Final report for WP4.3: Enhancement of design methods ... - Upwind

ated eigenfrequency is shifted into the upper 3P range - was formulated in [20]. This is not confirmed for
the torsion, as the extreme values of the torsional moment as well as the corresponding DEL in the tower
base are even reduced with the super-elements. The side-to-side loads are increased in terms of extremes,
but the more important corresponding DELs do not show results that are that clear.
For the DELs in the tripod itself, investigated at 11 different output positions, the results are not straightforward
to interpret at a first glance. Some loads increase, others decrease and some remain unchanged.
The step from a beam to a super-element model activates effects increasing or decreasing local loads
and - dependent on the position and load situation - one or the other effect may dominate. However the
changes that do occur are significant, and the load paths in the statically over-determined structure seem
to change globally. However, the largest changes of DEL with significant values show the clear tendency
of decreased moments using the super-element model and these results show again the large differences
in the DEL calculated with the two models.
To conclude, more detailed studies are essential to fully understand the influence of the super-element
approach for the loads simulation of OWT with tripod structures under realistic conditions. But it is obvious
that the more detailed joint modeling with super-elements in the fully-coupled simulation leads to remarkable
differences in the results and should therefore be applied for this type of structure.
5. Development of design requirements
As offshore wind farm sites become larger and there is greater site variability, the optimization of support
structure design and efficiency of the design process becomes increasingly important. In terms of the IEC
61400-3 international design standard both normative requirements and informative methodologies need
to be updated and improved in accordance with the advancement of the industry.
Section 5.1 presents a summary of recommendations for the development of the international IEC 61400-
3 design standard for bottom-mounted offshore wind turbines. As part of WP4 an interim review of the first
edition of the standard was performed [22], including recommendations for the development of future editions.
A review of various models for irregular, non-linear waves suitable for design purposes was also
performed, in order to judge their relevance for future offshore wind farms (see [21]).
Section 5.2 presents the results of reliability-based investigations into the required safety factor / Fatigue
Design Factor (FDF) values to be used for fatigue design of steel sub-structures for offshore wind turbines.
Design and limit state equations are formulated and stochastic models for the uncertain strength
and load parameters are described.
Section 5.3 presents recommendations for the implementation of a reduced set of offshore wind turbine
design load cases according to the IEC 61400-3 standard for the preliminary design of jacket support
structures. The number of design load cases required for full offshore support structure design is potentially
very large, and this can become even more impractical when complex multiple-member support
structures are considered.
Section 5.4 presents results from a design load case parameter analysis performed for a jacket support
structure, in order to investigate the driving fatigue and extreme load cases for this support structure type.
5.1 Input to development of IEC 61400-3 standard
The IEC 61400-3 standard [69] provides a set of international guidelines for the design of bottom-mounted
offshore wind turbines. The standard, which was developed by a working group of international experts,
provides design requirements for offshore wind turbines and their support structures, and provides much
needed international consistency in a rapidly growing industry.
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