Foundations

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This is the first post after a long silence (more than 6 months), fortunately due to good news (the birth of my first son, who reduced dramatically my free time).

I want to thank José Ramon, one of our experienced Project Manager, who pushed me to start again ;-)

This post is about geotechnical parameters relevant for wind foundation. I’m in debt with Ana from Esteyco, a very good geologist who explained me how this work.

She explained me that the most important parameters that we need define in a geotechnical survey for wind turbines foundation design are 2: bearing capacity and deformability.

 

Bearing capacity is defined by c (cohesion), ϕ (friction angle) and qu (unconfined compressive strength) and it is used in Ultimate Limit State (ULS) design.

In this type of verifications, loads such as extreme wind, abnormal loads or earthquake are used to check the foundation for overturn, slide and collapse (soil failure).

If we are working with soils, we normally define these parameters using in situ tests such as SPT (in sands), CPT, Pressuremeter test and Vane test, or with laboratory tests  such as triaxial (CD, CU, UU) , direct shear test or CBR.

In rock, we could use a compressive strength test, rock quality designation (RQD) or triaxial, although the bearing capacity of rock (even heavily fractured and weathered) is normally so high that these tests are normally not necessary.

Deformability is defined by E (Young's modulus) and G (Shear modulus).

These parameters are used to check if the foundation is compliant with vertical settlement, differential settlement and, above all, rotational stiffness.

These verifications are done with a Serviceability Limit State (SLS) design.

In case of soils, these parameters are defined with in situ test like SPT, CPT, down hole, cross hole (normally not used because we don’t have boreholes nearby normally) and pressuremeter. If we define them with laboratory test, we would use triaxial or oedometer test.

In both case, we prefer field tests to laboratory tests, mainly because it can be difficult to have undisturbed samples with their in situ characteristics.

It is important to highlight that in some cases, we could meet the bearing capacity requirements but not the deformability minimum conditions and vice versa. For instance, soft sandy soils with some combination of WTG and tower can offer a reasonable bearing capacity but an excessive deformability.

I’ve recently discovered the existence of precast foundation for wind turbines. Strangely enough, this solution isn’t having a big success, at least as far as I know.

By the way, there are several clear advantages: first of all, an important time saving.

According to the developer brochure, only 2 days are needed to complete the foundation: the first to install the pieces and the second to connect the tower tensioning the bolts.

Then, as with all serial products, there is the advantage of tighter controls on the quality of the materials and the production

The manufacturer also suggest that the excavation volume is reduced, although I don’t understand why (in the end, it is still a gravity foundation, so the dimensions should be similar).

I also don’t see what happen with the conjunction element (embedded ring or anchor cage in the newer models): I suppose that it will be substituted by bolts enclosed in the precast modules, but I can’t visualize how it will work without the lower flange.

I don’t know how many companies are actives in this business: the picture below are taken from a presentation of Artepref, a Spanish company specialized in precast components.

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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).

 

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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.

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The purpose of the remedial works in a damaged wind turbine foundation is to provide an alternative load path. Other operation that are normally done are injection of mortar or resins in the cracks and water sealing.

Here you have an example of remedial works in case of foundation failure and subsequent tower movements. The works was done several years ago in Ireland by Densit, allowing keeping the existing foundation (that is, saving around 90% of the cost of a new foundation).

If you wonder how much does it cost a WTG foundation, the figure is around 100.000€ plus turbine disassemble / reassembly plus monetary loss due to stop of production.

The remedial work was carried out in 5 stages:

1. Arresting the movements by fixing the tower. Before remedial works movement was around 5 cm.

2. Drilling of injection holes on both side of the tower and inspection.

3. Construction of a new flange and prepare the under casting

4. Injection into the foundation using Ducorit IQ

5. Under cast of the new flange with Ducorit S5

Due to the lack of bearing capacity of the embedded flange, a new flange is welded on the outside of the tower to withstand the compressive forces.

First of all, several clamps are installed all around to arrest tower movements while grouting.

