July 2012

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Here you have my collection of errors I frequently see when I check wind farm project developed by external companies.

Being a quite peculiar sector is no surprise that I normally found several mistakes: here you have the most commons.

 

Error #1: two levels crane pad / foundation area designed without considering the slopes of the foundation pit.

Here the problem, as you can see in the transversal section, is that the slope of the foundation pit “enters” inside the crane pad and the road nearby, reducing the available space.

The only way to build something similar is with a "2 steps" constructive approach, i.e. to build the foundation, to close the hole and after to build the crane pad.

An example of this 2 steps approach can be seen in the following drawings, taken from a real wind farm. The ground was very steep, so first of all we calculated the elevation of the bottom of the foundation pit to ensure the necessary soil covering that help balancing the overturning moment. In several cases the result was that the center of the foundation was too far away from the border of the crane pad (around 16 meters), making difficult the work of the main crane (a standard distance is around 10 meters).

So we had to approximate the border of the platform to the foundation filling the area in between until the required distance is reached:

 

Error #2: crane pad and foundation on an embankment.

The problem here is that the foundation must be realized below the natural ground for stability reasons. Normally the depth of the bottom of the foundation pit is around 3 meters, calculated from the lowest point of the terrain around the circumference of the foundation.

But if in the project the WTG is shown on an embankment, probably the stair or even the tower door will be below ground. In the transversal section you will see clearly the problem.

 

 

Error #3: road and crane pad at different levels

Here you have a top view of a platform and the access road. Everything looks fine:

But then, when we check the longitudinal profile, we discover that the road is going down (while obviously the platform stays at the same level, 646 meters). The only way to build something like that is to use a wall to retain the earth inside the crane pad, but this is obviously not the case.

At the end of the platform, the height difference is almost 5 meters:

In the longitudinal profile above you can see the platform in yellow and the road in dark blue.

 

Error #4: insufficient vertical transition curve

Sometimes the Kv parameter for the vertical transition curve, defined as  is not adequate.

This happens when it’s lower than 400 – 500: in these cases, the truck can remain “stuck” because it touches below.

 

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In general, the geotechnical study of a wind farm should describe the terrain in order to provide the necessary information to develop the following works:

  • Turbine foundation project: definition of the allowable soil bearing capacity and the most appropriate type of foundation (e.g. shallow, semi-deep, foundation on piles, etc..).
  • Wind farm access roads and hardstands project: determination of the characteristics of subgrade and its possible improvement if necessary.
  • Substation Project: Design of building foundations, along with the design of the earthing network (analysis of the electrical resistivity of the soil).
  • Determination of the suitability of the material from excavation for use in embankments, determination of the angle of stability for slopes.

The company responsible for the geotechnical study should collect all available geological information about the area (geological maps, photo geology, and visual inspection). The purpose of this phase is to provide a first approximation of the type of materials to be able to find and confirm the type of field and laboratory tests more suitable, and if necessary to propose other type of tests or additional studies.

With this initial information, it must be prepared an exhaustive proposal for the geotechnical survey detailing all the work to be executed and the time schedule.

The type of field tests to be performed will depend on the geological nature of the materials in the area.

 Field tests

In soft soils:

Boreholes drilling to a depth of 30 meters, including standard penetration tests (SPT) every 2 to 3 meters in not cohesive layers, extraction of undisturbed samples in cohesive layers, determination of the water table and graph with the geological and geotechnical profile, photographical report of the samples and of the boreholes, diameter and type of drilling at each depth, rate of recovery of specimens, presence of water.

The number of boreholes depends on the type of materials, being at least:

  • In favourable terrains, where is possible to predict homogeneity and continuity of the layers: boreholes in 40% of the positions of the wind turbines.
  • In geologically complex areas: boreholes in 100% of the positions of the wind turbines.

Trial pits using a backhoe. Includes sampling for posterior laboratory test, photographic report, carrying out of the appropriate in situ tests and classification of soils for engineering purposes using unified soil classification system. Also shall be indicated the position of the water table and a subjective estimate of the consistency and permeability.

