Civil Works

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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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BoP is an English acronym that stands for Balance of Plants. In the wind farms sector, it means everything but the wind turbines.

Balance of Plant

Basically there are 3 types of Wind Farm contracts commonly used:

  • Supply only. It include WTGs, SCADA, Installation supervision and Commissioning.
  • Supply and Installation, including all the items in Supply only plus WTGs transport and cranes for the installation (basically, it adds the assembly of the machine).
  • Turnkey (full EPC), including all the above plus civil and electrical works.

The sum of wind farm civil works and electrical works are usually called Balance of Plant (BoP).

Often it is done by a company different from the wind turbine supplier, and sometimes even 2 separate contractors are used, one for the civil works and one for the electrical works.

BOP civil engineering scopes of work include roads and drainage, crane pads, turbine foundation, meteorological mast foundations, cable trenches and buildings for electrical switch gear, SCADA equipment, and a maintenance/spare part facility.

BOP electrical work scopes include underground cable networks (medium voltage cables, copper cables and optical fibre cables) and sometimes even an overhead transmission lines, electrical switch gear to protect and/or disconnect turbines or other equipment from the system, and grounding and connections for control rooms, maintenance facilities and point of connection equipment to feed the wind farm’s power generation into the electrical grid. Transformers and switches for individual turbines are normally located within the turbine and they are provided by the turbine supplier.

In the EPC wind farm contract, three phases can be distinguished:

Engineering: A detailed engineering project is developed to fit the machines to the actual conditions of the site. Conditions can vary dramatically, depending on topography, geotechnical conditions, grid connection requirements, permits, local regulations an so on. In this phase technical choices are made, and drawings are produced together with the bill of quantities.

Then comes the Procurement phase, where civil and electrical works are subcontracted to one or more companies. Potential subcontractors are contacted, their quotations are received and compared.  Short listed contractors are screened and the work is finally awarded to the company with the best price/quality ratio.

Finally, the phase of Construction begin, where the works of the subcontractors are monitored to see if they comply with the technical specification and if there are delays respect to the signed chronogram.

 

 

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