Off-shore wind turbine towers in high-strength concrete

Tech-wise has investigated the possibilities of using high strength concrete (CRC) for the towers of offshore wind turbines. Theninvestigation is part of Elsam's research efforts regarding offshore wind turbines and is funded by the so-called PSO funds supplied by Eltra. Background: With the wind turbines being erected offshore, the accessibility for maintenance of the surface coating of steel towers is becoming increasingly difficult and costly. Furthermore, the height and slenderness of the structure are increased in offshore applications which in the case of steel towers increases the need for a structural vibration absorber placed in the tower or - alternatively - imposes restrictions on the working range of the wind turbine. Objective: The objective of the project is to investigate the technical, environmental and economic aspects of using high strength concrete for offshore wind turbine towers rather than steel. Technical aspects: The project is based on the material CRC - Compact Reinforced Composite - a concrete with a large content of small fibres and steel reinforcement. CRC is developed by Aalborg Portland A/S in 1986 and based on a large number of research projects. CRC is now sold and marketed by CRC Technology. The research projects indicate that it can be an advantage to use CRC in offshore applications: Its high strength and large ductility makes it highly suitable for structures subjected to impact; The durability against chloride is extremely high; It responds well to fatigue. Since the material has not been used for structures as large as towers for wind turbines there is some uncertainty as to the demands that the CRC has been developed at Aalborg Portland to meet these requirements, and some model tests have been performed and documented in the project. To prepare a realistic scenario for the towers they are presumed implemented on Horns Rev Offshore Wind Farm in the North Sea. A Vestas V80-2MW is used - the same as ordered for the site. The loads are calculated by means of the FLEX5 program, which includes wave loading. On this stage a high-strength concrete tower and a reference steel tower is designed, the two of which are investigated and compared throughout the report. The strength of CRC is considerably higher than normal concrete. A characteristic compressive strength of 100 MPa is used. The tensile strength of the concrete is significant in CRC contrary to what is observed in normal concrete, but it is not taken into account in the report. The dimensioning property of the tower is the fatigue strength of the reinforcement, while the strength of the CRC-concrete is not used to its full extent due to the minimal thickness restrictions for a proper production. The assembly must be given serious attention, since assembly could be a weak point in the structure from a maintenance point of view. Two examples of assemblies are given in the report: A bolted connection and a so-called joint-cast connection. The most rational production of towers in CRC for offshore applications is considered to be in relatively large sections to minimise the assembly works offshore. A method of producing wind turbine towers in CRC is introduced in the report, but CRC structures have not previously been produced on such a large scale and different techniques can come into consideration and would be relevant for further investigations beyond the scope of the project. The site installation routine is not very different form that of a steel tower - even though the weight of high strength concrete towers is higher. The maintenance costs of the tower are reduced to a minimum compared to a steel tower. Environmental aspects: The environmental impact of the two choices of technology has been analysed in a life cycle assessment, a so-called cradle-to-grave analysis. The analysis shows that for both types of towers the environmental impact is more severe for the production and disposal phases, whereas the environmental impact from the maintenance of the steel tower is insignificant. The possibility of recycling the high strength concrete tower is vital to the economy of using CRC. If the tower is not recycled the environmental is considerably higher than for a steel tower, but if it is recycled it gives a smaller contribution to most of the environmental aspects than a steel tower. The possibilities for recycling the CRC tower are briefly described in the report. Mechanical disintegration into gravel and recyclable iron bars is not possible for CRC because of the high strength of the concrete and the content of steel fibres. But there are other possibilities for reusing a CRC tower in full or partially: land based towers for wind turbines; harbour and coastal protection; tunnels; silos; water reservoirs. Conclusion: The investigations have found promising qualities of towers in CRC compared to steel towers: Significantly lower maintenance costs; Increased stiffness of the tower, which eliminates the need for a structural vibration absorber; Lower influence on most of the environmental effects, but only if the tower is recycled; The product costs are assessed to be on the same level as those of a steel tower, bearing in mind the lack of experience with manufacture of high strength concrete towers. The excellent durability of CRC could be a problem, however, as its duty life is likely to exceed far beyond the point where technology of the wind turbine is outdated. As it is vital for the environmental impact of the high strength concrete tower that it can be recycled, it is important to examine the recycling possibilities in connection with the planning of wind turbines using high strength concrete