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

Guideline, DLC 1.5). Furthermore, the rotor start positions shall vary from 0 deg to 90 deg (in 30 deg
steps).
Fatigue Load cases
An extreme load analysis shall be carried out for the fatigue load cases.
5.4 Design load case parameter analysis for jacket structure
The design of offshore wind turbine support structures is strongly driven by parameters relating to the external
conditions at the site at which the turbine is to be located, as well as the properties of the structure
and turbine itself. For the purposes of offshore wind turbine support structure design it is important to understand
the relative importance of the various external conditions applied to the structure in the design
process. These external conditions include the following:
• Wind speed
• Wind direction
• Wave height
• Wave period
• Wave direction
• Tidal levels
• Current velocity and profile
• Current direction
• Turbulence intensity
If all possible combinations of these factors were accounted for in the design of the structure, the number
of simulations required would be huge and exceed practical limits for the designer. In order to achieve an
economic design it is therefore important to know which of these factors are the most significant in determining
the loading on the structure, so that the amount of computational time and effort can be minimised
whilst maintaining accuracy of results.
The effects of changing the above parameters on monopile support structures are reasonably well known.
However, as suitable shallow water sites in European waters become more limited jacket support structures
are increasingly being used as a preferred solution for offshore wind turbines. Therefore, this section
uses the UpWind reference jacket support structure [54] to test the relative influence of a number of key
design load case parameters affecting offshore wind turbine jacket support structure design.
General considerations
In order to account fully for the dynamics of an offshore wind turbine the combined wind and wave loading
on the whole structure must be modelled in a non-linear time-domain simulation. This ensures that the
aerodynamic damping of wave-induced motion is properly captured, which can be an important effect. The
importance of integrated analysis for jacket structures has also been shown in [52], in which a significant
interaction between rotor rotation and local jacket brace vibration was demonstrated.
The GH Bladed software tool [9] is used to perform integrated load calculations and analysis. The UpWind
5MW reference wind turbine [52] is used for the analysis, mounted on the UpWind reference jacket support
structure [54] shown in Figure 5.5. This structure was optimized using the UpWind 50m design basis
[55], so the external conditions from this document are used as a baseline. Local joint flexibilities are not
included in the model, as it has been shown that for jacket structures modelling flexibility in the joints does
not typically affect the magnitude of forces and moments [66]. A full description of the geometry, mass and
stiffness of the jacket support structure can be found in [54].
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