Self-erecting turbines: the Elisa / Elican project

Elisa floating offshore wind turbine

The Elisa / Elican Project is a multimillion, full-scale prototype of a self-erecting offshore concrete tower.

The tower is coupled with a buoyant foundation – it floats and can be transported to the installation site where it is ballasted and sunken to the final position.

Once in place the tower self-erection can start, saving money on one of the most expensive items in offshore installation: the specialized vessels with cranes usually used.

The wind turbine is installed in the harbour, and as the tower is still “folded” a smaller crane can be used.

After the installation of the WTG an auxiliary floating system is used to stabilize the structure and the foundation is towed with tugboats. The auxiliary element is the yellow structure that can be seen in the picture below.

Elisa floating offshore wind turbine towed in place

This solution is applicable for a water depth in the 20 to 50 meters range.
The prototype has been founded with 3.5 ML€ by the EU and it is developed by a consortium led by Esteyco, a Spanish engineering company from Spain that developed several others very interesting projects such as the braced foundation.

The development of the solution started in 2015 and went through several stages (numerical modelling, tank tests campaigns, working prototype).

The prototype is equipped with a 5MW WTG (from the pictures I would say a Siemens-Gamesa) and it is obviously equipped with numerous sensors (inclinometers, accelerometers, etc.).

According to Esteyco, he two main distinctive features of the project are:

A self-erecting telescopic tower, which brings down the center of gravity during the temporary installation stages, enabling ground breaking possibilities in the installation process and providing full independence of costly and scarce offshore heavy-lift vessels which have become a bottleneck for the sector both in terms of capacity and availability.

An economic foundation base which (…) can temporarily act as a self-stable floating barge over which the complete system can be pre-assembled in-shore at low drafts and low heights and effectively towed to the site, where it is ballasted to rest on the seabed.

As you can see in the following picture, the tower is a hybrid solution (concrete & steel).

Hybrid offshore tower

While this is the first time that I see a full scale buoyant foundation that is subsequently sunken there are other self-erecting concepts being developed in the market – see for instance the Nabrawind idea.

Cable stayed wind turbines towers

The use of cable stayed wind turbine towers is somehow unusual.

They are however relevant when very high steel towers are used (from 100+ meters to the current record, 175m).

An example can be seen in the picture below.

They have been taken by Peikko, a Finnish company in charge of the design and construction of the wind turbines’ foundations.

The engineers at Peikko are very experienced and they have developed several interesting projects (see for instance their rock anchor solution).

They are also very nice and easy to work with (and if you wonder what is the logo of the company, a Peikko is a mythical creature, similar to a troll).

Cable stayed wind turbine

The way a cable stayed structure work is similar to a beam with a certain number of intermediate supports. This shortens the span of each element.

The pictures below are taken from a book on bridges from Javier Manterola, one of my favourite structural engineers.

The first is a cable stayed bridge:

The same concept is applicable to a suspension bridge:

The main difference between both concepts is that the suspension bridge requires a heavy overhanging cable (in red in the image).

This cable “collects” the vertical forces from the suspension cables.

By doing so a suspension bridge avoids the horizontal forces - one of the main problems of the cable stayed structures.

In the case of bridges we are supporting the structure from the top with tensioned cables – a very efficient solution.

In the case of wind turbines towers we are supporting a structure from the bottom.

The main force acting over a wind turbine is a massive bending moment, created by the horizontal loads of the wind on the blades.

The tower is behaving as a vertical cantilever fixed at the foundation.

Using cables we are creating a fixity point in the middle of the tower. This reduce the bending moment at the bottom.

The structural scheme would be as follows ("before" the cables on the left and "after" the cables in the right):

Structural scheme of a cable stayed WTG tower

The fixity point created in the tower by the cables reduces significantly the bending moments on the tower.

This allows using smaller diameters for the towers, with strong savings in material and transport costs.

It also makes possible using towers with very high hub heights.

However, there are not only advantages.

Some problems with this solution are:

  • Additional foundations for the cables are needed. These satellite foundation for the cables are taking horizontal and vertical (upwards) loads, complicating its design.
  • An interface element is required to anchor the cable in the tower.
  • Cables need to be pre-stressed - this could create delays the installation.
  • The topography in the area needs to be flat to avoid stiffness differences between the cables (this could happen is they have very different lengths).
  • Aerolastic phenomena in the cables need to be considered in the design.

