Loadout Ballasting

Loadout Ballasting

Hydrostatics, Stability, and Ballasting

Seakeeping

Jacket/Deck Installation

Mooring

Using Our Tools

This is a basic example for computing hydrostatics and ballasting of a barge. The data set has complete compartmentation and load groups defined so that it could be used for a jacket loadout; i.e. load groups are defined which correspond to the forces that the jacket will exert on the barge at when it is at a specified location.

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 entire analysis is done within the Hydrostatics menu which is entered via the command:

$
$*********************************************      ENTER HYDROSTATIC MENU
$
HSTATICS

Setting the inital condition is accomplished with:

$
$*********************************************      initial ballast
$
   &SUBTITLE NORTH TOW UNDER POWER LINES
   &INSTATE -CONDITION 27.5 0 -1.62
   &APPLY LIFT-OFF
   &COMPARTMENT -PERCENT  8/W 100 7PW 31.82 7SW 32.38 9// 100 9PW 87.75 \
                    10PW 32.51 10SW 33.06

The subtitle is set to "NORTH TOW UNDER POWER LINES" with the command &SUBTITLE. This command was discussed as one of the commands common to the "beginning" of files.

The barge condition is set with the command &INSTATE -CONDITION where the three numbers specify the draft, roll, and trim respectively. The loads associated with the name "LIFT-OFF" are applied with the command &APPLY. Having loaded the barge and placed it in the desired condition, we set the initial ballast of the compartments with the &COMPARTMENT command. Ballasting is specified by pairs of data, a compartment selector and a percentage full, e.g. 7PW is a compartment name and 31.82 is the percentage full. We have used some wild characters, /, to select compartments. A / "matches" any character, thus here, 8/W selects tanks 8SW and 8PW to be filled 100 percent.

One of the primary things one wishes to accomplish here is to create a ballast plan which minimizes the deflection in the barge. Thus, we next investigate the longitudinal strength of the barge in this condition with:

$
$*********************************************      long. strength
$
   EQUI
   MOMENT -ALLOW 24.00 1.
      REPORT
      VLIST
      PLOT 1 4 -RAX 5
      1
      STRESS RATIO
      2
      DEFLECTION
      -1
      3
      LONGITUDINAL LOCATION (FT)
      4
      STRESS RATIO
      5
      DEFLECTION (INCHES)
      -1
   END

For the longitudinal strength results to be meaningful, we first find the equilibrium position due to these loads with EQUI. Accelerations are within a prespecified tolerance. Longitudinal strength is calculated with the command MOMENT. The option -ALLOW specifies the allowable stress, 24 ksi, and the allowable deflection, 1 inch. At the conclusion of this command the user is placed in the Disposition menu. The standard longitudinal strength report is produced with the command REPORT, and it gives for each longitudinal location: the shear force, the bending moment, the bending stress over allowable, and deflection over allowable.

The command VLIST, short for view list, writes a numbered list of the variables available for selection to the screen, e.g.

                 The Variables Available for Selection are:
                 ==========================================
  1 Long Location           3 Moment                    5 Deflection Ratio
  2 Shear                   4 Stress Ratio

This is quite handy since it allows one to specify what will be plotted, view, etc. by using the variable's number. The PLOT command produces a plot with variable 1 (longitudinal location) as the independent variable, with variable 4 (stress ratio) on the left axis and variable 5 (deflection ratio) on the right axis. By using two axes, both curves will "fill up" the plot.

Before the program produces the plot it queries the user about the legend and titles it is going to use. The following is an example of the program queries:

                                 The Legend Is:
                                 ==============
 
        1  Stress Ratio
        2  Deflection Ratio
 
                        Enter No. Of Line To Be Changed
                        ===============================
 
?2
                                 Enter New Line
                                 ==============
 
?Deflection
                                 The Legend Is:
                                 ==============
 
        1  Stress Ratio
        2  Deflection

Here, the program shows what it will use for the legend and then asks the user if there are changes to be made. If so you should specify the number of the line you want to change. If you do not what to change anything, simply issue an blank line, or give it a number less than 1. By specifying the number 2 we tell the program we want to change the legend which reads "Deflection Ratio". The line following the number 2, "Deflection" is what the legend will now read. This is a reasonable change since our allowable was 1 inch, and thus the ratio is really the deflection. After queries about the legend, the program will also enter into a dialog about the titles. Both dialogs can be omitted by specifying either a title or a legend on the PLOT command.

