Automatic cost estimator - the Holy Grail of BoP

Yesterday I had the pleasure to drink an overpriced coffe (2,90€ for an Espresso? Really?) with my good friend José Ramón. He told me that I’m not writing on the blog often enough so I’ve decided to make an effort and find some time to write this post.

The subject I have selected is an evergreen topic, the Holy Grail of BoP – the possibility to create a tool that could calculate quickly the cost of the BoP of a wind farm.

There is already a good amount of material on the subject online, for instance this website of the University of Strathclyde (Glasgow) that present a model created in collaboration with SgurrEnergy (now part of the Wood Group's Clean Energy business).

You can download the tool from their web or from this link for your convenience: BoP estimator tool

I have decided to take it as starting point to show why the task is not so easy and probably me and the other engineers in the team will not be substituted by an Excel file anytime soon.

The ultimate purpose of such models is to pick a small number of input (to make the tool usable) without losing to much in accuracy. The guys at the University decided to go for an extreme simplification and selected only 6 inputs:

  1. Number of WTGs
  2. Turbines Rating
  3. Km of new roads
  4. Km of existing roads
  5. Km of cabling to substation
  6. Km if cabling to grid

That is a very, very extreme oversimplification.

For instance, the model doesn’t keep into account the topography of the area (flat, hilly, mountainous) or other relevant factors (poor soils, inundable areas, etc.) and link the cost only to the rated power of the turbine. As a consequence the calculation of the crane pads cost show a big dispersion in prices (from 5.000 to 42.000 Pounds) and a very low R Squared value (0.26 – that is, the model isn’t explaining the correlation).

Additionally, the model doesn’t consider any monetary input but the output is monetary. I believe it’s rather hard to accept this simplification: for instance, around 50% of the price of cables is in raw materials like copper, that have a high volatility. This could easily bring a multi million inaccuracy.

Also, there is no such a thing as a standard substation – and this is why we have very good electrical engineer in the team. Even without considering the peculiarities of the local grids (something hard to ignore when they are weak, like in Australia) different customers have also different needs. A customer interested in business certainty will ask for redundancy in the substation – 2 main transformer instead of one, emergency “cold” spare transformer, etc.

Same for the foundations: there are currently so many technical solution in the market (precast, with rock anchors, braced, P&H, etc.) that it is really hard to find a correlation between wind turbine MW and foundation cost. There is so much money in foundation and so much pressure on prices that project specific foundations nowadays are the norm, not the exception.

Plate bearing test for wind farm roads & hardstands

Plate bearing test elements. Copyright image

Plate bearing test (also known as “plate load”) is one of the in situ investigations most frequently used during the construction of wind farms.

Its objective is to confirm that roads and hardstands meet the minimum requirements for compaction and bearing capacity.

Basically, it is performed pushing a steel plate against the material to be tested (usually the subgrade or the surface layer of roads and hardstands).

Three different standard plate sizes are available – the larger the diameter, the greatest the depth reach by the test. The biggest plate (762mm) is the one usually used in wind farms.

The plate is loaded by hydraulic jacks and it is connected to a counterweight (usually a truck or a construction equipment with a weight of several tonnes).

The jacks push the plate and the settlements are then measured at set increments of the loads.

Two different load cycles are performed (that is, the plate is loaded, unloaded and subsequently loaded again).

The results of the test are two “load / settlement curves”, one for each load cycle. With the curves two value called “Strain modulus” can be calculated – one for the first loading cycle (Ev1) and one for the second (Ev2).

I will not enter in the detail of how Ev1 and Ev2 are calculated (nothing too complex by the way – the formula is relatively simple).

Ev1 and Ev2 gives you a value in MN/m2 linked to the bearing capacity, while their ratio (Ev2/Ev1) will tell you if the compaction is sufficient.

Usually this test is done in wind farms road (every 500m or every Km are two standard values) and in the cranes hardstands (at least one test for hardstand, but frequently I have seen 4 tests, one below each of the crane legs).

The test is relatively slow: usually only a few positions can be tested every day. If needed the results can also be correlated with CBR values.

Technical requirements and tests for wind farm roads and hardstands

Two readers asked me how to assess the quality of the civil works in a wind farm.

This is a very broad topic and it requires some previous knowledge in geotechnics and road construction.

However, I think that someone might found useful an introductory article with some of the requirements that I would recommend to include in the technical specifications and test during construction.

The objective of these requirements and tests is to confirm that the materials used are appropriate and that the works have been properly executed.

As the readers of this website are from different geographic areas, I will not suggest a specific national or international norm such as ASTM, AASHTO, UNE, etc.

Finally, I also intentionally did not specify the value that I consider appropriate for each parameter.

However, if you need support to find an adequate standard or to define a specific value, please feel free to contact me.

For the embankments, I would recommend to define:

  • Maximum grain size. This requirement will avoid the presence of rock and boulders in the wearing surface of the road.
  • Appropriate gradation by sieve analysis (that is, the “granulometry” or particle size distribution). This requirement will ensure an appropriate interlocking between the particles and it is linked to almost all other property (e.g. stiffness, fatigue resistance, permeability, etc.).
  • Atterberg limits (Liquid limit and Plasticity index). You do not want to use plastic material behaving like clay.
  • Low organic matter and soluble salts content. This requirement will insure that only appropriate materials are used in the construction of the embankment.
  • Very low swelling or collapse potential.

The material should be properly compacted. Compaction grade is defined by the percentage achieved compared to the “optimum Proctor value”.

You also want a low deformability. This is usually defined the “Strain modulus” (or “Ev1” and “Ev2”) – basically the deformation of the material under two load cycles as resulting from a Plate Load Test.

I would also recommend defining a minimum CBR (“California bearing ratio”) value for the top and the core of the embankment.

If possible it is always a good idea to build a test section of the road, for a full-scale trial of the construction procedure. It can also help to determine the most appropriate compaction moisture – the actual value could be different from the value given by the laboratory test.

Graded aggregate is the material used in the upper layers of the internal roads.

It is made of well-graded crushed stones. The typology of arid to use and its granulometry depends on the local availability of materials.

In addition to the parameters defined for the embankments I would recommend to specify:

  • Sand equivalent (because you want as little clay as possible).
  • Loss by abrasion (because you want to use for the wearing course a material that can last for several years, ideally more than 20).

It is also advisable to define the maximum difference between the actual roads finished surface compared to the theoretical surface defined in the project.

Finally, compaction must be verified. Again, the standard option is the plate bearing test.