Interestingly, in this project both cable stayed towers and helical strakes are used.

Another company who has worked at this concept is Mervento (curiously, another Finnish company).

Mervento cable stayed tower

Site visits and information management tools

This post complete the information of the previous article on how to plan a site visit for a renewable energy project.

I will try to answer this question: what is the better tool to manage the information obtained on site?

There are several very useful mobile apps that can help us finding the target locations and geolocate the position of all relevant elements of the project (wind turbines, substation, roads, crane pads, etc.).

They can also help us obtaining and storing information in an efficient and organized way.

Although none of these software has been specifically created for the wind business´s needs, they offer very useful features that could make our lives easier during a site visit.

As they are mobile apps, we should make sure we have enough battery and that is why I always recommend bringing a portable charger with you.

Google Earth / Google maps

These two apps are by far the most popular. Anybody who is familiar with the desktop software will rapidly recognize their strengths integrated in a very powerful GUI.

They allow the user to load kml files with very valuable information such WTG positions or road alignments or even connect to maps edited and stored in our personal Google account

I see three main drawbacks here:

  • The basemap loaded in the background is limited to the basic formats: Street, Terrain, Traffic, Satellite, etc. Unfortunately they do not offer the chance to insert our customized maps as a background.
  • The geolocation is based on the device´s built-in GPS, but the background map data is loaded only if network is available – and this could be a big risk in remote areas.
  • They are not really prepared to create and store points of interests with comments and link personal photos taken in the area of study – or at least, not as I would like.

Avenza maps

This app uses the device´s built-in GPS so that the correct positioning does not depend on the availability of the network.

One of the main advantages compared to the traditional GPS mapping apps like “Gaia gps”, “Google Maps” or “Google Earth” is that Avenza maps allows the user to import our own customized maps in different file formats such as geopdf, geotiffs.

These maps can be any kind of project drawings created with GIS tools such as QGIS or ArcMap.

This app can be used for free. With the free version we can download up to a certain number of maps at a time (usually 3). There is also is also Pro version which includes more features and increases the possibilities.

It also includes the option to create a store account. It is not required but is recommended though.

For more information about this tool, I recommend you go directly to the official web.

Maprika

This is a tracking map app used mainly by hiking lovers.

Initially, it looks like a very simple app without any special charm. However, it offers similar functionalities to “Avenza maps” and I personally consider it gives a good service to the actual needs in a site visit.

The main advantages are:

  • We track our routes and the elevation information (longitudinal profiles) is registered as well.
  • The photos taken along the routes are georeferenced and can be visualized over the route.
  • There is the option to add “Places of interest” with comments.
  • We can create our customized maps either by using the app or from the desktop tool called “Maprika map designer”. See below a link with a tutorial explaining how to do it.

As a drawback, we can considerate the lack of confidentiality: any map uploaded containing relevant wind farm information would be available to any user from the server.

Furthermore, it is quite easy to upload a map to the server but deleting is not as straightforward.

Here is the link to the website:https://www.maprika.com/

Road AI from Vaisala (old Vionice)

I leave to the end which, from my point of view, is the most promising from all the apps listed here and, surprisingly, maybe the less popular: Road AI

The first time I heard about this tool was in a Finish project I was involved in.

One of the contractors used to work with it and it ended up being a really nice discover.

Although addressed to cover infrastructure management, it can be “recycled” to work as an information management tool for site visiting. The mobile application provides a dynamic and flexible way to collect, manage and deploy data in a user friendly environment.

The main strengths are:

  • The user can record videos and make photos of the construction site which will directly be stored and available in a cloud service. E.g. We can make videos using a phone holder on the windscreen to record routes and the time and the GPS location will also be saved.
  • Make annotations associated to the audio-visual material recorded on site.
  • View all of the recorded routes and sites together with the annotations and metadata using the map interface available through a normal browser application.
  • We can share this information with anybody inside your organization or with a client.
  • It also allows to use filters and requests to visualize data following a certain criteria with a similar philosophy than in a GIS environment.

Surprisingly, it is difficult to find any reference to this application out from the local Finnish market. Only a few references are found in works at UK.

