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Energy Prediction

The energy prediction step is essentially the same as for onshore predictions.  There is generally only minor predicted variation in wind speed over a site.  Given the absence of topography offshore, measurements from a mast can be considered representative of a much larger area than would be possible onshore.

For large offshore sites, wake losses are likely to be higher than for many onshore wind farms.  The wake losses are increased due to the size of the project and also due to lower ambient turbulence levels - the wind offshore is much smoother.  There is therefore less mixing of the air behind the turbine, which results in a slower re-energising of the slow moving air, and the wake lasts longer.  Observations from the largest current offshore wind farms have identified shortcomings in the classic wind farm wake modelling techniques, due to the large size of the projects and perhaps due to specific aspects of the wind regime offshore.  Relatively simple amendments to standard wake models are currently being used to model offshore wake effects for large projects, but further research work is ongoing to better understand the mechanisms involved and to develop second generation offshore wake models.   

There is likely to be more downtime of machines offshore, which is primarily due to difficult access to the turbines and delays in mobilising crane vessels for major repairs.  If a turbine has shut down and needs maintenance work, access to it may be delayed until there is a suitable window in the weather.  This aspect of offshore wind energy is a critical factor in the economic appraisal of a project. Increasingly sophisticated Monte Carlo-based simulation models are being used to assess the availability of offshore wind farms, which include as variables the resourcing of servicing crews, travel time from shore, the turbine technology itself and sea state.