Tripod Support Structures

Article published on Wednesday, 05 June 2013

The tripod structure is considered to be a relatively lightweight three-legged steel jacket compared to a standard lattice structure. Under the steel central column, which is below the turbine, there is a steel frame which transfers the forces from the tower into the three steel piles. Piles are installed at each leg position to anchor the tripod to the seabed. The three piles are driven 10-20m into the seabed. The tripod can also be installed using suction buckets.

The tripod foundation has good stability and overall stiffness. However it's not suitable at water depths less than 6-7m, as this causes problems to the vessels approaching the foundation as sufficient draught is need to clear the steel frame.

Tripod Structure

Tripod Structure

The foundation is anchored into the seabed using a relatively small steel pile (typically 0.9m diameter) on each corner.

Proponents state the advantages of the tripod are the suitability for greater water depths, and a minimum of preparations required at the site before installation (assuming absence of large boulders etc.). Erosion is not normally a problem associated with this type of foundation. They go on to cite the foundations potential to save costs compared to a more complex jacket design. The effect of scour can be less significant when compared to the monopile support structure.

Others argue that tripods are not suited for locations with uneven sea beds with large boulders, and that the complex main joint has a potentially greater risk of fatigue from the large impact of wind and waves.

The tripod support structure is pre-assembled in an onshore construction yard. The entire structure is placed on a suitable vessel such as a barge and transported to the offshore location where it is slowly lowered onto the seabed ensuring that the structure is entirely level. Mud mats may be used at the three corners of the tripod ensure that the structure settles onto the seabed in a stable manner, while providing support until the foundation piles are in place. The foundation pin piles are each driven through pile sleeves at the three corners at the bottom of the structure using a submersible hammer. When the piles are at the required depth, a connection between the top of the pile and the pile sleeve is made by filling the annulus with grout or by means of a swaged connection.

Scour protection can be reduced if the foundation piles of a tripod support structure are loaded mainly in the axial direction. No separate transition piece is required, as the requirements for pile driving do not apply to the tripod and the appurtenances can be connected directly to the tripod support structure.

The tripod foundation draws on the experiences gained in the oil and gas industry where light weight three-legged steel jackets have been used for marginal offshore fields

Until now, the tripod foundation has been only used at Alpha Ventus, Borkum Phase 1 (under construction), Global Tech I (under construction) but will also be used at MEG Offshore I and Borkum Phase 2 offshore wind farms.

References

Offshore Standard: ‘Design of Offshore Wind Turbine Structures, DNV-OSJ101’, Det Norske Veritas (2010) Project UpWind Foundation Types - E.ON Windpower Offshore Windpower.Org

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