the turbine, including its nacelle and rotor blades, and cope with maximum wind speeds, wave heights and currents. Offshore wind foundation design and manufacture is a complicated process and the complexity increases at greater water depths. A monopile foundation consists of a steel pile driven under the seabed in order to support a tower placed on the transition piece.

Gravity foundations consist of large bases, made either from concrete or steel, which are placed on the seabed.
Some of the major manufacturers active globally in the last five to six years include Bladt Industries, Sif Group, EEW Special Pipe Constructions, Cuxhaven Steel Construction and WeserWind. A number of design companies are also active in the wind power market, such as MT Højgaard, Ramboll Group, ISC Consulting Engineers, Per Aarsleff and Bilfinger Berger Ingenieurbau.

Substantial investment must be made in this area, as the industry increases its use of advanced foundation technology; the expansion of offshore installations in deeper seas around the world is expected to increase demand for jacket, tripile and even floating-type foundations.

Solid foundations
The main foundation types used by the offshore wind industry in recent years have been monopile, gravity, jacket and tripile. Of these, monopile was the most widely deployed in 2013, dominating the market with a 62% share. It has succeeded because it is best suited to the sort of shallow waters in which most offshore wind farms are located.

However, the industry is expected to register an increase in the number of offshore wind farms with higher-capacity turbines installed in deeper waters, as this will help to boost productivity, thereby increasing demand for gravity-based and jacket foundations. In 2011, a new, floating type of substructure was installed so that its performance might be evaluated for the offshore industry.

Depth and distance from the shore
The average water depth and distance from the shore for wind farms installed in 2013 was 20-30m and 20-40km respectively. It was observed that, as offshore wind farms move further from the coast, they tended to receive higher and more consistent wind speeds, resulting in increased electricity production. Against this backdrop, construction and planning is under way for an increasing number of offshore wind farms, which are to be are built further from land and at greater water depths.

"Annual investment in the offshore wind turbine foundation market is expected to increase to $8.88 billion by 2020."

Most offshore foundations constructed to date are located near the shore at a water depth of around 20m. In countries such as the Netherlands, the US and China, major sites at this depth will be viable in the medium term. However, for countries such as Germany, every important location is at a depth of around 40m. The foundation design is primarily dependent upon local conditions and the turbine selected for the farm. As the size of the machinery increases, the design of the foundation becomes more complicated in order to withstand the greater weight of the nacelle and the higher rotor diameter.

The selection of a foundation has a major influence on the cost of construction. The foundation accounts for about 20-25% of the total cost of a farm, and therefore influences investment calculations.

At greater water depths, monopile foundations are preferred, as they reduce expenses significantly. The turbine tower is placed on the foundation, usually with a transition piece in between. Selection of foundations should also take into consideration factors such as maximum wind speed, wave heights, currents and soil properties. However, in terms of foundation selection, the depth of the sea is the major criterion for consideration.

There are five main types of foundation for offshore wind turbines:

  • monopile
  • gravity base
  • jacket
  • tripile
  • tripod.

The monopile foundation is the preferred choice for water depths of less than 20m. Its benefits include low cost, simplicity and proven performance in shallow water. Jacket foundations are the next preferred technology. These have proved their worth in the oil and gas industry, and can support large turbines with capacities in excess of 5MW that can be installed in water deeper than 40m. The design is more complex, than that of a monopile, and is more expensive in terms of procurement and installation.
Gravity-base foundations have large bases and were only used at depths of no more than 15m until recently. However, the Thornton Bank farm in Belgium successfully deployed a gravity foundation at depths of around 25m in 2008.

Tripile foundations are a new adaptation of the monopile and feature three piles driven into the seabed. This increases the strength and footprint of the turbine, allowing installation in waters of up to 50m.

Finally, a tripod foundation is a monopile design with three legs that sit on the seabed. These are relatively complex structures and are time-consuming to manufacture.

Monopile foundations were the most frequently installed foundation type by 2013, at slightly over 5GW, or 70.95%, of cumulative offshore wind capacity. Gravity foundations accounted for 11.32% of the cumulative capacity installed, followed by jacket, tripile and multijacket foundations, with respective 6.97%, 5.7% and 0.68% shares. Foundation types, such as tripod, suction bucket, floating concrete construction and hybrid gravity accounted for a combined small percentage share of 0.61%. Other foundation types accounted for a 3.76% share. Figure 1 (page 9) illustrates foundation type according to cumulative offshore capacity installed by 2013.

Global annual investment, 2006-2020
The offshore wind turbine foundation market has grown rapidly over the last five to six years, with annual investment increasing from $0.08 billion in 2006 to $1.55 billion in 2013 at a CAGR of 52.9% – see figure 2, above right. This amount is expected to increase significantly with the commissioning of projects at great water depths and at a substantial distance from the shore, which are likely to use gravity, tripile and floating technologies.

Against this backdrop, annual investment in the offshore wind turbine foundation market is expected to increase from $1.61 billion in 2014 to $8.88 billion in 2020 at a CAGR of 28.3%. The following figure illustrates annual investment in the offshore wind foundation market between 2006 and 2020.

Grouting failures
Grout helps to transfer the load from the transition piece to the monopile and is critical in preventing downward movement, or misalignment. Many European offshore turbines have witnessed design flaws in grouting whereby the material injected during installation has broken down, causing turbines to shift upon their monopile foundations. Around 600 of Europe’s offshore turbines are facing the issue of dissolved grouting causing shifting within monopile foundations.

Re-strengthening affected turbines cost the industry about £25 million ($38.6 million) in 2010. During installation, a hollow steel rod is driven into the seabed to a depth of around 40m. A tapered transition piece is then placed over the monopile, and the tower is plugged into the top.

Grout is then injected into the gap between the transition piece and the monopile, and is secured by a rubber seal. In order to rectify grouting failure, international classification company Det Norske Veritas (DNV) has amended its guidelines.

The following amendments were made, with a focus primarily on reducing costs and increasing safety:

  • new design requirements for grouted connections without shear keys
  • corrosion protection to be applied in different corrosion zones, with stipulations in place regarding corrosion allowance, cathodic protection and coating
  • establishment of standards for the necessary level of inspection in large wind farms and provision of a choice between periodic inspections and inspections according to a risk-based inspection plan for owners.

The grouting design fault is not expected to delay any new projects. DNV has suggested that in order to avoid failure, conical joint shapes should be applied and additional plates must be incorporated to support vertical loads (bypassing the vertical load).

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