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Line statics: Which method to use |
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Line statics: Which method to use |
The settings to use for the line step 1 and step 2 statics methods depend on the type of system being modelled and the type of static position desired.
The default settings are catenary for step 1, followed by full statics for step 2, and these are often a good choice. The catenary method is fast and usually gives a good initial estimate of the equilibrium position. Using full statics for step 2 then refines this position to account for the effects that the catenary method omits, such as bending and torsional stiffness and interaction with solids.
In some situations, you might need to choose different methods. Some specific cases are described below, but first here are some general points to bear in mind.
You should choose full statics for step 2 if you want to obtain the true equilibrium position.
If step 2 uses full statics, the initial starting shape for the full statics calculation is that calculated by step 1 so, in principle, the choice of method for step1 is not important: the final static position will be the equilibrium position, irrespective of the initial starting position for the calculation.
There are, however, some situations where the choice of step 1 method is important. In cases where there may be more than one equilibrium position, the full statics calculation will tend to find the one that is closest to the initial starting position found by step 1. Also, the full statics calculation is iterative and may have difficulty converging at all if the step 1 position from which it starts is a long way from the true equilibrium position. In both of these situations, you should choose the statics method that gives the best initial estimate of the desired equilibrium position.
The analytic catenary and catenary methods are both iterative and may fail to converge, in which case OrcaFlex will automatically use the quick method as a fallback. As an alternative, you could instead consider choosing the spline method.
Full statics may also fail to converge. The convergence process in this case is controlled by the statics convergence parameters on the line data form: you might be able to obtain convergence by adjusting some of these parameters.
The problem may, however, be due to the starting position (obtained from step 1) being a long way from the true equilibrium position. If so, it may be necessary to choose the spline method for step 1 and define control points that give a good starting shape for step 2.
| Note: | When setting up spline control points, it can be useful to first set the step 2 method to none. This allows you to refine the spline shape, by repeatedly running the static analysis and adjusting the control points, to get the spline close to the desired shape. You can then reinstate full statics for step 2 to obtain the true equilibrium position. |
The catenary and quick methods both ignore contact with solids and so they may well give a poor starting position for step 2. As a consequence, the full statics calculation may fail to converge, or converge to the wrong equilibrium position (e.g. one in which the line is on the wrong side of the solid).
In both of these cases, it may be better to use the spline method with appropriate control points (as above). This approach is not always very robust, however, and if you are analysing multiple load cases it might not be possible to find a single spline shape that will give the desired static solution for all load cases. In this situation it is often more effective to use a tether to guide the line to the desired contact surface.
For a pipe lying on the seabed there are usually many equilibrium positions, since seabed friction will often be able to hold it in the shape it was originally laid. For pull-in analysis, this as-laid shape is generally known and can be represented by the prescribed step 1 method.
You can then choose whether or not to calculate full statics in step 2. Usually, you would not. If you do, it will have no effect if the prescribed position is already in equilibrium – i.e. if friction is sufficient to hold the pipe in that position. But if friction is not sufficient then full statics will tend to find a nearby position that is in equilibrium.