Environmental Engineering Reference
In-Depth Information
4.6.2 Design situations and load cases
The load cases relevant to the design must be defined based on the conditions at the
site and the design principles as well as the operational and safety concept of the
offshore wind turbine [57]. The load cases must include all those cases necessary for
verifying the structural integrity of the wind turbine structure. Basically, we distin-
guish between loads for verifying the structural durability and extreme loads for
verifying the general stability (strength, stability, external stability). The structural
durability loads must be representative of the operation of the offshore wind turbine
over a design working life of at least 20 years. The extreme loads must encompass all
events that lead to the highest loads when considering the probability of their
simultaneous occurrence, for example “50-year gust”, “50-year wave”, extreme
oblique angle of attack for the rotor, ship impact (service vessel), ice pressure, and
so on.
After defining and evaluating the design load cases, the loading calculations, taking
into account the complete structural dynamics, must be carried out and submitted to the
certification body/specialist for checking. The certification body/specialist will check
the plausibility of the loading assumptions and the results by comparing them with
typical calculations.
The design working life of an offshore wind turbine can be represented by many design
situations during the structural design in order to cover the most significant conditions
that experience has shown a turbine is exposed to. In principle, the design load cases for
determining the structural integrity of an offshore wind turbine structure may be
derived from the following combinations [11]:
- Normal design situations with normal external conditions
- Normal design situations with extreme external conditions
- Fault design situations with the appropriate external conditions
- Design situations for transportation, installation and maintenance work with the
appropriate external conditions
Normal external conditions refer to a return period of one year, whereas extreme
external conditions are generally based on a return period of 50 years. If there is any
connection between an extreme external condition and a fault situation, then a realistic
combination of the two should be considered as a design load case.
The external conditions are made up of combinations of wind, sea state, ice, current and
sea level conditions. Scatter diagrams from the long-term statistics, which reflect wave
heights, wave periods and wind speeds (see Section 2.5.7), should be used to analyse
the loads due to the interaction of wind and waves. The extreme external conditions
(wind, sea state, current, sea ice and sea level) should be combined in such a way that
they result in extreme environmental effects on the structure with the specified
combined return period (1 year or 50 years).
If no long-term statistical data is available, then one and the same storm event with a 3 h
duration can be assumed for the extreme external conditions. Both the average wind
speed and the average current velocity plus the significant wave height are extrapolated
