EXHIBITIONS
the better. The rotor diameter plays a major role for the output and yield of a wind turbine, as the surface of the rotor determines the share of the available wind flow and the share that can be converted to electric power by the turbine. Moreover, depending on the location, an increase in height also means a higher wind speed. As the output generated by the wind is in proportion to the cube of the wind speed, the hub height has a major impact on the yield of the wind turbine.
More wind at sea than on land On the other hand, building an offshore wind farm means spending far more time and money than the onshore equivalent. The increasing attractiveness of offshore turbines for the energy sector is partly due to the lack of acceptance of onshore facilities, though not only. There are also other plausible reasons. First of all, the wind at sea is not just stronger but also more constant and regular than on land – as has been confirmed by the 2014 Wind Energy Report for Germany of the Fraunhofer Institute for Wind Energy and Energy System
Technology (IWES). Although both types of locations display seasonal differences (more in winter, less in summer) in the wind power supply, offshore wind is not as much dependent on the time of the day as onshore. This is because thermal convection has less of an impact on the open sea. In total, therefore, offshore wind turbines deliver far more energy than corresponding onshore wind farms. This is particularly true for the more recent farshore facilities which are situated further from the coastline. A simple principle applies to
turbines in general, whether onshore or offshore: the bigger
Trends towards more performance and bigger turbines This is accompanied by an increase in the nominal output of the wind turbines. According to IWES, the average nominal output of a newly installed offshore wind turbine has globally increased from 1.9 MW in 2000 to 3.6 MW in 2014. During the same period the hub height rose from about 60 to an average of 85 metres. Above all, there has been a continuous increase in the lengths of rotor blades on offshore facilities. In 2014 wind turbine rotors had an average diameter of 115 metres, compared with about 75 metres in 2000. New models in the 6-MW class even have diameters of 150 metres or more. In fact, the next generation of offshore wind turbines, which is currently under development, will have rotor diameters of over 160 metres, with a nominal output of 6 to 7 MW. The foundation of an offshore
wind turbine is impacted not only by a trend towards larger dimensions, but also by the water depth at the point of installation. Whereas the first wind farms were still built relatively close to the coast, where the water was quite shallow, today’s offshore facilities are situated 21.5 kilometres off the coast, reaching 15.5 metres into the water. In 2014 turbines were built at about 21.1 kilometres from the coast, at a depth of 32.3 metres. According to IWES, German offshore turbines are situated, on average, 65 kilometres from the coast, reaching a depth
www.internationalmetaltube.com
of about 29 metres. Seen globally, this means that they are at the furthest distance from the coast. As all offshore wind turbines operate under different conditions, a variety of solutions have been developed for the relevant foundation structures. In terms of quantity, the biggest share has always been that of monopiles and gravity foundations. Other construction methods are high-rise high pile caps, used in Asia, and, above all, timber-framed support structures (jackets), three-part foundations (tripiles, tripods) and floating foundations. To put it in simple terms, the structure of an offshore wind turbine depends on its location. Gravity foundations, high-rise pile caps and monopiles are largely used in coastal and shallow waters. Tripod and tripile foundations are particularly suited at long distances from the coast and in deeper waters. Obviously, the boundaries between these categories are fluid and indeed continually shifting, particularly with regard to monopiles.
Considerable strain on offshore wind turbines Offshore wind turbines generally need to be more stable and robust than onshore. This is because they are exposed to more and bigger forces. As well as their own weights and high wind speeds, they need to cope with waves, currents (including high and low tides) and floating ice. Their foundation structures must be designed and dimensioned in such a way that they can withstand all the forces to which they are exposed, and indeed over several decades and without tilting. The simplest way to anchor
an offshore wind turbine in the seabed is to use monopiles – a long cylindrical pipe which is usually driven into the bottom of the sea by an installation vessel. Monopiles can be installed within a very short period of time – i.e. several hours – and are therefore very cost-effective. As a rough guideline, about half of the pile
IMT June 2016 15
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36