About Francesco Miceli

I’m a civil engineer specialized in wind farms civil works (usually called BoP, that is Balance of Plant). I’m currently based in Madrid, Spain and I’m developing several interesting project all around the world – southern Europe, North Africa and above all central and south America.

Offshore wind turbines foundation types

Offshore is one of the fastest growing sectors in the renewable energy business in Europe. In 2010 more than 300 new turbines have been installed, reaching a total of more than 3000 MW connected to the grid.

The number is not huge, but the offshore agenda is quite busy: more than 20.000 MW are expected to be installed at the end of 2015. The majority of these projects will be in the UK, Germany and the Netherlands.

On a worldwide basis, Europe is accumulating more than 96% of all the installed capacity, with Great Britain as the biggest player right now, followed by Denmark and Netherlands.

The interest for offshore development has several reasons: bigger wind potential (over 4.000 full load hours vs. 2.000 full load hours onshore), bigger wind turbines (>3MW, up to 7MW) and wind farms (from 50 to 1000 MW of installed capacity, while the average onshore wind farm is around 50 MW).

The drawback is the enormous investment needed: billions of euros, due to the rough marine conditions where everything is more expensive: wind turbines, cables, substation, and of course foundations.

Several foundation types are available for wind energy offshore towers:  gravity-type, monopile, jacket-pile, tripod and suction caissons.

The type of foundation used depends mainly on water depth and sea bed conditions: there is no “standard” concrete foundation as in the onshore wind farms. The solution used more often is the monopile.

Gravity foundations are used preferably in waters with a maximum depth around 30 meters, are made of precast concrete and are ballasted with sand, gravel or stones.

Monopile foundations are used in water with a maximum depth around 25 meters.

They are made of steel, and they are driven into the seabed for about 30 meters with a hammer (similar to the one used to build offshore platforms)

Tripod is used in deeper waters (up to 35 meters). It’s made of different pieces welded together and it’s fixed to the ground with three steel piles.

 Jacket if used in deep waters (more than 40 meters). It is made of steel beams welded together, weighting more than 500 tons.

Unmanned aerial vehicle (UAV) topography

This is a new technology we are going to use in a wind farm we are going to build in Chile.

The area that we need is too small to make LIDAR topography cost effective, but too big to use standard field topography: with this new solution we can have the needed data for a reasonable price.

The vehicle comes in 2 shapes: airplane like (fixed wing) and helicopter like (rotatory wing). The fixed wing plane is launched with a sling.

The vehicle weight around 3 kilos, and it can fly for about 1 hour at a height of a couple hundred meters, with a speed of about 75 Km/h.

It is possible to obtain several useful outputs:

  • Cartography
  • Digital Model of the Terrain
  • Aerial pictures
  • Thermography
  • Multispectral images
  • Video

It normally flies alone without any input, but it can be used with a remote control as well.

The main advantage is that it is clearly cheaper: you don’t need to book a flight, wait for a days without clouds (because you can fly lower) and it’s quick and safe.

 

Wind Farm LIDAR topography

LIDAR topography is the best available solution available nowadays.

We have used it in several wind farms and I’m enthusiast of the results. It can be used to define earthworks with great precision.

Basically it is made with an airplane flying with a laser, a digital camera and a GPS.

The laser “sweep” the requested area and a receiver on the plane register the laser beam waves reflected by all the surfaces. At the same time high quality digital pictures are taken, and the position of the plane is registered thanks to the GPS.

This is how the equipment used looks like:

After the flight, the cloud of point obtained is filtered: first of all points are divided in classes, depending on the type of reflection (“echoes”). Groups of points representing trees, bushes, buildings and so on can be defined.

In the second filtering the intensity of the reflected beams is considered (vegetation and ground reflect with very different intensity).

The next step is to calculate a mean between different laser passages, as the same point may appear in slightly different coordinated.

This kind of mistake introduces a noise in the results, and contour lines may results as broken lines instead of smooth lines:

As a result of this data cleaning process, all the point can be classified as belonging to a type surface (trees, grass, building, and ground) and it’s possible to visualize them with a different color.

You can see in the following image some examples. In the first, an agricultural area is mapped. In green you can see the trees and in brown the ground:

In this other example, we mark in green the ground, in violet the trees and in brown the buildings:

In this final example, you can see as a whole town would look like in a cloud of points:

Control point are manually selected identifying them in the picture. They are used to put the data in the correct absolute coordinates:

Finally, a digital terrain model is produced using the Delaunay triangulation, to maximize the area of the triangles.

As you can see, the results are often exceptional:

Wind farms projects using Google Earth topography

One of the biggest problems in wind farms preliminary projects is the lack of a reliable topography.

