An offshore wind turbine must also be more robust than its onshore brethren. It has to be designed for the rigours of a saline climate and wind and wave interactions on the entire structure. Although wind turbulence is lower offshore, it builds up in large arrays, so spacing between individual units must be chosen with care, so fatigue loads do not rise unduly.
Turbine manufacturers are now offering machines specifically for the offshore market, mostly in the 1.5 to 3 MW size range (with Enercon and GE being the exceptions with bigger machines). Design modifications include sealed nacelles, corrosion protection, special access platforms for helicopter drops, and location of transformers and switch-gear inside the turbine towers. In-built cranes and provision for temporary cranes are increasingly part and parcel of the offshore turbine package: the NEG Micon 2.75 MW/92 metre rotor machine has a 25 tonne crane on top of the nacelle.
Most manufacturers are also developing 5 MW machines, with rotor diameters around 125 metres, as reported in more detail in Windpower Monthly's sister publication, WindStats Newsletter, late last year.