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.

 

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.

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).