Cargo Force Post-Processing Cargo Force Post-Processing

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, in the same output report table.

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 cargo 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. 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 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

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

     -CSTEEP, YES/NO

and the options are defined above. 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.

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 and the options are defined above.