Anchor cage design standard assumptions: is there room for optimization?

Since some years ago almost all wind turbine manufacturers (“OEM” - I hate acronyms) have modified their tower to foundation interface.

The previous technical solution to connect tower and foundation was based on an embedded steel section (like a “ring” inside the foundation).

It did not work properly and the issues caused by this element might be easily subject of several articles, about the problems caused by the ring and on the solutions developed to fix those problems (i.e. retrofitting and repairs works necessary to ensure the necessary lifetime of the turbine foundations).

In the last years (I would say since around 2010) the embedded ring has been replaced with a pre-stressed anchor cage, as shown in the following picture:

The design methodology for this element is usually based on simplified hypothesis:

The first assumption is that the tensile/compression strength on each element is calculated assuming a uniform load distribution, usually using formulas such as:

T = 4Md / (n*D) + N/n

Being

T = Maximum tension force on the more loaded bolt

Md = Bending moment from the tower

n = Number of anchors

D = Average diameter of the anchor cage

N = Axial force

This is usually known as the “Petersen approach”.

Petersen is a German engineer who wrote a book about steel structure design appropriately titled “Stahlbau” (“steel construction” in German) where this calculation method is presented.

The second assumption is that the tensile force is distributed between concrete and steel if there is no decompression.

If decompression happens (something that will always happen under ultimate limit state factored loads) all the tensile force will be taken by the steel.

The only problem with this approach is that the first assumption is only true in case there is no decompression.

This approach leads to conservative results, as it does not account for the force re-distribution due to the stiffness change when decompression occurs.

However, it is very easy to obtain the maximum tension the more loaded bolt or to calculate the needed number bolts or their dimensions for a given tension.

When decompression occurs the stiffness in the “compressed” side and in the “tensioned” side stops having the same value, as the concrete stops providing its stiffness (this is the magic of pre-stressing, before de-compression the concrete is somehow taking some tension, a thing that concrete rarely does).

In the compressed side we will have an area of concrete under compression and bolts in tension (due to the prestressing), that take the compression by de-tensioning.

In the tensioned side we will only have the bolts in tension.

Similarly to a hyper static structure the stiffer side (in this case the one under compression) is able to take more load.

This works like a beam supported by springs:

In the picture on the left all the springs have the same stiffness. This would be the current design model, as in the formula shown above.

In the picture on the right the tension springs (right side of the beam) have only half the stiffness.

This is just to show how stiffness affects the force distribution, in a real anchor cage the loss of stiffness when decompression occurs might be over 80% as the concrete area contribution it is much bigger than the bolts area (total stiffness would be Es*As+Ec*Ac, being Es and As the area and elastic modulus of steel, and Ec, Ac the ones from concrete).

The “softer springs” on the right take less load, that is redistributed to the more rigid area on the left.

Please note that this would not happen in an isostatic structure (with only two supports)

As the neutral axis moves, the redistribution of forces changes. This lead to a non-linear calculation.

To perform this analysis we can implement a model with a homogenized concrete-steel section, and with variable parameters depending of the location of the neutral axis. Using this type model, we would be able to obtain the maximum stress on concrete and the tensile force on the pre-stressing element.

This way the anchors size may be adjusted, and we will get a more accurate value for the concrete compression which is slightly underestimated with the current models.

I am not going more deep into this boring details about calculations but I think that it is interesting to know that there is still room for optimization in anchor cage design.

IEC 61400-6:2020 Tower and foundation design requirements: a new Design Code is in town!

The IEC (acronym of International Electrotechnical Commission) has just released a new design code. More precisely it is a new section of an existing code, the IEC61400.

The IEC is an international organization that prepares and publishes international standards for all electrical, electronic and related technologies, including energy production and distribution devices.

The IEC 61400 is a set of design requirements developed specifically for wind turbines – to be sure that they are appropriately engineered against damage from different type of hazards within the planned lifetime (currently, 20 to 30 years). If you are familiar with the wind business you will probably know that this is one of the key international standards.

The IEC 61400 has several sections.

Section 1  deals with the wind turbine loads (more precisely, “design requirements”) in most of the world. A relevant exception would be Germany and some of neighbouring countries, where DIBT is used.

