For the second time in 2 years I’m working at a wind farm with a potential karst problem.

If you’re reading this post you are probably aware of what is karst – basically, the result of dissolution of rocks such as limestone and gypsum, that can produce beautiful landscaping results (such as caves, sinkholes and dolines) but also several problems for the foundations of structures.

If the size of the cavities is reduced (a couple of centimetres) the will not be a problem because the foundation will not notice them. However in unlucky cases, you can dig and open the foundation and discover a huge hole that can be several cubic meters big.

I’ve heard stories of civil subcontractors who pumped the load of several concrete mixers in the holes without filling them (in some cases you can find a network of connected cavities).

Relocation of the WTG is a solution, but is usually not the best due to several reasons (maybe the adjacent land plot is not available, or you have a tight layout with very close turbines, or there are environmental constraints).

Also, it’s usually not free: depending on your contract, the civil subcontractor could be entitled to ask for a variation order.

Which are the available technical solution?

As far I’m aware of, MASW, electrical tomography and ground penetrating radar (GPR). I’ve used the last one successfully in a project in Jamaica and it worked well.

Basically, the GPR investigation is a non-invasive technique that can detect anomalies (locations, depth and approximate size of cavities) and spot  any inconsistency or variation in the expect soil profile across each site.

Resuming shortly how it works, GPR generate and send electromagnetic waves into the surface with an antenna, moving along the inspected surface.

Whenever a radar pulse strikes a boundary interface of contrasting dielectric (basically, between different materials), a portion of the pulse is reflected back to the surface to a receiving antenna.

The contrast in properties between clay (the weathered material which usually fills the holes) and limestone help to identify zones of subsidence. Conversely, a lack of contrast indicates relatively uniform stratigraphy.

Subsurface profiles will be generated displaying the resulting echoes of individual pulses.

Integrating this technique with standard boreholes will decrease the geotechnical risk of your project.

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This week I've had the pleasure to meet (virtually) Tom, an electrical engineer specialized in SCADA.

Tom developed an interesting software called "SCADA miner".

Basically, the software automatically dig the available data from various sensors and cross check the information to spot actual or potential problems that might go unnoticed, "lost in the sea of other alarms and event codes" to use his words.

When something goes wrong the software automatically send an email to the people included in a distribution mail list, alerting them.

One of the advantage of the system is that no new, dedicated hardware is needed: the calculations are made by remote servers.

In his blog you can find several real word examples, such as high main bearing temperature, met mast failure, wind vane misalignment and several others.

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I’ve just received a short technical summary of the works performed in Raki (Chile) from Terratest, the company who has made the gravel columns.

Unfortunately it’s in Spanish, but the pictures are beautiful ;-)

About 9000 meters of gravel columns have been put in place. The diameter is 80 cm, while the length changed depending on the position (from 9 to 13 meters).

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A couple of months ago I’ve been invited by Esteyco to the erection of a prototype auto lifting precast wind tower.

Although I was unable to attend due to other meetings I want to describe shortly their invention, which looks interesting.

Basically, the idea is to create a tower that can “lift itself” without the use of a main crane.

The first step is the assembly all the pieces of a precast concrete tower at ground level (such as an onion, or a matryoshka doll).

Assembly is done from the inside out, with the most internal section being the top one holding the nacelle.

When assembly is completed lifting can begin – starting from the center (the top tower section with nacelle and blades) and moving toward the exterior.

The tower “self-lift” itself using heavy lift strand jacks. These are commercially available equipment, currently used in several industries such as heavy construction or offshore.

The strand jacks will be positioned on an auxiliary platform at the first level.

This solution save you the main crane: only a smaller, 350t to 500t crane will be needed to assembly all the components.

To give you an order of magnitude, to reach 120 m you will need 4 concrete levels that in fold position will have a height of approximately 40 meters.

The expected time for assembly and lifting of all components would be around 3 days.

As an added bonus, due to the increased weight of the tower (acting as a stabilizing load) the WTG foundation would be somehow smaller compared to a steel tower of the same height.

 

A spectacular video of Raki wind farm, a beautiful project I’ve worked at.

Owned by Rame Energy (formerly Seawind) It is a 15 MW wind farm in the Bio Bio region (more or less in the middle of Chile).

WTGs are 5x V112 3MW turbines, and civil works have been developed by CJR.

The earthworks were not especially complicated, while the geotechnical study for the foundation required a more detailed, in depth study.

I’ve recently worked on a slope stabilization preliminary project.

Among other action, the use of Vetiver grass for stabilization purposes has been suggested by the customer, who had used it successfully in another wind farm in the same area.

Although this technique is used in Italy and other European country I was not an expert, so I’ve done a search to see the peculiarities of this solution and I want to share the key points I’ve discovered.

Basically, Vetiver is the colloquial name of a plant originating from India and South East Asia.

There are 3 species:

  • Chrysopogon zizanioides (formerly known as Vetiveria zizanoides), from southern India: this is the one that you’re going to use – deep roots, sterile and not invasive.
  • Chrysopogon nemoralis, from Laos and Vietnam: shorter roots and not sterile.
  • Chrysopogon nigritana, from South and West Africa: not sterile.

 

Vetiver can be used both on cut and fill slopes, up to a 1:1 gradient (45 degrees).

The number of plant that you will need will vary – the horizontal distance between plants should be from 10 to 15 cm, while the distance between rows (measured on the slope) will vary from 1 m. (highly erodible soil) to 2 m. (more stable soil).

First and last rows should be planted at the top and toe of the slope.

As cuts and embankments are not fertile (or at least, they shouldn’t be) fertilizer and watering at regular intervals will be needed. Later on regular cutting will be needed.

