La globalización y sus escalas

101 This section gives requirements for combination of environmental loads in the operational condition. 102 The requirements refer to characteristic wind turbine loads based on an investigation of the load cases

specified in Tables E1 and E3.

103 For design against the ULS, the characteristic environmental load effect shall be taken as the 98%

quantile in the distribution of the annual maximum environmental load effect, i.e. it is the load effect whose return period is 50 years, and whose associated probability of exceedance is 0.02. When the load effect is the result of the simultaneous occurrence of two or more environmental load processes, these requirements to the characteristic load effect apply to the combined load effect from the concurrently acting load processes. The subsequent items specify how concurrently acting environmental loads can be combined to arrive at the required characteristic combined load effect.

104 Environmental loads are loads exerted by the environments that surround the structure. Typical

environments are wind, waves, current, and ice, but other environments may also be thought of such as temperature and ship traffic. Each environment is usually characterized by an intensity parameter. Wind is usually characterized by the 10-minute mean wind speed, waves by the significant wave height, current by the mean current, and ice by the ice thickness.

F 200 Environmental states

201 Environmental states are defined as short-term environmental conditions of approximately constant

intensity parameters. The typical duration of an environmental state is 10 minutes or one hour. The long-term variability of multiple intensity parameters representative of multiple, concurrently active load environments can be represented by a scattergram or by a joint probability distribution function including information about load direction.

F 300 Environmental contours

301 An environmental contour is a contour drawn through a set of environmental states on a scattergram or

The environmental states defined by the contour are states whose common quality is that the probability of a more rare environmental state is p = T_S/T_R where T_S is the duration of the environmental state and T_R is a specified return period.

Guidance note:

The idea of the environmental contour is that the environmental state whose return period is TR is located somewhere along the environmental contour defined based on TR. When only one environmental process is active, the environmental contour reduces to a point on a one-dimensional probability density plot for the intensity parameter of the process in question, and the value of the intensity in this point becomes equal to the value whose return period is TR.

For an offshore wind turbine, the wind process and the wave process are two typical concurrent environmental processes. The 10-minute mean wind speed U10 represents the intensity of the wind process, and the significant wave height HS represents the intensity of the wave process. The joint probability distribution of U10 and HS can be represented in terms of the cumulative distribution function FU10 for U10 and the cumulative distribution function FHS|U10 for HS conditional on U10. A first-order approximation to the environmental contour for return period TR can be obtained as the infinite number of solutions (U10, HS) to the following equation

valid for TS < TR

in which Φ –1 _{denotes the inverse of the standard normal cumulative distribution function.}

The environmental contour whose associated return period is 50 years is useful for finding the 50-year load effect in the wind turbine structure when the assumption can be made that the 50-year load effect occurs during the 50-year environmental state. When this assumption can be made, the 50-year load effect can be estimated by some high quantile, such as the 90% quantile, in the conditional distribution of the load effect in that environmental state among the environmental states of duration TS along the 50-year environmental contour that produces the largest load effect. The environmental state is characterised by a specific duration, e.g. one hour. Whenever data for U10 and HS refer to reference periods which are different from this duration, appropriate conversions of these data to the specified environmental state duration must be carried out.

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401 In a short-term period with a combination of waves and fluctuating wind, the individual variations of the

two load processes about their respective means can be assumed uncorrelated. This assumption can be made regardless of the intensities and directions of the load processes, and regardless of possible correlations between intensities and between directions.

402 Two methods for combination of wind load and wave load are given in this standard:

— Linear combination of wind load and wave load, or of wind load effects and wave load effects, see F500. — Combination of wind load and wave load by simulation, see F600.

403 The load combination methods presented in F500 and F600 and the load combinations specified in F700

are expressed in terms of combinations of wind load effects, wave load effects and possible other load effects. This corresponds to design according to Approach (1) in Sec.2 E202. For design according to Approach (2) in Sec.2 E202, the term “load effect” in F500, F600 and F700 shall be interpreted as “load” such that “design loads” are produced by the prescribed combination procedures rather than “design load effects”. Following Approach (2), the design load effects then result from structural analyses for these design loads.

F 500 Linear combinations of wind load and wave load

501 The combined load effect in the structure due to concurrent wind and wave loads may be calculated by

combining the separately calculated wind load effect and the separately calculated wave load effect by linear superposition. This method may be applied to concept evaluations and in some cases also to load calculations for final design, for example in shallow water or when it can be demonstrated that there is no particular dynamic effect from waves, wind, ice or combinations thereof.

According to the linear combination format presented in Sec.2, the design combined load effect is expressed as

in which S_wind,k denotes the characteristic wind load effect and S_wave,k denotes the characteristic wave load effect. It is a prerequisite for using this approach to determine the design combined load effect that the separately calculated value of the characteristic wave load effect S_wind,k is obtained for realistic assumptions about the equivalent damping that results from the structural damping and the aerodynamic damping. The equivalent damping depends on the following conditions related to the wind turbine and the wind load on the turbine:

— whether the wind turbine is exposed to wind or not

) 1 ( ))) ( ( ( ))) ( ( ( 1 _{10 10} 2 1 _{| 10} 2 1 R S S U H U T T H F U F + Φ _S =Φ − Φ− − − k wave f k wind f d S S S =γ _{1 ,} +γ _{2 ,}

— whether the wind turbine is in operation or is parked

— whether the wind turbine is a pitch-regulated turbine or a stall-regulated turbine — the direction of the wind loading relative to the direction of the wave loading.