This is a link to the first of a series of video tutorials explaining the way to use and take most of the system (sorry it is in Finnish):

All in all, the proposals presented here are only a sample among a big offer presented in the market. Any tool with similar properties would work well enough used in a proper way. It is just a matter of personal preference as long as an ad hoc and convincing system is deployed and released into the market. Maybe one day…

Wind assessment: uncertainties in the horizontal and vertical extrapolation

Behind every wind farm project there is a technical feasibility study, made to assess the potential for electricity generation of the selected site.

At the beginning of the wind farms era wind speed and wind direction was estimated using data from existing weather stations.

Due to large differences between predicted energy production and actual generated energy (which was usually overestimated) meteorological met mast are now designed and installed to have better estimates.

Any estimate of the Annual Energy Production (AEP) contains several uncertainties and paramethers that can vary such as:

Measurement CampaignDuration, tower setup, anemometer quality.
Long term adjustmentsSelection of a long term wind data source, correlation and long term prediction methodology.
Flow modelLinear model, computational fluid dynamics models, mesoscale for estimation from met mast into none measured positions.
Horizontal and vertical extrapolationHorizontal and vertical distance from the meteorological mast(s) to each Wind Turbine Generator (WTG) position and hub height.
Wind speed to energyUse of a wind turbine calculated/measured power curve together with the site conditions wind speed to estimate energy (if within constrain inflow angle, wind shear, etc).
Technical lossesCalculated losses i.e. electrical, environmental, curtailment, wake model selection.
Uncertainties and paramethers affecting wind farm energy production estimates

Giving all these project uncertainties, an AEP can be calculated with different levels of confidence in the results.

For instance, what is called a "P50" is a production expected to materialize in 50% of the cases.

It is possible to estimate more conservative energy productions, such as for example P99 (a lower production value, that should materialize in the 99% of the situations).

The wind site resource assessment is a careful identification and evaluation of different risks and uncertainties sources that are unique for each site. 

In this article we will focus on two uncertanties, arising from the horizontal and vertical extrapolation of data.

For the vertical extrapolation we can identify two sources of uncertanties:

The difference between measurement height and hub height.

When using a lower met mast compared to the selected WTG hub height different methods can be used to estimate the wind speed either by extrapolating for example with a power law/log law method.

The altitude difference (flow model)

It is possible to reduce the first vertical uncertainty by installing a met mast with a top anemometer at the same hub height.

Alternatively remote sensing devices such as a Sodar/Lidar can be used on site.

It is recommended to have a wind speed measurement of at least 2/3 of hub height to keep uncertainties at an acceptable value, as any methods for hub height wind speed estimation (i.e. power law or flow model) will add uncertanty to the calculation.

A typical range for this uncertainty is in the 1 to 4% range, varying depending on the type of terrain and the total vertical distance (for example, 0.5% per every 10 meters vertical difference).

Vertical difference between met mast and wind turbine hub height

The Horizontal Extrapolation is heavily influenced by the terrain and surrounding vegetation.

Measnet (Evaluation of Site-Specific Wind Condition V2 April 2016) recommends different maximum distances between measurement position and wind turbine generator (WTG) for a simple and complex terrain.

One of the reasons is that certain orographic traits (plain, rolling hills, mesas, mountain ridges) and roughness (native vegetation, agricultural, forest, lakes) at the measurement position should be “similar” to the WTGs position for the flow to behave in the same manner.

A typical range for this uncertainty is from 1 to 4%.

According to Measnet the data from a met mast are representative only for few km - up to 10km for a simple terrain and around 2km for a much more complex terrain.

After the assessment of 200 projects DNV-GL identifies the horizontal and vertical extrapolation to be responsible for approximately 35% of the total energy uncertainty.

For projects with a high spatial variation (i.e. with turbines very far away from each other) the value can be as high as 51%.

More information on the topic can be found here: Reducing Uncertainty in Wind Project Energy Estimates with Triton

Depending on the wind farm total size, terrain characteristics and mesoscales effects this values can be even higher.

It interesting to note that even if two projects have the same P50 AEP, the one with lower uncertainties and therefore a higher P99 AEP will have better chances to be built being more "bankable". 

Conclusion: very early in the project, after just a few months of measurements, the Horizontal and Vertical uncertainty should be calculated and simulated in cost to benefit financial model to find out the best quantity of measurement locations to have.

A factor to consider this important topic in the initial period is to be able to carry out correlation between measurement locations and assess flow model cross-prediction errors that will further reduce project uncertainties.

