# Mooring in AQWA

Discussion in 'Software' started by Mikhail, Jan 10, 2011.

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### CWTeebsAnomalyGenerator

If you use the quasi-static solution for composite catenary lines (as
opposed to cable dynamics) this defines the upper bound for motions above
the initial position of the connection point on the vessel. See section
2.4.1 of the ANSYS Help for AQWA.

I circumvent this problem by never having cables attached directly to the sea bed. I create a fictitious body near the sea bed that has no diffracting panels and is not allowed to move. I create the connection point in design modeler by creating a primitive->cone as a surface body.

It's sometimes difficult to get the initial cable properties just right. You need to have realistic values for stiffness, maximum tension and weight per unit length of the cable.

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### omidNew Member

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### omidNew Member

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### JaqenNew Member

Mooring yaw stiffness

Hi everybody,
i'm an Italian engineer. and I've got a question about the mooring problem in AQWA. I'm trying to simulate the behaving of a small hull, anchored with a steel chain with a buoy. I've run many simulations with standard sea characteristics, but every time te hull starts to rotate along its z-axis, in an uncontrollable (and I think irrealistic) yaw that invalidate any other result of the simulation. I've tried many combination of mooring line (even the simple elastic line) and even with double lines on oth sides of the hull, but the problem persist.
So I'm asking if there is a way to insert a certain yaw stiffness in the model trough the mooring line, as it happens in reality.

Andrea

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### CWTeebsAnomalyGenerator

If the yaw motions are truly unimportant you can either deactivate that degree of freedom using the DACF keyword, passing 6 as an argument indicating yaw is to be deactivated. Alternatively you can use a type of hinge constraint.

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### JaqenNew Member

I need to specify that I'm using AQWA through Ansys Workbench, so I don't know where to insert keywors like DACF.
My problem is that I want to know if the yaw is really unimportant with my mooring line, since now I have an unstable behaving of the hull with the yaw, that in my opinion it's not realistic. I would like to give a pre-tension to the cable, or something that may limitate the yaw and stabilize my hull movement. I was looking for someone that may have encountered the same problem and maybe knows how to solve it.

Andrea

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### CWTeebsAnomalyGenerator

I just downloaded and tested Ansys WB v14, and it allows you to insert articulations for use in a time domain simulation from workbench. So, if you have access to v14 you could try inserting a hinge articulation. Otherwise, I strongly suggest learning the native AQWA keywords as a ton of AQWA functionality isn't even possible from Workbench.

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### onaindiNew Member

Help with mooring lines

Hi aqwa users! I am a begginer in aqwa.
I am usin aqua 12. An I am making a project of a catamaran with 3 suction anchors to reduce de roll and the pitch motions. I would like to obtain the graphs of tension in each mooring line. But unfortunately the tension is 0 in the three lines.
First I created an AL....dat file and define the nodes for the mooring lines in the bottom of the sea with jonhswap spectrum, then I created an AF....DAT file for the mooring lines and after run aqwa line again with the AF...DAT. When I open aqwa gs to see the graphs there is no tension in the mooring lines.
You can have a look at the attachment AF...dat file to see if is something wrong/missing in this message
Thank you very much
Alex

View attachment AQWA LINE BARGE.txt

View attachment AQWA FER BARGE.txt

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### khaled79Junior Member

Question in mooring analysis using AQWA..

I'm trying to learn how to model the mooring lines. I've started with a simple barge mooring, I always face a problem while modeling the mooring lines as a non linear catnary. I always get the error :"Excessive tension in cable, ca not calculate accurate initial position". I don't get that error if the mooring line is modeled as a linear type.
The following are my questions in the AQWA mooring analysis.

1. What is the proper type of AQWA mooring for a conventional chain or wire rope mooring line ? Is it non-linear centenary ? what are other types are supposed to used for ?
2. In the non linear centenary type, I've defined a centenary section data first (mass/unit length , equivalent cross section, stiffness (EA), maximum tension , equivalent diameter as per given example data from offshore handbook), and then I've defined the type of the cable lines as per the previous type and gave it length with a proper calculated value (I assumed double the in plane distance from the barge to the anchor), I assumed positive dZ range = 5m, and I did not know where to write the intial tension data. I always get the error : "Excessive tension in cable, ca not calculate accurate initial position".
- Any advice for that error ?
- How can I define the initial tension value ?
3. For the Non-linear polynomial, how can i define the values of the coefficients A,B,C,D and E ? How can I define the initial tension value ?

Regards,
Khaled

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### CWTeebsAnomalyGenerator

AQWA is picky about initial properties of the non-linear cables. The length must be a realistic value. It appears you're defining a scenario where the strain induced from the initial starting positions (i.e. the distance between the anchor and fairlead) results in a stress greater than what the catenary can handle.

You do not get this problem when using a linear cable, but you do need to correctly specific the unstretched length.

If you're modeling a non-linear polynomial I believe you calculate the ABCD coefficients by choosing a displacement strain dx and set it equal to the force at that strain by solving:

Ax + Bx^2 + Cx^3 + Dx^4 = Force

I am not logged into my work computer at the moment and I need to check that formula to make sure it's correct (i.e. I'm not sure if one of those is actually a damping term or something).

If you're doing this inside Workbench the initial tension is calculated by AQWA based on the quasi-static catenary equation (meaning, in this case, you don't specify it, although there are winch elements where you do specify tension).

I only use non-linear cables when absolutely necessary, i.e. when there's clump weights/buoyancy elements, large expected vessel excursion, or when the client needs to know the cable touch-down point.

In the aqwa_reference.chm there is a cable dynamics verification study which includes some typical properties of steel wires and chain.

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### khaled79Junior Member

I'm performing a motion analysis of two floating objects that are connected with a long pipe located above sea water level, using AQWA. I've used two universal connections, each one connects the pipe with one of the floaters. The problem that I can not perform the diffraction analysis due to the error : " Non diffracting structure can not be in an interacting set.. What is your advice ?
Also what is your advice about solving this problem using stiffness connection? How can I calculate the stiffness matrix between two mechanical connected floating bodies ?

regards,
Khaled

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### digijtalJunior Member

Realistic Mooring Values

Sometimes one would not have any mooring data for a concept in development, so errors in ansys are inevitable as the model would not make sense.

Could anyone post some catenary section property values which worked for him with a defined water depth and fixed point distance from the floating structure? That would be really helpful. Thanks in advance.
Digijtal

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### digijtalJunior Member

Khaled79, since your structure is above the water line you must remove the structure from the interaction set. Go to Hydrodynamic diffraction -> Structure Selection and remove the structure above the waterline from the interacting set. What is the structure you're trying to model as I am doing a similar project at the moment. I am analysing a Versabuoy Platform which connects 4 cylinders to a topside by universal joints, similar to what you are doing. Maybe we can help each other. Keep me posted. Cheers.

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### khaled79Junior Member

Hi,
Did u perform any float over analysis before ? How did u determine the damping coefficient of the leg mating unit ?

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### digijtalJunior Member

Nope sry khaled79 I've never done that. Sorry, wouldn't know what to tell you. I was used to using workbench to solve simple problems in aqwa but now that I am finding possible bugs and limitations I am learning the AQWA SUITE programs. Maybe you could know; When using AQWA-LINE I am getting a flat 0 added mass for all structures I model for all frequencies. The tubes all have an added mass coefficient of 1 and I have also added some disc elements with a coefficient of 2. It seems like the added mass is not being taken into consideration in the program. Would you or someone have a solution to that?
Cheers

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