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The foundations must be adequate to support the turbine under extreme loads. Normally the design load condition for the foundations is the extreme, ‘once in 50 years’ wind speed. In Europe this wind speed is characterised by the ‘three-second gust’. This is a site-specific parameter, which will normally be determined as part of the wind speed measurements and energy production assessment for the site. For most sites this will lie between 45 and 70 m/s. At the lower end of this range it is likely that the maximum operational loads will be higher than the loads generated by the extreme gust and will therefore govern the foundation design.
The first step towards the proper design of the foundations is therefore the specification of a load. The turbine supplier will normally provide a complete specification of the foundation loads as part of a tender package. As the turbine will typically be provided with reference to a generic certification class (see discussion under Introduction), these loads may also be defined with reference to the generic classes, rather than site-specific load cases.
Although extremely important, the foundation design process is a relatively simple civil engineering task. A typical foundation will be perhaps 13 m across a hexagonal form and might be one to two metres deep. It will be made from reinforced concrete cast into an excavated hole. The construction time for such a foundation, from beginning to end, can easily be less than a week.
The site roads fall well within normal civil engineering practice, provided the nature of the terrain and the weather are adequately dealt with.
For wind farms sited on peat or bogs, it is necessary to ensure that the roads, foundations and drainage do not adversely affect the hydrology of the peat.
The wind farm may also need civil works for a control building to house electrical switchgear, the SCADA central computer, welfare facilities for maintenance staff and spare parts. There may also be an outdoor electricity substation, which requires foundations for transformers, switchgear and other equipment. None of this should present unusual difficulties.
For upland sites, it is often beneficial to locate the control building and substation in a sheltered location. This also reduces visual impact.
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