How to plan a site visit for a renewable energy project

Today many resources can make the engineer life´s easier helping create a renewable energy project in any spot of the world.

Powerful GIS and CAD software and an incredible amount of data available from either public or private entities make possible designs with a sufficient level of accuracy to have good cost estimations, even at early stages of the project.

However no tool is good enough to give you the amount of information a site visit can provide. This is a step that can give a boost of extra quality to the design process.

Whenever possible, doing a visit to the project area is highly recommendable.

There are many circumstances which could make it impossible, such as aggressive delivery dates, excessive travel distances or lately pandemics.

Also priorities matters: we cannot compare the preliminary work needed for a tender phase in a very early stage with the final detailed execution design of a constructive wind farm project.

In this article I collected some tips to successfully plan a Site Visit:

Transport

Hiring a 4x4 car (4 Wheel Drive or similar) is a must. Take into account that we will usually find unpaved roads in uncertain conditions or even no roads at all.

Depending on the actual state, it would be wise sometimes pulling over, park the car and go to the targeted place walking.

Furthermore, companies are already offering already internally a 4x4 off road driver training. We can find a lot of difficulties and a skilled drive behind the wheel is a blessing. See below the state of a track in a recent site visit… Yes, we were able to arrive home safe.

Food and breaks

It could be the case that the site is in the middle of nowhere without direct access to any populated village. Even if that is not the case, we will have to assess the convenience of having lunch in a restaurant or canteen with the risk of losing valuable time in the travel. Anyway, we must prepare and take with us some food (e.g. sandwich and fruit would be a good combination) and enough bottled water for the whole journey. Short breaks every 3-4 hours to have some rest and eat some snacks is also highly recommendable. A well planned visit should have account for this moments and there should not be excuses for skipping them.

Clothing and HSE

For a good feet protection, construction security boots with reinforced toe will serve well enough. If construction has not started yet, hiking boots could be another option. Waterproof resistant trousers or, alternatively, with resistant fabric are also recommended. On the other hand, we should follow some common sense rules such as taking a hiking cagoule (raincoat) if we are expecting rain or a cap, sun glasses and sun protection for hot and sunny days.

A careful study of particular conditions at the project site shall be done to avoid any surprise.

Reflective vests were needed in a visit I made to Sweden because the wind farm was in an hunting ground. Anybody without proper clothes hiking in the mountains was in danger of being shot.

Another example was a visit I recently made to Australia. It was in December and the wave of forest fires was at its peak. Of course, temperatures raised easily to 35-40ºC and the initial temptation was to use short sleeved T-shirts. The fact is that the project area was full of ticks and a complete protection of the body was required to avoid any undesired surprise. I found myself shaking some of them out of my shoulders.

Visit planning

To make the most of our trip, we need to carefully plan our journeys beforehand.

Here goes a proposal with some key aspects. How to organize them will depend on the particular circumstances of the project:

  • If a route survey has already been done, follow the route sketched from the point of discharge (usually a maritime port) of the WTG components to the wind farm site and make sure that the report matches the reality. If there is not a route survey in place, try to find alternatives on site. If there is more than one option, we can use one route to go and other route to return.
  • Visit to the nearest towns or villages to assess the best place to stablish central headquarters and/or employee’s accommodations. A project execution phase will normally take, in the best scenario, several months. Finding a place within a one-hour radius to the construction site with leisure offer and good services will contribute to a keep the teams spirit up. As a rule of good practice, it is recommendable talking with the locals, as they will always be able to provide valuable information.
  • Go to commercial quarries close to the site. It is always good to know quality and properties of the available material, unit costs and availability. In general, no appointment is required but it would be a rule of good practice calling before to know whether anybody can receive us.
  • If not already defined, search for possible places to stablish the site compounds.
  • Inspection of suitable areas that may serve as storage points for the WTG components.
  • Check all the WTG positions one by one. As a rule of thumb, experience says that a realistic planning should contemplate a properly assessment of a range between 10 to 15 WTG locations. Of course, it will always depend on the site conditions, the state of the existing roads and how far the positions are from the nearest driveable path.
  • Visit to the substation area and the point of connection to the existing electrical network.

Of course, there will be always unexpected events that we will have to handle once on site. However, taking the good habit of following these rules before travelling reduces the odds of facing undesired surprises as well as gives the engineer the chance to work in a more efficient way and even enjoy the journey.

