London Array: the world s largest operational offshore wind farm

Transcription

1 Case Study 1st published in January 2015 ISSN : the world s largest operational offshore wind farm Operations & Maintenance Base Abstract The is the world s largest offshore wind farm, consisting of 175 turbines with a combined 630 MW capacity producing enough power for nearly half a million UK homes. It extends approximately 100 km 2 and cost 2.2bn to build (Fig. 1). From its position in the outer Thames Estuary, 20 km from the Kent and Essex coasts, the is displacing tonnes of CO 2 and playing a major role in helping the UK meet its targets on carbon reduction and sustainable energy generation. It also pioneered the way for Britain to build offshore wind farms on a much larger scale, helping to create a secure energy future, while tackling climate change (Fig. 2). Introduction Project development The was born in 2001, after a series of environmental studies in the outer Thames Estuary confirmed the area was a good place for an offshore wind farm. Two years later, The Crown Estate, who own the UK s shoreline and offshore seabeds to a depth of 60 m, awarded the project a 50-year lease for the site and cable route to shore. It was given planning consent in 2006, with the onshore elements being approved in Work on the project began in July 2009 and was completed in December It has four international owners with differing shares: three world-leading renewable energy companies E.ON (30%), DONG Energy (25%) and Masdar (20%) and La Caisse de dépôt et placement du Québec (25%), a global investor and one of the largest institutional fund managers in Canada and North America. Project definition In addition to its 175 turbines, the consists of more than 400 km of sub-sea cables; two offshore substations with 20 m 20 m topsides, each weighing approximately 1250 tonnes; and an onshore substation at Cleve Hill that transforms the power generated to grid voltage for the Canterbury Kemsley 400 kv overhead power line. The offshore substations one at a water depth of 6 m, the other at a depth of around 12 m aresituatedoneither side of the site, nearly 6.5 km apart. They have 12 array cables each, connecting them to the turbines, and two export cables. The onshore substation sub-surface is made of recycled railway ballast, reflecting the project s commitment to sustainable construction materials. For more information on the components and testing of offshore wind farms in general, see A Guide to an Offshore Wind Farm published on behalf of The Crown Estate: /guide_to_offshore_windfarm.pdf. Technology/environmental assessment As part of the planning and licensing process, the completed an EU-required Environmental Impact Assessment (EIA) to determine how the wind farm could potentially affect birds, fish, mammals and the ecology of the seabed, as well as shipping and navigation, coastal processes and the surrounding landscape. For example, 112 marine or estuarine fish species have been recorded in the greater Thames Estuary including ten species of sharks and rays as well as harbour porpoise, grey seal and common seal. The area is also a spawning area for sole and the Thames Estuary herring. All environmental studies conducted as part of the EIA showed it was possible to build and operate a wind farm without causing any major negative impacts on the environment. The Array was one of the first offshore wind farms to use the Rochdale Envelope approach, giving the wind farm consent for a wide range of development 1

2 substation, crossed the railway bridge in Graveney with only 60 mm to spare! but, as one might expect, it was offshore where high winds and rapidly changing weather and sea conditions posed some of the biggest construction challenges. Fig. 1 Aerial view of the Source: Reproduced with permission The technical challenges of installing and burying array cables in extremely shallow water locations required a comprehensive suite of solutions including bespoke modifications to vessels and equipment to ensure their suitability for the. The cables were laid using five specialist vessels and remotely operated underwater vehicles over 14 months in water that varied in depth from inter-tidal to 25 m. The export cables crossed shipping lanes and other cables. As the cables approached the shore, careful planning and vessel selection was required in the intertidal areas. The cables were then taken through ducts that had been horizontally drilled under the sea wall, before connecting to the onshore substation. Also of note, as the onshore substation site was used as a dummy port in World War Two to divert German bombers, some of the more than 1000 unexploded ammunitions left behind needed controlled explosions before building work could safely begin. Fig. 2 Map showing position of the Wind Farm Source: NASA/ options, while keeping environmental impact to a minimum. This approach is an acknowledged way of dealing with a development application comprising an environmental impact assessment, where details of the project have not been resolved at the time the application is submitted, which allows for the results of the assessment to alter the existing application. In total, was granted 12 key consents and licences during the planning stage. Environmental monitoring and protection will continue throughout the lifetime of the project to understand what effect, if any, the will have on the environment. Construction and challenges Land transport was sometimes difficult the transformers, the largest single item on the onshore Testing A 35-person team controlled and managed electrical safety and testing at the during both construction and commissioning of the wind farm s 175 wind turbines and three substations. The team was responsible for high voltage (HV) electrical installation throughout the project: attending factory tests, making sure the electrical equipment used at the Array was working correctly before it arrived, and testing the substations, export and array cables both before and after the wind farm started producing power. The team was responsible for making sure the Array complied with the National Grid regulations for new connections before the wind farm began feeding power into the UK electricity network. Over a very difficult winter, s turbines were energised and commissioned in just 95 working days, setting a new benchmark for wind turbine commissioning. The last turbine was switched into service in April Foundation issues On average, it took one to two days to install each of the 177 foundations. The foundations were made up 2

