Wood towers for wind turbines

I always believed that wood towers for wind turbines were a solution possible only in small, domestic WTGs (somewhere around 10 kW to maybe maximum 100 kW). There are several example available, for instance this product of InnoVentum.

Well, I was wrong: I see that some years ago (2012) a Vensys 77 1,5 MW turbine has been installed on a 100 meters tower. That is quite a number: a 77 meters rotor is considered small for today standard, however it fully qualify as a “utility scale” solution.

This full scale prototype followed a 25 meters test tower built by the same companies some years before.

Developed by 2 German companies (TimberTower & TiComTec) it has been built near Hannover. The foundation is standard (concrete) and the connection between the tower and the foundation is made trough 4 meters long steel rods.

With a somehow unusual octagonal cross section the tower diameter is comparable to a standard concrete or steel solution. I see however that other geometries are possible (hexagonal or dodecagonal).

The life span of this solution looks similar to the steel alternative (20 years). Unfortunately I haven’t been able to find information regarding the cost. For the sake of clarity it is not 100% wood – few steel elements are used inside the tower.


Long term instrumented monitoring of wind turbine foundations cracks evolution

Fiber Bragg grating sensors (Image copyright fbgs.com)

Some weeks ago I have discovered that, as I am currently enrolled as a university student (getting “slowly but steady” a second degree in Economics) I have full access to the Elsevier database.

This is an enormous amount of information, including all the best scientific papers and technical articles published by industry journals.

I am using this possibility to learn more and stay updated on several niche topics that I found interesting – from recycling of wind turbine to bird strikes to foundations pathologies.

Browsing the database, I recently stumbled upon an interesting article published by Jack McAlorum et al. from the University of Strathclyde (Glasgow).

The paper is called “Deterioration of cracks in onshore wind turbine foundation”.

The authors instrumented an octagonal slab foundation (sometimes called “star foundation” or “wall foundation”) to monitor the evolution of existing cracks.

I already wrote a couple of posts on foundation cracks: they can be due to a variety of root causes, such as:

  • Design mistakes
  • Errors in the composition of concrete mix
  • Extreme temperatures
  • Errors in the execution of the wind turbine (for instance, concrete poured in different batches creating construction joints)
  • Failures due to the use of an embedded can (this is a frequent failure reason for older wind turbines)

The paper does not specify the reason for the cracks. However, as typical, the most severe cracks were in the side of the wind turbine facing the wind (as the concrete is in tension there).

What it is interesting is the fact that the behaviour of the foundation has been monitored for a very long period (over 9 months) and under standard operating conditions. This is very unusual: while other key component of the turbine like the gearbox are constantly monitored and the data is collected trying to detect problems and predict failures, I have never heard of such monitoring for the foundation.

Additionally it is interesting the type of sensor used: instead of standard accelerometers or strain gauges the researchers used a strain sensor based on fibre-optic called “fibre Bragg gratings” (FBGs).

Basically it is a sort section of optical fiber treated in a way that some specific wavelengths are reflected and some are transmitted. They can be used as a strain sensor because when they are deformed the transmitted and reflected wavelengths shift, allowing a calculation of deformation.

Cracks can evolve with 3 different displacement type:

  1. Opening (the crack becomes wider)
  2. Sliding (one face of the crack slides on top of the other)
  3. Tearing

Through the monitoring period no significant evolution of cracks was observed. Basically, the wind turbine owner was lucky: cracks did not deteriorate and no intervention was needed.

Unfortunately, the cost associated with the monitoring are not shared, so it is difficult to make a business case (cost of immediately repairing the cracks with grouts or epoxy resins vs. cost of monitoring to see if the intervention is needed).

I also see that this solution only allow monitoring visible cracks. This is a strong limitation, as several failures originate in a non-visible area of the foundation.

Said that the idea is certainly interesting and useful, above all considering that some turbine are kept in operation for a very long time, even exceeding the design life of the foundation (usually 20 years).


Readers' questions: may I use an anchor cage somewhere else?

I’ve just received this question from a reader. As I believe it’s an interesting topic I’ve decided to answer with a post instead of an email or in the comment section.

“I'm custom broker and I have to classify under HS Code an anchor cage. I have consulted to Classification Office of Argentine Customs Service and they ask me if the anchor cages are designed to be used exclusively in the construction of wind generators, or if they could be used in other constructions, for instance an antenna tower.
I would appreciate if you could help me on this matter.
Kind regards.”

The answer is no – they can’t  be used somewhere else.

There are several applications for foundation cages: power transmission pylons, light poles, mobile phone antennas and other type of towers.