Intact stability is computed via:

$
$*********************************************      intact stability
$
   &SUBTITLE INTACT STABILITY NORTH TOW
   RARM 2.5 25 -ECHO N -WIND 100
      REPORT
      PLOT 2 5 7 -RAX 9
      2
      WIND HEEL ARM
      -1
      3
      ROLL (DEGREES)
      4
      RIGHTING ARM (ft)
      -1
   END

Righting arms are computed with the command RARM. The numbers The two numbers specify an increment, 2.5 degrees, and a number of increments, 25, for which arms will be computed. At each rotation the program will iterate an equilibrium position for the other degrees of freedom and then compute the righting and wind heeling arms. The option -ECHO N tells the program that we do not want to see each equilibrium iteration, and the option -WIND tells the program to make wind heeling calculations with a wind of 100 knots.

The command REPORT creates the standard righting arm report. This report will show table condition (draft, roll, and trim), minimum height of down flooding points, righting and heeling arms, areas under the righting and heeling arm, the ratio between the two arms, and the net arm. In the table, all the columns will have values which are not consistently zero, except the minimum height of down flooding points column. The down flooding points column will be zero because we did not define down flooding points. The PLOT command produces a plot of the stability results.

Damaged stability is done via the following commands:

$
$*********************************************      damaged stability
$
   &SUBTITLE DAMAGED STABILITY NORTH TOW ..TANK 3PW FLOODED
   &COMPARTMENT -FLOOD 3PW
   EQUI
   &STATUS B_W -HARD

The command &COMPARTMENT tells the program to flood tank 3PW, and then we find the resulting equilibrium with EQUI. A report of the current buoyancy and weight is obtained with &STATUS. Here the B_W is short for buoyancy and weight, and the option -HARD instructs the program to put the report in the output file.

Our next task is to compute a new ballast condition to get back to our original condition with 3PW flooded. This is accomplished with:

   &SUBTITLE COMPUTE BALLAST TO REGAIN TOW CONFIGURATION IN DAMAGED CONDITION
   &INSTATE -CONDITION 27.5 0 -1.62
   &STATUS B_W -HARD
   &STATUS COMPART -HARD
   &EVENT_STORE 1
   &SELECT :COMP -SELECT 7/W 10/W 8// 12//
   &CMP_BAL H109 :COMP
   &STATUS B_W -HARD

The &INSTATE command sets the barge back to where we want it to be, and we report the current buoyancy and weight as we did before. The next &STATUS command reports the current compartment properties. We store this situation as event number 1 with the command &EVENT_STORE command. By storing the configuration we will be able to return to it with &INSTATE -EVENT 1. This comes in handy because we now only need to remember one number, the event number, instead of the entire state.

We create a selection criteria called :comp to be used in the ballasting. It selects 7 and 8 wing tanks, and all of the 8 and 12 tanks. The &CMP_BAL command automatically computes the amount of ballast in the selected tanks so that the barge will be in equilibrium at its current location. Here, H109 specifies the body to be ballasted, and :comp defines the compartments to ballast.

Again, we look at longitudinal strength calculations with:

   MOMENT -ALLOW 24.00 1.
      VIEW -RECORD 1 15  -SEL @
      REPORT
   END

The only thing new here is the command VIEW which instructs the program to print records 1 through 15 of the variables selected by, @, to the screen. This wild character, @, stands for any arbitrary number of characters, so this selection results in all variables being printed.

In the final set of commands we return the barge to the intact condition via the following:

$
$*********************************************      restore condition
$
   &SUBTITLE RETURN TO INITIAL CONFIGURATION   NORTH TOW
   &INSTATE -EVENT 1
   &COMPARTMENT -NO_FLOOD 3PW
   EQUI
   &STATUS B_W

The command &INSTATE -EVENT 1 returns to that saved as event 1, and &COMPARTMENT -NO_FLOOD 3PW tells the program not to flood tank 3PW.

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