3 of monopiles, or long cylindrical steel tubes, each up to 68 m tall and 5.7 m wide and 650 tonnes in weight and transition pieces, including ladders and platforms up to 28 m tall weighing between 245 and 345 tonnes. Approximately tonnes of primary steel was used in the foundations, and all 177 foundation pieces were designed specifically for their location on site. For the turbine foundations a technique known as soft start was used to hammer the foundations into place to warn away fish and marine mammals. For the onshore substation foundations, earth and rocks totalling m 3 were excavated and re-used to make screening mounds to reduce visual impact. Transmission infrastructure The has 187 array cables laid on the surface of the seabed, each between 720 m and 3200 m in length, which connect the turbines to each other and to the offshore substations. They contain power and fibre optic cores so the wind turbines can be monitored and controlled remotely. Each metre of array cable weighs around 50 kg. Fig. 3 Foundation installation plan for the Source: ATION-INSTALL-PLAN.png Four 150 kv export cables are the single heaviest item on the wind farm, each measuring more than 50 km in length and weighing more than 4500 tonnes. Each one was laid and buried into the seabed in continuous lengths, using a plough from the specialist cable installation vessel. They connect the offshore substations to the 400 kv onshore substation at Cleve Hill, where power is fed into the national electricity network (Fig. 3). Onshore and offshore infrastructure The two offshore substations, which reduce electrical losses by increasing the voltage before export of power to shore, were built over three levels of structural steel decks and weigh 1250 tonnes. The lowest deck is a cable deck, the middle houses containerised HV switchgear and the top deck is home to the HV transformers. The platforms are supported on specially designed transition pieces, connected to the monopile foundations (Fig. 4). The power is generated by 175 Siemens SWT wind turbines. The turbines blades capture the power of the wind, causing them to rotate. Each blade has its own independent pitching mechanism capable of changing the angle of the blade or feathering Fig. 4 Platforms are supported on specially designed transition pieces, connected to the monopile foundations (Siemens AG, Source: inno-2013/inno_fo_1319_2-4.jpg 3

4 Fig. 5 Power is carried from the offshore substations by four large export cables to the onshore substation (Siemens AG, com/press) Source: jpg under any operating condition. This optimises power output when the turbines are generating and minimises stress from wind when they are shut down. The turbines are turned into the wind by a motorised yaw mechanism. The blades rotate the rotor to which they are fixed, which in turn spins the main shaft. Made of forged alloy steel, the main shaft is attached to the rotor with a flange connection. The shaft goes into a three-stage gearbox to increase speed before the generator converts the rotational energy into electrical energy. The electricity produced then passes through the turbine s transformer and is stepped up to 33 kv for export to the offshore substations. Finally the power is carried from the offshore substations by four large export cables to the onshore substation (Fig. 5). Operation and maintenance The turbines start generating energy when wind speeds reach around 11 km/h and reach full power at about 46 km/h. For safety reasons, the turbines will start shutting down when wind speeds approach 80 km/h, equivalent to a force 9 gale. Each wind turbine has a swept area of m 2 equivalent to an area over one and a half times the size of Wembley Stadium s football pitch. To help protect smaller boats that may sail near the wind farm, the tip of each turbine blade will always be at least 22 m above the maximum sea height at high tide, even at its lowest position. Each of the turbines is designed to run 24/7 for at least 20 years, operated and maintained by a 90-person team. The Operations and Maintenance base located at the port of Ramsgate includes computerised monitoring and control facilities and maintenance vessels. The base is built on a 8000 m 2 site and it achieved BREEAM (BRE Environmental Assessment Method) Excellent status, was built using sustainable materials and has an extensive grass roof and a Combined Heat and Power (CHP) facility. Turbines are serviced regularly by site technicians. If repairs are required, they can be facilitated either remotely or on-site. Reduction in the levelised cost of electricity The UK government is aiming to cut the cost of offshore wind to 100 per MWh by 2020 and is being supported by the renewables industry and the supply chain to achieve this. In particular, the London Array s unprecedented size allows for economies of scale: it will help to reduce the cost of building offshore wind farms in the future so they are cost comparable with other traditional technologies and to reduce the cost of energy overall. The Crown Estate Pathways Report (May 2012) points out that the key 4

5 Table 1 Technical specification of the Complete Farm capacity 630 MW investment 2.2bn annual energy yield 2200 GWh capacity factor 40% completed Dec 2012 CO 2 reduction 925 k tonnes Source: ID=4750&T=9. opportunities for cost reduction are generated by the introduction of turbines that are larger and have both higher reliability and energy capture and lower operating costs (for more information, < www. thecrownestate.co.uk/media/305094/offshore-windcost-reduction-pathways-study.pdf >). Contribution of the energy generated to the grid As well as producing enough power for nearly half a million homes, displaces tonnes of CO 2 annually, equivalent to taking cars off the road. As the world s largest operational offshore wind farm, the will help the UK meet its targets (targets may have changed by the time of publication) on climate change and renewable energy to lower CO 2 emissions by 34% by 2020 and by 80% by 2050, and to generate 15% of all energy from renewable sources by 2020, including a minimum of 14 GW (gigawatts) from offshore wind farms. On-going challenges and the future The location of the offers an inherent challenge the turbines located above sea level at low tide become harder to reach for operations and maintenance (Table 1). The wind farm has had a successful first year of operation. In the six months from October 2013 to the end of March 2014, s 175 turbines produced 1.5 TWh of clean power equivalent to the annual consumption of around British households. The good performance was helped by December and January s unusually windy weather which, combined with high availability, meant the turbines were able to capitalise on the near-perfect conditions to achieve some very high load factors. On 6 January 2014 alone, some 15 GWh was produced. The success of has helped to demonstrate that large scale wind offshore wind is both practical and has a significant role to play in securing the UK s energy supplies while also meeting renewable energy targets. 5