Blades repair – how to fix it

I have discussed in other posts why wind turbines blades are prone to different type of damages and how to detect them.

But what happen when a problem is identified?

Changing the blade is usually the “last recourse” option: in addition to the cost of the blade itself there are the transportation costs plus the need to fine a main crane for the job (and it is not easy to find a free slot for a crane: due to their cost owners try to keep them busy 100% of the time).

Therefore efforts will be made to repair the damaged blade with a short downtime - ideally one or two days.

The technique used to fix the blade will depend on numerous parameters such as the entity of the damage (cosmetic, structural or affecting the efficiency of the blade), the region of the blades that suffered the damage (some areas such as the leading edge are more critical) are and the type of problem (cracks, debonding, impact damage, etc.).

Among the repair techniques currently used it is worth mentioning:

  • Filling and sealing (also called "dill & fill"). With this solution small superficial cracks, delamination and other similar non-structural problems can be repaired injecting the appropriate material (usually resin, or special fillers or gel). To do it injection holes going to the depth of the defect are created. Often the resin is pre-heated before being injected with manual guns or pneaumatic tools using compressed air. The material injected can have a curing time of several hours or even a day. Curing can be at ambient temperature or at a higher temperature depending on the chemical properties of the substances used. In case higher temperatures are needed heating blankets or similar tools such as ultraviolet lamps (UV) are used.

 

  • Coatings, tapes or shields. These solutions are especially designed for leading edge erosion repair and protection – a classic and frequent problem. Basically the idea is to use an additional layer to protect the leading edge.

 

  • Plug/patch and scarf repair for major damages. This solution involve removing the damaged region, leaving a straight, stepped or, if possible, tapered hole. Subsequently the patch is applied to close the hole. There are different alternatives for the patch: it can for instance be formed from a pre-impregnated composite fibre tape cut to shape, applied in layers using intermediate layers of adhesive or preformed to the correct shape and subsequently bonded.

Wind turbne blades repair patch. Image from Report on Repair Techniques for composite parts of Wind Turbine blades
(D. Lekou)

The adhesive used in blades repair is usually especially designed for this type of application. It has to be resistant to fatigue and cracks and have a short curing time (although for wind turbines located in extremely hot environments a slow curing adhesive can sometimes be a better choice). Two-component adhesive are frequent (basically the blade repair technician mix the two different components of the adhesive, activating the reaction).

Different repair works may be needed if the damage has be caused by lightning (a frequent occurrence) or if the problem is with the ancillary elements of the blade (like the vortex generators).

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

Wind turbine blade damage detection systems

Wind turbine blades damages. B.F. Sørensen, E. Jørgensen, C.P. Debel, H.M. Jensen, T.K. Jacobsen, K. Halling, et al., Improved design of large wind turbine blade of fibre composites based on studies of scale effects (phase 1). summary report.

Wind turbine blades are rarely subject to catastrophic failures, although you can find a bunch of videos on YouTube with blades flying away or falling into pieces.

They are however subject to several problems, such as cracks, debonding of the various layers, internal delamination, etc.

As an order of magnitude blade failures are accountable for approximately one fifth of the problems usually seen during the operational life of the wind farm.

Such problems have a direct impact on the profitability of the plant and can be extremely expensive to fix.

Some issues can be fixed leaving the blade attached to the rotor. However not all problems can be solved without dismantling the blade, and the fee associated with the cranes needed for the dismantling can be very high.

In the worst case scenario a blade substitution will be needed, and the transportation cost will add up.

A certain number of failure is attributable to design error (insufficient material strength under fatigue or extreme loads) or manufacturing problems.

However the overwhelming majority of issues seem to be attributable to extreme weather – basically strong winds, thunderstorm (including lightning strikes), ice, pollutants, etc.

The effect of ice accumulation is especially dangerous in case of asymmetrical accumulation, because it can create unbalanced loads in the rotor.

Pollutants, usually in the form of airborne particles (e.g. sand) can shorten the life of the blades accelerating the erosion of the superficial layer. A gel coat usually cover the outside of the blade to protect it from ultraviolet degradation, water, etc.

Therefore the erosion of the external layer may have two impacts: lower the efficiency of the blade (this will have a direct impact on energy production, and ultimately on money) and accelerating the degradation of the blade.

Some areas of the blades are more prone to damage: tip, root and joints accumulate the majority of reported damages. The distribution of failures has however a typical pattern: erosion is more frequent in the tip side (due to the higher local speed) while cracks are more frequent toward the root.

Blades crack and erosion regions. Zhang, H., 2016. Reducing Uncertainty in Wind Turbine Blade Health Inspection with Image Processing Techniques. Iowa State University, Ames, Iowa

A variety of methods has been developed to detect and monitor damage. There is a consensus on the fact that early detection of a problem mean usually lower maintenance costs.