However, anchor cage are dimensioned to fit a specific type of tower. For instance, different wind turbine models have different anchor cages (both the number of bolts and their diameter might vary). You can’t take a generic anchor cage and put it below another tower: the number of bolts, diameter of the tower and size of the bolt would not match.

Some years ago there has been a famous mistake in a wind farm in Brazil – the wrong anchor cages have been shipped (and embedded in the concrete of the foundations). The mismatch between tower bottom and anchor cage was millimetric, so the installation crew tried to install the towers for hours before discovering the mistake. It was the anchor cage for a different model of tower for that specific wind turbine model.

It’s interesting to note that at least a wind turbine manufacturer offer a range of anchor cages with different bolt lengths compatible with a specific wind turbine model. This allow for a greater customization of the foundation and savings in material.

Peikko rock adaptor foundation

I've been asked by a reader of this website why there are no references to the various technical solutions available for wind turbines on rock.

The truth is that I’m not a specialist on this topic. However I’m learning, due to the fact that I’m currently working at several projects in northern Europe where it can be applicable.

To solve the problem I’ve decided to start with a video, that can be better than a 1000 words.

It’s an example of foundation on rock without anchor cage – one of several possible technical alternatives when the turbines are above shallow, unfractured rock.

Basically the tower rest on a steel “adapter” plate on top of a reinforced concrete block, and the turbine is fastened to the ground with dozens of post-tensioned anchors several methers long (figures above 9 or 10 meters are not unusual).

There are some very clear benefits with this solution if the geology is favourable: for instance less excavation, almost no blasting and lesser use of materials.

This video has been done by Peikko, a Finnish company specialized in steel elements. They have an interesting, unusual business model, as they do the engineering for the foundation and provide the steel but not the concrete or the manpower – therefore the foundation has to be built by another company.

Here a screenshot with the main elements of this solution:

One more video (possibly more detailed) on this technical solution here:

BoltShield® anchor bolts rust protector cap

Some weeks ago I’ve been contacted by a company developing an interesting product – a tailor made protector cap for anchor bolts.

I’ve notices that in some wind farms corrosion of the exposed side of the anchor bolt can be a problem. For instance, it’s not unusual to observe this phenomenon in areas with high salinity (e.g. Chile, or near the sea in the Netherlands).

If rusty, the bolt need to be cleaned before being tensioned. In theory this solution could improve the situation.

The solution, called Boltshield, is a metal cover cap available in several materials like aluminium and carbon coated steel (other similar products are made of plastic).

This cap should protect the upper part of the bolt, the nut and the washer from possible damages.

Additionally, coupled with paste or corrosion inhibitor, should prevent corrosion.

It’s a specific product line for the wind energy sector and apparently is already used in wind farms in  several countries (Italy, Finland, Scotland, Lithuania).

They claim that the market response is particularly interesting for the innovative screw-on system that allows an easy and safe screwing on the tie rod.

I didn’t had the opportunity to test this product (and obviously I’m not affiliated or compensated by them) so I can’t assure you that it delivers what promise. If you do have experience please drop me a line.

 Wind turbine foundations cracks – an update

I already discussed in another post a frequent problem of with turbines foundation, the appearance of cracks.

In general, my impression is that the new foundations with anchor cages are much more reliable that the previous technical solution (the “embedded ring” – the industry standard some years ago).

However, every now and then I still hear story of foundations that need some kind of  intervention due to mistakes during design and/or execution.

Unfortunately there is a lot of secrecy on this issues. Unlike other civil engineering products (e.g. roads, dams, etc.) problems with wind turbines foundations are generally hidden, probably due to the fact that they are mainly private investments and probably the companies experiencing an expensive problem prefer to have as little publicity as possible.

From several studies I’ve been able to found on the topic it seems that towers and foundations are accountable for less than 5% of WTG failures – being blades, gearbox and generator much more frequent sources of problems.

However,  while replacing a blade or a gearbox is “business as usual”, replacing a foundation is not  really an option – and any intervention will probably be quite expensive.

Problems in the foundations usually materialize as cracks in the concrete.

In many cases they are caused by the cyclical nature of foundation loads – with a lifespan of 20 to 25 years the foundation can be exposed to millions of loads cycles.

These cracks can be radial or circumferential, and appear both in the pedestal (the visible part of the foundation, where the tower connect to the foundation) and in the buried part of the foundation.

Usually these cracks tend to appear soon (1 or 2 years) and they doesn’t pose a danger to the stability of the wind turbine. However, water could infiltrate them damaging the reinforcement bars.

The position of cracks can be defined with ultrasonic devices.

These technology use the echo of sonic waves to create tridimensional images of the foundation. In practice a crack will appear as a discontinuity, reflecting the wave to  the receiver.