The new section released is the IEC 61400-6:2020 Tower and foundation design requirements.

If you are a wind turbine foundation designer, you are already aware that there is not really and internationally accepted design reference for wind turbines: there are some national references (such as the French CFMS Recommendation, or the Chinese FD 003-2007), some guidelines from certification bodies (such as the DNV guidelines), and recommendations from associations (AWEA for example has a recommendation for foundation design, but not a specific code for wind turbine foundation).

If we assume a similar applicability of this code as the one from the IEC61400:1 my opinion is that this is going to be one the more relevant technical reference (if not the most important) in the market for the next few years.

I am not going to enter deep into the technical detail of this standards, but there are a few points I would like highlight:

  • The new standard specify that foundation gapping does not need to be the limiting factors for foundations in all the cases. This opens the option to reduce the foundation size importantly when the soil is good enough.
  • Specifies the applicable codes for concrete design and provide guidance in how to perform some calculations (for instance cracking, dynamic shear modulus, etc…)
  • Has a set of very interesting annexes providing specifics about seismic calculation, strut and tie modelling, rock anchors, etc…
  • Specifies that there should not be decompression of the tower flange under the extreme (un-factored) loads.
  • Provides guidance about how to apply the sub pressure and perform the equilibrium verifications (this may modify some existing practises in some countries).

There are several interesting sections in this code, and many about towers and concrete towers that I have not yet analysed deeply but it seems that we might see some changes in the way we design at the moment.

It looks somehow unusual that this code has been issues by an Electrotechnical Commission – given the subject, it looks more like a code that should have been created by an institution of civil/structural engineers.

However I also believe that this type of reference and guidance was much needed in the sector, so I am happy that the IEC had taken the initiative of releasing such code.

Nabrawind Transition Foundation

Some days ago I have been contacted by Miguel, Sales And Marketing Manager at Nabrawind.

Nabrawind is a Spanish company working at several interesting breakthrough concepts – including a modular blade that I will try to describe in another article in the future, a self erecting tower and the innovative “Transition Foundation”.

Miguel asked me if I was interested in receiving material about the Transition Foundation solution they have developed.
I was obviously very happy to accept his offer and share with you what I have learned.

This alternative foundation use a 20 meters tall transition element in steel and cast iron in the lower section, at the bottom of the tower.

A detail of the transition element

The transition element is connected to the ground through three “feet” that allow different technical alternatives for the foundation: the standard solution (“shallow” or “gravitational”) plus two cheaper options – piled foundations (for standard soils) and rock anchors (when bedrock is very near to the surface).

The three alternatives solutions for the Nabrawind foundation - shallow, piled and with rock anchors

For a 4MW wind turbine the piles are expected to have a diameter of 1.5 meters with a depth in the 15 to 20 meters range.

A 15 meters pile would need approximately 80 m3 of concrete and 21 Ton of steel in total (that is, for the three piles).

This figure indicate very substantial potential savings in the amount of concrete and important reductions in the quantity of steel as well.

The piles are connected to the transition element via anchor cages (obviously smaller than the normal anchor cage used with standard solution).

In addition to the savings in the quantities the other main benefit of the solution is the speed. You will need only one or two days to drill the hole for the pile, and the installation of the reinforcement bars and concrete pouring is very quick as well (both operation should last between 2 and 4 hours in total).

The anchor cage variant promise to be even faster, needing only three concrete blocks (one for each “foot”) to level the surface and distribute the loads and 6 post-tensioned rock anchors with a length in the 15 meters range.

The Transition Foundation is more than a concept – the first foundations using this solution have been built in a wind farm in Morocco for a 3.6MW wind turbine on a 144m tower.

The have 24 meters long piles with a 1.2 meters diameter, for a total of only 81 m3 of concrete and 25 Ton of steel – a remarkable result.

How the foundation looks like (notice the 3 elements)

In the last picture you can see a detail of the completed works for the foundation.

How the completed foundation look like

Lift me up: the braced foundation

The braced foundation is a partially precast foundation that lift the wind turbine some additional meters above the ground.

