Design of support structures for offshore wind turbines - Wind Power Generation and Wind Turbine Design - page 587

5.8 Optimising
The design steps described in this section have been treated on a high level only.
The design process will involve several repetitions in which structural properties
change in one step and require checking in all other steps again. Furthermore,
some steps have not been treated here: the design of secondary steel and its impact
on support structure loads and stress concentrations; the drivability analysis of the
pile and the associated fatigue (the pile loses 25-30% of its fatigue resistance during
installation); the impact of scour, corrosion and marine growth, etcetera.
6 Design considerations
6.1 Offshore access
The majority of the maintenance activities that are required during the entire life-
time of an offshore wind farm consist of simple repairs rather than the replacement
of turbine parts. Therefore, the accessibility to be treated here will involve person-
nel and light equipment only. The accessibility of a wind turbine depends fi rst of
all on the chosen access method. In the offshore industry there are two means of
transportation used to reach offshore structures: helicopters and vessels.
6.1.1 Helicopters
Helicopters are used regularly to gain access to various offshore installations since
they provide a fast means of transportation for personnel and light equipment at cruise
speeds up to 250 km/h. Another big advantage of using helicopters is that both travel
and access operations are not limited by wave conditions. If an offshore structure is
equipped with a helicopter landing deck, the helicopter can land on this deck and pas-
sengers can safely board or exit the helicopter. However, mounting a landing deck on
an offshore wind turbine would be unpractical. Instead, a hoisting platform can be
placed on the turbine nacelle. The transfer of personnel from helicopter to turbine is
then achieved by having the helicopter hovering above the turbine and hoisting people
from the helicopter down to the platform on top of the turbine. Although this method is
fast, disadvantages are the high costs of operation and the fact that a hoisting platform
is required on each turbine. In addition, most exploiting parties are not eager to use this
method due to the risks involved using helicopters: in case of a crash, the risk of casu-
alties is high. In fact, the Horns Rev wind farm, located in the North Sea 14 km west
of Denmark, is the only wind farm where helicopter hoisting is applied as a means of
access. Furthermore, this method only allows transferring personnel with a very lim-
ited amount of tools and safe fl ying can be hampered by limited visibility and too large
wind speeds. The accessibility of a helicopter is therefore determined by the percentage
of the time that both wind speed and visibility are outside the restricted values.
6.1.2 Vessels
The use of vessels is a more cost-effi cient and probably safer way of accessing
offshore wind turbines than using helicopters. Currently, the most commonly used