It growths very fast (up to 3,5 meters in 12 months).

This solution has several advantages:

  • Above all, it’s cheap. This is particularly true in countries where labor wages are low. Also, there are no heavy long term maintenance costs.
  • It’s a natural erosion control measure (at least, more natural than a plastic geotextile and less ugly than a rip rap).
  • It’s very effective.

 

There are also some potential problems:

  • Intolerance to shading (but this can be an advantage if you use it only as a pioneer plant for initial stabilization).
  • Roots don’t penetrate well below water table.
  • Need an initial establishment period of 3 to 6 month depending on the climate. During this phase it must be protected from livestock.

 

You can read more on the subject on this interesting book from the Vetiver System network.

Having the pleasure of studying dozens of subcontractors offers each month I’ve already seen a huge number of disclaimers.

Some of them are reasonable, some of them can be removed (basically, giving a price tag to the subcontractor risk excluded by the disclaimer) but some are “deal breakers”.

Which are the disclaimers that I warmly suggest not to include in your turnkey contract?

Here you have my personal “top 3” list.

Disclaimers on the quantities when a project is available

Basically, the message here is “Give me the contract. Later we’ll discuss.”

It unloads a huge risk on your side (be sure that the quantities will increase, or when they decrease you want get savings).

Also, it’s against the philosophy of a real turnkey.

Finally, it means that they haven’t studied the project, otherwise they would know with a reasonable approximation the quantities.

Disclaimers on the percentage of rock when a geotechnical survey is available

What we’ve got here is a subcontractor that wants to win easily. Even in cases when there is geotechnical information (trial pits, boreholes, etc.) they want to reserve the right to increase the price if the percentage of rock that they have estimated is wrong.

Don’t accept this one if you’re providing enough information.

Disclaimers on the presence of quarry & landfills

I hate this too. Not only for the possible discussions about alleged extra costs: the problem here is that this disclaimers means that the subcontractor hasn’t studied enough the project and has no idea about where he may buy the material, or unload the earth he don’t need.

This post is a follow up of my other post on the blade lifter.

When I’ve been discussing about this solution with the other guy in the business 3 or 4 years ago, they told me it was something utopic, a “nice to have, but still a dream”.

Well, apparently now the blade lifter arrived, and it’s here to stay - and with a more aggressive configuration.

We’ve used it in a project in central America together with GES, a company specialized in wind and solar project, who bought one.

I’ve also discovered that another transport company in Italy (SIA – they are specialized in WTGs components transport) bought one in August 2014.

Clearly, this unlock the power of the blade lifter in Europe: I’m sure that in some complicated situations, it will make sense to move it from a country to the other.

There is a very good post on the company web describing this solution – have a look.

They’ve also done a very professional video (embedded in this post).

Resuming the key features:

 

  • Produced by Scheuerle
  • Maximum angle 60°
  • Maximum capacity is 25 tons
  • Maximum slope: 23% (paved road) / 18% (dirt road)
  • Wireless wheel remote control

 

Enjoy the video!

Having the opportunity of working at several project worldwide I already discovered that the idea of what is “cheap” or “expensive” may vary a lot depending on the country.

Also, huge countries like Brazil or Chile may show a relevant price dispersion depending on the area (Patagonia or Atacama desert?).

A third consideration is that, due to inflation, market situation, other project being constructed at the same time, etc. the prices may move a lot (downwards, but normally upwards) from one year to the following.

However, here you have my favorite top 5 of the most expensive price I’ve ever seen. They are totally crazy, even with all the above considerations.

 

Concrete: Mozambique (900€/m3)

Steel: Morocco (15€/Kg)

Embankment: Mexico (241€/m3)

Crushed stones base layer: Mozambique (450€/m3)

Overheads: Chile (50% of the project cost)

 

On the cheap side I'd like to mention Portugal: a country with a lot of skilled civil works companies, used to the renewable business - and with great food as well! ;-)

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Among the numerous fenomenous that follow a Pareto distribution (i.e., 80% of the effects come from 20% of the causes) there is the price of civil works in wind farms.

Through the years we’ve developed a really completed, exhaustive documentation for tenders. Our beautiful Bill of Quantities includes hundreds of items.

However, how many of them have a real impact on price?

Well, just 5.

Basically, around 70%-80% of the total price is driven by the following items:

  • Concrete
  • Steel
  • Cut
  • Fill
  • Crushed stones for base layer

I’m not saying that it would be a good idea to ask only these 5 prices to the subcontractors.

There are many reasons to produce a complete and accurate Bill of Quantities – for instance, to be sure that you are on the same page with the subcontractor.

Being a niche sector, often smaller local companies have an idea somehow distorted of what is included in a wind farm.

They might ignore the existence of something (“You need 50.000 kg of grout? For what?”) or overprice one or more items (20.000€ to assembly an anchor cage is a good example).

However, at the end of the day what will move the total price will be the 5 items listed above.

Concrete is usually the heaviest item. I’m including in it all types of concrete that might be found in a wind farm (lean concrete, foundation concrete, concrete used in roads, etc.).

Although obviously the numbers will vary depending on the project (a mountainous area with a lot of rock will have expensive earthworks, while a project with dozen of piled foundations will rise steeply steel and concrete price) I’ve seen that they can be used as a rule of thumb and are a useful guideline when I skim through the offers.

As I don’t know if it’s better to present a graphical example with a pie chart or bar diagram (and I enjoy playing with Excel) I attach both.

It’s a 50 MW wind farm quoted by a subcontractor who charges a lot Overhead Costs (which is perfectly fine for me: I don’t want to have them “scattered” where they don’t belong).

71% of the total comes from the 5 key cost drivers.

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