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*Time

Purpose

To define time parameters for an analysis.

Theory

Refer to Time Variables for further information on this feature.

Keyword Format

One of the two blocks of lines below.

Block of two lines for a fixed time-step analysis:

STEP=FIXED

Start Time, End Time [, Time Step] [, Ramp Time]

Block of lines for a variable time-step analysis:

STEP=VARIABLE

Start Time, End Time [, Ramp Time]

Suggested Time Step, Min. Time Step, Max. Time Step [, Step Length]

[Suggested Time Step, Min. Time Step, Max. Time Step, Step Length, Section Start Time]

The variable time-step definition will typically have 3 lines only, so the same maximum and minimum time step values apply for the full duration of the simulation. Should you wish to adjust these variables for different sections of time over the course of the simulation, one or more additional lines may be included (as shown above). Step Length defaults to 0.055.  

Fixed Time Step

Table Input

Notes

(a)In a static analysis, the time variables are largely notional, and are typically from 0 to 1 second, or possibly from 1 to 2 seconds in a restarted static run. Normally, static loads and displacements can be applied in a single step, and if this is the case, no input is required for Time Step - by default it is the difference between start and end times. If a time step is specified, then the static loads and displacements are built up to their full values at the end of the analysis over the specified number of steps. So for example in a static analysis from 1 to 2 seconds with a time step of 0.1 seconds, the loads increase linearly to their full value over 10 steps.

(b)The Ramp Time entry is a time duration - it begins at the Start Time and ends Ramp Time seconds later. It allows control over the build-up of dynamic loads and displacements in an analysis with waves and/or vessel motions. For example, wave loads in a regular wave analysis are typically ramped on over 1 wave period, and the solution then proceeds for a further number of wave periods to achieve a steady state solution. If a Nonlinear ramp is used (and this is the default), a half cosine ramp function is used (as opposed to a linear function).

Variable Time Step

Table Input

Notes

(a)The variable time step option is usually confined to dynamic analyses, although it may be necessary in some extreme static cases.

(b)The program chooses an optimum time step based on two main criteria, namely i) the number of iterations required for the last three convergent solutions, and ii) the instantaneous current period, which is a measure of the dominant period in the response at any particular instant. The exact details are immaterial here, except to note that the process is efficient and largely transparent to the user.

(c)The program calculates the analysis time step, when this is based on the current period, by multiplying the instantaneous current period value by the factor specified under the heading of Step Length. So for example the default Step Length value of 0.055 means the time step is approximately 1/18th of the dominant period in the dynamic response at any time. The default value is adequate in almost all cases, but may occasionally be increased (say to 0.1 or 1/10th) by experienced users who feel the resulting time step is too short.

(d)The Ramp Time entry allows control over the build up of dynamic loads and displacements in an analysis with waves and/or vessel motions. For example, wave loads in a regular wave analysis are typically ramped on over 1 wave period, and the solution then proceeds for a further number of wave periods to achieve a steady state solution. If a Nonlinear ramp is used (and this is the default), a half cosine ramp function is used (as opposed to a linear function).