The importance of the geophysical survey for a Wind Farm

Today we speak about geotechnics…yes!, geotechnics, that activity that sometimes sounds strange but is so essential for the development of a proper wind farm project.

More specifically, we will see in detail tomographic refraction tests and MASW analysis.

This information is key for wind turbine foundation design but also can be needed for several other activities, such us for instance estimation of excavation (earthworks cost is linked to the hardness of the materials) and seismic design.

Geophysical works for a Wind Farm in Egypt, 2021
Geophysical works for a Wind Farm in Egypt, 2021

The analysis of the elastic modulus had been typically done with laboratory tests until a few years ago, when geophysics techniques began to be widely implemented (sometimes with unclear goals and misunderstood results).

The main advantages of these on site tests are:

  • Characterization of real terrain conditions taking into account the large magnitude of the structure it will support and its three-dimensional behaviour, rather than extracting small samples that could be not representative of the system as a whole
  • The possibility to obtain the dynamic parameters of soil stiffness, which are often the dimensioning factor due to the need to achieve a certain rotational rigidity for the wind turbine

It is important to highlight that some soils don´t seem to be apt to withstand the ever-larger loads of today's wind turbines if they are tested using “classical” methods (odometer, triaxial, etc…).

However, the same soils show a better performance when geophysical techniques are used to analyse them, mainly because they are less conservative, more realistic and more appropriate for dynamic loads and quite relevant foundation dimensions.

One more comment on dynamic parameters of soil stiffness: it´s worth noting that correlations that were widely used to estimate dynamic values from static values used to imply, depending on the type of terrain, wide uncertanties.

In some cases the values were unfavourable and did not allow to optimize the dimensions of the foundation. In others cases, unrealistic values reduced the level of safety in designs.

For all these reasons geophysical survey is strongly embedded in the way wind turbine foundations are currently designed, so the IEC 61400-6:2020 has made changes in its latest edition (see Annex L) to improve and adjust the calculation methodology.

Variation of shear modulus with soil strain

As a summary, take into account that optimization and accuracy within geotechnical calculation can lead into an optimization of the foundation, so be aware of the importance of geophysical survey for a wind farm and opt for a more comprehensive geotechnical study, appropriate to each case, modern and based on international regulatory updates.

Once More, with Feeling: Timber Towers

Modvion laminated wood tower. Image copyright Modvion

Approximately one year ago I wrote a post about a full scale prototype of a wind turbine tower made of wood.

It has been built in 2012, but after that the idea seemed to have stalled without progress: one of the companies involved in the construction, TimberTower GmbH, disappeared from the radar shutting down their website and I suspect they went out of business.

However, I see that someone else has taken up the challenge: Modvion, a Swedish start-up.

They could be more successful in moving from the prototyping phase to the industrialization for at least two reasons: they are coming from Sweden, a country with an extensive know how and network of companies active in wood construction, and they successfully went through a round of Venture Capitalists, Business Angels and European Union founding.

They target is to be ready to market in 2022 cross laminated timber towers in the 100 – 150 meters range. This means that they want to enter in the 4 to 5 MW segment, the current standard for utility scale WTGs. The prototype that they have just installed is 30 meters high.

Such tower could have several benefits – solve the current transportation problems (steel towers with diameters over 4 meters have huge transportation challenges due to bridges, cables, etc.), lowering the carbon footprint and possibly even be cost competitive against the current technologies (steel, concrete and hybrid).

I have no idea of the behavior of this solution from the resonance point of view although I suspect that the increased diameter at the base improve the situation. I also ignore how this tower would behave in case of fire: I have personally seen a fire very near to the wind turbines some years ago in Portugal.

Unfortunately their website does not share many technical details. I understand that it is a modular solution, with the total number of modules obviously depending on the tower height being a standard solution in the 30 - 40 modules range.

The tower section is circular, unlike the TimberTower solution that was octagonal.

Among the materials used for the tower I see glued laminated timber and laminated veneer lumber - basically a mix of wood and adhesives, with superior technical properties and more uniform characteristics as it is produced in a controlled environment.

The modules are joined together using double-treaded fasteners, preassembling on site 4 or more tower sections with a bottom diameter of 6+ meters (that is, more than a standard steel tower).

This concept is similar to some concrete modular tower solution with a key difference – the modules are assembled horizontally, so I guess that the need of big crane support is limited.

I also understand that the internals of the towers would be similar to the ones currently built (with elevator, ladder, space for transformer, etc.).

Printable 3D concrete wind turbine towers

Yes, you're reading right – you can print your concrete tower.

I have discussed in many previous articles how I see some evidence that we are reaching the maximum size for steel towers, mainly because of transportation issues.

For higher towers concrete towers could help solving the problem, as they can be transported in pieces and assembled on site.

Among the different technologies available for concrete I have just discovered this exiting evolution: a Danish company specialized in 3D concrete printing, COBOD, partnered with GE Renewable and LafargeHolcim to develop a large printable tower.

COBOD already printed some years ago a full scale building, a small but beautiful 50 m2 office with curved walls.

For this interesting evolution they already made a prototype about 10 meters tall.

The concrete is extruded by the machine in a sort of ribbon, and the internal and external sides of the tower are reinforced by a “wavy” central section.

Currently the solution that they are targeting is a hybrid tower (that is, with the top section made of steel) with an on-site printed base.