September 2012

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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.

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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

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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.

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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.

 

 

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