Developed and patented by Esteyco (a Spanish engineering firm) is a technical solution validated, certified and used in several wind farms worldwide.

This solution increase the hub height up to 5 meters, which usually results in a significant increase of the annual energy production.

The “braces” are elements of precast concrete – basically double beams with a rectangular section transmitting the loads from the tower and stiffening the foundation.

They are on top of a cast in situ circular concrete slab that transmit the loads to the ground. This slab has a circular edge beam below, whose function is to absorb bending moments and contribute to the overall stiffness.

In the middle there is a central ring, while the tower rest on a smaller upper slab.

The main benefit of this solution is the increase of energy production – 5 meters of additional hub height can bring an annual increase in the 1% to 2% range depending on local wind condition.

Although this could look like a small number, compounded over 25 to 30 years it can really make a difference for the economics of the investment.

You first question could be something like “why not to use an higher tower”?

Generally, towers are designed, manufactured and sold with specific heights. Each wind turbine manufacturer has a portfolio that include only some heights (e.g. 90m, 100m, 110m, etc.).

Therefore you could find yourself in a position where the project could theoretically use a different hub height not offered by the wind turbine manufacturer.

Although every now and then project specific tower are designed and built this is not the standard and it has several implication in terms of time, cost, etc. Therefore it could be better to go for an off the shelf solution that gives you those additional few meters that your project need.

According to Esteyco this solution is also quicker to execute, at least in big wind farms. I do not have real world feedback to comment on this, although my impression is that the number of precast or partially precast foundation solutions used in the market is increasing.

This solution as a certain versatility because it can be used with different soil condition, including difficult geotechnical situation that needs piles.

It also use less material due to its geometry. I do not have actual figures to comment on the final cost, however my impression is that the real benefit will come from the additional production and that the saving in materials will be offset by the increased manufacturing complexity.

This solution has already a certain track record. I see that it has been used in Italy, Mexico, India, China and Saudi Arabia (in Dumat Al Jandal, a wind farm that I tendered 8 or 9 years ago – this gives you an idea of how long it may takes for a project to materialise).

It has also been certified by DNV-GL and TUV, undoubtedly a strong plus.

All the pictures are stolen from the presentation that Esteyco has given at India Windergy 2017.

Once More, with Feeling: Timber Towers

Modvion laminated wood tower. Image copyright Modvion

Approximately one year ago I wrote a post about a full scale prototype of a wind turbine tower made of wood.

It has been built in 2012, but after that the idea seemed to have stalled without progress: one of the companies involved in the construction, TimberTower GmbH, disappeared from the radar shutting down their website and I suspect they went out of business.

However, I see that someone else has taken up the challenge: Modvion, a Swedish start-up.

They could be more successful in moving from the prototyping phase to the industrialization for at least two reasons: they are coming from Sweden, a country with an extensive know how and network of companies active in wood construction, and they successfully went through a round of Venture Capitalists, Business Angels and European Union founding.

They target is to be ready to market in 2022 cross laminated timber towers in the 100 – 150 meters range. This means that they want to enter in the 4 to 5 MW segment, the current standard for utility scale WTGs. The prototype that they have just installed is 30 meters high.

Such tower could have several benefits – solve the current transportation problems (steel towers with diameters over 4 meters have huge transportation challenges due to bridges, cables, etc.), lowering the carbon footprint and possibly even be cost competitive against the current technologies (steel, concrete and hybrid).

I have no idea of the behavior of this solution from the resonance point of view although I suspect that the increased diameter at the base improve the situation. I also ignore how this tower would behave in case of fire: I have personally seen a fire very near to the wind turbines some years ago in Portugal.

Unfortunately their website does not share many technical details. I understand that it is a modular solution, with the total number of modules obviously depending on the tower height being a standard solution in the 30 - 40 modules range.

The tower section is circular, unlike the TimberTower solution that was octagonal.

Among the materials used for the tower I see glued laminated timber and laminated veneer lumber - basically a mix of wood and adhesives, with superior technical properties and more uniform characteristics as it is produced in a controlled environment.

