First of all, you mention macros. In 9.4 we got rid macros in the post-processing spreadsheet, primarily so that we could take advantage of multi-core machines.

If you are post-processing extremely large simulation files then this change can be counter-productive because you may end up in a situation where you have multiple threads trying to read disk files simultaneously. This can in turn result in performance problems.

There are usually ways to solve any such problems but it’s impossible for me to advise you without knowing more about your problem and the files you are working on.

I suggest you e-mail us at orcina@orcina.com with more details and best of all a sample input data file. I’m sure we’ll be able to help you solve the problem.

I would like to know if it is another way (faster) to extract results rather by the Orcaflex macro spreadsheet. For the model that I had run it is practically taken days to extract results (dynamic with 3 hours of simulation) which make unrealistic. I am still fine to move from Flexcom to Orcaflex, due in Flexcom should be impossible to run the irregular dynamic cases.

Thanks for your attention.

Alberto

First of all thank you for your kind comments. We are always looking for ways to make OrcaFlex faster and it’s nice to hear that our users are reaping the benefits.

It’s a little hard to answer the question – for a start we are in no position to make any comment on Flexcom’s performance characteristics. I will try to answer as best I can, but I apologise if I cannot include as much detail as perhaps you would like.

I believe OrcaFlex’s finite element formulation to be very efficient. We have also spent much effort optimising the convergence algorithms leading to few iterations per time step. Good convergence benefits statics too and OrcaFlex also provides a number of techniques to aid convergence in statics which is, in general, a harder problem than convergence for dynamics.

Another area that we are have worked very hard on is multi-threading which has been particularly rewarding with the recent advent of many-core machines.

I hope this offers some insight, but if you would like to learn more then I suggest you contact us by e-mail.

I am new in using Orcaflex, I have been working with Flexcom since 2006, so I have enough experience with it. But due several reason the project that I am working now requiere to use Orcaflex. Comparing analisys time spent in running static or dynamic condition between Orcaflex and Flexcom, it has been been notice that Orcaflex is much , but much faster than Flexcom. I am really curious in search which are the reason behide it. Do you have any document where can I find any answers?

Thanks on advance.

Alberto

Slow drift (and vessel calculations in general) can be a complex area, so I have simply given some broad outlines below. If you wish to discuss any of these in more detail then please feel free to contact our support email address (orcina@orcina.com).

Slow drift is modelled in OrcaFlex through QTFs (Quadratic Transfer Functions) – these are specified on the vessel type data form. The QTF data (like vessel RAOs, added mass and damping etc) need to be calculated outside of OrcaFlex (usually they are output from a diffraction analysis or model test).

You mention that your analysis concerns very shallow water. At the moment, OrcaFlex uses something called Newman’s approximation in the QTF calculation, however, this approximation is known to become significantly less accurate in shallow water. To get more accurate slow drift loads in shallow water you need to enter the full QTF matrix and we are currently developing OrcaFlex to allow this in the future.

You also asked about set-down. OrcaFlex will automatically determine the set-down if you use a 6DOF Calculated vessel as then the vessel response will be calculated from the loads acting on the vessel (mooring lines, hydrostatic stiffness etc).

It is also worth considering the effect of set-down on the vessel response. A large set-down could be sufficient to cause a significant change in the diffraction response of the vessel. This would mean that you actually need to use different vessel type data (RAOs, QTFs etc) for different degrees of set-down. However, OrcaFlex does not allow you to change vessel type part-way through a simulation; the vessel type data used must remain constant throughout the simulation.

Having said all this, if the set-down motion is relatively slow in comparison to the other motions of interest then you could simply assume that the vessel mean position stays constant throughout your simulation. You can then analyse different degrees of set-down, each as a separate short simulation.

I hope that these thoughts help but please do get back to us if you want to discuss anything in more detail.

Hi,

I have 2 problems:

(1) I have a mooring model to analysis in shallow water of around 20m. As it appears that there is set-down phenomenon in the shallow water and that the drift effect is more prominent. How do we take this into account in Orcaflex?

