Offshore wind turbine design - Wind Power Generation and Wind Turbine Design - page 377

most suitable foundation types for approximately 15 m sea depth in 1996-1997.
This was viewed at that time as the next offshore foundation challenge. The study
identifi ed the steel monopile as the most suitable for this depth. The monopile
installation requires hammering a steel pile into the soil. Then a so-called “tran-
sition piece” is grouted on top to correct for any lack of pile verticality. Tower
and turbine top components are erected on top of the transition piece. Since its
introduction in Europe, the monopile has become standard for sea depth rang-
ing from 10 m up to approximately 20 m. Figure 6 shows the pile hammering
operation at the Arklow Bay, Ireland offshore power plant that features seven
GE 3.6 MW wind turbines.
As water depths increases, the capability of monopile is stretched to the limit
primarily because of the manufacturability and constructability limitations. For
sites deeper than 25 m, the monopile diameter reaches 5.5 m with thickness up to
90 mm, which could weigh upward of 250 metric tons. This level of dimensions
and weights are near or exceeding the maximum capability of the fabrication
factories, hammers and the installation vessels across the world.
To take advantage of the offshore wind energy in U.S., the wind industry must
break the 25-m barrier in a profound way with technology development.
One recent trend practiced by some EU OEMs is to use tripod, quadpod and in
general jacket type foundation structure to extend the service water depth to 50 m.
These structures utilize O&G platform and shipbuilding welding technologies and
tend to be very expensive in fabrication, transportation, and installation. We still
need to wait and see if the new effort such as the REpower 5M jacket foundation
in Beatrice as shown in Fig. 7 [12] and the new innovations such as the REpower
5M casted steel multiple-direction joints depicted in Fig. 8 will help to reduce the
cost drastically and make this type of foundations economical.
Figure 6 : Pile hammering at Arklow [ 3 ].