After, injection holes are drilled down to the anchor plate in the concrete foundation to a depth of approximately 1.7 meters

8 injection holes are outside the tower while other 8 injection holes are inside of the tower, and a camera survey allows inspection of foundation condition.

Then, an ultra-high performance grout is injected and distributed in the foundation through the inlet holes and along the voids around the embedded flange and tower.

This product hardens quickly, obtaining around 50% of the compressive strength after only 48 hours at 10°C (the warmer the climate, the quicker the grout hardening) allowing the removal of the clamps.

Finally, a reinforced concrete ring is casted around the foundation below the new upper flange.

The reinforced bars are seen on the picture.

A shuttering is constructed on both sides of the reinforcing bars and concrete and the high resistance grouting is casted under the new flange.

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The appearance of fissures in wind turbine foundations is a rather common event.

There is no consensus in the sector about the root cause of the cracks. You can find a comprehensive family tree of fracture types below:

From my experience it seems that old foundations with embedded ring are more prone to fissures compared to the new foundations with anchor cage.

Among the foundations with embedded ring, high pedestal foundations like the one in the following picture are the most problematic:

Visiting old wind farms I’ve observed both radial cracks and concentric superficial cracks in the visible superficial concrete around the tower.

However it seems that the more dangerous fissures are the ones appearing below the lower flange. Apparently the load concentration below the steel plate, in combination with cyclic loads, can damage the concrete inducing small movements. This is an area where is very complicated to pour correctly the concrete: the air can remain trapped, lowering the resistance of concrete.

After several cycles, these movements can pulverize the concrete below the flange and increase from few millimeters to a centimeter or even more.

At this point, the superficial sealing between the embedded ring and the concrete won’t be able to withstand the displacement and will break. Subsequently, water will be able to enter the foundation: this will lead to a pumping effect, possibly even freezing and expanding in low temperature areas.

Several side effects will start, such as carbonation and possibly to reinforcement bars corrosion. Subsequently, it will be necessary to stop the wind turbine and undertake remedial works.

Regarding the root cause, the explications frequently quoted can be categorized in 2 groups:

  • Design error: fatigue loads and stress concentration below the lower flange were underestimated, wrong steel design.
  • Construction error: the concrete below the lower flange was not of the proper quality, or it wasn’t correctly vibrated or cured. Sometimes even concrete pouring at very low or high temperature or improper installed reinforcement is quoted as a possible cause, together with pouring sequence problems (joint between concrete layers which are already hardened due to delays in concrete supply).

It is more unusual to have environmental problems, such as water aggressiveness or other chemical problems.

As solution, sufficient vertical reinforcement and anchorage length should be provided. Some designers also like to position the bottom flange as high as possible to increase the thickness of the concrete below.

You can read more here:

Cracks in onshore wind turbine foundations

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Grout is an extra high properties material that is normally used in wind farm construction just below the tower flange, above the foundation.

It is a transition material, designed to have a very high fatigue resistance. All the dynamic loads need to be transferred and absorbed by the grout connecting the tower to the foundation structure.

Grout is volume stable and his resistance grow very quickly (it is faster than normal concrete in achieving a relevant percentage of his final value, and it has a very low porosity and water absorption.

Thanks to the rapid strength build-up, it allows earlier pre-stressing of anchors at all temperature ranges (it is normal to work between 2 and 30 degrees): a short hardening time permit a short overall installation times and earlier operation of the wind farm.

In the picture you can see how it is prepared. We are working by night, because the temperature was real high during the day (more than 38 degree). In cold wind farm we need to use thermal blanket to keep the grout warm.

First of all pipe are lubricated with standard Portland cement slurry. Then, grouting is prepared in the mixing machines. We are using 2 machines in parallel because mixing time (minimum 7 minutes) is about 2 times the time needed to empty the mixer. Water consumption was 2.25 liters for every 25 Kg grout bag.

After the grout is tested: a “pizza” is made to see if the viscosity is right (not too dense, not too liquid). The technical name of the test is ASTM C230 ring.