Due to the tight budgets developers are often working with, it is often impossible to obtain a good topography (like the ones you can get with a LIDAR instrumented flight) or at list a decent one (as the standard field topographic surveys).

One of the possible solutions is to work with the Google Earth topography. Software like AutoCAD civil 3D makes it possible to download a cloud of points with coordinates and elevations and work with them.

The question is: how good is this info?

Unfortunately it’s impossible to answer univocally. The base grid, covering almost all the inhabited surface of the planet, has been obtained with a space mission (the NASA SRTM, Shuttle Radar Topography Mission).

The points are approximately spaced 50 meters, so it is a very rough starting point.

This base has been integrated with a “mosaic” of different DEM (digital elevation models) freely available material, so there are states in the US with a 1 meter contour line, or even cities with a very dense cloud of points (even a point every 20 centimeters).

It is currently impossible to know from where the points you are using are coming from, and if a point is “true” or interpolated. It is possible to have an impression seeing the shape of the contour lines (if you “zoom out” from the topography you will often see a pattern of squares coming from the available points).

This data is normally reasonably acceptable for a somehow preliminary project. Several commercial plug in are available to enhance the results.

For instance Plex Earth allows you to import Google Earth data (pictures and points) in different coordinates systems, importing contours in an area with any shape specified. It can also be used for preliminary volume calculation, to export objects from AutoCAD to visualize them inside Google Earth or doing the opposite (that is, importing a Google Earth KML file).

Wind farm project financing

Project financing is a financial structure where debt providers rely on revenues generated by the wind farm to service and repay borrowing, and where the financing is secured by the project assets.

It is a complex technique, that sees several parties involved in the project (equity, debt providers, power purchaser, land owner, WTG provider, etc.) working together.

The economic initiative (investment) is carried out by the promoters (Sponsor) through the establishment of a  SPV (Special Purpose Vehicle), which allows the economic and legal separation of the investment.

Project finance allows investments of significant size, even exceeding the "credit capacity" of the companies involved.

The key concept is the free cash flow, the cash available for debt service, and the fundamental ratio is the annual debt service cover ratio (ADSCR), the ratio between operating cash flow(that is, "unlevered free" cash flow) and debt service during any one-year period. Other control ratios frequently used are Loan Life Cover ratio (LLCR) and Project Life Cover Ratio (PLCR): they will help top define the level of required equity.

Normally equity providers, debt providers and WTG suppliers will work with different financial models.

Among the various steps to be fulfilled to close the project finance, the most relevant are:

  • Development of the economic and financial model
  • Definition of the optimal mix of funding sources (debt and equity)
  • Preparation of the optimal financial structure
  • Definition of the collateral constituting the security package of the operation
  • Finding of the sources of funding needed to meet the financial needs

In a onshore wind farm, equity is typically around 20%-30% of total funding, with ordinary or preference shares. Standard leverage vary from 70/30 to 85/15: if the project can give a steady, reliable cash flow it will probably be on the lower side.

Other sources of debt are unsecured junior (mezzanine) debt and senior debt.

A standard distribution can be something like this:

In case of event of default, lenders can step in and control the SPV.

Several risks have to be mitigated:

  • Pre-construction risks (licenses, land lease rights, permits, social acceptance, country risk etc.)
  • Construction phase risks (Costs exceeding budget, EPC contractor problems)
  • Technological risks (WTG technological reliability)
  • Operation phase risks (WTG availability,  energy price)

For this reason a security package is prepared. There are contractual guarantees (obligation of the SPV to organize and handle the project diligently) and real guarantees (normally shares of the SPV).

The list of contracts to be negotiated normally includes:

  • Contract with the General Contractor
  • Contract with a WTG manufacturer for the turbines supply.
  • Operation & Maintenance Contract.
  • Insurance policies / Deposits.
  • Grid connection contract.
  • Land acquisition contracts.
  • Agreements with municipalities.
  • Contracts for the sale of electricity (PPA) and green certificates.

Wind turbine foundation remedial work: an example

The purpose of the remedial works in a damaged wind turbine foundation is to provide an alternative load path. Other operation that are normally done are injection of mortar or resins in the cracks and water sealing.

Here you have an example of remedial works in case of foundation failure and subsequent tower movements. The works was done several years ago in Ireland by Densit, allowing keeping the existing foundation (that is, saving around 90% of the cost of a new foundation).

If you wonder how much does it cost a WTG foundation, the figure is around 100.000€ plus turbine disassemble / reassembly plus monetary loss due to stop of production.

The remedial work was carried out in 5 stages:

1. Arresting the movements by fixing the tower. Before remedial works movement was around 5 cm.

2. Drilling of injection holes on both side of the tower and inspection.

3. Construction of a new flange and prepare the under casting

4. Injection into the foundation using Ducorit IQ

5. Under cast of the new flange with Ducorit S5

Due to the lack of bearing capacity of the embedded flange, a new flange is welded on the outside of the tower to withstand the compressive forces.