Should cracks on a foundation worry you?

It depends.

It’s important to note that not all cracks are created the same: shrinkage cracks or cracks in the grouting due to an excess of material are usually less critical than the appearance of voids (for instance below the load spreading plate or the bottom flange of the anchor cage).


Piled foundation for wind turbines

I’ve noticed last week that the blog has no post on piled foundation and decided to write a couple of words on the topic on a flight back to Hamburg.

Piled foundations is a broad term that include several technical solution aimed at increasing the bearing capacity of the soil when design requirements such as bearing capacity, limited differential settlement and/or necessary rotational stiffness can’t be met designing a standard shallow foundation.

Piled foundation are a type of “deep foundation”, a concept opposed to the standard “shallow foundation” solution - other type of deep foundation solutions are for instance soil substitution and soil injections.

They are relatively frequent. The number of piled foundation that you will see are very dependent on the countries you are working in – probably almost 100% of the new WTGs in the Nederland are piled, while turbines installed in Portugal on the top of a mountain ridge are usually on a shallow foundation.

Piled foundation are usually expensive – as a rule of thumb, they can cost between 1.5 and 2.5 times the cost of a standard shallow foundation for the same turbine type.

The selection of a solution will  usually depend on 2 main criteria:

  1. Cost: unsurprisingly, the main concern is usually to limit the impact of special foundations on the project budget.
  2. Constructability: sometimes the “best” solution can’t be selected due for instance to a shortage of machinery (a problem more frequent that you might think, above all in good times of strong economic growth), environmental constraints or other potential impacts (for instance, some piles are hammered and they can induce vibration on other building and structures nearby).

The main categories of piles are

Precast piles: this solution use concrete (or more infrequently, steel) piles fabricated somewhere else. Piles are driven into the ground by a hammering machine that measure the resistance of the soil to each blow. They say that “a driven pile is a tested pile” – and if you are not reaching the needed capacity you can keep hammering the pile to a greater depth as they are modular. Precast piles are usually more expensive and as mentioned they generate noise and vibrations.

In situ piles: this type of piles are fabricated on the spot. Augercast piles, also known as continuous flight auger piles, are drilled by a machinery that first drill the hole to the requested depth and after, while retracting the auger, inject cement ground in the ground. After concrete reinforcement bars are inserted in the pile. Drilled piles are similar – first the hole is drilled, after the walls of the excavation are kept into place using a fluid like bentonite or a steel casing. After, as in the previous case, concrete is pumped and concrete bars are inserted in the hole. The last type are rammed aggregate piles (also called impact piers), where instead of concrete gravel is inserted in the hole and after hammered by the machine. By doing so a higher density is achieved for the  gravel and for the soil around it. This solution is specially effective in seismic areas, were concrete piles can be broken by earthquake.

The pictures above are coming from a wind farm I’ve worked at many, many years ago (I think it was called Zarzuela, with 22x V90 2MW… now the WTGs in the market are over 4 MW!).

The piles shown were done by a company called Rodio Kronsa.

If I remember correctly they were built pumping directly the concrete, without bentonite. The concrete was pumped from a central pipe, while in parallel the earth was extract by the rotatory driller (basically, a huge Archimedes' screw).

After, the reinforcement bars were inserted.

Nabrawind Technologies self erecting tower

Image copyright of Nabrawind

Some years ago I posted an article about a self-lifting wind turbine tower.

The idea was to use using heavy lift strand jacks already available in the market to lift the concrete sections of a wind turbine tower. It's a project developed by Esteyco, a Spanish engineering company.

Well, I just discovered that it’s not the only idea currently under development on this topic – another company (curiously from Spain as well) is studying a somehow different concept with the same objective: avoid using big cranes, above all in areas of difficult access.

The product is called Nabralift - you can read more on the company webpage.

Basically it's a tower with a mixed technology "standard" tubular steel + lattice where the steel section is erected by a standard auxiliary crane (like in the pre-erection of the first section) while the lattice elements are "pushed" little by little from the bottom by hydraulic jacks (this is a similarity with the solution discussed in the other post).

Apparently the solution doesn't use an anchor cage - instead what I see from the pictures look like an adapter between the steel and the lattice section.

The company stated that they are undergoing a 6 months long fatigue test on a full scale model already erected in northern Spain.

Theoretically, there are 3 possible sources of saving here:

  1. Installation cost (no main crane)
  2. Tower cost (the lattice segment is proportionally cheaper than a standard steel one of the same height
  3. Foundation cost (apparently this solution will need a smaller foundation).

Last but not least, the increased stiffness of the lower part could lower the self resonance risk due to the passing blades, frequent in the very high towers currently in the market.

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.