Ramboll’s towering achievement

In today’s onshore wind market, some projects require wind turbines with large hub heights to yield a viable return on investment. The introduction of new +4 MW onshore wind turbines also requires larger hub heights.

Building on about 70 years experience in the design of radio masts, together with work on transmission towers and around 30 years of involvement in the wind industry, Ramboll has developed a guyed tower concept for the construction of tall wind turbines.

“We initially conceived the concept in 2003, but back then there was no market demand for such tall towers. Around 2012, more suitable hub heights came into the market and we picked up the development. With this concept, which we call the Guyed Wind Turbine Tower concept, hub heights of more than 200 m are not a problem,” says senior tower specialist at Ramboll, Ulrik Støttrup Andersen.

The greatest advantage of slimmer towers is the lower consumption of steel for the structure, which reduces costs.

Apart from reducing the cost of the towers, the concept also significantly reduces the cost of foundations, which are generally smaller and simpler than for self-supporting towers.

The tower sections can also more easily be transported to site as the sections are well within onshore transport restrictions, which drives down costs even further.

Calculations suggest costs are cut “by more than 30% when using the Guyed Wind Turbine Tower concept instead of alternative tower concepts,” says Ulrik Støttrup Andersen, who believes that the concept will be a game- changer for the onshore wind industry and pave the way for the production of more wind energy.

“Due to the cost-effectiveness of the concept, more sites can be viable, and the small foundations required also makes repowering of small turbine sites with power upgrade feasible,” he believes.

The Ramboll-developed tower concept strengthens the traditional steel tubular tower by introducing guy ropes (stays) attached to the tower just below the rotor of the turbine. This means that taller towers can be constructed with a smaller cross section than that of traditional tower concepts. Combined with the installation concepts used for telecommunications masts, the significantly greater hub heights become possible. Comprising six identical plate elements, which are inserted between flanges of the standard tubular tower sections, the attachment is “lean and simple to manufacture”, with excellent fatigue behaviour. Having two guys in each of the three directions is partly to provide redundancy should a guy rupture and also means the guys are of more manageable size. As well as optimising the tower, the concept also significantly reduces the cost of foundations, which are generally smaller than for a conventional self-supporting tower, and make use of Ramboll’s seven decades of experience with guyed mast foundations for broadcast and telecoms applications.

The smaller foundations and their design also facilitate the use of prefabricated concrete elements as an alternative to in-situ cast foundations, notes Ramboll. At sites with bedrock, the guy ropes can be directly anchored to the rock.

As an independent engineering consultant, Ramboll says it can perform “cost benchmarks and technical due diligence including risk profile against other tower technologies, such as traditional tubular steel, concrete/steel hybrid or lattice steel towers, without conflict of interest.”