First of all, several clamps are installed all around to arrest tower movements while grouting.

After, injection holes are drilled down to the anchor plate in the concrete foundation to a depth of approximately 1.7 meters

8 injection holes are outside the tower while other 8 injection holes are inside of the tower, and a camera survey allows inspection of foundation condition.

Then, an ultra-high performance grout is injected and distributed in the foundation through the inlet holes and along the voids around the embedded flange and tower.

This product hardens quickly, obtaining around 50% of the compressive strength after only 48 hours at 10°C (the warmer the climate, the quicker the grout hardening) allowing the removal of the clamps.

Finally, a reinforced concrete ring is casted around the foundation below the new upper flange.

The reinforced bars are seen on the picture.

A shuttering is constructed on both sides of the reinforcing bars and concrete and the high resistance grouting is casted under the new flange.

Update 29/12/2019: I have received some question from a reader. I will try to answer them below.

Was there a concrete pedestal that you demolished and rebuild or it was a new part?

I did not take part to the remedial works – I have only seen the description of the actions afterwards. The existing pedestal has been demolished as part of the works.

Was the upper flange new? Because you mention the new flange at step 3.
If both the above answers are yes, was the demolition of the pedestal and the movement fixity of the tower done partially (let's say for every quadrant) or as a whole?

I have read again the post and I have to admit that it is not very clear. What is happening is that, in addition of pumping high resistance materials in the cracks and holes inside the foundation, a brand new structure made of reinforced concrete is built all around the tower. Such structure work as a “collar”, basically retaining the tower in its position.

Since there was movement how did you recenter the tower to be vertical? Using the bolts inbetween the UPNs for example?

Unfortunately I don’t know how they have re-centred the tower.

Cracks in onshore wind turbines foundation

The appearance of fissures in wind turbine foundations is a rather common event.

There is no consensus in the sector about the root cause of the cracks. You can find a comprehensive family tree of fracture types below:

From my experience it seems that old foundations with embedded ring are more prone to fissures compared to the new foundations with anchor cage.

Among the foundations with embedded ring, high pedestal foundations like the one in the following picture are the most problematic:

Visiting old wind farms I’ve observed both radial cracks and concentric superficial cracks in the visible superficial concrete around the tower.

However it seems that the more dangerous fissures are the ones appearing below the lower flange. Apparently the load concentration below the steel plate, in combination with cyclic loads, can damage the concrete inducing small movements. This is an area where is very complicated to pour correctly the concrete: the air can remain trapped, lowering the resistance of concrete.

After several cycles, these movements can pulverize the concrete below the flange and increase from few millimeters to a centimeter or even more.

At this point, the superficial sealing between the embedded ring and the concrete won’t be able to withstand the displacement and will break. Subsequently, water will be able to enter the foundation: this will lead to a pumping effect, possibly even freezing and expanding in low temperature areas.

Several side effects will start, such as carbonation and possibly to reinforcement bars corrosion. Subsequently, it will be necessary to stop the wind turbine and undertake remedial works.

Regarding the root cause, the explications frequently quoted can be categorized in 2 groups:

  • Design error: fatigue loads and stress concentration below the lower flange were underestimated, wrong steel design.
  • Construction error: the concrete below the lower flange was not of the proper quality, or it wasn’t correctly vibrated or cured. Sometimes even concrete pouring at very low or high temperature or improper installed reinforcement is quoted as a possible cause, together with pouring sequence problems (joint between concrete layers which are already hardened due to delays in concrete supply).

It is more unusual to have environmental problems, such as water aggressiveness or other chemical problems.

As solution, sufficient vertical reinforcement and anchorage length should be provided. Some designers also like to position the bottom flange as high as possible to increase the thickness of the concrete below.

You can read more here:

Cracks in onshore wind turbine foundations

Wind farm internal roads bends additional widening

One of the problems frequently found in wind farm project and construction is that road bends with a reduced radius often needs an additional widening to allow the passage of the trucks with the WTGs components.

Normally the biggest problems come with the blades. I suggest using a commercial software such as AutoTURN to estimate the actual additional road widening needed, because the tables provided by wind turbine manufacturers are almost invariably inaccurate and often based on wrong hypothesis.

The point is that there are too many variables to simplify the problem and give a single "one size fits all" value:

  • Angle between entry and exit tangent of the bend: the lesser the angle, the bigger the widening.
  • Bend radius:  smaller radii means greater widening.
  • Type of vehicle (number of wheels, center of turning circle, dimensions).
  • Different possible trajectories chosen by the driver.
  • Use of real wheels independent control.

