Design of support structures for offshore wind turbines - Wind Power Generation and Wind Turbine Design - page 560

structures found at sea. The design is not only dependent on turbine loads and
associated overturning moment, the wave and currents add signifi cant loads too.
For the design of these structures wind, wave and current loads need to be assessed
as acting on the offshore wind turbine system as a whole.
The development of offshore wind began in the 1970s and 1980s with studies
and assessment of the potential wind resource offshore. In the 1990s several pilot
offshore wind farms were constructed in the European waters, which helped
develop knowledge and new technology. In 2002, the fi rst large offshore wind
farm Horns Rev was constructed in the North Sea off the Danish coast: 80 turbines
with an installed capacity of 160 MW. In the years that followed, other countries
followed with the construction of these large, commercial offshore wind farms.
The EU target for 2020 is to have 40 GW of installed capacity.
2 History of offshore, wind and offshore wind development
of offshore structures
2.1 The origin of “integrated design” in offshore wind energy
During the 1970s, 1980s and early 1990s, a number of studies were conducted in
the fi eld of offshore wind energy. Offshore and shipbuilding as well as renewable
energy groups drafted reports on how to effectively harness the offshore wind
energy potential. The fi rst designs were mainly based on the multi-megawatt pro-
totype turbines built in the 1970s: 3 MW and more. The structures were large,
heavy and stiff: based on the accumulated experience of offshore construction in
the North Sea for oil & gas exploitation. Figure 1 shows examples of a design from
the British RES study and a Heerema tripod design [1, 2].
The design did incorporate combined wind and wave loading, but only on a basic
level for extreme load case calculations. The stiffness of the structure prevented
heavy dynamic response, so fatigue was not a big issue. For the subject operation
and maintenance a direct copy of offshore platforms was made: the addition of a
complete helicopter deck.
Figure 1: Offshore wind turbine design from the RES and the Heerema study.