Use of STAMOD

The main purpose of STAMOD is to read and check the system description from SYSFIL and to write those data to a direct access file, INIFIL, which will be used in DYNMOD and OUTMOD. This file contains a complete system description, definition of the environment to be applied and the initial positions for dynamic simulation.

 I->  STAMOD main menu
 I
 I      1 : Read system description file
 I      2 : Read initial condition file
 I      3 : Modify/present system
 I      4 : Print static condition
 I      5 : Equilibrium calculation
 I      6 : Eigen value calculation
 I      7 : Mooring system optimization
 I      8 : Write initial condition file
 I      9 : Write file for visualisation
 I     99 : Terminate
 I
 I->  Select option >

1. Read system description file

The SYSFIL is an ASCII file containing a complete description of the system to be analysed. For a brief description of the sys-file layout and file syntax, see System Description File. A detailed description of the file contents is given in Appendix A.

The SYSFIL is read twice, the first time only to find dimensioning parameters for the work array, the second time to put all data into the work array.

If an error occurs, messages will be written both on the print file and in some cases also on the terminal. Error messages on the print file are found by searching for the string * ERROR from the beginning of the file. If a non-fatal error is encountered, reading of SYSFIL will continue until the maximum number of errors are found.

If errors are found during the first scan of SYSFIL and the print file does not give any meaningful indications on the problem, the amount of output to the printfile can be increased by the MAIS-command @ FILE 5.

1.1. Symmetry codes

Wind force coefficients, current force coefficients, wave drift force coefficients and transfer functions are given as functions of directions relative to the body.

In the calculations, extrapolation on directions are not allowed. In order to avoid problems with program termination due to attempted extrapolation, it is advised that both 0 degrees and 360 degrees should be represented in the range of directions on the input file.

If symmetry codes are used, this will automatically be taken care of. For example, if symmetry code is 2 and the directions 20 degrees and 70 degrees are specified, they will be mirrored to -20, 20, 70, 110, 160, 200, 250, 290, 340 and 380 degrees.

NB: It is strongly advised that values are specified for the symmetry lines. For the example above, 0 deg and 90 deg should also be included as input.

2. Read initial condition file

An INIFIL that has been written by STAMOD during the present or a previous run may be read. If no changes have been made to the system as defined on SYSFIL, this is an alternative which is much faster than reading SYSFIL.

3. Modify / Present system

The menu for system modification is:

 I->  STAMOD modify/present system
 I
 I      0 : Return
 I      1 : Select environment
 I      2 : Initial positions
 I      3 : Eliminate degrees of freedom
 I      4 : Positioning system
 I      5 : Restoring force (incl. environmental forces)
 I      6 : Dynamic positioning
 I
 I->  Select option >

3.1. Select environment

Several regular and irregular wave conditions, current conditions and wind conditions may be defined on SYSFIL (or INIFIL). The default wave condition will be the present condition. If no condition has been selected, the first one defined on SYSFIL will be the default. Waves, current and wind can be removed individually.

3.2. Initial positions

New initial positions for all bodies may be specified. Present positions will be default.

3.3. Eliminate degrees of freedom

See User manual for details

3.4. Positioning system

The initial tension may be changed. This is done by moving the anchor position. Print of positioning element locations and plot/print of line characteristics are available. It is also possible to get the line profile printed at the prs-xxx.lis file. To obtain this, the parameter ICPRO in data group CATENARY SYSTEM DATA has to be set to 1. In addition, the user has to give the macro file command @RESU 5 before command 9 : Write file for visualization is given.

3.5. Restoring forces

The horizontal restoring force for translation in any direction or rotation may be specified. Output is presented on plots and tables.

3.6. Dynamic positioning

The following data may be modified:

  • Reference position and heading, including the point on the body to be positioned

  • Start value on bias forces that are not measured. If static forces have been calculated, the default values will be updated.

  • Kalman filter data or PID controller data

4. Print static condition

Initial positions for all bodies will be written to the terminal (standard output) and, if selected also to the print file.

Static forces acting on bodies are written to the terminal and optionally to the print file. The following forces are presented:

  • Average wave drift force calculated for the present heading, including wave-current interaction if specified

  • Wind force (linear interpolation between directions)

  • Current drag force based on linear and quadratic current coefficients (linear interpolation between directions)

  • Munk moment caused by current velocity and different magnitude of added mass specified in individual degree of freedom

  • Positioning element forces

  • Thruster forces

  • Coupling element forces

  • Forces from general line systems

  • Hydrostatic stiffness force

  • Gravity and buoyancy forces, also including small volume hydrodynamic force with wave particle velocities and accelerations set to zero. The forces contain contributions from current, gravity force and any static soil reaction forces.

  • Gravity forces due to time dependent added mass at time zero

  • Gravity and buoyancy force on slender elements and fixed body elements

  • Current drag forces acting on slender elements and fixed body elements

  • Specified forces at time equal to zero

  • External forces, i.e. forces from any special force subroutines linked into the program

If a dynamic positioning system is defined, thrust demand will be calculated according to the thrust demand equation.

5. Equilibrium calculation

The user can choose between two different algorithm in order to compute the equilibrium position of the system: transient method or Newton-Raphson method.

See User manual for details

6. Eigenvalue calculation

See User manual for details

7. Mooring system pretension optimization

By this option it is possible to adjust line lengths (or move anchor position) in an optimal way in order to counteract average environmental forces. The sum of squared differences between pretension and actual tension in each line is minimized.

8. Write initial condition file

After all modifications have been made with the system, the initial condition must be saved. The initial condition file will be read by DYNMOD. It can also be read by STAMOD if more modifications are required.

9. Write file for visualization

A file for 3D visualisation by the stand-alone program SimVis can be written whenever the system description file has been read successfully. This means that the system can be visualized before or after equilibrium condition calculation has been made. Visualisation directly after reading the system description file can be advantageous as a preliminary control of the system geometry.