﻿ Line statics

# Line statics

The line statics calculation is one stage of the overall statics analysis in OrcaFlex. The aim of line statics is to provide a good starting position for the final iterative stage – whole system statics – of the overall statics calculation, by determining an equilibrium configuration for all the lines in the model while all other objects are held in their initial positions.

Line statics is itself a two-step process, for much the same reasons: step 1 of line statics is aimed largely at providing a good starting position for the iterative step 2 calculation.

There are two levels of control over the line statics calculation. Global line statics policies govern which lines are included in each step overall, while each individual line has independent data defining its particular line statics calculation methods.

## Line statics policies

### Step 1 policy

Of the available step 1 policies, none is only rarely used. It has the effect of skipping step 1 statics entirely and moving to step 2 with the lines in their reset configuration. This is not usually desirable, but it may, in some situations, be more productive to move straight from the reset configuration to whole system statics. This option, and the corresponding one for step 2 make that possible.

All lines included is the usual choice here, and will generally give good results. In some cases, however, particularly those involving child lines connected to other parent lines, the parent lines excluded policy gives the best results; for an example, see our detailed blog post on this subject.

### Step 2 policy

The step 2 policies are similar, and again none is only used in very specific circumstances. Systems which include lines connected together, either directly or indirectly, will usually be best solved by the solve coupled systems policy. Under this policy, OrcaFlex detects distinct groups of lines which are coupled together and calculates step 2 statics for each of these groups separately. Such groups are identified by examining both the connectivity between objects and any potential stiffness terms between the degrees of freedom of the model.

For such a group, we must define the statics convergence parameters for the step 2 calculation. We take them to be those of the first line in the group, as listed in the model browser (viewed by types), that is not a child of any of the other lines in the group. This line will be the first-named in the description of the group as it appears in the statics progress window.

## Line statics calculation methods

### Step 1 method

Step 1 calculates a configuration of the line (i.e. positions for all the nodes on the line), using the specified method. This step does not, in general, find a true equilibrium configuration of the line. The primary purpose of step 1 static analysis is to provide a good starting point for step 2.

Step 1 statics is carried out for all lines which require it, under the step 1 policy, before moving on to step 2.

### Step 2 method

The step 2 method is either none or full statics. Full statics calculates the true equilibrium position of the line iteratively, starting from the configuration resulting from step 1. If the overall step 2 policy is to solve coupled systems, then the full statics calculation may also encompass other lines (which is why all eligible lines must have had their step 1 statics completed before any begin step 2).

If the step 2 method is none, then the line is simply left in the configuration found by step 1. You should be aware that this might not be an equilibrium position.

 Note: If you are having a problem with the static analysis of a particular line, and there are a lot of lines in the model, then you can make investigation of the problem easier by excluding the other lines from statics.

### Unstable equilibrium

Sometimes, line statics can find an equilibrium configuration that is unstable. An unstable equilibrium for a line can usually be detected by the presence of large curvature spikes on a range graph. Statics and dynamics results from such unstable equilibria are invalid, and you should modify your statics analysis to obtain a stable equilibrium configuration.

## Summary

Remember that the line statics is only one part of the overall statics calculation, used to obtain an initial configuration to be used by the whole system static analysis. So, even if an inappropriate choice of policy or method leads line statics to come up with a suboptimal configuration, the subsequent whole system statics analysis will usually still converge consistently: it may, however, take longer to do so and, in some cases, be less reliable. The line statics policies and calculation methods are primarily intended to enable the calculation of a good initial configuration to feed into the whole system statics analysis. The aim is to make statics convergence more robust and reliable, not to change the ultimate equilibrium position.