The commands discussed here apply to "dynamic forces" acting on pieces of cargo located at specified points. These forces, however, are derived completely from the motion of the body, and are quite useful in estimating the load which will act on any cargo. Here, the frequency response of the forces consist of two parts: 1.) The forces required to produce the given accelerations, and 2.) The component of weight which arises due to change in angle. These forces do not contain any static contributions such as the vertical component of weight, or any force on the body due to immersion in the water. Since all computations involving these forces have the correct phase relationship between the acceleration and the angular motion, they will be less conservative than adding the two components after an irregular sea computation.

The commands here are essentially the same as those discussed previously for the motions of a point, except that the results will be forces instead of motions. 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 a SRESPONSE command, then no environment, nor options can be specified.

The FR_FCARGO command is used to produce the frequency response of the dynamic forces acting on a rigid body whose CG is located at the last position specified on a FR_POINT command. The form of this command is:

     FR_FCARGO, WEIGHT, RX, RY, RZ

where WEIGHT is the weight (bforce) of the body, and RX, RY, RZ are the X, Y, Z radii of gyration (feet or meters) of the body. 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. If no data is specified on the FR_FCARGO command, then the data will be generated so that one get the "G" forces on the cargo and the angular accelerations, and a hard copy report will have different headings

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

     ST_FCARGO, 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, -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 statistics here are of the forces which resulted from the last FR_FCARGO command. If one is computing statistics of "G" forces, the angular accelerations should be the same as the angular accelerations produced from statistics of the motions. They may, however, differ due to numerics. The ones produced from the motions are "computed better". If the difference is too large to suit, you need more periods.

The details of the results depend upon additional options. If either the -NPERIOD option was omitted, or if the number of periods was specified to be one, then the RMS, H1/3, H1/10, and H1/100 statistical results for this period will be reported. In this case, there is actually no data to plot, since there is no real independent variable. If there is more than one period, then the situation is somewhat different. Here, the statistical result for the statistic specified with the last -PROBABILITY option on an &DEFAULT command is calculated for each period. Now, period becomes the independent variable, and plots illustrating vessel response for each period can be produced. 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 &DEFAULT. 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 dealing with irregular seas, it is often of interest to know the variation of the sea and response spectra with frequency or period. To obtain results of this nature, one should issue:

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