The modules are joined together using double-treaded fasteners, preassembling on site 4 or more tower sections with a bottom diameter of 6+ meters (that is, more than a standard steel tower).

This concept is similar to some concrete modular tower solution with a key difference – the modules are assembled horizontally, so I guess that the need of big crane support is limited.

I also understand that the internals of the towers would be similar to the ones currently built (with elevator, ladder, space for transformer, etc.).

Printable 3D concrete wind turbine towers

Yes, you're reading right – you can print your concrete tower.

I have discussed in many previous articles how I see some evidence that we are reaching the maximum size for steel towers, mainly because of transportation issues.

For higher towers concrete towers could help solving the problem, as they can be transported in pieces and assembled on site.

Among the different technologies available for concrete I have just discovered this exiting evolution: a Danish company specialized in 3D concrete printing, COBOD, partnered with GE Renewable and LafargeHolcim to develop a large printable tower.

COBOD already printed some years ago a full scale building, a small but beautiful 50 m2 office with curved walls.

For this interesting evolution they already made a prototype about 10 meters tall.

The concrete is extruded by the machine in a sort of ribbon, and the internal and external sides of the tower are reinforced by a “wavy” central section.

Currently the solution that they are targeting is a hybrid tower (that is, with the top section made of steel) with an on-site printed base.

Dancing in the wind

I have discussed in other post the phenomenal growth of the dimensions of wind turbines in the last 2 decades. Bigger rotors, taller towers and more MW has been the industry trend year after year.

There is some evidence that we are reaching the limit – blades of more than 50m length pose significant logistic challenges, while steel tower more than 100 meters tall can be subject to strong vibrations and dangerous oscillations under certain circumstances.

Such vibrations can be induced by several external sources such as an unbalanced rotor, an earthquake or the wind itself.

They are dangerous because they can damage the turbine due to fatigue loading (the weakening of materials due to cyclical loads). Some type of foundation can also partially lose stiffness – for instance monopile foundations.

Additionally, these vibrations can also trigger resonance phenomenons in the tower – you can follow this link to see of how “soft soft” and “stiff” tower are designed based on the blade passing frequency.

You can see a good full scale example of this problem in the video above and read here more about wind turbine vibrations.

There are several technical solutions currently being studied to dampen the tower reducing the vibrations.

Among the most interesting concept that I have seen I would mention tuned mass dampers – basically an auxiliary mass connected to the structure with spring and dashpots (viscous friction dampers), friction plates or similar energy dissipating elements.

These dumpers are called “tuned” because they have been designed keeping in mind the natural oscillation frequencies of the structure they have to protect. The two main parameters are the spring constant and the damping ratio: by varying them it is possible to damp harmonic vibrations.

I do not know if tuned mass dampers that can work with the first fundamental frequency of  industrial size wind turbines (below 1 Hz) are currently available – however I have found quite a lot of  studies on the topic.

A similar technological solution is the tuned liquid column damper. In this case a liquid inside an U shaped tank. By varying the geometry of the tank and the depth of the liquid different damping frequencies can be achieved.

The main benefits of this solution are the geometrical flexibility (you have to put the dumper somewhere inside the tower or the nacelle – I can assure you that the space there is very reduces) and low cost.

Another variant is the pendulum damper. In this solution, the length of the pendulum is calculated to match the fundamental frequency of the WTG.

Mass Damper (a) and Pendulum Damper (b)
Copyright O. Altay, C. Butenweg, S. Klinkel, F. Taddei
Vibration Mitigation of Wind Turbine Towers by Tuned Liquid Column Dampers
Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014

Where have all the wind turbine gone? Foldable towers

Perima foldable wind turbine tower - folded. Copyright Pantano et al., Springer

In previous post some years ago I have described two alternative solution for the wind turbine tower that should help solving the problem of the huge cranes that are currently needed for the erection of the wind turbines components.

One is the self-lifting precast tower developed by Esteyco, a Spanish engineering company that developed several interesting technical solution.

The other is the Nabrawind solution – again, a Spanish company that developed a self-erecting tower. They also have another interesting product, modular blades that can be assembled.

