Dynamic loads on wind turbines

I discussed in another post the relevance of resonance analysis in the design of steel tower for wind turbines and the different technical solutions currently in the market.

But what are the loads that could induce resonance in the wind turbine?

There are several different type of dynamic loads:

Unbalanced rotor. This basically means that the blades have not the same weight. This problem can happen for several reasons: accumulation of snow, ice or simply variance in the production of the blade itself in the factory. The consequence is that the centre of gravity will move with the rotation of the blades inducing a centrifugal force in the system.

Tower shadow effect. It is also known as dam effect – basically, the tower affect the speed of the wind nearby. This will affect the blade passing in front of the tower, unbalancing the rotor.

Wind shear. The wind will usually have a different speed increasing from the bottom of the rotor to the top.

Errors in the configuration of the turbine. In this category I include any type of asymmetry generated by design mistakes, assembly errors, software errors, etc. For instance, a difference in the pitch of the blades could create a dynamic load.

Type of towers – stiff, soft or soft soft?

In the last month I spent a lot of time discussing about “soft soft” towers.

But what does it exactly means?

Steel tower for wind turbine are classified as stiff, soft, or soft soft based on the relative natural  frequencies of tower, rotor and blades.

You obviously want to avoid that your tower is excited by dynamic loads and start resonant oscillations.

The primary sources of dynamic loads on the tower are the rotational speed of the rotor (usually indicated with P) and the blade passing in front of the tower. The blade passing speed will obviously be 3P. I think that it’s worth mentioning that rotational frequency loads will arise only when the blades are unbalanced.

We call “stiff” (or “stiff stiff”) a tower whose fundamental  natural frequency is higher than that of the blade passing frequency. This is a very good thing (the tower is unaffected by  the rotor) but a bigger mass is needed – therefore the cost can be very high. Additionally, a stiff tower tends to radiate less sound.

“soft” is a tower whose fundamental frequency is lower than the blade passing frequency, but above rotor frequency.

“soft soft” is a tower whose natural frequency is below BOTH rotor frequency and blade passing frequency.

“stiff stiff” design is not usual.

Currently, towers in  the market are either “soft stiff” or “soft soft”.

Soft towers are usually lighter (= cheaper) but require more dynamic analysis.

WTG tower – concrete or steel?

One of the key decision in a wind farm is the type of tower that will be used to reach the desired hub high.

In the infancy of the wind industry, lattice towers where used – you can still see them in very old wind farm, for instance in southern Spain.

However, this technology was not really a good fit when the hub high reached 50+ meters. The following step has been to switch to tubular steel tower with a circular section, which has been (and still is) the standard technical solution.

In parallel, the concrete tower solution has been developing. This can be either hybrid (the lower part of the tower is made of concrete and the upper section of still) or a full concrete solution (except for a small element on top of the tower that act as a sort of adapter between the last section and the nacelle.

The components of the tower can be either precast in an existing factory or cast in situ in a factory specifically built for the project, usually in the wind farm area. Obviously, this second alternative make sense in big wind farms, with dozens of wind turbines.

Regarding the assembly process, there are different technical solutions in the market. However, in general each tower section is composed by several elements (usually from 2 to 6) that must be assembled together with vertical joints to compose a complete tower section.

After, the different tower sections are assembled together and united with horizontal joints.

The joints are usually filled with grout, and a system of cables run through the tower usually all the way down to the foundation.

The foundation of  a concrete tower is usually smaller and different from the standard shallow foundations used for steel towers.

Is a concrete tower a good choice?

As often, the answer depends on many factors.

From an economical point of view, to simplify the problem, concrete towers are usually competitive when the wind turbine is high (100 m. and above).

From the technical perspective, there are several advantages of concrete over steel:

  • No restriction in the geometric design
  • Greater stiffness (good for resonance) and damping
  • Greater maximum hub height possible
  • Smaller foundation due to increased weight

Self lifting precast wind towers

A couple of months ago I’ve been invited by Esteyco to the erection of a prototype auto lifting precast wind tower.

Although I was unable to attend due to other meetings I want to describe shortly their invention, which looks interesting.

Basically, the idea is to create a tower that can “lift itself” without the use of a main crane.

The first step is the assembly all the pieces of a precast concrete tower at ground level (such as an onion, or a matryoshka doll).

Assembly is done from the inside out, with the most internal section being the top one holding the nacelle.

When assembly is completed lifting can begin – starting from the center (the top tower section with nacelle and blades) and moving toward the exterior.

The tower “self-lift” itself using heavy lift strand jacks. These are commercially available equipment, currently used in several industries such as heavy construction or offshore.

The strand jacks will be positioned on an auxiliary platform at the first level.

This solution save you the main crane: only a smaller, 350t to 500t crane will be needed to assembly all the components.

To give you an order of magnitude, to reach 120 m you will need 4 concrete levels that in fold position will have a height of approximately 40 meters.

The expected time for assembly and lifting of all components would be around 3 days.

As an added bonus, due to the increased weight of the tower (acting as a stabilizing load) the WTG foundation would be somehow smaller compared to a steel tower of the same height.

Geotechnical parameters for WTG foundations design

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.

Wind Turbines Precast Foundations

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.

Gestamp iConkrete wind turbine foundation

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

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 turbine foundation remedial work: an example

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.

Cracks in onshore wind turbines foundation

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