Monopile foundation with a resistance plate

In the currently constructed offshore 5 MW wind power plants with a stationary tower one cannot use a monopile foundation, because the diameter of the foundation they require would be more than 5 m and there is no drilling equipment on the market which would be able to drill a foundation of such diameter into the sea bottom.

EKO-PRO presents a solution employing a monopile foundation with a resistance plate, where the diameter of such foundation does not exceed 4.75 m. The resistance plate is made on land, from heavy concrete rings having the mass of 50 - 100 tons. After the foundation is drilled into the sea bottom, the resistance plate rings sink and settle on the sea bottom; finally, gravel is poured into the free space between the foundation and the rings and the foundation is ready. The proposed solution is presented on Drawing 4.
It may be applied in 5 MW wind power plants with a fixed tower, installed at depths up to 35 m.

The above solution has the following advantages:

• It requires less equipment and staff during the assembly as compared to other contemporary solutions, which results in considerable savings;
• It allows to save on expensive steel, which is partially replaced by cheap heavy concrete;
• It allows to apply a monopile foundation which is shorter by approx. 1.5 of the height of the resistance plate; hence, additional steel savings are achieved;
• At small water depth, it permits to apply a monopile foundation of a smaller diameter, allowing to use yet less steel.

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How a monopile foundation with a resistance plate works

The enclosed Drawing 4 explains how a monopile foundation with a resistance plate works.
The pressure force of the wind on the rotor is transferred, appropriately increased, to the top of the foundation at the height of H₂, in the form of the horizontal force F₁ and the bending moment M₁, resulting in equivalent reaction forces which act on the ground.
All the elements of the monopile foundation with a resistance plate co-operate with each other so that one part of the reaction forces ΣR₁ consist in cutting forces in the ground around the monopile foundation and the other part of the reaction forces consist in the friction force R₂ appearing at the junction of the sea bottom with the bottom part of the resistance plate.
It is assumed that in the proposed solution the surface of the monopile foundation will counterbalance part of the cutting forces in the ground to the limit of their permissible values, while the resistance plate will quite independently counterbalance the rest of the cutting forces in the ground, which the monopile foundation is unable to counterbalance due to its insufficient diameter.

The value of the friction force shall depend on the roughness and weight of the resistance plate.
The static friction coefficient was determined empirically (Photo 1).
The resistance plate should have big mass density, amounting to at least 3 g/cm³.
The best material for that purpose will be heavy concrete with an addition of approx. 25% of ferrous scrap.
Gravel or other hard material, put between the monopile foundation and the resistance plate, transfers the horizontal force from the monopile foundation to the resistance plate and at the same time does not disturb the resistance plate's settling on the sea bottom.

EKO-PRO is able to define the technical parameters for a monopile foundation to be constructed at a specific location using the method of extrapolation of computer calculation results for a similar monopile foundation, with a degree of accuracy sufficient to make a prototype foundation and test it.
It is also possible to appropriately modify the software used to calculate the parameters of a specific monopile foundation with a resistance plate.

Possible applications of a monopile foundation with a resistance plate

The application of a monopile foundation with a resistance plate shall bring the maximum advantages in the construction of 5 MW wind power plants with a fixed tower.

However, in case of smaller units with power output of 3 or 3.6 MW, it may be more advisable, particularly in shallow waters, to apply a smaller diameter of a monopile foundation with a resistance plate; then expensive steel will be replaced by cheap heavy concrete and the cost of the drilling will be smaller. As a consequence, this solution will be cheaper.

If one intends to erect a 5 MW wind power plant with a rotary tower, then probably the application of a monopile foundation with a resistance plate will not be necessary; however, it may be recommendable for financial reasons.

A prospective wind power plant with a rotary tower of power output of 2x5=10 MW will most certainly require the application of a monopile foundation with a resistance plate.