Nabrawind Transition Foundation

Some days ago I have been contacted by Miguel, Sales And Marketing Manager at Nabrawind.

Nabrawind is a Spanish company working at several interesting breakthrough concepts – including a modular blade that I will try to describe in another article in the future, a self erecting tower and the innovative “Transition Foundation”.

Miguel asked me if I was interested in receiving material about the Transition Foundation solution they have developed.
I was obviously very happy to accept his offer and share with you what I have learned.

This alternative foundation use a 20 meters tall transition element in steel and cast iron in the lower section, at the bottom of the tower.

A detail of the transition element

The transition element is connected to the ground through three “feet” that allow different technical alternatives for the foundation: the standard solution (“shallow” or “gravitational”) plus two cheaper options – piled foundations (for standard soils) and rock anchors (when bedrock is very near to the surface).

The three alternatives solutions for the Nabrawind foundation - shallow, piled and with rock anchors

For a 4MW wind turbine the piles are expected to have a diameter of 1.5 meters with a depth in the 15 to 20 meters range.

A 15 meters pile would need approximately 80 m3 of concrete and 21 Ton of steel in total (that is, for the three piles).

This figure indicate very substantial potential savings in the amount of concrete and important reductions in the quantity of steel as well.

The piles are connected to the transition element via anchor cages (obviously smaller than the normal anchor cage used with standard solution).

In addition to the savings in the quantities the other main benefit of the solution is the speed. You will need only one or two days to drill the hole for the pile, and the installation of the reinforcement bars and concrete pouring is very quick as well (both operation should last between 2 and 4 hours in total).

The anchor cage variant promise to be even faster, needing only three concrete blocks (one for each “foot”) to level the surface and distribute the loads and 6 post-tensioned rock anchors with a length in the 15 meters range.

The Transition Foundation is more than a concept – the first foundations using this solution have been built in a wind farm in Morocco for a 3.6MW wind turbine on a 144m tower.

The have 24 meters long piles with a 1.2 meters diameter, for a total of only 81 m3 of concrete and 25 Ton of steel – a remarkable result.

How the foundation looks like (notice the 3 elements)

In the last picture you can see a detail of the completed works for the foundation.

How the completed foundation look like

Peikko rock adaptor foundation

I've been asked by a reader of this website why there are no references to the various technical solutions available for wind turbines on rock.

The truth is that I’m not a specialist on this topic. However I’m learning, due to the fact that I’m currently working at several projects in northern Europe where it can be applicable.

To solve the problem I’ve decided to start with a video, that can be better than a 1000 words.

It’s an example of foundation on rock without anchor cage – one of several possible technical alternatives when the turbines are above shallow, unfractured rock.

Basically the tower rest on a steel “adapter” plate on top of a reinforced concrete block, and the turbine is fastened to the ground with dozens of post-tensioned anchors several methers long (figures above 9 or 10 meters are not unusual).

There are some very clear benefits with this solution if the geology is favourable: for instance less excavation, almost no blasting and lesser use of materials.

This video has been done by Peikko, a Finnish company specialized in steel elements. They have an interesting, unusual business model, as they do the engineering for the foundation and provide the steel but not the concrete or the manpower – therefore the foundation has to be built by another company.

Here a screenshot with the main elements of this solution:

One more video (possibly more detailed) on this technical solution here: