4.6.7.1 When information is not available to produce the required characteristic combined load effect directly, the required characteristic combined load effect can be obtained by combining the individual characteristic load effects due to the respective individual environmental load types. Table 4-9 specifies a list of load cases that shall be considered when this approach is followed, thereby to ensure that the required characteristic combined load effect, defined as the combined load effect with a return period of 50 years, is obtained for the design. Each load case is defined as the combination of two or more environmental load types. For each load type in the load combination of a particular load case, the table specifies the characteristic value of the corresponding, separately determined load effect. The characteristic value is specified in terms of the return period.
Guidance note:
Table 4-9 forms the basis for determination of the design combined load effect according to the linear combination format in [4.6.5]. Table 4-9 refers to a characteristic combined load effect with a return period of 50 years and shall be used in conjunction with load factors specified in Sec.5.
When it can be assumed that a load effect whose return period is TR occurs during the environmental state whose return period is TR, then the tabulated recurrence values in Table 4-9 can be used as the return period for the load intensity parameter for the load type that causes the particular load effect in question. With this interpretation, Table 4-9 may be used as the basis for determination of the characteristic combined load effect by linear combination, in which case the analyses for the particular load cases of Table 4-9 replace the more cumbersome searches for the characteristic load effect on environmental contours as described in [4.6.3].
When the direction of the loading is an important issue, it may be of particular relevance to maintain that the return periods of Table 4-9 refer to load effects rather than to load intensities.
For determination of the 50-year water level, two values shall be considered, viz. the high water level which is the 98% quantile in the distribution of the annual maximum water level and the low water level which is the 2% quantile in the distribution of the annual minimum water level. For each load combination in Table 4-9, the most unfavourable value among the two shall be used for the 50-year water level.
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4.6.7.2 Every time a load combination is investigated, which contains a load effect contribution from wind load, the load combination shall be analysed for two different assumptions about the state of the wind turbine: — wind turbine in operation (power production)
— parked wind turbine (idling or standing still).
The largest load effect resulting from the corresponding two analyses shall be used for design. Guidance note:
It will usually not be clear beforehand which of the two assumptions will produce the largest load effect, even if the blades of the parked turbine are put in the braking position to minimise the wind loads.
In a ULS situation where the characteristic wind load effect is to be taken as the 50-year wind load effect, the calculation for the wind turbine in operation will correspond to calculation of the load effect for a wind climate whose Table 4-9 Proposed load combinations for load calculations according to [4.6.5]
Environmental load type and return period to define characteristic value of corresponding load effect
Limit state
Load combinatio
n
Wind Waves Current Ice Water level
ULS
1 50 years 5 years 5 years 50 years
2 5 years 50 years 5 years 50 years
3 5 years 5 years 50 years 50 years
4 5 years 5 years 50 years Mean water level
intensity is somewhere between the cut-in wind speed and the cut-out wind speed. For stall-regulated wind turbines, the cut-out wind speed dominates the extreme operational forces. For pitch-regulated wind turbines, the extreme operational forces occur for wind climates whose intensities are near the 10-minute mean wind speed where regulation starts, typically 13 to 14 m/s.
For the parked wind turbine, the calculation in a ULS situation will correspond to the calculation of the 50-year wind load effect as if the wind turbine would be in the parked condition during its entire design life.
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4.6.7.3 When it can be established as unlikely that the wind turbine will be in operation during the wave, ice, current and/or water level conditions that form part of a load combination under investigation, the requirement of [4.6.7.2] to analyse the load combination for the assumption of wind turbine in operation may be too strict. When such an unlikely situation is encountered, the fulfilment of this requirement of [4.6.7.2] may be deviated from in the following manner: The load combination under investigation shall still be analysed for the assumption of wind turbine in operation; however, the requirements to the return periods of the wave, ice, current and water level conditions that the wind load effect is combined with may be relaxed and set lower than the values specified in Table 4-9 for the particular load combination, as long as it can be documented that the return period for the resulting combined load effect does not fall below 50 years.
Guidance note:
When the fetch is limited and wind and waves have the same direction in severe storms, then the wind climate intensity is likely to reach its extreme maximum at the same time as the wave climate intensity reaches its extreme maximum, and it may be unlikely to see wind speeds below the cut-out wind speed during the presence of the 50-year wave climate. Likewise, it may be unlikely to see the 50-year wave climate during operation of the wind turbine. When the topography, e.g. in terms of a nearby coastline, forces the extreme maximum of the wind climate to take place at a different time than the extreme maximum of the wave climate intensity, then it may be likely to see wind speeds below the cut-out wind speed during the presence of the 50-year wave climate. Likewise, it may be likely to see the 50-year wave climate during operation of the wind turbine.
