Offshore wind turbines are based on the same technology as their onshore counterparts and their expected lifespan is the same in that they require refurbishment after around 20 years. The main difference is their size. Sheringham Shoal Offshore Wind Farm will use Siemens 3.6MW turbines, which at 132m high, are around the same height as the London Eye.
The Siemens turbines have three blades mounted on a nacelle that houses the generator, gearbox, controller, shaft and other components. Sitting upwind of the tower, the 52m (170 feet) blades are made of tough fibreglass-reinforced epoxy resin designed to withstand the rigours of offshore conditions. The nacelle is mounted on an 80m (262 feet) tapered tubular steel tower with an internal lift enabling technicians easy access to the main workings.
The two installation vessels, Endeavour and Sea Jack, transport two turbines at a time from Great Yarmouth to the site where they jack up to undertake a highly technical lifting procedure. They first raise and install the 80 metre tower, and subsequently the nacelle, followed by the three 52 metre blades.
These 90 giant monopile structures, as well as the transition pieces which join the turbines to them, were fabricated by tubular structure specialist, the Sif Group, at their plant in Roermond, the Netherlands. Each foundation was made to individual specifications. They are between 44 and 61 metres long, with a 4.2-5.2m diameter and weigh from 375 to 530 tonnes.
Contractor Seaway Heavy Lifting used the state of the art crane vessel the Oleg Strashnov to drive foundation piles 23-37m (75-121 feet) into the seabed and mount the yellow-painted transition pieces on top, to support the installation of the two substations and 88 wind turbines. The foundation construction was completed in August 2011 and the installation of the elastometric spring bearings to reduce the vertical load on the grouted connection between each inner monopile and the outer transition piece was concluded two months later.
The wind farm features two 1000 tonne offshore substations. Each of the Sheringham Shoal substations is 30.5 metres long, 17.7 metres wide and 16 metres high. Offshore construction specialist Heerema fabricated the substations at their yard in Hartlepool in the North East of England. The complex logistics operation to transport them to the Sheringham Shoal site started in November 2010 when they were transferred from the fabrication hall to an outside location. The operation to transfer the substations out of Victoria quay and on to Dina Launcher, the barge which brought the substation to site, required careful planning and precise manoeuvring.
The substations were lifted onto their individual foundations by the state-of-the-art crane vessel Oleg Strashnov. Both substations were installed at their locations in May 2011.
The offshore cables were produced by global cable expert, Nexans in Norway at its manufacturing plant in Halden, south of Oslo, while the fibre optical elements were produced in Rognan, in the northern part of Norway. The power and optical cables are bundled together into one unit.
There are two long export cables carrying the power from the wind farm to landfall - one 23 kilometres and one 21 kilometres in length, with a weight of 77 kilograms per metre. Thatís a total weight of 3,388 tonnes!
There are two different types of infield cables connecting the turbines and the offshore substations. Type one (27kg/m) has a total length of 26 kilometres and will be used to connect the turbines closest to the substations, while type two (18kg/m) has a total length of 56 kilometres and will connect the turbines further out. Both cable types have been cut into actual lengths during installation by contractor Visser Marine Global Contracting.