Pressure Data Pressure Data

To compute the sea pressures on the vessel, the program must know the form of the vessel below the water. This is communicated to MOSES by a set of vessel description data, defined earlier, and the current condition of the body. To initiate the pressure computations discussed above, one must issue:


     G_PRESSURE, BODY_NAME,  PKT_NAME -OPTIONS

and the available options are:


     -HEADING, H(1), H(2), ...., H(n)

     -PERIOD, T(1), T(2), ...., T(n)

     -MAX_DIST, DIST

     -MD_TYPE, METHOD

When this command is issued, MOSES will take the system in its current configuration and compute frequency domain pressures and a total hydrodynamic database for the body BODY_NAME. This data will be stored by the name PKT_NAME in the database.

The -HEADING option is used to change the default values of vessel heading for which results will be computed. H(i) is the ith value of heading (deg.) to be considered. Here, heading is measured as an angle from the X axis, positive toward Y. Hence, for a vessel described with the origin at the bow, head seas are 180 degrees. The -PERIOD option is used to alter the default values of encounter period, and T(i) is the ith value of encounter period in seconds. As many of these options as needed can be input, so long as the number of periods does not exceed 200. The defaults for heading and period are set with options on &DEFAULT.

When the G_PRESSURE command is issued, MOSES takes the vessel description and the condition defined, and converts the vessel description into one describing the vessel below the waterplane. The program will then compute the added inertia and damping matrices, and the applied forces on the vessel. These computations will be performed for the periods and headings defined by the command. In converting the model, the two options -M_DISTANCE of the &PARAMETER command are used to refine the computation. Use of these options allows one to define a quite crude mesh and have MOSES automatically refine it to achieve any desired degree of precision. Also, the option -MAX_DIST provides a way to get approximate solutions to large diffraction problems with reduced computational effort. This option defines a maximum distance (feet or meters) for panel interaction. Any two panels which have a distance between them greater than DIST will have a zero for their coupling terms in the diffraction matrix. For very long slender bodies, this option can be used quite effectively to save computer effort. For a body 4000 ft long with 6000 diffraction panels, answers within 10% of the exact ones can be obtained in 66% of the time.

The -MD_TYPE option allows the user to choose a method for calculating the mean drift forces on the body. The default is the pressure integration method which is selected with the parameter NEARFIELD. The Salvesen method can be selected with the parameter SALVESEN. Alternatively the user can select to use the momentum method with the parameter FARFIELD, however this will only produce mean drift forces for the horizontal components (X and Y) and the vertical moment (RZ). Note that the mean wave drift parameters will be reset by any calls to the deprecated G_MDRIFT command; it is therefore highly recommended that all options and parameters for G_PRESSURE are always explicitly specified.

Once one has a pressure database, it can be examined in the Disposition Menu. In particular, the command


     V_MATRICES, BODY_NAME

will gather the added mass and damping matrices for the pressure packet currently associated with body BODY_NAME and place the user in the Disposition Menu. Alternately, the command


     V_EXFORCES, BODY_NAME

will do the same thing with the wave excitation forces. For both of these commands, the results are about the origin.

As mentioned earlier, it is possible to define the hydrodynamic pressure distribution and/or the total hydrodynamic database via commands. The same device is used if one has computed hydrodynamic pressures and wished to "save" them for use later. This capability can be quite useful when analyzing a large diffraction model. For instance, one could alter the mooring lines of system, and use the importation feature to save time in the reanalysis. Caution should be used here, so that only changes to the model that do not effect hydrodynamic properties are performed. To emit a hydrodynamic database, one issues:


     E_PRESSURE, BODY_NAME -OPTION

where BODY_NAME is the name of the body for which hydrodynamic databases will be emitted. This command will export a pressure database for the packets currently associated with the body BODY_NAME. Here, the only option is -NOTE, which is just like the same option on model definition} commands. In particular, you need to specify all five characters of the option, and the note that you attach will be included as a comment in the emitted file. Also, the title and subtitle will be included if they are not blank. No drift or total data will be emitted since it is recomputed when the pressure data is imported. It is possible to export (and subsequently import) a total hydrodynamic database. This is accomplished with:


     E_TOTAL, BODY_NAME -OPTION

The resulting output is substantially smaller than the pressure database. Here the options and titles are the same as for the pressure database. This file, however, consists data for both Total force and drift force. For simulation purposes, this is all that is necessary, but it cannot be used for computing structural loads. When such a database is input, a single warning to this effect will be given.

