Motion and Force Statistics

Hydrostatics, Stability, and Ballasting

Seakeeping

Jacket/Deck Installation

Mooring

Using Our Tools

In this example, we add commands to calculate and report the cargo motions and forces statistics to the previous analysis. (Click here for a review.) The output that is generated will contain cargo statistics of motions, velocity, acceleration and forces - quantities which will help us determine the viability of the transportation.

Most of our examples contain a common set of "beginning" commands as well as a common "ending" command. Click here to get documentation for these commands. They will not be discussed directly. The files which are discussed here are:

The rest of the commands are the same until we get the the Frequency Response menu. Here, after computing the response operators at the vessel cg, we find an additional command:

     FR_POINT *CAR_CG

In other words, we have now asked for the response operators of the center of gravity of the cargo, "*CAR_CG". Again, notice in the log file that the program informs us of the coordinates of the point *CAR_CG. Following the FR_POINT command we are placed in the MOSES Disposition menu. We do not ask for a report but simply END. Now you may ask why did we go to the trouble of computing the RAOs of the center of gravity of the cargo if we were not going to report them? We did it so that we can use these raos in the next steps! In other words, the commands that calculate the statistics of motion and forces work with the last set of RAOs computed. (For details, click here.)

To compute statistics, we need an environment. Thus, we define one with the command:

     &ENV S_90 -SEA ISSC 90 8 4

Here, S_90 is the name we will assign to the environment. The option -SEA defines the type of spectrum used (ISSC), the direction of the sea (90 degrees), the significant height of the sea (8 feet) and the period of the sea (4 seconds). The values used here were chosen for convenience and are not intended to represent any part of the world. (For details, click here.)

Now that we have an environment we can use it to compute statistics with the command:

     ST_POINT S_90 -NPERIOD 1 12

The name is intended to stand for "statistics of a point", i.e. statistics of the motions computed via the last FR_POINT command. It produces statistics of motions, velocity and accelerations for the sea "S_90". The option -NPERIOD instructs the program to generate results for seas of several different periods. If this option is omitted, then a single period, the period associated with the environment name (S_90), will be used. By using this option, we can look at the statistics for a range of sea periods from 4 to 16 seconds. You will notice in the log file that the program informs us of the environment it is using and places us in the Disposition menu. While in the Disposition menu we report the values calculated and END.

The next command produces the frequency response of the dynamic forces acting on a rigid body whose CG is located at the last position specified by the FR_POINT command. The command used is:

     FR_FCARGO 1000 16 16 20

Here, 1000 is the weight of the body, and 16, 16 and 20 are the X, Y and Z radii of gyration of the body. The program informs us of the weight and radii of gyration it is using for calculations. (For details, click here.) At the conclusion of the calculations, we are again placed in the Disposition menu. For this step we do not report the frequency response dynamic forces. We simply END out of the Disposition menu. The RAOs of the forces may be of interest in themselves, but we are interested in only using them to obtain statistics with next command:

     ST_FCARGO S_90 -NPERIOD 1 12

Notice that the syntax of this command is identical to the one for computing statistics of motions. The only difference is that _POINT has been changed to _FCARGO. After the command has been entered, the program informs us of the environment it is using and at the conclusion of the calculations, places us in the Disposition menu. Within the Disposition menu we ask for the standard report and then END out of the Disposition menu, END out of the Frequency Response menu and finish the analysis with the command &FINISH.

Now let's talk about the output we have produced. (Click here to view it.) Page 1 of the output contains the righting arm results. Pages 2 through 4 contain the RAOs at the body center of gravity. Remember we did not ask for a report of the RAOs at the cargo center of gravity. Pages 5, 6 and 7 were the results of the ST_POINT command and page 8 is the result of the ST_FCARGO command. The output generated with the command ST_POINT contains motion statistics, velocity statistics and acceleration statistics. The star box for the three pages contain the vessel condition, the GM value, the radii of gyration and the parameters of the environment. For all three pages the values are reported for the six degrees of freedom. As an example, if we knew that our equipment located at x=90, y=0 and z=20 could not take more than a 20 degree roll, we could conclude that for seas with an 8 foot wave height and mean period ranging from 4 to 15 seconds, we are safe. The highest roll value in our motions table is 19 degrees. There are usually more constraints to a motions analysis than just the magnitude of the motion. There are also acceleration constraints.

For accelerations we have two tables to look at, the table on page 7 and the table on page 8. The acceleration values listed in page 7 do not include a g*sine(theta) term, the force values listed on page 8 do include a g*sine(theta) term. The constraints placed on your analysis will determine whether you want to use the accelerations from the ST_POINT command or the forces from the ST_FCARGO command.

With the commands shown in the .cif file you can perform a standard motions analysis for one piece of cargo. For many transportations you will be transporting two pieces of cargo, for example a deck and a jacket, or a deck and a flare boom. You can see that with the command we have presented so far, you would need to either repeat some steps or perform the analysis twice.

In the following step we introduce some concepts that help save time by automating some of the commands.

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