When a large fetch is present, there may be a phase difference between the occurrence of the extreme maximum of the wind climate intensity and the extreme maximum of the wave climate intensity, and it may be likely to see wind speeds below the cut-out wind speed during the presence of the 50-year wave climate. Likewise, it may be likely to see the 50-year wave climate during operation of the wind turbine.
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4.6.7.4 Every time a load case is investigated, which contains a load effect contribution from ice loads, loads from moving ice shall be considered as well as loads from fast-frozen ice and loads due to temperature fluctuations in the ice.
4.6.7.5 Load combination No. 5 in Table 4-9 is of relevance for structures in waters which are covered by ice every year. Investigations for Load combination No. 5 in Table 4-9 can be waived for structures in waters which are covered by ice less frequently than every year.
4.6.7.6 When a load case is investigated, which contains a load effect contribution from wave loads, loads from wave trains in less severe sea states than the sea state of the specified return period shall be considered if these loads prove to produce a larger load effect than the sea state of the specified return period.
Guidance note:
Dynamic effects may cause less severe sea states than the sea state of the specified return period to produce more severe load effects, e.g. if these sea states imply wave trains arriving at the wind turbine structure with frequencies which coincide with a frequency of one of the natural vibration modes of the structure. The possibility that waves break at the wind turbine structure may play a role in this context and should be included in the considerations.
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4.6.7.7 Co-directionality of wind and waves may be assumed for calculation of the wave loads acting on the support structure for all design cases except those corresponding to the wind turbine in a parked (standstill or idling) design situation. The misalignment of wind and wave directions in the parked situation is to be accounted for.
Guidance note:
Allowance for short term deviations from the mean wind direction in the parked situation should be made by assuming a constant yaw misalignment. It is recommended to apply a yaw misalignment of ±15°.
In areas where swell may be expected, special attention needs to be given to swell, which has a low correlation with wind speed and wind direction.
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4.6.7.8 The multi-directionality of the wind and the waves may in some cases have an important influence on the loads acting on the support structure, depending primarily on whether the structure is axisymmetric or not. For some design load cases the load calculations may be undertaken by assuming that the wind and the waves
are acting co-directionally from a single, worst case direction.
4.6.7.9 Characteristic extreme wind load effects are in this standard defined as wind load effects with a 50- year return period. 5-year wind load effects form part of some load combinations. When only wind load effects with a 100-year return period are available, the 100-year wind load effects have to be converted to 50-year values. This can be done by multiplication by a conversion factor. Likewise, to the extent that 5-year wind load effects are needed in load combinations and only 50-year values are available, the 50-year values have to be converted to 5-year values for use in these load combinations.
Guidance note:
The ratio Fwind,100/Fwind,50 between the 100- and 50-year wind load effects depends on the coefficient of variation in the distribution of the annual maximum wind load and can be used as a conversion factor to achieve the 50-year wind load effect Fwind,50 in cases where only the 100-year value Fwind,100 is available. Unless data indicate otherwise, the ratio Fwind,100/Fwind,50 can be taken from Table 4-10. Table 4-10 also gives the ratio Fwind,5/Fwind,50 between the 5- year wind load effect Fwind,5 and the 50-year wind load effect Fwind,50. This is useful in some load combinations that require the 5-year wind load effect. Table 4-10 is based on an assumption of a Gumbel-distributed annual maximum wind load effect.
The conversion factors are given as functions of the coefficient of variation of the annual maximum wind load effect. There is no requirement in this standard to document this coefficient of variation.
Note that use of the conversion factor Fwind,5/Fwind,50 given in Table 4-10 to obtain the 5-year wind load effect from the 50-year wind load effect will be nonconservative if the distribution of the annual maximum wind load effect is not a Gumbel distribution and has a less heavy upper tail than the Gumbel distribution.
Note also that for a particular wind turbine, the coefficient of variation of the annual maximum wind load effect may be different depending on whether the wind turbine is located on an offshore location or on an onshore location. For offshore wind turbines the coefficient of variation is assumed to have a value of approximately 20 to 30%.
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