The format of an exported pressure database is the same as the one used to define the data directly to MOSES. It begins with the command:


     I_PRESSURE, BODY_NAME, PKT_NAME,  DISPL,  -OPTIONS

which places the user in a submenu. Here, BODY_NAME is the name of the body for which the database is being generated, PKT_NAME is a desired packet name, DISPL is the displacement at the condition being defined, and the options are:


     -PERIOD, T(1), T(2), ......

     -HEADING, H(1), H(2), ......

     -CONDITION, DRAFT, ROLL, PITCH

Even though these items are called options, the first three of them are necessary to properly define the database. The -PERIOD option defines the periods (sec) for which the database will be defined, and -HEADING defines the headings (deg) for which the exciting forces will be defined. The -CONDITION option defines the vessel condition for which the database is defined and DRAFT, ROLL, and PITCH are the draft (feet or meters), roll (deg), and pitch (deg) defining the condition. The remaining options were described previously.

Once the menu has been entered, several commands are available. First, the command:


     FP_MAP, PANEL_NAME, :PNT_SEL(1), :PNT_SEL(2), .......

defines a how the structural loads on the panel PANEL_NAME will be mapped to all points matching :PNT_SEL(i). The command:


     FPANEL, PANEL_NAME, AREA, XC, YC, ZC, NX, NY, NZ, WLLEN

defines a panel. Here, PANEL_NAME is the name of the panel, XC, YC, and ZC are the coordinates ( feet or meters ) of its centroid, NX, NY, and NZ are the components of its normal, and WLLEN is the length of the intersection of the panel with the waterline ( feet or meters ).

After a panel has been defined, the pressures acting on it are defined through a set of velocity potentials with commands:


     FPPHI, PER, RPRX, IPRX, RPRY, IPRY, .... \
     RPRRZ, IPRRZ, RPDH(1), IPDH(1), ...

Here, PER is one of the periods T(i), and the remainder of the data are velocity potentials per unit wave amplitude (feet or meters). These commands must be in decreasing order of period. In other words, the value of PER for a given command must be less than the value of PER for the previous one and greater than PER for the next one. The velocity potentials are pairs of real and imaginary numbers. The first six pair (twelve numbers) are the radiation potentials, and the remainder are diffraction potentials. The diffraction potentials correspond to the headings H(i), and are in the same order.

The final type of data for a panel is defined with:


     FDELP, PER, RPRXX, IPRXX, RPRXY, IPRXY, .... \
     RPRRZZ, IPRRZZ, RPDHX(1), IPDHX(1), ...

These quantities are the gradients of the potentials defined with the FPPHI command, and thus there will be 3 times as many values as on the FPPHI command. If these are viewed as complex numbers, then the first three numbers are the derivatives with respect to X, Y, and Z of the first potential on FPPHI. The first 36 values (18 complex numbers) are gradients of the radiation potentials. The gradients of the diffraction potentials follow for each heading.

After all of the panels have been defined, the menu is exited with an END_I_PRESSURE command.

To define a "total hydrodynamic database" one first issues:


     I_TOTAL, BODY_NAME,  PKT_NAME, DISPL,  -OPTIONS

One can then describe the hydrodynamic data, and issue END_I_TOTAL to exit the menu. Here, BODY_NAME is the name of the body for which the database is being generated, DISPL is the displacement at the condition being defined, and the options are:


     -PERIOD, T(1), T(2), ......

