19.10. The LBRK Data Record - Mooring Line Break

The LBRK data record allows you to specify that one or more mooring lines should be broken (de-activated) during an analysis. The format of the LBRK data record is as follows:

      2    7    11   16   21        31        41        51
     - ---- ---- ---- ---- --------- --------- --------- ---------
    |X|    |LBRK|    |    |XXXXXXXXX|         |         |         |
     - ---- ---- ---- ---- --------- --------- --------- ---------
             |     |    |                |          |        |
             |     |    |                |          |        |_(4) Breaking tension at 2nd structure (AQWA-DRIFT/NAUT)
             |     |    |                |          |
             |     |    |                |          |_(3) Breaking tension at 1st structure (AQWA-DRIFT/NAUT)
             |     |    |                |
             |     |    |                |_(2) Time at which to break the mooring line (AQWA-DRIFT/NAUT)
             |     |    |
             |     |    |_(1) Mooring line number to break (I5)
             |     |
             |     |_Spectrum number (AQWA-LIBRIUM/FER) (max 20)
             |
             |_LBRK Data Record Keyword

(1) The mooring line number is taken from the order in which the lines are defined.

(2) The time has two meanings.

If no tension is defined the line will break at this time.

If a tension is also defined, the line will break the 1st time the tension exceeds the specified value, after this time.

(3) The mooring line will break when the tension at the 1st structure on the LINE/NLIN/NLID data record is greater than this value, if the time is greater than the specified breaking time (2).

(4) The mooring line will break when the tension at the 2nd structure on the LINE/NLIN/NLID data record is greater than this value, if the time is greater than the specified breaking time (2).

Note:

Logic for breaking lines.

   TIME               TENSION 1         TENSION 2        RESULT
not given or 0     not given or 0    not given or 0    Always broken
not given or 0           F1          not given or 0    Breaks when tension 1 > F1
not given or 0     not given or 0         F2           Breaks when tension 2 > F2
not given or 0           F1               F2           Breaks when tension 1 > F1 or tension 2 > F2
    t1             not given or 0    not given or 0    Breaks at time=t1
    t1                   F1          not given or 0    Breaks when tension 1 > F1, after time t1
    t1                   F1               F2           Breaks when tension 1 > F1 or tension 2 > F2, after time t1

In Aqwa-Fer and Librium the line is always broken; the time and tension values are ignored.
The normal usage of the LBRK data record for Aqwa-Fer/Librium is for a specified spectrum rose (e.g. N,NW,W,SW,S etc). An initial run is performed to determine the worst line tensions and then a second run is performed with identical data apart from the LBRK data record. Thus for, say, 8 spectra, the equilibrium and significant motions can be determined in two runs (each) for all 8 spectra.