2   5  7   11          21        31
    - --- -- ---- ----------- --------- ---------
   |X|   |  |TIME|           |         |         |
    - --- -- ---- ----------- --------- ---------
       |  |   |           |       |         |
       |  |   |           |       |     (3) Start Time (F10.0) (default zero)
       |  |   |           |       |
       |  |   |           |       |_(2) Value of Time-step (F10.0) (default 0.1s)
       |  |   |           |
       |  |   |           |_(1)Number of Time-steps (I10) (default 10)
       |  |   |
       |  |   |_Compulsory Data Record Keyword (A4)
       |  |
       |  |_Optional User Identifier (A2)
       |
       |_Compulsory END on last data record in Data Category (A3)

(1) This number (NT) governs the length of real time simulated, i.e. simulation time is given by (NT-1) × DT (see (2)).

(2) This important parameter (DT) governs the accuracy of the integration of the equations of motion. This value must be small enough to enable an accurate representation of the highest frequency present in the motion of a structure. Failure to do so will at best give an inaccurate simulation, and at worst will cause divergence of the integration scheme and the program will abort.

For pure sinusoidal motion, this value should not be greater than 1/10th of the period of that motion. For non-sinusoidal motion, the user should consider how accurately a set of discrete points (whose interval is the value of the time-step) would represent the motion of the structure (see the Table of Time Integration Parameters).

(3) This is the time (ST) at the start of the time-history simulation period, so that the time at the end of this period is given by ST + (NT - 1) × DT. It is normally left blank or set to zero except when starting the simulation from a previous analysis or when the user wishes to alter the initial phase of frequency-dependent parameters.

To change the time-step during the analysis, multiple TIME data records may be used, up to a maximum of 10.

General Points Regarding the Time Integration Parameters

The values of all of the time integration parameters are dependent on the type of analysis, but with experience the user should have no difficulty in estimating their value for any particular problem.

In addition, it should be pointed out that more program automation of these values (e.g. the automatic variation of time-step based on accuracy of the integration of the equations of motion) has deliberately been avoided. This is intended to make the user more aware of the approximations necessary when representing discontinuities in the motion of a structure which are typically present in non-linear simulation analysis.

The following table shows typical values of time integration parameters for a large barge or tanker. This must not be considered as an accurate guide but an indication of values to be input by the user. If the user's values are considerably different from these, then it is likely that an error has been made in their estimation.