1. Discretisation Why Worry? Check Static Model Visually
Too fine and the run takes longer
Too coarse and behaviour is missed or wrong
Check Static Model Visually
Does it look like too much or too little?
Make finer in regions such as touchdown, near solids, near step changes in the properties
Make coarser in regions where little happens, deep water catenary suspended lengths for example
These two topics have been put together because they produce what we call “How long is a piece of string” questions. We are often asked how much segmentation is needed or how do we get the model to converge.
We cannot give an absolute answer because it depends so much on the system being analysed. Those that have asked these questions will be aware the standard reply is “Send it in and we’ll have a look.”
This presentation will show you some of the things we check.
First judging the segmentation on your model. It is important because too little can result in an inaccurate model and too much can result in a longer run with no additional benefit, see 2004 speed-up presentation on the website.
Check visually when you have first built your model. Is there any place where the line curves or a discrete change is expected?
Touchdown region. Exit of a bend stiffener. Lift off from an arch. Near a solid. These areas will need more detail.
Remember to reduce segmentation sometimes too. In deepwater catenaries there are long suspended lengths doing very little. They can get far coarser than the touchdown region.

2. Discretisation Check Dynamic Model Visually Check Results
Are lines passing through solids, catching on the edge or failing to follow the profile?
Does the line seem to kink rather than curve
Are nodes bouncing on the seabed?
Check Results
Tension Time History: ‘Spiky’ or high frequency variation
Tension Range Graph: Euler limit infringed and high compression
Curvature Range Graph: Smooth variation
Look at the animation. Is it behaving as expected?
Is solid contact behaviour right. Remember they contact the nodes only. Look out for nodes catching on the edges of solids or the solids passing between the node gaps. Also that the line can curve to follow the solid profile.
Similarly a line that appears to kink may be trying to form a curve but with insufficient segmentation.
Look for big nodes bouncing on the seabed as the touchdown moves - the hinge laydown where it acts like a series of sticks.
Check the quality of the results. Don’t use it as a ‘black-box’.
Tension - you would expect smooth variation with time. Spiked or ‘noisy’ responses should be checked but remember they may be real. Euler limit checks and high compression in range graphs are a warning. See the 2004 compressive wave presentation on the website.
Curvature - a smooth variation with arclength unless a known discrete change. Spikes could be insufficient segmentation to follow the curve fully. Noise could be high frequency waves from a discrete event.
@5mins - go to examples for 10 mins

3. Statics Convergence Where to get information Message Box
Statics Progress Window
Out-of-Balance Loads (Shift+Ctrl+Y)
Your eyes
We now move onto static convergence. I am assuming you would all start with a reasonable estimate of where things are.
Convergence can be a dark art. OrcaFlex has lots of facilities to help but you need to know when and how to use them.
First where do you get information on where static convergence is going wrong.
The message box will tell you how the statics on the object being processed is getting on. It moves fast and clears when statics is done.
Click on it and you get the statics progress window. This will tell you how all the objects are progressing through each system iteration. It updates through the run and the information will stay there until the next static run.
The out-of-balance forces will show you the net load on any vessels and buoys in the system.
Most important are your eyes. Look at the view while you run statics. What is the model doing through each static iteration.

4. Statics Convergence What to look out for
Object overshoot - more Min 1-1.5, Max in steps OR bad 1st guess
Statically indeterminate Buoys - Check DofF or give an out-of-balance force
Circling solution but never quite reaching it - increase tolerance (care!)
Multiple dependent objects - fix some, run simulation, then save final positions. Then start again with all free.
I will show you some typical problems in minute but lets run through things to look out for.
Objects throwing themselves around, overshooting. Probably more damping needed. You will need more iterations as a result.
Buoys statically indeterminent. They sometimes need a good out-of-balance load to start them off. It gives OrcaFlex a clue of the right direction to head. Or you may have forgotten to restrain a degree of freedom.
Rattling between two tolerances but nearly there. This is common with long lines on seabeds. Imagine children holding hands while they jump on a trampoline then someone shouts stop. It can also happen with stiff systems. Increasing tolerance slightly, it will settle dynamically. With stiff systems take care.
A similar one is the non linear bend stiffness for a bend limiter. If the slope is too steep it can do this. It is skipping between locked and free.
Multiple dependent objects. Fix some and run the others. When solved, save their positions and run with more free. This may take a couple of iterations.
@10mins so 5mins examples

5. Statics Convergence If all else fails?
Fix the objects and let it settle dynamically
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