Offshore blades: The cutting edge – LM Wind Power
LM Wind Power is the largest and most well-known blade manufacturer in the world but its origins are perhaps surprising. In an exclusive interview with Offshore WIND Peter Hansen, Senior Project Manager LM Wind Power, Technical Account, outlines the development of the offshore blade and the journey that took the company from making mobile fish tanks to the world’s largest rotor blade. Headquartered in Kolding, Denmark, LM Wind Power has produced more than 140,600 blades over more than 30 years, which represents around 5,605MW of installed wind power capacity.
Even though its turnover is now 100% dedicated to blade manufacture, service and brakes solutions, LM Wind Power evolved from making furniture. Starting in the fifties, the company eventually turned into making fibreglass products. In fact the first fibreglass piece ever made by the company was a transportable fish tank. Caravans, and then boats emerged from the production halls later on. The company is still renowned in Denmark for its LM Boats.
The company didn’t enter the wind market until the seventies when the company installed its first blades on a turbine in 1978 in the Orkney Islands. Peter Hansen quips: “The last I heard is they were still rotating so it shows we build for longevity!” The first offshore order was for Denmark’s first offshore wind farm, Vindeby, which was commissioned in 1991 and for which the company delivered 33 sets of 17m blades.
A long way
Of course, blades have come a long way since those early days. He tells Offshore WIND that essentially even today, the fundamentals remain the same. But the approach is radically different and much more sophisticated as turbines get bigger and the industry gets more professional. “There was nowhere near the level of detail needed compared to today. In those early days it was more an industrial architect working with fibreglass and glue until we found something that worked. As the industry gets more competitive, there is more pressure to get costs down compared to the overall performance of the rotor.”
These days, we have a team of aerodynamicists, a wind tunnel and specialists in several continents. Everything is much more scientific based, with sophisticated measurement equipment and very skilled people involved, he stresses.
Originally, LM Wind Power looked to the aeronautical industry as a starting point for the shape of the blade, considering both aeroplane wings and helicopter blades. “We benefited from the knowledge built up in the aeronautical industry over decades because essentially turbine blades have to do a similar job.” Although of course, the weight requirement for a blade is not so extreme as with an aircraft, and have to be designed for much longer inspection intervals, he adds.
A fundamental difference concerns the thickness which is related indirectly to the weight of the blade. This is something that has changed over the years of blade development. “If looking at an aircraft wing say of 10m long and 3m wide, the thickness is instead of if typically 40-60cm – it is a ‘very thin’ construction.”
When blade manufacturers are considering building something similar to a wing and looking at weight issues, it is certainly tempting to increase the thickness to facilitate weight savings, he says. As a wing will bend upwards with the load forces, a blade will bend towards the downwind direction and it can bend towards the tower, which could be catastrophic, says Mr Hansen. Increasing the relative thickness is an effective way to increase the stiffness. Therefore, LM Wind Power strive to design its blades with the highest possible stiffness. How to achieve this rigidity, with the consequent increase in thickness, without affecting the energy yield, he admits is a dilemma. “We always have to find the balance.”
“The balance between thickness, rigidity, material consumption and performance has always been, and still remains, a challenge.”
Although the increase in thickness is perhaps not so obvious, the increase in blade size certainly is. LM Wind Power’s first blades were about 8m long and now their longest blade, launched last year, comes in at a huge 73.5m long for Alstom’s new Haliade 150-6MW offshore wind turbine which was installed on a prototype site in France in March this year.
Mr Hansen is reluctant to be drawn on where the industry will reach the limit of blade size, adding that many have fallen in that pitfall before him. “Vestas has announced its 7MW, 164m diameter turbine and we will probably see the 10MW. Yes, it will go bigger but I don’t know how fast and don’t want to predict it, technology changes all of the time.”
Ultimately, he says, it doesn’t automatically pay off cost-wise to go bigger and bigger. “For blades, the investment needed increases dramatically when you go up in size, just when considering the size of the factory alone and even the logistics in that factory – moving a blade from one part to another, let alone the transport costs involved.” All the costs soar, he stresses “As a manufacturer, you only have your own capacity to work on for cost reductions, therefore you can’t get economies of scale.”
“The costs of going bigger then become a problem because there are fewer suppliers. This effect can restrict the growth of turbine size.”
However, he believes that if the offshore wind industry really picks up and hundreds of turbines are built (perhaps in the Round 3 projects), then specific factories for very large blades could be justified and some of those supply issues could be reduced. Mainly because of this issue LM Wind Power manufactures all over the world and has facilities in China, India, Europe, the US and Canada. “We have to build where the market is because it just becomes too expensive to transport the blades.”
Offshore versus Onshore
Offshore blades have also developed over the years from being essentially onshore blades out at sea. The shape is roughly the same, but naturally there are differences in the cost structure that can lead to a different optimal configuration. “It is different to land installation, offshore foundations and cables are so expensive this naturally leads to larger sized turbines.”