(2) How do we take into account the effect of slowly drift? In my understanding, I should obtain RAO with range of frequency covering natural frequency of the floater.

Thanks in advance.

-Ika-

Many thanks for your comment and questions.

On your second question, consider inputting the data via the script facility (so linking directly to your spreadsheet calculations) or using text format data files (.yml). With text format data files you can use the IncludeFile facility so that your .yml file only has the changes you want to make, so it is easier to QA. Quite a few clients take advantage of the text file format to print off a file that they can read and annotate away from the computer in order to review the input data. This option would also satisfy those QA procedures that require a signed-off paper copy of a file. If you look into this kind of procedure, it would be worth obtaining a text editor that ‘understands’ the YAML format – we recommend the free Notepad++.

On your first question, I imagine that you are modelling a complex structure using a 6d buoy with other objects (e.g. single-segment lines) attached to it to give the right distribution of mass, drag, added mass etc. A useful OrcaFlex technique for checking a compound object like this is to clamp it rigidly to a vessel, using a very stiff single-segment line, and clamp it so that the object origin is at the vessel origin. The connections loads on the vessel will then tell you various useful things about the properties of the compound object.

The clamp line should be single-segment, include torsion, have all the connection stiffnesses set to Infinity, and have line type properties that give it very high stiffness (e.g. 1.0e10 for bending, axial and torsional stiffnesses) but otherwise zero properties (or 1e-10 for those that aren’t allowed to be zero). This gives you a clamp that holds the buoy to the vessel pretty rigidly, and so transmits all the buoy loads to the vessel, but doesn’t add any extra loads of its own. And with suitable connection angles you can clamp the buoy at any given orientation you want.

Here are some examples of things you can check using this approach:-

- If you place the vessel such that the object is in air, then the statics results vessel connection force will be in the global Z direction and will equal that total weight. And the X and Y components of connection moment, when divided by that weight, will tell you the horizontal offset of the CG from the vessel origin (and therefore also from the buoy origin). This gives you the x and y offset of the CG. To get the z offset, you can turn the object through 90 degrees (adjusting the clamp connection angles to match), so that the object is now clamped to the vessel with its z-axis horizontal, so that the X or Y connection moment (divided by weight) now tells you the CG z offset.

- You now cancel out the weight force by applying to the buoy a global applied Z-force upwards equal to the weight and applied at that CG. If you then you place the vessel so that the object is fully submerged in the water, then the connection load will now tell you the buoyancy force, and again the X and Y components of connection moment, when divided by the buoyancy, will tell you the offsets of the centre of buoyancy from the origin.

- You can do similar things with drag and added mass, by running a short simulation with the vessel given a suitable simple motion and then looking at the time histories of vessel connection loads. For example you can set the vessel to have Prescribed Motion and give it either a Constant Speed of 1m/s (for analysing drag) or a Speed Change that gives a constant acceleration of 1m/s^2 (for analysing added mass).

- You can check moments of inertia in a similar way, by using vessel Harmonic Motion to give the vessel some angular acceleration about the vertical. This time the connection loads will vary harmonically, but you can take the results at a given time and divide by the vessel angular acceleration at that time. (You should use a short time step for this, but the simulation can be quite short and so very quick.)

I hope these suggestions help. If you’d like clarification, then email orcina@orcina.com for support.

Since an OrcaFlex model can be complex with many components and properties, it may contain hidden errors that can have big effects on the model’s behavior or loads. Plus the user can accidentally enter a wrong value. Can you please recommend:
1) Some basic reality checks that should be performed on any completed model, such as turning off the environment and checking the model’s still water draft
2) Some methods to extract and document all the user choices that went into the model, such as a routine to generate a report on the dimensions and properties of each model component

Also is there a way to display or calculate the CG of a model made up of many components, rather than the CG of each component?

Thanks!