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

Rainflow evaluation
The load cases shall be evaluated in rainflow counts according to two different assumptions for the availability
of the turbine (100% and 85% or equivalent conservative value based on failure statistics). Combined
with the two support orientations, that means four rainflow counts and weighting analyses have to be
performed. The setups are as follows:
1. 100 % availability: rainflow count assuming DLC 1.2 and DLC 6.4 for the complete lifetime, Jacket orientation
0°
2. 100 % availability: rainflow count assuming DLC 1.2 and DLC 6.4 for the complete lifetime, Jacket orientation
45°
3. 85% availability: rainflow count assuming 85 % of lifetime DLC 1.2 and DLC 6.4, 15 % of lifetime DLC
7.2, Jacket orientation 0°
4. 85% availability: rainflow count assuming 85 % of lifetime DLC 1.2 and DLC 6.4,15 % of lifetime DLC
7.2, Jacket orientation 45°
The highest resulting loads shall be used for jacket predesign.
Step 2: Wind and waves misaligned
The load case definitions of step two are, but for the wind-wave misalignment, equal to the load case definitions
given in step one. As step two considers site specific directional distribution with a defined orientation
of the support structure regarding north, the support structure orientation must not be varied. For DLC
7.2 no further load cases have to be set up, as assuming wind aligned with waves is conservative with
regards to missing aerodynamic damping. Besides that, the IEC standard does not request consideration
of wind-wave misalignment for DLC 7.2.
The rainflow evaluation of step 2 includes both the dimensioning step 1 rainflow extended by the directional
distributions of step 2 simulations.
Wind-wave misalignment can be considered via several approaches of different complexity, the wind bin
based wind-wave rose combination (exact method) or methods of reduced complexity. The exact method
assumes that a wind direction – wave direction scatter diagram exists.
This section additionally addresses an approach based on the assumption that wind-wave misalignment is
independent of wind speed and wind direction (wind independent method). As wind and wave data for the
present site are sufficient to include wind-wave misalignment via the exact method, a comparison to the
results of the less complex wind independent method may be useful.
Exact method
The exact method assumes wind-wave misalignment to vary with three parameters: wind speed bin, wind
direction and wave direction. Thus, one misalignment has to be computed for each wind bin, wind direction
and wave direction combination, resulting in 4032 simulations (2 support structure orientations · 14
bins · 12 wind sectors · 12 wave sectors = 4032) for DLC 1.2 and DLC 6.4 if wind and wave seeds and
wind misalignment are varied within. It is assumed that one seed for every single case is statistically sufficient
due to the high number of load cases.
Wind independent method
To reduce the sheer amount of simulations and the effort of simulation setup, the wind independent
method is based on the assumption that wind-wave misalignment is independent of wind speed and wind
direction. First, wind and wave roses are tabulated as probability distributions and the misalignments with
according probabilities of misalignments are derived. This is illustrated in the following table:
54