Is anyone familiar with the natural period of the system of a floating structure in ANSYS AQWA after analyzing hydrodynamic diffraction & hydrodynamic time response?
I have modeled a Tension Leg Platform for a wind turbine in ANSYS AQWA and have analyzed the structure for hydrodynamic diffraction & hydrodynamic time response. Also I want to know which type out of the 4 types of mooring cables should I select ? Cable non-linear polynomial or non-linear steel wire ? How to select the stiffness and un-stretched length.
Hello Yeshwant! What is the issue you have with natural periods? AQWA calculates them based on the hydrodynamic stiffness but I do not recall whether it takes into account also the stiffness of the mooring.
Regarding the type of mooring line, it depends on the design of your platform. Standard O&G TLPs employ steel tubes but I suppose in your case you are probably thinking of steel wire ropes so the latter option might be more suitable.
Thank you so much sir for your reply. I am new to ANSYS AQWA. I found out that the undamped natural periods (without the mooring cables) is shown in .LIS file. The structure is actually a TLP floater for wind turbine (ref. attached file).
Q.1> Why is the natural period of the system is shown zero always in surge, sway and yaw ?
The depth of water for my model is 200m. The model is 32m deep inside water. I am using non-linear steel wire as the mooring cable.
Q.2> I don't understand how to decide the values for unstretched length, asymptotic stiffness, asymptotic offset. Where can I find more details about definition of these terms ?
Based on time history motion simulation I found out arbitarily ifm y unstretched length is around (200-32) = 168m with asymptotic stiffness = 10e6 N/m and asymptotic offset = 1m the motion is not completely haphazard but I get output info as "solution diverged after certain time" ( ref. attached file ). If I try to increase the stiffness I get time step error ( ref. attached file ) so I decrease the time step.
The user manual says :
T = k (x - d (tanh(x/d)))
where
T = Tension in mooring line.
x = extension of mooring line
k = asymptotic stiffness (constant)
d = asymptotic offset (constant)
The names of the constants k and d arise from the fact that, at large values of extension, tanh(x/d) tends to unity and the equation tends to the asymptotic form:
T = k (x - d).
Kindly give some insight.
You can also send me message on [email protected]
Thankyou.
HI Yeshwant,
Sorry for the delay in replying.
Regarding your two questions:
Q.1: The natural frequency in surge, sway and yaw of a freely floating structure is zero (the natural period should be infinity as a result). This is due to the fact that there are no restoring forces on a floating structure in those degrees of freedom and indeed if you had no mooring system, your platform would drift away and would not be capable of maintaining its position in the horizontal plane (whereas it would maintain approximately the same position in the vertical plane). A mooring system in place induces some stiffness and therefore there are indeed natural periods in the motions that are associated to the stiffness of your mooring system. You should check whether AQWA can provide you the natural periods that are dependent on the moorings. If it cannot, then you should derive the natural period by doing some calculation based on the stiffness of your system. For a TLP with purely vertical tendons, the natural period in surge/sway is dependent on the square root of the length of the tendons and inversely proportional to the square root of the net positive buoyancy (which is going to be equal to the pre-tension of your system). If you need more details, I can refer you to some books or even explain better via private message.
Q.2: I am not really sure on the use of the "asymptotic offset" here. I would think the value of stiffness is not very high and you could potentially use something larger. I would suggest to select the asymptotic offset based on the pre-tension you want to reach at equilibrium. If you have established the configuration of your platform (i.e. mass, inertia and draught) then you should be able to compute the net positive buoyancy that will generate the tension in your tendons. You should use that value to identify the most appropriate offset.
I hope this helped. It has been some time since I used AQWA but I will try to have a look at it.
@Pierpaolo Ricci I have sent you a private message on researchgate, waiting for your reply. Or you can ping me on [email protected]
Dear Yeshwant, I'm currently having a problem exactly the same as yours, noted in these pictures. Did you solve it? What was wrong? Your problem accuses errors exactly like mine.
You should not confuse the natural period of an ocean / offshore body in ANSYS AQWA. The natural frequency of a freely floating structure is zero because there are no restoring forces on a floating structure in those degrees of freedom. However, if there is no mooring system, it will move /drift some distance off. Thus, the natural period should be infinity as a result of this
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