Durations Durations

The purpose of a DURATION is to associate a time of exposure to an environment. With mooring line fatigue this is done directly on the &ENV command itself. For all other cases it is done in the &DATA DURATION Menu. This is done in one of two ways depending on the type of analysis performed.

For either type of analysis, the following command is used.


STRPOST

     &DATA DURATION, DURATION_NAME, PRC_NAME,  -OPTION

where the only available option is:


     -TDOM, DUR_TIME

Here DURATION_NAME is the "duration name". When asking for fatigue results one selects the durations to be applied, and PRC_NAME is the process name for which the duration will be applied. The -TDOM option signifies that one wishes to perform time domain fatigue on the process PRC_NAME and that the total duration of this process is DUR_TIME in days. This is all that is required for a time domain process.

When DURATION is used without the -TDOM option, a sub-menu is entered where frequency domain fatigue data will be defined. In other words, for frequency domain the DURATION command is followed by a series of records of the form:

     WTIME(i),  -OPTIONS

Where the available options are:



     -SEA, SEA_NAME, THET, HS, PERIOD, GAMMA


     -A_SEA, SEA_NAME, THET, HS, PERIOD, GAMMA

Here, WTIME(i) is the time (days) which the system will be exposed to the seastate defined by the remainder of the record. Here the parameters SEA_NAME, THET, HS, PERIOD, and GAMMA are the same as those used to define a sea with the &ENV command. If a duration environment is defined with more than one spectrum then MOSES provides two ways to compute the "average period". The choice is governed with the -T_AVERAGE option of the &PARAMETER command.

When all of the duration has been defined, one exits the sub-menu with an


     END_DURATION

command. Notice that one can perform both time domain fatigue and frequency domain fatigue in the same run, and both types of fatigue can be computed for the same process.