Gap or no gap? The new IEC61400-6

How to optimize the design of WTG foundations?

As wind turbine loads and foundations size keep increasing year after year sharpening the geotechnical calculations and modelling correctly the interaction between soil and foundation is becoming a priority.

The cost of foundations can represent a significant percentage of the investment in a new wind farm - even more in 2021, when steel and concrete are becoming every day more expensive.

An important topic that is becoming the focus of detailed studies is the soil bearing capacity degradation under cyclic loads.

This subject has been incorporated in the new version of the standard IEC61400-6 on Wind energy generation systems in Part 6: Tower and foundation design requirements.

Now, under certain conditions, a certain amount of "gap" below the foundation may be allowed.

“Gap” means that under certain situations the ground below part of the foundation might become uncompressed – as if the foundation was partially lifted, creating a "gap" (i.e., a separation between the structure and the soil).

This is something that previously was not allowed (unless the foundation was on rock).

The reason is that if the soil goes through several cycles of compression and decompression its bearing capacity might deteriorate. Basically the bearing capacity becomes lower and lower, putting at risk the stability of the structure.

This is a relevant change, as the IEC standard is one of the most important document (if not the most important) used in wind turbines foundation design.

The key idea behind the change is that if the soil below the turbine is not susceptible to the phenomenon of degradation under cyclic loads a certain amount of gap can be allowed.

Removing this “no gap” requirement means that a significant reduction in the diameter of the foundation can be achieved.

This happens because otherwise the foundation would have been bigger only to keep the soil below it always compressed.

The “no gap” requirement used to be one of the dimensioning constraints in wind turbines foundation design when the soil was good.

The key to allow some gap in the foundation design (and as a result, a smaller foundation and savings in concrete and steel) is to be able to justify that the soil characteristics will not will not degrade under cyclic loads.

This involves dynamic tests, which are time consuming, expensive, difficult to implement on site, unusual for most geotechnical companies and difficult to post process and interpret.

In some cases, even with a robust testing campaign, additional finite elements models have to be created to validate the design.

Will we see smaller foundations after this change in the IEC? We will need to wait several months to answer this question.

Tower cranes: a real alternative to lattice boom cranes?

The constant search for higher rated power, taller towers and longer blades has pushed wind turbine manufacturers in an arms race to secure a position in an extremely competitive market.

Today in the onshore market there are machines with rated power close to 6MW, hub heights in a range of 150-165m and blades longer than 80 m. Several projects are currently under development considering these massive sizes.

Lately I have had the opportunity to analyse in depth a new solution which is emerging as an alternative to the traditional lattice boom cranes: the tower cranes.

I have analysed two scenarios, one with the standard lattice boom crane and an alternative scenario with the tower crane.

Standard scenario: lattice booms crane

Lattice boom cranes such as the Liebherr LG1750 have been the standard solution for the installation of the latest generation of turbines, with a tower height in a range up to 140+m.

This type of crane can be moved fully assembled between positions under certain assumptions (such as a very low road longitudinal slope and minimum road width of more than 6m).

If the crane has to be dismantled a substantial area for the boom assembly and disassembly process will be needed (in red in the image below).

Other characteristics that can have an impact on the project are:

  • A mountainous landscape: in this case the boom assembly area would be even more essential. This will have a substantial impact of civil works cost.
  • Very high installation rates (such as 3 or 4 turbines per week). The limited stock of lattice boom cranes suitable for this hub heights worldwide create a risk: either you book the cranes two years in advance (giving up the possibility of changing the schedule) or you wait - with the risk of losing the crane availability slot.

Alternative scenario: tower cranes

The idea of using tower cranes for wind turbines installation is not new in the onshore sector.

Said that, as far as I was aware of, the use of this typology of cranes has been negligible in the last few years.

Not so many references can be found in America or Europe. One example would be the installation of Gamesa’s G114 of 2,5MW 156m steel tower at the Borja Wind farm (Spain). The crane used was the Liebherr 1000 EC-B 125.

Common sense tells me that the experience turned out not to be very positive (otherwise I presume that the concept would have been replicated, while that does not seem to be the case).

More recently, new models from Krøll Cranes have been used in wind farms at the opposite side of the planet, in Thailand and Australia.

Big players like ALE are suggesting that this new concept is reliable.

One of the main references is the Theparak wind farm project in Thailand, where 60 V136-3.0 MW where installed using this crane.