The “perfect”, ideal method should be able to assess the full scale of the blade without contact, allowing for continuous remote monitoring.

Unfortunately such methods does not exist yet. Among the most promising techniques it is worth mentioning:

The use of strain sensors, either embedded in the blade or on the surface. It is the same type of fibre-optic called “fibre Bragg gratings” (FBGs) that I have described in another post. As each sensor measure only direct strain and shear strain only in one point of the blades many sensors will be needed to have the full picture. Another problem is that the sensor can give a "false positive" result if it fails due to creep, fatigue, etc.

Measuring the acoustic emissions of the blade is another extremely interesting method to find damages. Unfortunately it is a complex system that need many sensors and physical contact with the blade of many sensors – something that could be difficult to achieve in operational wind farms. Sensors operate on an extended frequency range (50 kHz to 1 MHz) and as in the case of the strain sensor are used not only for the blades but also for the other components (gearbox, bearings, etc.)

Ultrasound is an extremely common non-destructive technique, that basically analyse the ultrasound waves reflected by the different layers of materials and by discontinuities in the materials. The main problem of ultrasound detection is that usually physical surface contact is needed. On top of that, there is also usually time consuming (and it can be hard to process the signal). It can however be extremely useful for an in depth analysis of the situation of the blade.

Thermography is a technique that use infrared cameras to assess the temperature distribution. If there is a defect in the structure the flow of the heat will be disrupted, causing temperature gradients. Thermography comes in two types, passive (made from the ground using the “natural” temperature) and active (heating the surface).

The last method, and possibly one of the most promising giving the quick evolution of computer computational power, is Machine Vision. One or more cameras are used to capture images of the blades subsequently analysed with image processing algorithms.

There are several interesting studies on machine learning – basically using two set of image, one of blades with known failures and the other of blades without problems, to “train” the algorithm to detect issues.

 

 

Taking over – it really matters

One of most the relevant milestones in the life of a wind farm is the taking over.

It happens when the contractual requirements for the wind farm are considered fulfilled by the subcontractor, except for smaller items that are noted in a “punch list” and have to be fixed as soon as possible.

The requirements for taking over are defined in the contract. They usually include a (very, very long) list of documents to be provided and all the tests to be performed.

The taking over is officialised by a taking over certificate. From this point delay liquidated damage stop accruing, and usually there is a reorganization of the bond structure (for instance the performance bond can be replaced by a warranty bond).

Additionally the clock for the defects liability period start ticking. Subcontractors have the obligation to replace defective items or equipment (for instance, a transformer) and this usually “reset” the clock for that specific equipment.

The obligations of the subcontractor are usually guaranteed by retention of payments for the punch list items and by the warranty bonds for the defect liability period.

Under FIDIC and FIDIC-like contract the subcontractor can make a claim if he consider that the employer is avoiding to issue the taking over certificate without a justified reason. This is particularly relevant if delay liquidated damages are accruing – in this case an independent third party expert is usually involved to solve the dispute.

“Sectional taking over” is another relevant concept – it means that the wind farm is not taken over as a whole but in smaller sections. Usually those sections match the wind farm internal circuits, but in theory even a single wind turbine (or even a foundation) can be taken over.

“Deemed taking over” means that if certain events happen (for instance, the wind farm start its operation) or a number of months elapse for when the takeover certificate is requested by the subcontractor the taking over is consider to have happened

How good is the wind farm you are working at? Some indicators

So, how good is the wind farm you are working at?

There are several parameters that can be used to assess a renewable energy project and to compare different projects.

Among the most used, it is worth mentioning the Capacity Factor, NPV, IRR and LCOE.

Capacity Factor is the ratio between the actual energy production of the wind farm (that is, GWh/year) compared with the theoretical production.

Expressed as a percentage it is usually a number somewhere between 20% and 50%. Wind farm with a capacity factor above 50% are usually regarded as quite exceptional.

It is basically function of two parameters, wind variability and wind turbine selected for the project. On top of that you will have several losses - for instance electrical losses, noise curtailment, wake losses, etc.

To calculate it you will simply divide the energy produced by the wind farm by the nameplate capacity by the number of hours. Due to the seasonal variability of the wind it makes sense to make an yearly calculation.

What is interesting is that Capacity Factor is fundamentally and economical decision. At the end of the day you want to improve your business case, so it could make sense to install wind turbines giving a lower capacity factor (but with an even lower total cost).

The Net Present Value (NPV) is today’s value of a future cash flow.

In the formula C is the cash flow (-C0 is the initial investment, C1 is the cash flow of the first year and so on until the last year, n) and r is the discount rate.

This metrics give priority to the absolute return of the investment. Basically it is useful if you have only one shot: if you put all your money in a single project you will prefer (ceteris paribus) the one bringing more money.