When it’s considered ready, the small channel between the tower and the foundation is quickly cleaned and the grout is pumped from the highest point of the circumference (it should be leveled, but there are always small difference in elevation.

It can’t be vibrated, so a steel rebar is used to shake it a little. We needed around 2 hours to fill the channel. At the end of the work, pipes are cleaned pushing a sponge inside them to remove the rests of grout.

The grout slowly fill the empty space below the tower flange:

Meanwhile several samples are taken to test it: only 24 hours after we had the astonishing result of 62 N/mm2. Here you have the expected hardening curves:

Testing is normally made with 12 75 mm cubes, although cylinders are considered acceptable.

There are numerous producer of this material. In this wind farm we used chemical giant BAST “Masterflow 9200” grout designed specifically for Vestas. We employed around 96 x 25 Kg bags (2400 Kg).

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Concrete towers are still an uncommon solution nowadays, basically they are still more expensive than they steel counterparts.

But the trend we see in the market is toward highest tower, of 100 meters and above, while current 2MW-3MW turbines normally have an hub height around 80 meters: this can lead to situation where an hybrid solution (concrete base plus steel top) or even a full concrete solution is finally economically profitable.

The reason for this increase in tower size is the need to increase the productivity (the wind speed increase exponentially with the eight) and to overcome surface friction, although it must be noted that almost every country has laws to limit the height of the tip of the rotor (on average, around 100 meters).

The biggest problem of tapered steel towers is that they maximum dimension in onshore wind farms is limited by transport issues: normally the biggest diameter allowed to circulate on public highways is below 5 meters, due to the free height of existing bridges. Tallest towers need bigger diameters, so there is a legal/technical limitation to the use of steel.

Several turbine manufacturers (for instance Enercon, GE and Nordex) have full concrete or hybrid towers in their catalogue, normally with eight above 100 meters. Often different sections are considered for the purpose of strength and stiffness design, fabrication and erection:

  • Base zone, made of thick walled precast concrete segments or in situ concrete. Here the thickness can be around 40-50 cm.
  • Middle zone: here the wall thickness is determined by concrete cover to reinforcement rather than by the necessary strength and stiffness, so a saving in material is possible.
  • Upper zone: here the wall thickness will be around 10 cm only. It normally includes a steel section of about 2 meters to connect the yaw ring and the nacelle.

Various configuration, techniques and details have been proposed, all of them normally considering the use of vertical prestressing. It is normal to see solution with concrete rings divided in 2, 3 or even 4 segments, assembled and joined together in situ with 2 type of joints (vertical and horizontal) using mortar, fishplates or other technical solutions.

The weight of the components is really high: can be 50+ tonnes (in some solution even more), so there can be cases where it is more difficult to lift a tower section than the nacelle.

Several documents with conceptual design of concrete wind tower are freely available on the web.

Check for instance:

Concrete_Windmills

Concrete Towers for Onshore and Offshore Wind Farms

Competitive_Concrete_Foundations_for_Offshore_Wind_Turbines

 

 

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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.

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Here you have several picture of an anchor cage we assembled a few days ago.

The anchor cage is a system conceived to transfer the loads to the foundation more effectively, anchoring the tower to the foundation.

It is made of two ring-shaped steel plates, an anchor plate and a load distribution plate, secured by anchor bolts.

Bolts are tensioned to a standard value, or sometime to different values depending on the calculation made for every foundation.

This system has been used for many years – I think that one of the first manufacturer to use it was Nordex in 2000.

As you can see we received the bolts and the distance pipes in several wood boxes, together with nuts and washers – all manufactured by Cooper and Turner in the UK.

Other items we received where the template to help the workers in the mounting operations, the foam to protect the top of the bolts during grouting and the legs of the cage.

The assembly operation take several hours: the two halves are assembled separately and than joined together below using a fastener and above using a fishplate (there is a picture with the detail of the operation).

There is a tolerance of a few millimeters, so the operation can be time consuming as several manoeuvres with the crane are necessary. We double checked each time using a laser level.

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