Long Term Environmental Conditions |
 
|
Long Term Environmental Conditions |
|
Long-term environmental conditions at the location in question are defined in terms of (i) the wave scatter diagram, and (ii) long-term directionality data. Although these inputs are strictly only required for the RAOs mode of operation in LifeFrequency, they are central to the overall operation of the software, and even though they are not explicitly required inputs for the Stress Spectra mode, they effectively determine the choice of seastates for this mode also.
The format for inputting the scatter diagram very much reflects how the actual wave scatter diagram is usually presented. Each cell represents a particular combination of Hs and Tz, and you input into a cell the number of occurrences (typically the number of “three-hour intervals”) of that particular combination during, say, a 10 or 20-year period. You can alternatively specify the scatter diagram in terms of Hs and Tp, the wave spectrum peak period.
In principle, a fatigue analysis should involve performing a Flexcom random sea analysis for each seastate in the scatter diagram for each direction of wave approach, and then using the results to calculate fatigue damage. This is the approach adopted by the Stress Spectra mode of operation, however this might be computationally expensive. Alternatively, when you are inputting the scatter diagram in the RAOs mode of operation, as well as inputting individual numbers of occurrences, you can also divide the scatter diagram into a small number of “blocks”, and nominate a seastate (a particular combination of Hs and Tz) within each block as the representative or “reference” seastate for that block. An example of a wave scatter diagram input in this way is shown in the table below. Here the scatter diagram is divided into 12 blocks, each with a reference seastate shown highlighted. The values input and the blocking scheme are largely arbitrary.
Sample Seastate Scatter Diagram
Tz |
|||||||||||
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
||
0.5 |
136076 |
252100 |
255434 |
235832 |
|||||||
2.0 |
321287 |
334477 |
298020 |
74558 |
|||||||
3.5 |
272100 |
362849 |
383204 |
148557 |
167188 |
||||||
5.0 |
174127 |
296125 |
308157 |
275432 |
110168 |
||||||
6.5 |
175352 |
264871 |
260697 |
126533 |
|||||||
8.0 |
117157 |
233065 |
231621 |
110175 |
|||||||
Hs |
9.5 |
111420 |
143196 |
102307 |
93240 |
||||||
11.0 |
83161 |
90355 |
80073 |
||||||||
12.5 |
67966 |
52943 |
|||||||||
14.0 |
32669 |
36253 |
|||||||||
15.5 |
11986 |
20976 |
|||||||||
17.0 |
13739 |
||||||||||
18.5 |
10200 |
||||||||||
20.0 |
1520 |
279 |
Long-term directionality effects are accounted for in LifeFrequency by considering storm directions from eight compass headings. For each direction, a percentage occurrence value is specified. Obviously, the sum of all occurrences must total 100%.
•*BLOCK is used to input the fatigue analysis wave scatter diagram, including the definition of blocks and/or reference seastates as appropriate.
•*DIRECTION is used to specify long-term directionality data.
•*SPECTRUM is used to specify the wave spectrum type to use for fatigue life calculations.