The discount rate reflect the fact that money in the future is worth less than money today – for inflation, cost of opportunity, etc.

Internal Rate of Return (IRR) is the discount rate that makes NPV = 0.

This metrics give priority to the percentage return. It could be useful for instance if you can pick several projects among many.

Levelized Cost of Energy (LCOE) is defined as (CAPEX + OPEX ) / AEP

CAPEX (Capital Expenditure) is the money that the wind farm developer will have to put in all the assets – not only the wind turbine itself but also the infrastructure (roads, foundations, substation, etc.), and the development costs (everything from land lease agreements to the engineering studies).

OPEX (Operational Expenditure) is what the wind farm owner will spend to have the wind farm up and running.

This include basically the maintenance of the wind turbines (they need new oil every now and then, pretty much like your car) and of the substation equipment. As the lifetime of such project is increasing from what used to be industry standard (20 years) to 25, 30 years and more.

Additionally the more the wind turbine gets older the more is likely that it would need major maintenance (for instance a new gear box).

LCOE makes a lot of sense when you are trying to compare energy produced by different technologies, for instance wind and solar photovoltaic.

Lazard (a huge private investment bank and financial advisory firm) distribute periodically a study on the evolution of LCOE. Currently available in version 13 it gives you some visibility on how much different forms of energy cost without subsidies.

I still remember when I started working in the renewables sectors about 10 years ago. Comparing the cost of solar and wind I believed that my colleagues who decided to work in solar were crazy as the cost per MWh of Solar PV was huge.

Well, it looks like I was wrong.

What do you call it? Basic terminology in wind farm construction

Lately I have found several high quality videos on YouTube with time lapses of wind farms constructions.

I have decided to take some screenshots and use them to create a very basic BoP visual dictionary. You can click on the pictures to make them bigger.

Enjoy!

Crane and auxiliary crane lifting a steel tower section:

A rotor fully assembled on the ground before erection. It start to be an outdated practice due to current rotors size and weight:

The different areas of a crane pad in a wind farm in Australia (Goldwind's Cattle Hill):

Main crawler crane and its elements:

A blade lifted by an auxiliary crane. One of the workers is under a suspended load - not a best practice:

Workers completing the anchor cage assembly:

Slingers & trenching machines in wind farms: a guest post by Christopher James

After posting this article on Slingers I have been contacted by Christopher James, an expert on the topic with an exceptional amount of real world experience.

He has been so kind to share is knowledge on the theme and I am thankful for that. I am sharing it with you in the very comprehensive post below with almost with no edits.

You can reach him via LinkedIn or following one of these links:

https://slingers.com

http://buckeyetrenchers.com

(Beginning of guest post)

This video link shows some of the applications of the Slingers working on wind farms.

Slingers are high speed material placing machines.  By introducing them to backfill cable trenches most companies can remove one piece of earthmoving equipment, one operator and two or more labourers.

While doing all of this they are able to increase production of over 10 times faster than some operations and while almost reducing waste of the imported material.  In the  video below these speeds of backfilling are real time speeds.  The machine was backfilling cable trenches on a wind farm in NSW, Australia.

 

Speed is obviously an advantage but it is the ancillary savings like reduced labour and equipment.  Also the waste of the imported material is huge with traditional methods.  In some cases on large scale wind farms we are able to off-set the cost of a Slinger to just the savings alone…all on one job.

These machines can move up to 3.5 cubic meters per minute.  This is in a perfect world and perfect conditions.  Generally speaking a good average to work on is 8 to 10 meters per minute on most 400mm wide trenches.  As you can probably tell this is huge production compared with some of the old methods.

This wind farm in NSW where we brought this TR-30 Slinger (rubber tracked with cab) they were using a 25 tonne articulated dump truck and a 20 tonne excavator.  The excavator would scoop material out of the truck and place into the trench.  We were around 15 times faster than this operation with the obvious one less machine and operator.

I have also added the below the link to a video by Buckeye Trenchers working on the cable trenches on wind farms in the USA.  We are agents for Buckeye Trenchers and these really are high speed trenching machines.  A Bucketwheel trencher is suited to soils or small stoney ground.  They are not suited to rock at all.  Once you hit rock you have 2 options, either a Chain Trencher or a Rock Wheel trencher.  Either way there are options.

A chain trencher is one of the most common trenchers around the world, as seen in the Vermeer video link below.  These units can range in trenching widths from 300mm up to over 2 meters wide.  Ranging in weight from 20 tonne up to over 200 tonne.

A rock wheel trencher as shown in the video below by Vermeer will generally only go up to 350mm wide trench.  Now I have seen up to 450mm wide but not a very common machine.

Next you step up into the “dig, lay, bury” machines.

Rivard make a unit in the attached video. These are great units for single pass operation.  From experience though if one link in the chain stops, like the trencher or the cable spooling and so on, everything stops.  This can be quite costly.

