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.

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

     -SP_TYPE, TYPE

     -SPREAD, EXP

     -NPERIOD, PER_INC, NUM_PERIOD

     -CSTEEP,  YES/NO

The options -SEA and -SPREAD are used to define the sea state to which the vessel will be subjected and are the same as 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. Here, MOSES will compute the statistics of the motions, velocities, and accelerations of the response operators which resulted from the last FR_POINT command.

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 -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.

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. The sea spectrum is defined with the -SEA, -SP_TYPE, and -SPREAD options, discussed with the &ENV -SP_TYPE, TYPE command.

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

     -NPERIOD, PER_INC, NUM_PERIOD

     -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 -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.

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

     -NPERIOD, PER_INC, NUM_PERIOD

     -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: the mean distance above the water, the statistical "clearance change", and the difference between the mean and the statistic.