Simple Stability

Simple Stability

Hydrostatics, Stability, and Ballasting

Seakeeping

Jacket/Deck Installation

Mooring

Using Our Tools

Here, a basic stability analysis is performed. Important basic concepts of the program are introduced while only minimal computations are performed. Although the topics discussed here are basic, the concepts that are introduced are fundamental for any analysis.

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:

There are four major parts to the analysis:

Read in the vessel model
Put the vessel in the desired condition (draft, roll and trim)
Define the weight and weight distribution properties, and
Compute the righting arms and wind arms

Here is the text for the file bstab.cif.:

   $
   $@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
   $@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
   $@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
   $
   $                 Sample Problem Showing - Basic Stablity
   $
   $@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
   $@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
   $@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
   $
   $
   $*********************************************      set basic parameters
   $
   &DIMEN -DIMEN FEET KIPS
   &TITLE Step 1 - Basic Stability
   $
   $*********************************************      READ MODEL
   $
   INMODEL
   $
   $*********************************************     set transit condition
   $
   &INSTATE -CONDITION 7
   $
   $*********************************************      compute weight for cond.
   $
   &WEIGHT -COMPUTE  5 32 85 85
   &STATUS B_W
   $
   $*********************************************      enter hydrostatic menu
   $
   HSTATICS
   $
   $*********************************************      stability trans.
   $
      RARM 2.5 10  -WIND 100
         REPORT
      END
   END
   $
   $*********************************************      all done
   $
   FINISH

The first 11 lines are comment lines. Anything after a $ is ignored. These comment lines explain that the file is intended to perform a stability analysis.

The first two real commands are preceded by the comment that states they are "basic parameters". These two commands are important because they define the dimensions which are used to communicate with the program and the title of the analysis. This title will be in each page of output.

The command:

     INMODEL

instructs the program to read in the data file. In this case the data file is bstab.dat. In this step, the data file is one line long:

     USE_VES CBRG180

This line simply tells the program to use barge "CBRG180" from the barge vessel library. Notice that here, you do not need the extention .dat, the root of the file CBRG180 is all that is needed. (For details, click here.)

After the vessel description has been read the desired condition of draft, trim and heel for the vessel needs to be defined. The vessel condition is set with the command:

     &INSTATE -CONDITION 7 0 0

The three numbers at the end of the command signify the draft, the roll, and the trim. For this analysis the draft has been set to 7, the roll and trim are set to 0. (For details, click here.) Once the vessel has been placed at the desired condition the necessary weight and raddi of gyration to keep it at that condition are calculated. The weight, vertical center of gravity (VCG), and the radii of gyration are set with the command:

     &WEIGHT -COMPUTE  5 32 85 85

This instructs MOSES to compute the weight needed to achieve equilibrium with the computed weight having the specified VCG = 5, KXX = 32, KYY = 85, and KZZ = 85. For stability, you do not need the correct radii of gyration, but they will be important in later steps. (For details, click here.)

The command:

     &STATUS B_W

instructs the program to report the current status of the buoyancy and weight. Having defined the condition and the weight of the vessel we can now proceed to perform specialized calculations. (For details, click here.)

In MOSES we have the concept of menus. In each menu a specific part of the analysis can be performed. You always begin in the Main menu. From there, there are several levels of menus. The function of the menus can be determined from the menu name, for example here we are interested in hydrostatics, so we issue the command:

     HSTATIC

to enter the HYDROSTATICS menu.

Within the hydrostatics menu, righting arm calculations are performed using the command:

     RARM 2.5 10  -WIND 100

This particular command will result in righting arm calculations being done every 2.5 degrees from the current position for a total of 10 calculations. The option -WIND will perform the wind heeling calculations with a wind of 100 knots. (For details, click here.)

At the end of the calculations the program will automatically place the user in the Disposition menu. You will find yourself in this menu many times when using MOSES. You can do many things with the results you have just created. (For details, click here.) Here, however, we will simply print them to the output file with the command:

     REPORT

The first END then exits the Disposition menu and the second END exits the HYDROSTATICS menu. When you exit the HYDROSTATICS menu, you return to the MOSES Main menu.

The last command:

     &FINISH.

finishes the analysis.

Now, we have a model defined in the file bstab.dat and a set of things for MOSES to do with the model defined in the file bstab.cif. To get MOSES to interact with this information, you simply issue the command:

    moses bstab

This will create a directory bstab.ans. Inside this directory will be several files with different suffixes. (i.e. the three characters after the "." will be different.) Of these files, the two which are normally of most interest are bstab.ans/log00001.txt and bstab.ans/out00001.txt. These two files contain the results of the anlysis you have just performed. (For details, click here.)

