Anchor Selection for Floating Offshore Wind

26.3.2. Anchor Selection for Floating Offshore Wind#

A very good overview of the use of the various foundation options specifically for Floating Offshore Wind is given by the paper “Anchor geotechnics for floating offshore wind: Current technologies and future innovations” It provides some indication of the sizes and capacities and clarifies which FOW system would benefit from which foundation option:

  1. Driven pile: The diameters of these piles vary from 2 to 5 m, with L/D up to 60. The horizontal & vertical efficiency (Force over Mass) ranges from 12 to 100 depending on the soil conditions. The driven pile is suitable for the whole range of angles shown in the picture.

  2. For the drag anchor it was already indicated that high capacities can be achieved. As an example, one of the large Offshore anchors is the Stevpris Mk5 anchor. For which the Ultimate Holding Capacity (30 mT anchor) is nearly 1700 mT in sand and hard clay. The Horizontal efficiency (force over mass) ranges from 30 to 300 depending on the soil conditions.

  3. With suction piles the diameter is mainly lower than 16 m, with a Length over Diameter of less than 1 in sand and 8 in clay. The horizontal efficiency ranges from 50 to 250 & Vertical efficiency ranges from 9 to 100 but whether vertical force can be accommodated is dependent on the soil characteristics.

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Fig. 26.8 Anchor geotechnics for floating offshore wind: Current technologies and future innovations http://dx.doi.org/10.1016/j.oceaneng.2023.114327#

Anchor choice for FOW applications depends on load requirements, soil conditions, installation feasibility, and removal considerations.

  • Driven piles: Flexible in soil type and capable across wide angle ranges; high noise during installation may be a constraint. The diameters of these piles vary from 2 to 5 m, with L/D up to 60. The horizontal & vertical efficiency (Force over Mass) ranges from 12 to 100 depending on the soil conditions. The driven pile is suitable for the whole range of angles shown in the picture.

  • Drag anchors: Highly efficient in cohesive soils for horizontal loading; limited vertical capacity. High capacities can be achieved. For the drag anchor it was already indicated that high capacities can be achieved. As an example, one of the large Offshore anchors is the Stevpris Mk5 anchor. For which the Ultimate Holding Capacity (30 mT anchor) is nearly 1700 mT in sand and hard clay. The Horizontal efficiency (force over mass) ranges from 30 to 300 depending on the soil conditions.

  • Suction piles: Favoured where vertical load capacity is needed and soil allows, with minimal noise and ease of removal. With suction piles the diameter is mainly lower than 16 m, with a Length over Diameter of less than 1 in sand and 8 in clay. The horizontal efficiency ranges from 50 to 250 & Vertical efficiency ranges from 9 to 100 but whether vertical force can be accommodated is dependent on the soil characteristics.

  • Torpedo piles: Best for deep-water, soft clay environments where installation logistics favour gravity drop.

  • VLAs: Optimal in layered clays requiring both vertical and horizontal holding.

For more detailed design guidance, including anchor sizing and performance envelopes for FOW systems, see Anchor Geotechnics for Floating Offshore Wind: Current Technologies and Future Innovations.