Motion Post-Processing Motion Post-Processing

The commands discussed in this section all deal with the frequency response of a point or relative motion of two points, and they allow one to obtain: the frequency response of the point, statistics of this response, and a time realization of the motion. Basically, one first finds the frequency response at a given point, and then the other commands discussed here deal with this response until the response at another point is obtained. As mentioned previously, the data for these commands depends on the manner in which the original frequency response data was computed. If it was computed with an RAO command, then all options and data discussed here are available. If it was computed with an SRESPONSE command, then only geometrical data can be input. In other words, no environmental data can be specified.

Many of the commands here compute statistics of quantities and as a result have many common options. In particular:


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

     -SPREAD, EXP

     -SP_TYPE, TYPE

     -E_PERIOD, EP(1), EP(2), .....

     -CSTEEP,  YES/NO

The statistical result is the statistic specified with the last -PROBABILITY option on a &DEFAULT command, and If the original response data was produced with the SRESPONSE command, then no additional sea data can be specified.

The remainder of the commands available for motions 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. The options -SEA, -SPREAD and -SP_TYPE are used to define the sea state to which the vessel will be subjected. The -E_PERIOD 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, EP(1), EP(2), ... will be produced. 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.

To obtain the frequency response at a point, one issues the command:


     FR_POINT, WHERE, -OPTIONS

Here, WHERE can be either the body coordinates ( feet or meters ) of the point in question, the name of a point, or the names of two points. If a single point is specified, then the results are the motion of the point. If two points are specified, then the results are the relative motion of the two points. The options are: -VELOCITY and -ACCELERATION. If no option is specified, then the motion response is produced, with -VELOCITY, velocity response is produced, and with -ACCELERATION, acceleration response results. If one specifies coordinates, MOSES assumes them to be coordinates of the current body. To remove confusion in multi-body situations, it may be a good idea to issue an &DESCRIBE BODY command to establish the current body before using coordinates.

When this command is issued, the program will compute the response at the specified location for every period and heading at which the original response was computed. When placed in the Disposition Menu, the results for all headings are available. The names of the variables are prefixed by HEDXXX where XXX is the heading angle in degrees. When using the REPORT command in the Disposition Menu, one can selectively report the response. If there is no data on the REPORT command, all headings will be reported. To report data for only some headings, one should specify the angles of the heading to be reported on the REPORT command.

To compute statistics of responses in irregular seas, one should issue:


     ST_POINT, ENV_NAME, -OPTIONS

where the available options are:


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

     -SPREAD, EXP

     -SP_TYPE, TYPE

     -E_PERIOD, EP(1), EP(2), .....

     -CSTEEP, YES/NO

The options were discussed above. When computing motions and using more than one period, there are three reports available with the REPORT command in the Disposition Menu. If the REPORT command is issued without data, then all three reports will be written. To select a subset of these three, the REPORT command should be given followed by: MOTION, VELOCITY, and/or ACCELERATION, in which case only the reports specified will be written.

When dealing with irregular seas, it is often of interest to know the variation of the sea and response spectra with frequency and period. To obtain results of this nature, one should issue:


     SP_POINT, ENV_NAME, -OPTIONS

where the available options are:


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

     -SP_TYPE, TYPE

     -SPREAD, EXP

The results produced here are based on the results of the last FR_POINT command and the options were discussed above.

The results obtained with the ST_POINT command consist of motions measured from a point in vessel coordinates. Often, one desires a global motion measured from a specified reference. Results of this type can be obtained via the command:


     PMOTION, :PNT_SEL, ENV_NAME,  -OPTIONS

where the available options are:


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

     -SP_TYPE, TYPE

     -SPREAD, EXP

     -E_PERIOD, EP(1), EP(2), .....

     -CSTEEP, YES/NO

Here, :PNT_SEL is a selector for the points whose motion will be computed, and the other options are the same as for the ST_POINT command. When this command is issued, statistics for the global "dynamic motion" of the Interest Points selected by the selector :PNT_SEL will be computed. In addition, MOSES will compute the mean global position of the point, and the mean "motion" of the point. The mean motion is the vector from the global position of the point the last time the &DESCRIBE INTEREST command was issued. The mean motion is added to the dynamic motion with the sign of the mean to produce a "total" motion away from the marked global position of the point. The options were discussed above.

A command similar to the above is:


     ST_CLEARANCE, :PNT_SEL, ENV_NAME,  -OPTIONS

where the available options are:


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

     -SP_TYPE, TYPE

     -SPREAD, EXP

     -E_PERIOD, EP(1), EP(2), .....

     -CSTEEP, YES/NO

The purpose of this command is to investigate the statistics of the relative motion between the points selected by :PNT_SEL and the water surface. Here, one will receive for each point selected: