Line data: Statics convergence

The static position is obtained by an iterative process, in which OrcaFlex tries to converge on the solution in a series of steps. This process is controlled by the statics convergence data, found on the lines data form.

Convergence control method

Either line search or mag. of std. error / change. We recommend using the default value, line search, but if this fails then the alternative mag. of std. error / change may succeed.

Do intermediate non-torsion full statics solve

This option is only available when torsion is included.

• When the option is checked, the full statics analysis is performed in two stages: line statics is first solved with torsion not modelled, and then solved again with torsion modelled. The result of the first stage is used as the starting point for the second.
• When the option is not checked, full statics is solved in a single stage with torsion modelled.

The static analysis is usually quicker and more robust if the intermediate non-torsion solve is performed. However, some lines, typically those involving significant amounts of twist or pre-bend, may converge more robustly if the intermediate non-torsion solve is not performed.

Max iterations

The calculation is abandoned if convergence has not been achieved after this number of steps. For some difficult cases, simply increasing this limit may be enough to obtain a solution.

Tolerance

This controls the accuracy of the solution. OrcaFlex accepts the line position as a static equilibrium position if the largest out-of-balance force component on any node is less than tolerance $\times$ total weight.

When torsion is included the static equilibrium position must also satisfy the condition that the largest component of out-of-balance moment on any node is less than tolerance $\times$ total weight $\times\ O\!D\urm{max}$ where $O\!D\urm{max}$ is the maximum diameter, outer contact diameter, stress diameter or normal drag diameter over the entire line.

Reducing the tolerance value will give a more accurate static equilibrium position, but it will take more iterations to get there. OrcaFlex may not be able to achieve the tolerance specified if it is too small, since computers have only limited numerical precision.

Min damping, max damping

For some cases it is necessary to control the convergence process by damping down (i.e. reducing) the step taken at each stage. OrcaFlex includes an automatic damping system that chooses a suitable damping factor for each iteration, but you can define the minimum damping and maximum damping factors to be used.

Normally the default values will suffice but for difficult cases the default values can be altered. For cases that appear to make the convergence unstable, try increasing the min damping factor to a value greater than 1, say in the range 1 to 10. You can also try increasing the max damping factor, say to a value in the range 10 to 100.

Mag. of std. error, mag. of std. Change

These parameters are only available when the mag of std. error / change convergence control method is selected.

For some cases it may be necessary to reduce one or both of these values from their defaults. Doing so is likely to slow down the convergence, but it may be necessary in order to achieve convergence.