Offshore wind turbine design - Wind Power Generation and Wind Turbine Design - page 366

the NEG MICON merger is in the works. GE holds a 3.6 MW turbine specifi cally
for the offshore market and just acquired a 3.5 MW direct-drive product through
merger with SCANWIND. REpower, Multibrid, and Enercon are known to be
testing designs in the 5-7 MW and >100 m rotor diameter range. Nordex has a
2.5 MW N90 offshore product. On the more advanced pace, Siemens is dem-
onstrating their 2.3 MW 93 m rotor turbine on a fl oating foundation suitable for
deep water and has a 3.6 MW 120 m rotor turbine in the pocket. Gamesa recently
introduced an advanced compact G10X 128 m rotor 4.5 MW turbine with a 120 m
concrete tower that might serve as a low cost offshore wind turbine in the future.
The current industry trend to increase turbine power and blade sizes is based on
two fundamental assumptions. First, the trend assumes that the cost of foundations
and other balance-of-plant items do not increase linearly with the turbine's power.
Second, the assumption is made that the cost of operating and maintaining a
smaller number of bigger turbines is lower than operating and maintaining large
number of smaller turbines.
In reality, offshore plants break both assumptions almost completely. With exist-
ing technology, foundation costs increases signifi cantly as the plant is placed at
deeper sites. Similarly, operations and maintenance costs also increase as plants
are placed further offshore. Therefore, when considering offshore plants, it is no
longer true that industry should blindly head toward larger turbines. With offshore
plants, turbine size is not the only factor of importance unless the push toward
increasing turbine sizes is coupled with signifi cant reductions in foundation,
operation and maintenance costs, as well as improvements in reliability and
availability, to achieve the potential and viability of offshore wind power plants.

4 Offshore-specifi c design challenges

Signifi cant wind turbine design innovations are required for offshore environment.
Characterizing the potential development barriers and identifying the technologies
to overcome them will be the important aspects of these innovations. Offshore
wind plants present challenges that can be broadly grouped into three categories:

• Economic challenges

• 25-m barrier challenge

• Design envelope challenge

4.1 Economic challenges

The capital cost of offshore wind plant foundations increases as sea depth increases.
While the current onshore wind plant foundations represent around 10% or less
of the total overall capital cost, the foundation cost jumps to 20% simply by plac-
ing the turbine offshore. As shown in Fig. 2, the foundation contribution to the
overall capital cost steadily increases as the sea depth increases. As foundations
do not directly contribute to any increase in generating power, the increased cost
of foundations directly increases the cost of electricity.