Final Report 4.1

UPWIND
Page 44 of 146
Figure 4.7: Normalized tower base overturning moment vs. upstream turbine yaw angle [30]
Figure 4.7 shows an example from the European TOPFARM project for wake condition at a
turbine distance of 6D and the effect on the tower base overturning damage equivalent fatigue
load for an SN exponent of 4 as relative change in loading compared to the free-flow conditions.
The plot illustrates that for full wake conditions, here at an x-axis value of 0 degrees which
corresponds to parallel rotors for the upstream and downstream turbine, the loading is
increased by a factor of 1.5 If the upstream turbine is yawing and thus the downwind turbine
experiences only a partly wake, the loading increases. The curve reaches its maximum with a
factor of 1.65 for conditions where the wind direction that contains the wake is at approximately
-8 degrees, which corresponds in this example to the conditions where the center of the
meandering wake is at the blade tip. This clearly indicates the importance of wake effects.
In conclusion it can be stated that for an optimized wind farm layout, several parameters have to
be taken into account. For an optimal layout in terms of loading, the selected sites and the
shape of the wind farm have the major impact. In order to reduce wake-induced loadings, the
wind farm layout target has to be to obtain as few as possible wake situations or at least highest
possible turbine spacings in the prevailing wind directions. However, for a final cost-effective
design solution, the cost of energy is leading the decisions and here aspects like the electrical
infrastructure play another important role [32].
4.5 Robust design
Within this work, the main emphasis is on advanced turbine design and control concepts in
order to achieve a cost-effective offshore wind turbine design. In order to complete the
conceptual evaluations, an opposite concept has also to be discussed. This concept excludes
all advanced systems and reduces the amount of components in the turbine. Therefore this
concept is called robust design. Due to the lower amount of components, less failure shall occur
or the investment costs shall be lower as well as costs for operations and maintenance. These
aspects are defined as design according to RAMS – Reliability, Availability, Maintainability and
Serviceability [33], where each of the four criteria shall be maximized. Several pre-studies have