April 2012

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Patrick Henderson foundation is a technology widely used in the States, with thousands of installed foundations, but far less common in Europe.

It’s a patented solution that consists of a large diameter, cast-in-place annular pier, with an average diameter of 4.5 m and a depth of about 9m, depending on the soil conditions and the loads.

Two concentric corrugated metal pipes (CMP) form the interior and the exterior of the concrete hollow annular pier: between the 2 corrugated pipe, an anchor cage is positioned and the bolts are post tensioned to ensure that the concrete of the pier remains always compressed.

The hollow interior of the pier is filled first with 1 m of concrete, than with uncompacted soil and finally with a 35cm thick structural slab.

Basically the P&H foundation works a lot like a beach umbrella.

Among the advantages, a smaller footprint and less material consumption: as an order of magnitude we are talking of approximately 160m3 of concrete and 20Tn of steel.

 

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A quick photographic overview of the different types of transverse drainage systems currently used in the wind farms.

In the following pictures, standard precast concrete pipes, cast in-situ concrete pipes and hard plastic.

Basically the choice is more an economical than a technical problem, considering that they are equally effective. The real problem, as I will comment in a separate article, is the (normally) inexistent maintenance of the drainage network.

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Cable trenches construction probably doesn’t looks like the most exciting things in life, but including for them there are things that is better to know to avoid mistakes.

They are used to connect the WTGs together with medium voltage cables, and they bring the earth and the optical fibres cables as well.

The standard construction sequence is as it follows:
After the opening of the trench, on the bottom a layer of sand 10 cm thick is spread, placing above it the medium voltage cables, the ground cable and the optical fiber cable.

Subsequent to the positioning of the cables another layer 30 cm thick of fine sand, washed and compacted properly, is extended.

Above this second layer it will be placed, throughout the entire length of the trench, a HDPE cover tile marking the presence of underground cables.

The cover tiles will be placed directly above the filling that covers the cables. It has a function of mechanical protection of the cables, signaling the proximity of the cables in case of reopening of the trench.

On this tape will be extended another layer 50 cm thick, using native material coming from the excavations free of stones, branches and roots, properly compacted in layers of no more that 20 cm in thickness of loose material before compaction.

After completing this layer it will be positioned a PVC warning tape informing of the existence of medium voltage electrical cables under it.

Finally, the backfilling of the trench will be completed with a layer of 30 cm using material coming from the excavations, free of stones, branches and roots, compacted properly. The last layer of backfill will be done with topsoil coming from the excavation and previously stored properly, in order to recover the natural environment of the area as soon as possible.

In the following pictures the main elements of the operation are shown.

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Wellpoint system is a solution that we used successfully in a wind farm with serious drainage problems.
The soil was made of silty sands, with a very high water table level due to a river nearby.
The system consists of several interconnected wells spaced along a trench. All of them are connected to a common header, and the water is moved thanks to one or several submersible pumps.
Our depth was about 2.5 meters, but the system works with depth up to 7 meters. With this solution, a dry bottom of excavation was obtained and the foundation was successfully completed.

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The following pictures are from a wind farm where due to the weak conditions of the subsoil a deep foundation with piles has been necessary, using a technique known as continuous flight augering (CFA).

As you can see, with the same machine we drilled and pumped the concrete in the hole.

A dry hole has been made, without the use of bentonite or others drilling fluids to maintain vertical the walls of the hole: the earth is removed using an Archimedes' screw, and than concrete is pumped inside the hole, slowing lifting the tool.

The machine use a manometer to know the pressure used to inject the concrete in every moment: this way, it is possible to detect if there are holes underground.

Than, the reinforcement bars are lifted using a crane and carefully inserted in the hole. Small plastic wheels are positioned at the sides of the steel structure to make easier to lower it in the hole.

When the concrete hardens, after about a week, the head of the piles is removed using drills, leaving the steel bars clean and ready for the construction of the foundation.

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These are several pictures that I took in 2010 during a trip to Tarifa, southern Spain.

It is one of the best spot for wind surf, kite surf and of course for wind energy production.

The WTGs are about 20 years old, but they look like remnants of a faraway past, with their lattice tower and noisy turbines.

Regarding the drainage system, very effective concrete ditches and a complete network of transversal drainage pipes has been used.

Also, the exit of the drainage pipes has been “hidden” using an interesting mix of concrete and local stones.

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San Anton is a small wind farm (4 WTGs) located in Lillo, Toledo (Spain).

I had the opportunity to follow this project from the beginning visiting the site very often. I collected several picture to show the main phases of a wind farm construction.

First of all a met mast is installed, and wind data is collected. If there is enought wind in the area and the investment is profitable, an authorizative project is developed and transmitted to the authorities. Normally this project is developed using existing information, that is without developing a new cartography or other field works.

When this project is approved and it start to become reality, than the first work on site is the geotechnical report, done to define the subsoil conditions trought several in situ and laboratory test: here in the picture a trial pit is open to see the materials in the subsoil

Trial pit

A trial pit opened using a backhoe

With this information and a document provided by the seller of the wind turbine with the load transmitted to the base (shear, moment and vertical loads), a preliminary (or sometimes directly a constructive project) of the foundation is developed.

Afterwards, a topographical survey is developed. This can be done using standard field topography, a flight or LIDAR (a laser system that can provide a very dense cloud of point).

The following picture show the first step of the construction phase: topsoil stripping, an operation that consist in the removal of the first layer of fertile vegetal terrain that will be stocked and piled separately and used at the end of the works to revegetate the site area.

Topsoil stripping

Topsoil stripping

The next step is what is called earthworks: the construction of embankments and cuts. For instance in the image below a crane pad is under construction. Several thousands of cubic meters can be moved during this operation.

Earthworks

Crane pad earthworks

In parallel, the foundation for the met mast is constructed. The mat foundation is smaller than a WTG foundations: it is around 7 x 7 meters. It has 3 pedestal where the met mast will be connected.

Met mast foundation

Met mast foundation

After, the circular holes for the turbine foundation are opened: here you can see the topographer checking the dimension and horizontality of the hole. Several days of work are necessary to open the hole, depending on the hardness of the excavated material.

Foundation hole

The internal roads are continuously washed with water, to avoid accumulation of dust in the air due to the traffic of heavy trucks and other machinery.

Roads watering

The following step is to pour the blinding concrete, to assembly the anchor cage and to position the steel reinforcement bars (rebars). More info here.

This is how it looks a completed foundation:

Meanwhile, the operation and maintenance building is built nearby. It will host the SCADA equipment and the medium voltage switchgear:

O&M Building

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Hello,

My name is Francesco and I am a newbie in the word of blogging.

Being an engineer working in a big sector in the wind sector, I decided to start this page to share with colleagues and other whoever may be interested what I learn projecting and building wind farm worldwide.

I work in a team dedicated to Balance of Plant (also called “Bop”): it means everything in the wind farm that is not the wind turbine, for instance the roads, crane pads, foundations for the civil part and the substation and medium voltage cables for the electrical part.

I'll try to post pictures, video and info regarding wind farm and wind farm construction, a job that I find really interesting.