The trial pits to be done are the following:

  • In all the WTG position where hasn’t been done a borehole: a trial pit in every position, up to the foundation depth or the bedrock.
  • In the access roads: a trial pit every Km, or where it is considered to be representative. The required depth is from 2 to 3 meters.
  • In the substation area: a trial pit in the building area, with an additional pit in the switchgear area if it is considered necessary. The required depth is from 2 to 3 meters.

In hard soils and rock:

Drilling up to a depth of 30 meters with continuous rotary drilling. Alternatives geophysical methods are acceptable (e.g. Seismic refraction profiling) but they must be technically justified. This is the case in situation where the area is clearly homogenous and made of solid rock.

In karst areas or when the presence of underground cavity is suspected an exploration of the ground using geophysical methods (micro gravimetric georadar, etc.) must be performed in order to determine the position of the cavities.

Resistivity tests. In order to provide information for the design of the grounding system of the substation electrical resistivity tests of the subsoil shall be conducted in the whole area of the substation.

The tests will be normally realized using Wenner four-electrode method arrangement or equivalent array as per ASTM D6431 - 99 (2010).

Laboratory tests

With the specimens obtained during the field works at least the following test will be done:

Classification tests:

  • Determination of the particle size distribution by sieving.
  • Determination of the Atterberg limits (liquid limit, plastic limit, plastic index).
  • Determination of the natural water content.
  • Modified Proctor test to determine the relationship between water content and dry unit weight of soils (compaction curve).
  • Soil classification.

Mechanical tests:

  • Direct shear test.
  • California Bearing Ratio (CBR).
  • In plastic, expansive or poorly consolidated clays: triaxial test.
  • In rock: geological classification of the sample as per ASTM D5878 - 08 using a suitable system of classification for Engineering Purposes. Rock quality designation, rock mass rating.

Chemical tests:

  • Determination of environmental aggressiveness and corrosion risk for concrete: pH and sulphate content in topsoil and water.

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Concrete towers are still an uncommon solution nowadays, basically they are still more expensive than they steel counterparts.

But the trend we see in the market is toward highest tower, of 100 meters and above, while current 2MW-3MW turbines normally have an hub height around 80 meters: this can lead to situation where an hybrid solution (concrete base plus steel top) or even a full concrete solution is finally economically profitable.

The reason for this increase in tower size is the need to increase the productivity (the wind speed increase exponentially with the eight) and to overcome surface friction, although it must be noted that almost every country has laws to limit the height of the tip of the rotor (on average, around 100 meters).

The biggest problem of tapered steel towers is that they maximum dimension in onshore wind farms is limited by transport issues: normally the biggest diameter allowed to circulate on public highways is below 5 meters, due to the free height of existing bridges. Tallest towers need bigger diameters, so there is a legal/technical limitation to the use of steel.

Several turbine manufacturers (for instance Enercon, GE and Nordex) have full concrete or hybrid towers in their catalogue, normally with eight above 100 meters. Often different sections are considered for the purpose of strength and stiffness design, fabrication and erection:

  • Base zone, made of thick walled precast concrete segments or in situ concrete. Here the thickness can be around 40-50 cm.
  • Middle zone: here the wall thickness is determined by concrete cover to reinforcement rather than by the necessary strength and stiffness, so a saving in material is possible.
  • Upper zone: here the wall thickness will be around 10 cm only. It normally includes a steel section of about 2 meters to connect the yaw ring and the nacelle.

Various configuration, techniques and details have been proposed, all of them normally considering the use of vertical prestressing. It is normal to see solution with concrete rings divided in 2, 3 or even 4 segments, assembled and joined together in situ with 2 type of joints (vertical and horizontal) using mortar, fishplates or other technical solutions.

The weight of the components is really high: can be 50+ tonnes (in some solution even more), so there can be cases where it is more difficult to lift a tower section than the nacelle.

Several documents with conceptual design of concrete wind tower are freely available on the web.

Check for instance:

Concrete_Windmills

Concrete Towers for Onshore and Offshore Wind Farms

Competitive_Concrete_Foundations_for_Offshore_Wind_Turbines

 

 

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