Some days ago, I have discovered another technical solution that share some similarities with these two concept but with an interesting twist: a group of Italian engineers has developed a “retractable” tower, basically a telescopic mechanism that can be folded bringing the blades down to the ground without using cranes or other equipment.

Theoretically it could be operated from a remote location, even if I guess that some kind of supervision during the operation is advisable.

Why should you want to make your wind turbine disappear?

The authors mention several reasons, for instance minimization of the visual impact (you can make your WTG almost invisible during the day and having it work at night).

I can also think at other uses – minimization of bird impact (folding the tower during the migration period) or increased safety during extreme wind (for instance during the monsoon season in south east Asia).

The idea is not only a concept –a working prototype has already been built in southern Italy.

Perima foldable wind turbine tower - erected. Copyright Pantano et al., Springer

It is a small wind turbine (55 kW), at least for what is today the standard in utility scale projects (3 to 5 MW). Additionally it has only 2 blades, which I think can help when you retract the tower.

However the hub height is 30 meters, quite a reasonable figure.

It is interesting to observe that this technical solution needs a deep foundation, basically with a depth equivalent to the hub height.

It is mentioned the possibility to modify the concept to use the foundation hole as a well to extract water. Quite an interesting side benefit I would say.

The authors are not sharing the cost of the tower and the ancillary elements, although I suspect they could be several time the cost of the standard, non-retractile  tubular steel tower.

Finally, it would be interesting to know the applicability of this solution to WTGs in the MW class.

The authors mention a dimensioning bending moment of around 300 kNm. Such value is two orders of magnitude lower of the values that are common in industrial size turbines, so it is not immediately evident that the idea can be scaled without major modification.

An additional problem would be the length of the foundation pit.  Reaching depths of 50 meters and below, although not impossible, introduce new issues – for instance the need of very specialized drilling equipment.

Perima foldable wind turbine tower - technical details. Copyright Pantano et al., Springer

Second life: the destiny of turbine foundations after decommissioning

I have discussed in another article the challenges associated with the disposal of the blades when a wind farm is decommissioned or repowered.

But what happen to the foundations?

The destiny of a foundation will depends on local regulations, on the environmental requirements that are normally given with building permits and on the wish of the owner of the wind farm and of the land.

As a general rule, foundations are at least partially dismantled. The first centimetres (20, 50 or even one meter) are removed and the rest of the foundation is left in place and buried below a layer of organic soil.

Sometime the entire foundation is removed. This is a complex activity, and blasting or at least many hours of hydraulic hammer are needed.

The third option is to bury the foundation below a small hillock.

In case a repowering is planned there is also a fourth and more interesting possibility: giving a second life to the old foundation integrating it in the bigger, new foundation. There is a group of company that is studying this possibility under the name “FEDRE” (Fondations d’Eoliennes Durables et Repowering – French for “Long lasting wind turbine foundations”).

The concept that is being developed is how to reuse part of the existing footing for the new foundation - adaptation the existing one on the short term and working with reusable foundations designed ad hoc on the long term.

In case the foundation has to be dismantled some difficulties may be experienced.

They are very “dense” in steel (on average a foundation can easily have more than 100 Kg of steel for each cubic meter of concrete). Due to the concentration of rebars in some areas of the foundation (above all, in the centre) it can be more difficult and time consuming to separate the steel from the concrete.

Usually steel is separated from concrete and melted again. In some countries the reinforcement bar are even used “as they are” without being melted and reformed (i.e. they are straightened and used again in another structure).

The presence of steel makes more complicate grinding the foundation in smaller elements to use it again as a construction material, for instance to build roads (in the nucleus of the embankment) or for earthworks as a filling material.

It is worth mentioning that some with turbine have concrete tower or hybrid (concrete + steel) solution. They could be equally difficult to recycle.

Repowering and decommissioning will probably gain momentum in the near future.

2018 and 2019 saw only a few hundreds of MW decommissioned (unsurprisingly mostly in Germany, where the installed capacity is huge). However the numbers should increase steeply in the next years when more and more wind farms will end their 20 years of supported tariff.

Unless they are able to close some king of PPA (power purchase agreement with a counterpart willing to buy the electricity at a certain price) it could prove to be not economically viable to sell the power at spot prices.

