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

Further, the effect of possible inspections during the design lifetime is investigated. It is assumed that inspections
are performed with three different levels of reliability modelled by exponential POD curves with
mean length of detectable cracks equal to 2, 5 and 10 mm. The results show that significant reductions in
required FDF values can be obtained if a good inspection quality is used and for example 3 inspections
are performed during the design lifetime. The costs of inspections and possible repairs should be compared
to the cost reductions in the initial material costs due to lower FDF values. It is noted that due to
correlations between similar fatigue critical details in substructures in wind farms information from inspection
of one substructure can be used to update the reliability assessment of nearby substructures.
The results shown in this section can be considered as representative, but more examples should be considered
before implementation in standards. Especially examples where both wind and wave loads are
important should be investigated.
5.3 Recommendations for offshore design load case implementation
The following section gives guidance for the implementation of offshore wind turbine design load cases
according to the IEC 61400-3 standard for the preliminary design of jacket support structures. To achieve
cost-effective support structure design it is important to make the design process as efficient as possible.
This is particularly true for the more complex braced support structures used in deeper water, as multiple
wind and wave directionality has also to be taken into account. Therefore a reduction in the number of
simulations required for the preliminary design stages can result in big savings in time, and also in cost.
The set of design load cases (DLC) listed in this section is intended for predesign with focus on a jacket
sub-structure. It is assumed that due to the space frame structure of the jacket, loads and thus wind and
wave orientation influence the overall design.
5.3.1 Fatigue Load Analysis
For fatigue predesign two methodologies may be applied:
1. Simplified method considering reduced directionality but two support structure orientations
2. Consideration of site environmental conditions for directional wind and wave distribution and directional
load analysis.
As described, the fatigue load analysis is divided into two steps, resulting in two load case sets for two
independent fatigue analyses. The first step represents a simplified, fast but conservative first approach
assuming wind and waves are aligned. The second approach assumes wind-wave misalignment. Thus, it
more precisely represents the site conditions while it reduces the amount of conservativeness by increasing
the computational effort.
Neither of the two approaches considers transient DLCs (start, stop, fault events) as they may be left out
for a preliminary load analysis that focuses on jacket predesign.
Step 1: Wind and waves in line
Two support structure orientations (0° and 45°) shall be analysed. The support structure orientation is defined
with regard to the rotor axis, while the rotor axis is assumed collinear with the wind direction (excluding
8 deg wind misalignment). For conservative simplicity it is assumed that the rotor axis points north.
The support is oriented accordingly to N (0°) or NE (45°). The following figures illustrate the 0° and 45°
orientation by giving top views on the jacket.
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