From my experience having crews working on multiple fronts at once limit your risk and usually allows for a more productive environment.  For example a bucketwheel trencher should get around 3 kilometres of trench done in a shift, the same goes for backfilling with a Slinger (depending on many factors).

If you were doing a single pass operation you would not achieve numbers like this.

It really comes down to how much production you really want, or you can achieve.

Each operation has its pluses and minuses, no doubt about it.  As I was always taught in pipelines, get the basics right.  Get the equipment right.  Lower your risk as much as possible and then find minutes to shave off each process.  Laying cables is as repetitive as it comes, just like pipelines and it is all about streamlining processes as much as possible to get as efficient as possible.

This is why we have always used trenching machines for pipeline works.  They replace many excavators (bucketwheel trencher can replace up to 10 x 25 tonne excavators).  Trenchers also make exceptional backfilling material, excavators just cannot do this.

As a general rule (very general as all ground conditions change dramatically) please see the below for trenching equipment production:

  • Bucketwheel in good, dry soil ground conditions: 3 kilometers up day digging 400mm wide trench at 1.5 meters deep
  • Chain trencher in firm, dry, small stone ground conditions: 800 meters to 1000 meters per day digging 460mm wide and 1.5 meters deep with a 45 tonne class machine

I cannot give production in rock as it is too much of an unknown with the hardness and so on.

I have had 45 tonne class chain trenchers take 3 days to cut 4.5 meters of pink quartz and then dig 600 meters of hard limestone. It is too much of a variant to give some solid figures.

Additional links that you might find useful:

https://www.linkedin.com/pulse/where-how-use-my-slinger-episode-10-wind-farm-christopher-james/

https://www.linkedin.com/pulse/7-reasons-why-bucketwheel-trenchers-bring-efficiency-your-james/

https://www.linkedin.com/pulse/where-how-use-my-slinger-episode-2-trench-backfill-imported-james/

https://www.linkedin.com/pulse/stop-wasting-money-when-backfilling-your-trenches-imported-james/

Can a rotor be smart? Passive optimization systems

The spectacular growth of the dimension of the wind turbine has led to the introduction of several interesting technical solutions. Different type of towers (concrete, hybrid, lattice, self-erecting, etc.) and new technical solutions for the foundations appeared in the last decade, trying to have the lowest possible cost of energy.

The rotor of the turbine has followed the same trend. With the impressive size of the blades currently in the market (today we are around 70 meters, the width of a football field) it is not surprising to see a variety of new concepts already in the market or under development.

One of the key issues of very long blades is that it is difficult to optimize them: finding the “sweet spot” for a component exposed to a variety of wind flow characteristics during its operational life is not easy, above all if such characteristics are not uniform along the blade.

The engineers designing the blades are trying to achieve several goals, such as:

  • Increase the energy production (maximizing the aerodynamic efficiency and the power extracted from the wind)
  • Reduce the loads on the structure
  • Create a solution that can easily be transported on public roads and installed with cranes already in the market
  • Extend the life of the blades (we are moving to the 25 or even 30 years mark)

All these objectives should be reached with the lowest possible cost.

The term “smart rotor” refer to a variety of technical solutions whose purpose is to increase the production or reduce the loads.

They belong to two categories, passive systems (not controlled by a software or an operator) and active systems.

Among the passive systems the most interesting are:

Vortex generators. Believe it or not, you can buy the elements generating the vortex from 3M (the same company that invented the Post It all around my desk). You can use them to retrofit pitch-regulated turbines sticking them near the root of the blade, where the air flow is separated by the blade (that is, it is “stalled”).

They basically work reducing the separation of the flow, increasing the production 1 or 2 percentage points (they can be a lot of money).

A second passive technology is the bend-twist coupling.

As you will guess from the name it creates a link between the bend and the twist of the blade, with the object to reduce fatigue loads created by sudden inflow changes during turbulent wind conditions.  

The wind blowing on the blade is creating 2 forces – “lift” (the one pulling up the blade and making the rotor turn) and “drag” (the one bending the blade backwards). As a general rule the engineers try to minimize the drag and maximize the lift, achieving a high “lift to drag” ratio.

With the bend-twist coupling the loads are reduced because the blades “adapt” its shape changing its shape and the angle of attach when deflected.

This coupling can be achieved changing the geometry of the blade (geometric coupling) or by changing the direction of the fibre inside the composite material (resin + fiber) that constitute the blade.

This interesting technology is currently being investigating by several entities, including a heavy weight such as the Fraunhofer Institute for Wind Energy Systems.

Image Copyright Mark Capellaro, 2012 Sandia Wind Turbine Blades Workshop

I also described some months ago the serrations. They have a different scope (reduce the noise) but I believe they can be consider a type of passive optimization system.

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.