The Challenges of Designing a Modular Blade
(This is an article released in Indian Wind Power Magazine.)
Modular blades are not something new in the wind industry. More than a decade ago, some companies such as Gamesa or Enercon released their own split blades. However, at that time other logistic alternatives were mass adopted for the transport of blades, limiting the implementation of these pioneer blades.
Nevertheless, nowadays the wind market’s trends reveal a renew interest in them. Almost all the OEMs are greatly investing in developing this technology as they asume it crucial for the future.
Still, the challenges of this kind of technology remain. Nabrawind has joined the race for optimizing the blade’s modularity and by doing so has developed an innovative solution that will be explained in detail in this article.
Before deepening in the technical part, a question arises and needs an answer: Are modular blades again a temporal trend or are they here to stay? According to current market forecasts (see Wood Mackenzie forecast, table below), it seems this time mass adoption of split blades will happen. In fact, Wood Mackenzie forecast establishes that by 2025 half of the blades market will be modular. In particular, the 38% of the market will correspond to 80-89.9 meters length blades, which will most likely require modularity to be transported. Furthermore, also in 2025, 90-99.9 meters length blades will represent the 25% of the market.
The transition to modularity will not stop there and by 2027 extremely long 90-99.9 meters length blades will cover the 40% of the market. Even more, by this year the first 100-109.9 meters blade models will be introduced in the onshore market.
All in all, modularity will exponentially grow in the following years until represent more than 50% of the market by 2025, again according to Wood Mackenzie forecast.
Modular blades alternatives
Although modularity seems inevitable, it is also true that several challenges remain concerning this technology. The main barriers for its mass adoption are, so far, related to the added weight of the joint, the cost, its maintenance requirements and the field assembly process, among others.
There are different technologies trying to solve these issues related to modular blades. The main alternatives that can be found in the market are the following:
This is a very well-known connection in standard blades as it is one of the most commonly used blade root connection since it was applied for the development of the DEBRA 25 (100 kW wind turbine) in 1981. A T-bolt consists on a threaded bolt, a barrel nut and a standard nut. A high number of T-bolts are equally spread along the circumference of the blade root in order to create the blade-hub connection. This alternative has been historically one of the most used for modular blades, with split joints located close to the blade root.
Another bolted joint alternative is the connection by means of metallic inserts. The inserts are metallic parts bonded into the blade laminate that enable the two blade segments to be bolted together by the use of intermediate connecting parts. Important wind industry players have developed this kind of technology and applied it in the blade segmentation.
Thirdly, there is the category of bonded joints, which are usually lighter than bolted joints, as they do not require metallic parts, but require to move a complex infrastructure to the wind farm in order to assure that the bonding process is made in controlled conditions.
Finally, there are also hybrid solutions, some of them recently implemented. These alternatives combine a bonded part with a mechanical union, but without using bolts. An example of this solution can be seen in the following figure.
So, there are already several alternatives in the market, confirming the industry interest in these segmentation solutions. However, we decided to think outside the box and that is how we came with an innovative solution called Nabrajoint.
Our proposal is based on a maintenance-free bolted connection using as basis metallic inserts. Differently to other insert options we designed a connection between blade modules by means of just a single stud. This reduces significantly the size and mass of the joint, and consequently its cost and load impact in the blade. Of course, in order to be able to do so, we needed a way to preload the stud. It was at that moment when our engineer team came up with the XPACER, a dispositive which preloads the stud.
But let’s take it one step a time. Three parts participate of our bolted joint:
First, a set of inserts bonded to the blade main composite structure, between each pair of which a stud is bolted. Secondly, the array of studs that connect the modules once they are threaded to the inserts. Finally, the preloading device.
The following pictures shows a general view of these elements.
The inserts are these metallic elements acting as the interface between the blade main composite laminate and the bolted connection in charge of joining the blade modules.
On the other hand, the studs, made of high strength steel, are bolted between each pair of facing inserts and are in charge of transferring the load between modules.
Then, the joint is completed by means of the patented expandable spacer. This device allows the precise preloading of the studs in order to improve their fatigue life. To that purpose, a preloading device is introduced between each pair of inserts in such a way that covers the central stud that joins them. A view of this device is shown below:
This preloading device is the key part of the joint, and it is formed by four elements: the Upper Wedge, the Lower Wedge, the Washers and the Tightening bolts.
In an initial state, the upper and lower wedges are not levelled with the washers, but they stand out above the washers’ external surfaces. When the bolts are tightened, however, the upper and lower wedges slide over the washers’ faces, and as a result of which:
- The external wedges and washers’ surfaces level up.
- The stud is preloaded with a tension force and is therefore elongated.
- The washers and wedges are compressed, acting as the clamped parts of a typical bolted preloaded joint.
- The system is locked after completing the preloading, assuring the maintenance of the stud preload and hence the non-necessity of re-tightening of the joint.
Thanks to the operative characteristics of the preloading device and to the high strength of the newly developed inserts, the number of elements needed for the joint are minimized, so the mass and cost are also reduced.
Moving to the field operation of the joint, in order to ease the assembly process, we have developed and patented an automatic tightening tool, which is purely mechanical and allows the quick and precise application of the required preload to the studs.
In addition to this, the engineering team had to deal with other issues related to the integration of the joint in the blades, using our vast experience in these design processes.
Firstly, we had to solve the integration of the joint in the blade shell. This is done by transition joints that are specifically tailored to the configuration of the blade to be joined with this technology.
Secondly, the connection between webs is completed by metallic plates that are assembled and bolted between both webs extremes, assuring the shear load transfer between both parts.
This solution has been successfully validated through a rigorous certification process and its corresponding testing campaign. The validation process covered all the required tests, including materials, subcomponent, manufacturing and testing of two blade segments. These specimens have been submitted to the standard blade testing scheme, including static tests, fatigue tests and static post-fatigue tests. These tests were completed successfully at the beginning of 2020, validating the structural performance of the joint. Furthermore, the required certification tests were extended with static tests in a Fail-Safe configuration, reaching 118% of the design load (limit of the test bench) without incidence, proving thus the robustness of the solution.
All in all, after analyzing so many modular blades alternatives, what might seem clear is that modular blades will play a key role in the near term. There are several interesting solutions already in the market that, hopefully, will help to ease the logistic constrains to the blade’s transport is already facing. We just hope to do our bit in this fascinating challenge that the wind industry has to solve.