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*Parameters |
 
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*Parameters
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*Parameters |
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To define miscellaneous parameters for use in timetrace postprocessing.
Refer to Timetrace Postprocessing for further information on this feature.
A block of two or three lines, the first of which is mandatory. The first line specifies data relating to the calculation of statistics and spectra by the Timetrace Postprocessor.
GENERAL=[Start Time] [, Time Step] [, No. of Ensembles]
Start Time defaults to the analysis start time. A Time Step value is required in a random sea dynamic analysis with a variable time-step, while it is ignored in the case of an analysis with a fixed step. A No. of Ensembles is required in a random sea dynamic analysis, while it is ignored in the case of a regular wave analysis.
The second line, which is optional, specifies the procedure to be used in the computation of extreme values by the Timetrace Postprocessor.
[EXTREME=Calculation Procedure]
The third line is required if the line with EXTREME= is present. The format of the third line depends on the Calculation Procedure specified – the options are RAYLEIGH (the default) and WEIBULL. For RAYLEIGH extrema postprocessing, the third line takes the following format:
DATA=[Storm Duration] [, Probability]
For WEIBULL extrema postprocessing, the third line takes the following format:
DATA=[Storm Duration] [, Probability] [, Threshold]
Storm Duration defaults to 3 hours and Probability to 0.01 (1%). Threshold defaults to 1.
To specify the start time for the calculation of response statistics.
Input: |
Description |
Start Time: |
The start time for the calculation of statistics. Any values before this time are excluded. Use this entry to exclude initial transients from statistics calculations. |
(a)The use of this table is optional. By default, the start time for the calculation of statistics is the first timetrace output time.
(b)The entry you specify here is also used as the start time for the calculation of response spectra in a random sea analysis.
To define parameters relating to the calculation of response spectra.
Input: |
Description |
No. of Ensembles: |
The number of ensembles to be used in calculating spectra. See Note (a). |
Time Step: |
The time step to be used when calculating spectra from the results of a random sea dynamic analysis with a variable time step. See Note (b). If the analysis used a fixed time step then this entry is not required, and any value you specify is ignored. |
(a)The procedure used by Flexcom in calculating spectra is as follows. Firstly, the output timetrace is divided equally into a number of smaller timetraces or ensembles. A spectrum for each ensemble is then calculated using the Fast Fourier Transform (FFT) algorithm. Finally, the actual spectrum to be output is found as an average of the spectra calculated for each ensemble. This standard procedure minimises the statistical error associated with the FFT process. You specify the number of ensembles to be used in this process using the No. of Ensembles entry above. This value should always be greater than 1.
(b)The FFT algorithm requires a record with a fixed time step. When you perform a variable step analysis, obviously such a record is not available, and so Flexcom must synthesise one by interpolating from the variable step record. The Time Step entry tells Flexcom what time step to use in the synthesised record.
To specify parameter values to be used in calculating extreme values with the Rayleigh distribution.
Input: |
Description |
Storm Duration: |
The storm duration in hours to be used when calculating extreme values. The default is 3 hours. |
Probability: |
The exceedance probability to be used when calculating extreme values. The default is 0.01 (= 1%). |
(a)Refer to Extreme Values for a detailed discussion of all aspects of specifying data relating to extreme value postprocessing.
(b)Note that the Extreme Values – Rayleigh and Extreme Values – Weibull tables are mutually exclusive. Specification of data in either table indicates the extreme value postprocessing approach which you wish to use.
To specify parameter values to be used in calculating extreme values with the Weibull distribution.
Input: |
Description |
Duration: |
The duration in hours to be used when calculating extreme values. The default is 3 hours |
Probability: |
The exceedance probability to be used when calculating extreme values. The default is 0.01 (= 1%). |
Threshold: |
The proportion of largest maxima /smallest minima to be used when calculating extreme values. The default is 1. See Note (b). |
(a)Refer to Extreme Values for a detailed discussion of all aspects of specifying data relating to extreme value postprocessing.
(b)A threshold equal to 1 denotes all the maxima/minima in the timetrace will be used to calculate the extreme values. A threshold equal to 1/n (e.g. 1/3) uses the upper 1/n of largest maxima and lowest 1/n of smallest minima to calculate the extreme value.
(c)Note that the Extreme Values – Rayleigh and Extreme Values – Weibull tables are mutually exclusive. Specification of data in either table indicates the extreme value postprocessing approach which you wish to use.