Connector Force Post-Processing

If connectors were attached to the system when the frequency response was computed, the frequency response of the constraint forces was also computed. To obtain the frequency response of the forces which the connectors exert on the first body to which they are connected, one can issue:

     FR_CFORCE, CONN_NAME

Here, the user is placed in the Disposition Menu with the frequency response of the connector which matches the selector CONN_NAME. He can then proceed to dispose of these results.

The remainder of the commands available for connector forces have a similar syntax in that the final portion of the command is identical to that of the &ENV command. In fact, these commands not only initiate the computation of quantities in an irregular sea, but are also &ENV commands. Thus, when one issues one of these commands with a non-blank ENV_NAME, he is altering the definition of this environment within the database. If ENV_NAME is omitted, then the environment used will be totally defined by the options specified. Again, if the original data was obtained with an RAO command, then all of the data discussed here can be specified. If instead, the original results were obtained with an SRESPONSE command, then no environment, nor options can be specified.

To obtain statistics of the forces which the connectors exert on the first body to which they are connected, one can issue:

     ST_CFORCE, :CONN_SEL, ENV_NAME,  -OPTIONS

where the available options are:

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

     -SP_TYPE, TYPE

     -SPREAD, EXP

     -NPERIOD, PER_INC, NUM_PERIOD

     -USE_MEAN, YES/NO

The command produces irregular sea results for the first connectors which are selected by the selector :CONN_SEL. This command works exactly as the ST_POINT command, except here the results are for constraint forces instead of motions. Additionally, one can use the -USE_MEAN option to instruct MOSES to add the mean value of the force to the computed deviation with the sign of the mean so that the reported force will be a measure of the total force. The options -SEA, -SP_TYPE, and -SPREAD are used to define the seastate to which the vessel will be subjected and are the same as if they were specified on an &ENV command. The -NPERIOD option can be used to generate results for seas of several different periods. If this option is omitted, then a single period of PERIOD will be considered. With the option, periods of PERIOD, PERIOD+PER_INC, ..., PERIOD+NUM_PERIOD*PER_INC will be produced. Here, PER_INC is the period increment, and NUM_PERIOD is the total number of periods. The statistical result is the statistic specified with the last -PROBABILITY option on a &DEFAULT command. As with the &ENV command, the meaning of PERIOD and PER_INC depend on either the setting of TYPE specified with the -SP_TYPE option or with -SP_TYPE specified on if they were specified on an &ENV command. If the original response data was produced with the SRESPONSE command, then no additional data can be specified. If -CSTEEP is specified with a YES/NO of YES, then the height of the wave will be altered so that all seastates have the same steepness as the initial one. Otherwise, the wave height will remain constant.

Fatigue can be computed on the connectors if one has computed frequency response with an SRESPONSE command. This is accomplished with the command

     FAT_CFORCE, -OPTIONS

where the available options are:

     -INITIAL

     -ACCUMULATE, :CONN_SEL

     -REPORT, TIME

This command was designed to accumulate fatigue for several different environments (SRESPONSEs). When the command is issued with the -INITIAL option all fatigue accumulators are zeroed. When it is used with the -ACCUMULATE option, cumulative damage is computed for all connectors which match :CONN_SEL, and this damage is added to that which exists. The duration used for the damage is that specified on the &ENV command. Finally, when the command is used with the -REPORT command, a report of the cumulative damage is written. The TIME (days) variable can be used to scale the damage from the time accumulated to TIME. In other words, if the sum of the durations was T1 and TIME was specified to be T2, then the damage will be multiplied by T2/T1 before reporting.

A command closely associated with fatigue is:

     COUNT_CF, -OPTIONS

where the available options are:

     -F_BINS, T(1), T(2),  .....  T(n)

     -ACCUMULATE, :CONN_SEL

     -REPORT, TIME

Instead of accumulating fatigue, however, this command accumulates cycles of tension in specified ranges. The ranges are specified with the -F_BINS option where T(i) is in bforce, and the other options function exactly as for the FAT_CFORCE command.

If some of the connectors are rods, then one has two additional commands available. These commands compute statistics of the internal forces and the stresses in the rod. To obtain the statistics of the internal forces, one should issue:

     ST_RFORCE, ROD_NAME, ENV_NAME,  -OPTIONS

where the available options are:

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

     -SP_TYPE, TYPE

     -SPREAD, EXP

     -USE_MEAN, YES/NO

Here, ROD_NAME is the name of the rod one wishes to investigate. If one is interested in a pipe assembly, then he should use &PIPE for ROD_NAME. When issued, the statistics for the rod internal forces will be computed for the sea defined by the -SEA, -SP_TYPE, and -SPREAD options. Also, one can use the -USE_MEAN option to instruct MOSES to add the mean value of the force to the computed deviation with the sign of the mean so that the reported force will be a measure of the total force.

Stresses in rods are computed via the command:

     ST_RSTRESS, ROD_NAME, ENV_NAME,  -OPTIONS

where the available options are:

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

     -SP_TYPE, TYPE

     -SPREAD, EXP

     -USE_MEAN, YES/NO

which operates exactly the same as the ST_RFORCE command.