Here we have a list of some of the projects installed in Thailand with tower cranes:

The main pros of the tower crane are:

  • Road width required: only 4.5 m (even as little as 3.5 m according some sources).
  • Cranes boom is only 70 m long.
  • Advanced crane bases allow savings in the critical path.
  • Lower minimum lifting radius compared to lattice boom areas.
  • Installation rates about 2 hours per component. An installation rate of 1 WTG every 4 days has been reached in the Thai projects.
  • Operational up to wind speeds of 15m/s.

On the other hand, cons would be:

  • Lack of experience of the operators with these new set of cranes and low offer worldwide.
  • Uncertainty on the actual installation rates due to insufficient track record.
  • Real installation costs are still unknown.

Depending on the characteristics of the project, Kroll cranes has available these models:

How would an hardstand layout adapted to both the tower crane and the new generation of turbines look like?

It seems that a tower crane could work using a standard hardstand without the boom area:

Some 3d models where created on real WTG locations to assess the actual impact on cost of this new configuration. Quantity reductions in topsoil stripping, excavation and fill material may lead to a cost reduction around 5000€ per hardstands (being conservative).

A nice image with a 3D model of the hardstand analysed is included below:

Even if potential savings in the civil works seem to be easy to achieve, a real total project cost reduction can be confirmed only considering the actual installation costs, which are not so clear at this moment.

Are tower cranes going to be a more mainstream solution in the future?

Only time will tell.

Addenda (21/04/2021)

I have received this email from Jasper from Lighthouse projects. I think it
can be useful to other readers.

Regarding the tower cranes in compare to Crawler crane I would like to share with you some experience.

I have worked 2 years ago at windpark Krammer which consists out of 34 Enercon E115 turbines on a dyk with no space for storage components or installation of the cranes.Therefore we have used 2 Liebherr EC1000 Tower cranes in the project to build the wind turbines.

My experience is that with the limited space on site it is easier to install the crane.  Another benefit is that crane capacity because the crane can lift up to 100t  we could pre assemble the rotor and generator up front and lift the generator in one lift which is efficient. In addition what we saw is that you can lift longer you can lift up to 12 m/s or more.

A disadvantage of this type of crane is that it must be extended at some point. Mast sections must then be placed in between so that the hook of the crane becomes higher.

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.

Nearshore wind turbines foundations

Near shore wind turbine foundations

Nearshore (or “intertidal”) foundations are not a usual type of foundations.

It is a hybrid solution, an on-shore foundation in a quasi-offshore environment.

I have heard about this type of foundations several times in my career. The first time it was a preliminary design that I have made about 10 years ago for a Chinese project.

In the last years I have seen it used in several Asian countries, for instance in Vietnam (in the Mekong delta, in a project appropriately called Dong Hai 1 “intertidal wind project”).

The typical application of this kind of foundations are the shallow waters of the continental platform, using an on-shore wind turbine nearby the coast.

During the low tide, the foundation is exposed to the air while it is partially submerged in seawater during high tide.  This is an extremely aggressive environment for both concrete and steel.

Additionally, in sandy beaches with muddy underground the foundations may require piles of exceptional length (>30 meters).

The technical solutions that can be used for these special locations are typically three:

  • Mono-piles
  • Pile foundations
  • Sheet-piling cofferdams

Monopiles are the standard foundation used in maritime structures. It is a driven steel pile with a diameter up to 6-8 meters.

The wind turbine tower can be bolted directly to the monopile element without a transition element (this is often the cheaper configuration).

This type of configuration has even been considered also for on-shore projects as an alternative to pile foundations.

It has, however, several risks related to the pile driving process and the type of equipment required.

Monopile wind turbine foundation

The piles foundation with an elevated pile cap is another interesting solution.

It consist on a set of driven piles (concrete or steel) joined together by a concrete pile cap.

Interestingly, in some cases the foundations are connected to the coast (or even between them) with walkways.

This help servicing the wind turbines without using ships.

WTG piled foundation with an elevated pile cap

Another alternative that has been used is using cofferdams to create something similar to a small island, and then build the foundation inside this element by means of a gravity or pile foundation.

This construction technique is inherited form bridge construction. Since the time of the Roman Empire sheet piles and cofferdams were used to build the piers of a bridge inside a river or lake.

Cofferdams WTG foundations

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.