Correct assumptions for the wind turbine and the wind load shall be made according to this list. The equivalent damping shall be determined in correspondence with these assumptions. Structural analyses by an adequate structural analysis model and based on this equivalent damping shall then be used to determine the characteristic wave load effect S_wave,k. The damping from the wind turbine should preferably be calculated directly in an integrated model.

Guidance note:

When the characteristic load effect S_wave,k is defined as the load effect whose return period is T_R, the determination of S_wave,k as a quantile in the distribution of the annual maximum load effect may prove cumbersome and involve a large number of structural analyses to be carried out before contributions to this distribution from all important sea states have been included.

When the assumption can be made that Swave,k occurs during the particular sea state of duration TS whose significant wave height HS has a return period equal to TR, then Swave,k may be estimated by the expected value of the maximum load effect during this sea state, and the analytical efforts needed may become considerably reduced. The assumption that Swave,k occurs in the sea state whose return period is TR is often reasonable, unless sea states exist for which the structure becomes more dynamically excited than by this particular sea state, for example sea states involving wave trains whose periods are close to integer multiples of the natural period of the structure.

When the structural analysis involves executions of a number of simulations of the maximum load effect that occurs during the sea state whose significant wave height has a return period TR, then Swave,k shall be estimated by the mean of these simulated maximum load effects.

The wind loads in the wind direction during idling and with the yaw system in function will be quite small and will consist mainly of drag on the tower and the nacelle cover. During this condition it is implied that the blades are pitched such that the blade profiles point in the direction up against the wind or in the wind direction. The largest wind loads in this condition will be the blade loads that act perpendicular to the wind direction.

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by simulation

601 The combined load effect in the structure due to concurrent wind and wave loads may alternatively be

calculated by direct simulation. This approach is based on structural analyses in the time domain for simultaneously applied simulated time series of the wind load and the wave load. By this approach, simulated time series of the combined load effect results, from which the characteristic combined load effect S_k is interpreted.

Guidance note:

The approach requires that a global structural analysis model is established, e.g. in the form of a beam-element based frame model, to which load series from several simultaneously acting load processes can be applied. Although this is here exemplified for two concurrently acting load processes, viz. wind loads and wave loads, this can be generalised to include also other concurrent load processes.

When the characteristic load effect S_k is defined as the load effect whose return period is T_R, the determination of S_k as a quantile in the distribution of the annual maximum load effect may prove cumbersome and involve a large number of structural analyses to be carried out before contributions to this distribution from all important environmental states have been included.

When the assumption can be made that Sk occurs during an environmental state of duration TS associated with a return period TR, then Sk may be estimated by the expected value of the maximum load effect during such an environmental state, and the analytical efforts needed may become considerably reduced. Under this assumption, Sk can be estimated by the expected value of the maximum load effect that can be found among the environmental states on the environmental contour whose associated return period is TR.

To simulate one realisation of the maximum load effect along the environmental contour whose associated return period is T_R, one structural simulation analysis is carried out for each environmental state along the environmental contour and one maximum load effect results for each one of these states. The same seed needs to be applied for each environmental state investigated this way. A following search along the contour will identify the sought-after realisation of the maximum load effect. In practice, it will suffice to carry out structural simulation analyses only for a limited number of environmental states along a part of the environmental contour. The procedure is repeated for a number of different seeds, and a corresponding number of maximum load effect realisations are obtained. The sought- after characteristic load effect S_k is estimated by the mean of these simulated maximum load effects.

When dynamic simulations utilising a structural dynamics model are used to calculate load effects, the total period of load effect data simulated shall be long enough to ensure statistical reliability of the estimate of the sought-after maximum load effect. At least six ten-minute stochastic realisations (or a continuous 60-minute period) shall be required for each 10-minute mean, hub-height wind speed considered in the simulations. Since the initial conditions used for the dynamic simulations typically have an effect on the load statistics during the beginning of the simulation period, the first 5 seconds of data (or longer if necessary) shall be eliminated from consideration in any analysis interval involving turbulent wind input.

The wind loads in the wind direction during idling and with the yaw system in function will be quite small and will consist mainly of drag on the tower and the nacelle cover. During this condition it is implied that the blades are pitched such that the blade profiles point in the direction up against the wind or in the wind direction. The largest wind loads in this condition will be the blade loads that act perpendicular to the wind direction.

The wave field must be simulated by applying a valid wave theory according to Sec.3. Simulation using linear wave theory (Airy theory) in shallow waters may significantly underestimate the wave loads.

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701 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 F1 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 F1 forms the basis for determination of the design combined load effect according to the linear combination format in item 501. Table F1 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 F1 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 F1 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 F1 replace the more cumbersome searches for the characteristic load effect on environmental contours as described in item 301.

When the direction of the loading is an important issue, it may be of particular relevance to maintain that the return periods of Table F1 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 F1, the most unfavourable value among the two shall be used for the 50-year water level.

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702 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 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.

Table F1 Proposed load combinations for load calculations according to item 501

Environmental load type and return period to define characteristic value of corresponding load effect

Limit state combinatioLoad

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

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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