Ansys Aqwa - Ship alongside quay mooring

Hi All. New here.

Current question relates to a 220m long ship alongside a quay with 13 mooring lines and 3 fenders.

I am new to Aqwa although having been using Ansys FEA for some time, so familiar with the interfaces.

I have done the catenary spread mooring in the tutorial and another actual 8 point catenary mooring with success.

I am now busy with a vessel alongside, and keep getting errors regarding the Hydrodynamic Interacting Structures moving apart etc etc

I eventually suppressed the ship and ran it for the quay only, but the quay also moves....although just once....it drops by 4.5m (-4.5m in Z dir). This inspite of being set to fixed, having its base surface set to non defracting, and having a rigid joint defined to a Fixed Point.

I then tried suppressing the quay and running for the ship only by setting up 4 fictitious (linear) mooring lines to fixed points, and suppressing all the actual mooring lines and fenders. Through a 180sec analysis the displacements are initially zero and then suddenly it starts moving dramatically (by 100's of meters).

Does anyone have experience in alongside analysis in Aqwa who can advise me. I would also ask my local Ansys support but they don't support Aqwa (only the other modules)

Many thanks

 

I would really need to look at the workspace to answer your questions. I have some ideas of what's going on as I've encountered many of the same problems before.

I don't use Aqwa in Workbench, I edit the data files directly and run in batch mode, basically because Aqwa inside Workbench is too buggy and excludes much of the functionality I've needed for various projects.

 

CWTweebs

I realise that that issue with the hydrodynamically interactive structures moving too much, is because the hydrodynamic diffraction was calculated for the vessel and quay very close to each other (in this case 1.2m apart) and with an offshore beam wind the vessel would hang well off the quay (by say 3 or 4 meters anyway) and then the pre-calculated HD is then not applicable, hence it throwing out the error. This is why it the Time Response runs for small beam winds but not high winds.

I don't see a way around this other than what I am trying now and that is to suppress the quay altogether and to attach the lines and fenders to fixed points.

I agree the WB Aqwa interface it very buggy. Forever hanging with blank screens only to eventually come back to life, but I was introduced to Ansys (FEA structures) only after WB was in play hence never learned the "hardcore" Classic route.

 

There is no limitation on the movements of the vessel if hydrodynamic interaction is turned off.

 

New Error - D_CATSEG: Negative Horizontal Force Illegal

Tweebs can you shed any light on the meaning of this error message.

I dropped the quay, and had a working model running.

I needed to turn my vessel around, ie moor it Port Side To, so shifted the connection points across, moved the Fixed Points across and to their correct position, by altering their coords, and re attached the cables.

Also had to shift the fenders across and reverse the X dir of the Fender Axis and Contact Axis

Now I get it failing on this error all the time.

Wind and Current Coefficients have remained unchanged.

Any help would be greatly appreciated.

 

Cable dynamics, i.e. simulation of non-linear moorings, is failing. Try adjusting the length of the mooring line. Slack moorings are not simulated well in Aqwa.

 

Cable Section Tension

Many thanks CWTeebs for the advise on slack centenaries throwing out that error. Spot on.

Another quick question if I may.

I get all the tension results I need from Whole Cable Forces, but I always get zero tension if I use Cable Section Tension, and then select the appropriate section of the catenary?

They obviously cant be zero so I am not sure what I am doing wrong here.

 

If you use the PMST option, which means Print Mooring Section Tensions, then by default it prints the fairlead tension and the anchor tension. You may have a scenario where the mooring forms a simple catenary, e.g. the only tension is due to its own weight, and the bottom tension, e.g. at the anchor, does indeed experience no tension.

Questions I'd be asking:
What is the laid length? How far away is the anchor from the fairlead? What's the total length of the mooring line? Which time domain module are you using? Is there any anchor uplift (z component on the bottom tension)?

 

Thanks CWTeebs

My catenaries are very long as the area in question has many subsea pipes and cables to clear.

Up 1500m catenaries, including some 500m of dyneema, floats, 76mm wire rope and 3" chain etc.

However the last 500m is 3" chain in all cases and the laid length remains more than 300m even at the breaking strength of the weakest section, hence there is never anchor uplift.

I get no tension in the graphs for any selected section of any mooring line when selecting the output "Section Tensions", even when the "Whole Cable Tension" of those lines are in excess of 200 tonnes.

I wonder if there is a flag somewhere that is switched off, hence not printing the section tension pe say?

BTW using Workbench

Thanks

 

statistical post processing

CWTeebs, I have another question, if I may.

As you know I have no local support on Aqwa hence if I dont find something in the documentation, I can be a bit lost.

I have a requirement imposed on me to meet the requirements of certain codes, which in this case require 3hr TD runs and statistical analysis of the results with separation of mean values, rms values and statistical max values, for low frequency and wave frequencies responses, in terms of offsets and tensions.

I have been informed that Aqwa has post processing tools which perform these statistical analysis, but cannot find any reference to it in Workbench or their reference documentation.

I assume it must be in AqwaGS?

Is it possible to use AqwaGS to post process results obtained in Workbench? If so how?

Where in AqwaGS (which I have not used before, being a strictly Workbench Ansys person thus far for FEA and Aqwa) would I find these tools?

Appreciate any help on this as always.

 

I have since managed to get some support from Ansys who did a webex session with me that covered the use of AqwaGS on a Workbench model and results, and statistical analysis.