The file bstab.ans/log00001.txt contains a command log. This is a record of each command MOSES executed and any messages or reports you asked to be printed to the screen. You should click here to look at the log. When viewing the log, you will see the message that the program begins in the Main menu. The program echos the commands to the log file. The command INMODEL is the first command where MOSES reports what it is doing. Notice that after reading in the model, MOSES reports the time used. For many of the commands the program will report the CP time required to accomplish the command. In most cases, this is printed to give some input as the analysis progesses, and after a while, to give you some comparison as to the size of this analysis versus others you have done.

Up to this point, the log has simply been a reports of what MOSES is doing. The command &STATUS B_W is the first command where MOSES reports something specific to our analysis. Here we find a table of the buoyancy and weight for CBRG180. This table summarizes the state, or status, of the system from a hydrostatic perspective. At the top, the draft, roll and pitch specified on the command &INSTATE are reported. The entry in the weight summary "LOAD_GRO" is the results of the &WEIGHT command. The radii of gyration and the metacentric heights are for the total system, not just that due to our computed weight.

The reamainder of the log reports when the menu changes. When we enter the hydrostatics menu MOSES reports:

                       +++ H Y D R O S T A T I C S +++
                       ===============================

and when we enter the disposition menu MOSES reports:

                        +++ D I S P O S I T I O N +++
                       ===============================

The remainder of the log is uneventful. It simply echos the commands, reports menu changes, and at the end report the "exit status". The end report for this analysis states "MOSES Finished Normally". In other words, it tells us that no errors were found and how much computer time was used for the analysis.

Now we turn to the output file bstab.ans/out00001.txt. You should click here to look at the file. (You may want to change the size of your browser window and/or font so that you can view the whole width of the file.) Each page of the output has, in general, four parts: a "star box", a page header, column titles, and a matrix of numbers. The first line on each page contains program identification, and the next several contain information enclosed in "*"s. The program identification tells the user which version of the program was used to generate the output. The star box will always contain the date the output was generated, a title, and a subtitle. For our analysis, we identifited the title as "Step 1 - Basic Stability". Since we did not define a subtitle, one does not appear. The other contents of the star box may vary and will be discussed when the contents change.

The rest of the page will vary depending on what is being reported. For this output, there are three types of pages: the echo of the data input, the results of the righting arm calculations and the index of the output. This file begins with an echo of the vessel data file. Notice that the echo of the input data is much larger than the file bstab.dat. This is because we told MOSES to use a vessel from the library. It is the definition of the vessel from the library which is being echoed. A discussion of the commands used to define the library vessel will be presented later.

The second to last page is the report of the righting and heeling arms. Here, the header defines the condition used to compute the righting arms.

            +++ R I G H T I N G   A R M   R E S U L T S +++
            ===============================================

Process is DEFAULT: Units Are Degrees,  Feet, and  Kips Unless Specified

  Moment Scaled By 3753.11, KG = 5.50, and Wind Speed  =  100.  Knots

                 Initial: Roll = 0.00, Trim = 0.00 Deg.

              Arms About Axis Yawed 0.0 Deg From Vessel X

The report header contains the units being used, the weight of the vessel, the KG value, the wind speed used, the inital roll and trim and the yaw axis. The weight and KG information are the same as was reported with the &STATUS command. The KG is the value for the combined lightship and the computed weight. Remember that with &WEIGHT we only defined the KG of the weight being added, not the KG of the total weight. The initial roll and trim tells us that the vessel was at an even keel when the roll increment of 2.5 was commenced. Below the header, we have the righting arm results. There are ten columns of numbers, but not all columns will be discussed in this step. The omitted columns will be discussed later. The first three columns are draft, roll and pitch. For example, for a roll of 12.5 deg the output file shows a draft of 6.60, and a pitch of .11. The table also reports the righting and heeling arm values for each roll increment.

For heeling arm calculations there are, in essence, four variables in the moment calculation: displacement, wind velocity, the heel angle and the wind area. Of these four variables, we know the program has a value for the displacement because it was listed in the buoyancy and weight table. We also know the wind speed is 100 kts because it was echoed in the report heading. This leaves the wind area as the only variable which could yield zero arm. Thus, we need to either figure out how to ask the program to tell us what it thinks the wind area is, or we can look at the model and determine if the wind area is there. In general the vessels in the vessel library do not have wind areas, therefore we can conclude that our model does not have wind area.

The last page of the output contains the index of the output file. There are three entries: the echo of the data file, the righting arms and the index itself. This, among other topics, will be discussed in the next step.

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