 

Good vibrations: wind induced resonance and turbines oscillations

Have you ever wondered why sometimes the wind turbines (and other similar tall structure) sometime vibrate?

Under some conditions the wind blowing on the tower create vortices.

These vortices appears regularly on both sides of the tower, creating low pressure zones first on one side and then in the other.

This beautiful sequence of vortices is called von Karman vortex street. von Karman has been a pioneer of aeronautics

For these reason the tower will start moving perpendicularly to the wind, first toward one side and after toward the opposite.

The tower has a very low structural damping – when the oscillation start its reduction is very slow because the steel tower has a limited capacity to absorb the kinetic energy.

It also has a very low natural frequency (the frequency at which the tower will tend to vibrate when subjected to external forces).

The vortices created by the wind will appear at a frequency that depends on the speed of the wind and the diameter of the tower.

The formula to calculate this frequency is very simple:

f = St · U / D

where

f is the vortex shedding frequency

St is a value called Strouhal number (in our case it is around 0,2)

U is the wind speed

D is the diameter of the tower

At a certain wind speed the vortices will appear and shed at a frequency equal to the natural frequency and the tower will resonate. The wind speed that start the resonance is called the critical speed.

If the wind blow at the critical speed for enough time the amplitude of the vibration will increase and you will see the tower oscillating.

This is a simplistic summary of a very complex phenomenon. It is however a limiting factor for the use of higher steel tower. In additional to the risk of catastrophic failure the wind induce vibrations will also generate additional fatigue loads, shortening the life of the tower.

It is also interesting to observe that a structure has more than a natural frequency.

Vibrations in the lowest natural frequency (first mode) will have this shape:

First wind turbine tower vibration mode

However, sometime a turbine can vibrate in what is called “the second mode” the second lowest natural frequency):

Second wind turbine tower vibration mode

Following this link you can see a real world example of how is a tower vibrating in the second mode.

What can we do to avoid these dangerous vibrations?

Some solutions are structural - basically aimed at increasing the damping of the tower or changing the way the mass is distributed.

It is not easy to change the mass distribution in a wind turbine tower (basically its an inverted pendulum).

Nevertheless it is possible (and it is becoming increasingly common) to install dampers in the wind turbines, either only temporarily during the installation or as a permanent feature.

Other solutions are aerodynamic - the idea is to change the shape of the tower adding elements that disrupt (or "spoil") the vortices. Conceptually they are similar to the spoilers used in cars or planes, in the sense that they are intended to mitigate an unwanted aerodynamic effect.

An example are helical strakes, sometimes called "coils" "ropes". This is a concept developed by Christopher Scruton and other scientist such as William Weaver in the fifties and sixities and used often in chimney and similar structures.

However sometimes they do not work as you can see in the video below.

In general, the effectiveness of this solution is driven by two parameters:

  1. Diameter of the strake (usually defined as a ratio of the diameter of the tower)
  2. Pitch (the distance along the cylinder axis that is needed to complete one full turn of the strake)

Definition of pitch and height

From experimental tests in wind tunnels it has been found that the optimum height of the strake is approximately 10% of the diameter of the tower (that is, around 40 cm for a standard steel tower with a diameter of around 4 meters).

For the pitch the results of the test shown an optimum value in the region between 5 and 15 diameters (that is, one full turn of the rope should be done between 20 and 60 meters).

Obviously the smaller the pitch, the more the strake is parallel to the tower.

There are however other secondary factors that influence the efficiency of this solution, such as the area of the cylinder covered by protrusions (usually called “strake coverage”) and the pattern of the ropes (usually three or four independent ropes are used to create the helix).

One of the biggest problems of the strakes is that they greatly increase the drag coefficient (the resistance opposed by the wind turbine tower to the flow of the wind).

The implication is that the loads at the bottom of the tower will increase as well, so that a bigger foundation could be needed.

For this reason strakes are normally used as a temporary solution only during the installation – above all in the most critical part of the procedure, when the tower is installed but there is still no nacelle on top.

Removable strakes are wrapped around the tower, either before the lift while the tower segment is on the ground or after installation.