Noise is also not a concern at sea but having to take this into account onshore can mean that there are limits on the turbine speed. Whereas if the blade can go faster, this reduces torque and leads to cost and weight savings in the turbine nacelle, he adds. The weight of the blade itself becomes a more important factor with blades getting larger, forcing the industry to address this, he adds. Again, the focus is on reducing the blade weight, and again this can justify a small drop in the aerodynamic performance.
Fibreglass still No.1
One thing that has been a stable factor in the development of the blade is the material used. Fibreglass remains the material of choice, and it looks like it will be for some years to come. He estimates that currently probably more than 90% of blades are made from fibreglass which is especially important offshore given its ability to tackle the worst conditions that sea salt can throw at it.
However, as turbines get bigger in the future, Mr Hansen can see that there could be more use of carbon fibre because it reduces the weight significantly. The problem still remains over the cost of raw materials because carbon fibre is a more delicate material with which to work, and requires a much more complicated manufacturing process.
Additionally, more different resins can be used with fibreglass currently. However, given these issues and the focus on reducing blade weight, this will lead to a stronger motivation to look at carbon fibre, he emphasises.
Maintenance is a constant issue but one that again takes on even more significance with turbines getting ever bigger and going further out to sea.
Uniquely, LM Wind Power uses a special gel similar to that used on boats, which seals the surface from ultraviolet rays and rain damage. The special polyester resin is incorporated into the LM manufacturing process, whereby the company paints the actual blade mould rather than painting the blade afterwards. When demoulded, the blade is painted and protected. This eliminates a process step and also leads to a very strong and durable coating.
However, when the rotational speed goes up and with the blades designed for at least a 20-year lifespan, the leading edge of the blade will eventually wear. At a certain point it comes to an end, he says, so LM Wind Power in some cases puts a “helicopter tape” on the leading edge of the blade, which as the name suggests was originally developed for helicopters. He adds that this is a “big challenge for the industry because the tape could eventually peel off within the lifetime of a blade”. “The industry has been using the tape for around eight years, so we still have not got the total picture for the lifetime of the tape.” LM Wind Power recommends that blades should be inspected every two years. “This is a visual inspection where engineers look for cracks. If there is an issue, our repairing teams can do quite a lot with rapelling technique but there is always the need for calm weather when repairs are being done.” The company also has its own “working platforms” which climb up the tower. This is a lot cheaper per hour than deploying a crane on the seabed.
Durability is the priority
“The number one priority of course, is to avoid having to go and do repairs, so quality and robustness are vital.” Durability is naturally one of the major concerns of the industry, he stresses. “We are designing for at least a 20 year lifetime.” Mr Hansen does think that blade monitoring will become more important in the future, perhaps including load monitoring, condition monitoring of the blades.
“In theory we can monitor the lifetime of the blade. We can use the turbine controller to reduce loads etc. but we have to be able to rely on the equipment and know it works. Imagine having to turn off the turbine for three days until it can get repaired and all for a faulty sensor in the monitoring equipment. Just a few days downtime can have a big impact. This equipment simply must not fail and so it does make sense to monitor the blades and have remote checks for evidence of cracks. I can see this happening.”
So for the future, does LM Wind Power think there will be a radically different blade emerge? “We optimise new blades for every turbine in a close cooperation with our customers. Our portfolio will continue to comprise a mixed portfolio of very specialized blades and “off the shelf” blades for a range of turbines.”
“But LM Wind Power will continue to focus on our track record and global footprint rather than a completely different blade type.” For instance, he points to the development of the two-blade rotor. This is perhaps easier to transport in one piece, which is a big advantage off shore and could prove to be a step in reducing the cost of energy but it is also a big step in terms of risk level, he says.
“We always want to focus on our track record, our proven concept. We want to optimise our products but ensuring that we know what we’re doing at all times. When we developed the 73.5m blade, we tried to stay as close to our standard materials and processes as possible to reduce risk and take advantage of our track record. Several new methods and concepts had to be developed for both design and manufacturing simply due to the size. To combine that with a fundamental change in technology would have been a major increase in risk which we were fortunately able to avoid.”
However, with the very large planned offshore projects there is much more likelihood that the industry could see more customisation – perhaps one blade design dedicated to a particular wind park, he says. “If Round 3 kicks off I think we might see a tailor-made blade for specific geographical areas to take advantage of the site conditions. We will also continue with more optimisation by taking some kilos out of the blade.”
Times are not the best for the industry at the moment, he says “but the long-term future looks good and I believe there will be a place for LM Wind Power”. Blade manufacturers are getting the benefits that globalisation brings however, he adds. “We are not so vulnerable to the politics of wind as it was the case for instance in the 80’s.”
“Like all other sub suppliers, the future of LM Wind Power will depend on our customers, but it is up to us to prove that we also in the future can add value to our customers and remain an attractive alternative to their own blade manufacturing.”