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Skirt Suction Applied to Offshore Wind Turbine (OWT) Foundations and Anchors

Development of Skirt Suction applied to offshore wind turbine (OWT) foundations and anchors
Contributing to cost and construction period of offshore wind turbines (OWTs), which will shift into high gear going forward

Obayashi Corporation has developed an ocean structure Skirt Suction, applied to offshore wind turbine (OWT) foundations and anchors.
Offshore wind power is expected to be more widely deployed going forward, driven by the purchase price newly established in the Feed-In Tariff (FIT) system from FY2014. The structure of wind turbines used in offshore wind power can be divided into the fixed type for which the columns of the wind turbine reach the seabed, and the floating type for which the wind turbine itself floats on the ocean. The fixed type is suitable for relatively shallow water, and the floating type is suitable for deeper water.
For the fixed type a variety of foundation systems are used including monopile, jacket and gravity-based. However, the issue is that as offshore wind turbines increase in scale and are installed in deeper water, their foundations also increase in scale and costs rise. Meanwhile, there are a variety of models of the floating type, including the semisubmersible type, the spar type and the TLP (Tension Leg Platform) type. It is forecast that going forward the floating type will be introduced in Japan, which has many sea areas where the water gets deep quickly, but at the present stage it is expected that the installation costs will be higher compared to the fixed type.

Examples of the fixed type offshore wind turbines
Examples of floating type offshore wind turbines

Therefore, Obayashi Corporation has developed Skirt Suction which enables foundations for the fixed type and anchors for the floating type with higher performance and lower costs than previously. Skirt Suction consists of the top slab and the cylindrical vertical wall (the skirt) leading down from the top slab, and fixes the offshore wind turbine (OWT) more firmly in place than the previous foundations and anchors, by having the skirt penetrate the seabed foundation. Furthermore, when it is installed on the seabed, large machinery, etc. is unnecessary, so it is possible to greatly shorten the construction period compared to previously.

The main features of an offshore wind turbine (OWT) applying Skirt Suction are as follows.

  1. It has approximately three times as much pullout resistance compared to conventional foundations (demonstrated in experiments in sea areas)

    A large pullout load acts on the foundation and anchor of offshore wind turbines due to wind and ocean waves, so it is necessary to obtain sufficient pullout resistance from the seabed. When the pullout load has acted, Skirt Suction increases pullout resistance due to the passive suction (the flow of water into the skirt is slower than the pullout speed so the pressure inside the skirt becomes lower than the hydrostatic pressure). This increase in the pullout resistance has already been demonstrated with clay soil, but the results were inconclusive on sandy soil, which is common in the sea areas around Japan, so Obayashi Corporation conducted experiments in two types of full-scale test units with different shapes in sea areas on sandy soil. The results of the experiments demonstrated that passive suction occurs with respect to the pullout load that acts in bad weather such as storms, and there is pullout resistance approximately three times that of ordinary foundations that resist with their own weight and friction resistance (the fixed type) and approximately five to eight times that of anchors that resist with their own weight only (the floating type).

    Behavior of Skirt Suction at the penetration and at the pullout load
  2. There is no need to use a large pile driver, so the cost of the foundation work for the fixed type offshore wind turbine is greatly reduced

    The installation of Skirt Suction on the seabed is not done by driving (hammering), but by using the suction that occurs due to dewatering from inside the skirt (the pressure of the inside of the skirt becoming lower than the hydrostatic pressure). The large machinery used to place monopile and jacket piles is not necessary, it enables construction of foundations without vibration and noise, and the costs of foundation work for fixed offshore wind turbines can be reduced by approximately 20%. Furthermore, the construction period can be shortened by approximately 40%.

  3. Reducing the cost of floating type offshore wind turbines

    It is possible to reduce installation costs by applying Skirt Suction to the anchors of floating type offshore wind turbines. In particular, the TLP type tautly moors offshore wind turbines vertically (anchor the floating structure on the seabed using the tensioning force generated by the surplus buoyancy), so the occupied area of the sea is small compared to other types, and it has the advantages that (1) the impact on organisms in the sea areas is reduced, (2) only a small amount of mooring materials are required, and (3) the shaking of the offshore wind turbines is small so power generation efficiency becomes higher, etc., while on the other hand there is the disadvantage that larger anchors compared to other models are necessary, so the installation costs increase. If Skirt Suction is applied to the TLP type, the materials used decrease compared to conventional anchors (concrete gravity-based anchors), so the cost of the offshore wind turbine itself can be reduced by approximately 20% and at the same time the costs required for the foundation work for the offshore wind turbine can also be reduced by approximately 30%, so it is possible to realize highly efficient offshore wind power with a small environmental load at minimal cost. Furthermore, there is also the advantage that the transportation of the equipment at the anchor installation becomes easier, so the construction period can be shorted to half of what it was previously.

    Case of offshore wind turbine applying Skirt Suction
    Full-scale Skirt Suction test unit used for the experiment

Pressure acting in still water. Skirt Suction resists with the difference in water pressure between inside and outside the skirt when the pullout load acts.

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