     -HEADING, H(1), H(2), ......

     -CONDITION, DRAFT, ROLL, PITCH

     -SCFACT, SCLEN, SCMASS, SCDRAG, SCFOR

Even though these items are called options, the first three of them are necessary to properly define the database. The -PERIOD option defines the periods (sec), for which the database will be defined, and -HEADING defines the headings (deg), for which the exciting forces will be defined. The -CONDITION option defines the vessel condition for which the database is defined and DRAFT, ROLL, and PITCH are the draft (feet or meters), roll (deg), and pitch (deg) defining the condition.

The option -SCFACT defines a set of scale factors which can be used to convert to the units required. In other words, all of the quantities input via the commands discussed below will be multiplied by the scale factors prior to being stored in the database. The manner in which these factors will be combined with the input numbers will be discussed with each command.

After the I_TOTAL command, the database is defined by a sequence of the following commands:


     H_ORIGIN,  OX, OY, OZ

     H_EULERA, EROLL, EPITCH, EYAW

     H_PERIOD,  T

     H_AMASS, AM(1,1), AM(2,1), ...., AM(6,6)

     H_DAMP, DAMP(1,1), DAMP(2,1), ...., DAMP(6,6)

     H_FORCE, H,  RFKX, RFKY, ... RFKYAW, IFKX, ..., IFKYAW \
     RDIX, RDIY, ... RDIYAW, IDIX, ..., IDIYAW \

The H_ORIGIN and H_EULERA commands define a change of coordinate system from the one being input to the one employed by MOSES. Here, OX, OY, and OZ are the components of a vector, in the MOSES body system from the origin in the local body system to the origin of the system in which the input quantities are computed. Likewise, the quantities EROLL, EPITCH, and EYAW are three Euler angles (deg). These angles, when applied as a yaw followed by a pitch, followed by a roll, define the direction cosine matrix which transforms the system in which the quantities are computed to the local body system. If either of these two commands is omitted, the corresponding transformation will be assumed to be the identity. Once a transformation has been defined, it will be used until it is redefined by another similar command.

The H_PERIOD command defines the period for all quantities which follow until a new H_PERIOD command is encountered. Here, T is the period (sec) and it must have been defined by the -PERIOD option on the I_TOTAL command. If the period defined by an H_PERIOD command is the same as one previously used, the data following will be added to the previous data for the same period. Thus, one can define the properties of a complicated body by inputting the properties for each piece of the body and letting MOSES combine them to form the properties of the body.

The remaining commands are used to actually define the hydrodynamic properties for the "current" period, and they can be repeated as many times as desired until an END is encountered. This marks the end of the hydrodynamic database definition for a given body.

The H_AMASS and H_DAMP commands define the added mass and linear damping matrices respectively. The data is input on the command by columns of the matrix. The values which MOSES needs for the added mass matrix are added mass divided by displaced mass, and the length units should be feet or meters. When an H_AMASS command is input, the top 3x3 is multiplied by SCMASS, the two coupling 3x3 matrices are multiplied by SCMASS*SCMASS, and the bottom 3x3 is multiplied by SCMASS*SCLEN*SCLEN, as defined with the -SCFACT option. It is the product of the input values times the scale factors which should have the dimensions defined above. The scaling for the damping matrix is similar to that for the added mass except that SCDRAG is used instead of SCMASS. The desired units for the product of the input values and the scale factors is damping coefficient divided by displaced mass.

The H_FORCE command defines the wave exciting force for the heading, H, which has been defined by the -HEADING option on the I_TOTAL command. The first twelve values define the Froude-Krylov force, the next twelve define the diffraction force. The first three values for each force will be scaled by SCFOR, and the last three will be scaled by SCFOR*SCLEN, as defined on the -SCFACT option. The product of the input values times the scale factors should be either bforce, and feet or meters, depending upon the last &DIMEN command.