Simulating software are great because they use real word data: the trucks are equipped with GPS equipment and the movements of the vehicle is registered and transformed in an algorithm that allow to replicate it in your AutoCAD project, with realistic results and cost effective solutions.

In the next image the trajectory of the different components of the truck in a bend with reduced radius are detailed. If the tractor unit follows a path in the center of the road, the rear wheels (orange lines in the drawings) will need an additional road widening both before the beginning of the bend and inside the bend.

It will also be necessary to clear an area appropriate for the transit of the blade tip (outside the bend, cyan line in the drawing) and for the truck body (inside the bend, green line in the drawing).

 

Figure 1: Standard wind farm internal road bend

It is also noteworthy that most of the trailers for WTGs components transportation allow orientating rear wheels independently from guiding front wheels.

For this reason, the road widening can be completely internal to the bend (using the steering control of the rear wheels) or external, sweeping the area outside the bend.

These solutions are normally more demanding in term of additional required area. They are used in situation where, due to existing constraints (buildings, structures, property boundaries, etc.) the standard widening cannot be used and a solution only inside or outside the bend must be found.

Here you have an example of a widening only in the interior side of the bend:

Figure 2: Wind farm internal road bend. Widening only in the interior.

This is a non standard solution, and as you can see it needs more space.

The third possibility is to use only the area outside the bend:

Figure 3: Wind farm internal road bend. Widening only in the exterior side.

 

This solution needs an enormous amount of space, and we use it only in exceptional situations.

Wind power plant concepts and performance indicators

Wind turbines are usually placed in clusters called wind farms, with sizes ranging from a few MW (sometimes even a single wind turbine is sold, for instance to a private investor or to give power to an energy intensive factory like a cement plant) up to several hundred MW.

These clusters are connected to the grid as single generation units, therefore the term wind plants is the best suited. Whereas initially the emphasis on wind farm design was mainly on efficient and economic energy production that respected the rules of the grid operators, nowadays, with increasing wind power penetration, the demands of the grid operators have changed.

In response to these demands, modern wind turbines and wind farms have developed the concept of the so called “wind energy power plant”. The concept is essentially a wind farm with properties similar to a conventional power plant, with the exception that the fuel injection is variable. The operation of a wind energy power plant is designed in such a way that it can deliver a range of ancillary services to the power system.

Its control system is designed such that the power can be actively controlled, including ramping up and down similar to conventional generation plants. Wind power plants can and do positively contribute to system stability, fault recovery and voltage support in the system.

The properties described above greatly enhance the grid integration capability of wind power. In order to achieve high penetration levels, active control properties are essential to optimally share the power supply tasks together with other plants and to enhance network security.

An essential difference between wind plants and conventional power plants is that the output of wind plants very strongly depends on the characteristics (mainly the local wind climate) of the site where they are installed. The rated power, also known as the nameplate power, is the maximum power, which is reached only 1% to 10% of time. Most of the time wind turbines operate at partial load, depending on the wind speed. From the point of view of the power system, wind turbines can be regarded as production assets with an average power corresponding to 20 to 40% of the rated power, with peaks that are three to five times higher.

Wind power performance indicators are related to the principal wind turbine specifications, that is rated power, and rotor diameter. The specific rated power is in the range of 300 - 500 W/m2, where the area is the "swept area" of the rotor. Wind turbine electric power output will vary with the wind: it is measured according to IEC and is graphically represented in a power curve (a graphical representation of the power output at several wind speeds).

This energy output can be standardized to a long-term (20 years) average energy output and to derive the power output in short-term forecasting from 10 minute average wind speed values produced by forecast models.

 

Here you have an example of range and typical values for several relevant technical characteristics both for wind turbines and wind farms:

 

Wind Turbine characteristicRangeTypical value
Rated power (MW)0.850-6.03
Rotor diameter (m)58-13090
Specific rated power (W/m2)300-500470
Capacity factor onshore18-40Varies
Capacity factor offshore30-45Varies
Full load equivalent onshore1600-3500Varies
Full load equivalent offshore2600-4000Varies
Specific annual energy output (kWh/m2 year)600-1500Varies
Technical availability onshore95-9997.5

 

 

Wind Farm characteristicRange
Rated wind farm size (MW)1.5-500
Number of turbines1-hundreds
Specific rated power offshore (MW/Km2)6-10
Specific rated power onshore (MW/Km2)10-15
Capacity factor (load factor) onshore18-40
Capacity factor (load factor) offshore30-45
Full load equivalent onshore1600-3500
Full load equivalent offshore2600-4000
Technical availability onshore95-99

 

Sources for this post:

Powering Europe. Wind energy and the electric grid (EWEA, November 2012)

IEC, 2005 Power performance measurements of electricity producing wind turbines

TradeWind 2009. Integrating wind – developing Europe’s power market for the large-scale integration of wind power.