Tag Archives: foundation design

Gestamp iConkrete wind turbine foundation

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Some weeks ago a document describing a new type of wind turbine foundation, the “iCK foundation”, landed on my desk.

Also known as “Gestamp Hybrid Towers” (GHT) it has been developed and patented by iConkrete and Gestamp. Essentially it is a shallow foundation made of a slab to achieve a uniform pressure distribution, a central reinforced ring with his pedestal and several reinforced beams below the slab.

You can see how it looks like picture here:

The basic idea is to obtain a T section, for a better use of materials: compression is distributed on the top of the T head, with a reduced depth of the neutral axis.

This geometry, according to the developers, gives and improved fatigue behavior for concrete and a higher resistance reserve.

Among the other potential advantages, a (partial) prefabrication of the foundation’s elements is possible. This lead to a “cleaner” work and to a saving in time.

Moreover, the smaller excavation promises other savings due to reduced earthworks: this foundation is more superficial than the standard one, with an average depth of almost 3 meters. The excavation volume avoided can be around 50%.

Last but not least, according to a test design made by Gestamp, a 15% of steel can be saved thanks to a better use of materials.

If the soil below the foundation has a low bearing capacity, a geotextile can be used.

This solution can be adapted to any type of tower (steel, concrete or mixed).

 

Offshore wind turbines foundation types

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Offshore is one of the fastest growing sectors in the renewable energy business in Europe. In 2010 more than 300 new turbines have been installed, reaching a total of more than 3000 MW connected to the grid.

The number is not huge, but the offshore agenda is quite busy: more than 20.000 MW are expected to be installed at the end of 2015. The majority of these projects will be in the UK, Germany and the Netherlands.

On a worldwide basis, Europe is accumulating more than 96% of all the installed capacity, with Great Britain as the biggest player right now, followed by Denmark and Netherlands.

The interest for offshore development has several reasons: bigger wind potential (over 4.000 full load hours vs. 2.000 full load hours onshore), bigger wind turbines (>3MW, up to 7MW) and wind farms (from 50 to 1000 MW of installed capacity, while the average onshore wind farm is around 50 MW).

The drawback is the enormous investment needed: billions of euros, due to the rough marine conditions where everything is more expensive: wind turbines, cables, substation, and of course foundations.

Several foundation types are available for wind energy offshore towers:  gravity-type, monopile, jacket-pile, tripod and suction caissons.

The type of foundation used depends mainly on water depth and sea bed conditions: there is no “standard” concrete foundation as in the onshore wind farms. The solution used more often is the monopile.

Gravity foundations are used preferably in waters with a maximum depth around 30 meters, are made of precast concrete and are ballasted with sand, gravel or stones.

Monopile foundations are used in water with a maximum depth around 25 meters.

They are made of steel, and they are driven into the seabed for about 30 meters with a hammer (similar to the one used to build offshore platforms)

Tripod is used in deeper waters (up to 35 meters). It’s made of different pieces welded together and it’s fixed to the ground with three steel piles.

 Jacket if used in deep waters (more than 40 meters). It is made of steel beams welded together, weighting more than 500 tons.

Wind turbines foundation design

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Wind turbines foundation design is an art (and a job) by itself. Here you have a very quick overview of the process for shallow foundation (Patrick & Henderson foundation, widely used in the USA, follow a totally different approach) .

The inputs for the foundation design are the result of the geological survey and the design loads.

The geological survey gives the structural engineer key parameters to define the soil: internal friction angle, cohesion, density, Young modulus, shear modulus, etc. The number and type of parameters will depend on the type of materials below the foundation: expansive clay will need different test compared to sand or rock.

Design loads are provided by the manufacturer of the WTG, who has standard documents with the different type of loads (usually available data includes operational loads, extreme normal loads and extreme abnormal loads, together with fatigue calculation data).

Numerous codes, national laws and international standard are used in the design process. These are the most commonly used:

 

 

Following the IEC-61400, the design load cases used to verify the structural integrity of a wind turbine shall be calculated by combining:

  • Normal design situations and appropriate normal or extreme external conditions;
  • Fault design situations and appropriate external conditions;
  • Transportation, installation and maintenance design situations and appropriate external conditions.

The design load cases must be analyzed at fatigue (F) or at ultimate loads (U). Ultimate loads are defined as normal (N), abnormal (A) or transport and erection.

Several load combination are checked: for every ULS (ultimate limit state) and SLS (serviceability limit state) the correct partial factor is used, to diminish stabilizing forces and incrementing destabilizing forces. The same load type can have a different partial load safety factor, depending on the verification made. For instance, in the case of overall stability, the bending moment at the tower base has a factor of 1.35 for normal loads and 1.1 for abnormal loads.

Partial safety factors are defined by the IEC as:

Seismic loads must be analyzed separately and they are normally combined with the turbine operational loads. It is of critical importance to recognize that seismic plus operational loads may in some cases govern the tower and foundation design.

The main requirements to be satisfied are:

 

  • Not to exceed the bearing capacity of the soil: the soil pressure must be lower than the allowable bearing pressure.
  • Not to overturn (i.e. there is no rotation around the edge)
  • Not to slide (i.e. there is no horizontal movement)
  • Not to exceed the maximum differential settlement provided by the manufacturer during the life of the structure (normally few millimeters/m). The differential settlements are normally calculated using a finite elements models software, simulating the composition of the different layers of materials below the foundation.
  • To comply with the minimum dynamic rotational stiffness given by the manufacturer (to limit the potential coupling phenomena with the rotating parts of the WTG).
  • The compressed area below the foundation is normally assumed to be 100% for operational load and at least 50% for other loads cases.

 

The first step is to define a tentative geometry: than, if the various checks are satisfied (overturn, slide, bearing capacity, etc.) a detailed analysis with a finite elements software is made, to define the amount of reinforcement needed: from nodal stress distribution along the most unfavorable positions of the cross section, necessary mechanic capacity is calculated,and the amount of steel (mechanical capacity) necessary to withstand the calculated stresses is compared with the amount of steel placed in the section. Clearly, the first must be lower than the second.

Soil reaction transmitted to the foundation is modeled with vertical non linear springs

The calculation can sometimes lead to the conclusion that a shallow foundation is not feasible, due to low bearing capacity, insufficient rotational stiffness or many other possible factors. In these cases, a soil improvement is made or a deep (piled) foundation is calculated. These alternative solution are normally quite expensive: depending on the country and the technology needed, the extra cost can vary from 50% to 100% and more.