# Spar Bouy - Thin Cylinder

Discussion in 'Stability' started by Eugene Mak, Mar 13, 2014.

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### peterengSenior Member

I think we need to clarify the use of "CoG". the CoG only refers to the centroid of the masses involved. The CoG of the waterplane and volume does not exist or the term is being used incorrectly. The correct term is the centroid (or geometric centre) for these positions. And yes as Tansl states this problem is not simple. Plus the answer you seek needs to be stated so you can either develop a method to approximate this or you need to do it accurately using FE. The main problem as I see it is the catenary load is non linear so depending on your cable geometry this will make the problem easier or harder. ie the further out you place the anchors the more the catenary becomes significant, the more difficult the probem becomes to solve manually. Regards Peter S

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### jehardimanSenior Member

Do not confuse a reference axis related to the body, to the global axis system. TANSL wishes to look only at the body, not how the body is moving in the global axis system. Are you looking to minimize the pitch/roll motion of the buoy, or to minimize the off station x/y location of the buoy? Based upon cross-coupling, these two are different. Let me get back to the office on Monday and I will check were to look in Berteaux.

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### TANSLSenior Member

Of course, the movements of the object always depend on the degrees of freedom we give it. This is obvious and would not need to explain. But any movement of the object can be reduced to a combination of a translation and a rotation. This is also obvious. The rotation can always be decomposed into three rotations around the three principal axes of the object, X axis, Y axis and Z axis Those axes, always pass through the center of gravity of the float at any times. This is what, may be not so obvious, but, in my opinion, is how the movement works.

(..... this is the plane that the bouy trims around : that is not possible)

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### peterengSenior Member

Hi Eugene - If you publish a drawing with all the required geometry and weights etc I'll consider running it as a FE model for your info. I can establish its stability condition if this is what you want. If you have a thrust on the turbine this can be included as well. You state that you may use synthetic cable for the anchors. Chains are used as a damper. If you use synthetic that is close to neutral bouyancy how do you pretension the system and maintain tension in the system
(due to tide?)? I've been involved in rubber bungy stability of marina pontoons and it works very well. I assume you will have tidal variation to account for again this is what the chains are used for. Cheers Peter S

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### jehardimanSenior Member

Try this paper, you can read it from that site, but you cannot print it. (...though whatever is on the screen is effectively yours anyway...)

https://dspace.mit.edu/handle/1721.1/46545

I looked in Buoy Engineering, and while the theory is there, there is not a clean worked example for a moored spar buoy. Perhaps a better text, which has a worked example for a moored structure, is Advanced Dynamics of Marine Structures by Hooft. The worked example is chapter 9 but it presumes a decent knowledge of hydromechanics. For a basic introduction to floating structure dynamics with lots of worked examples I would recommend Dynamics of Marine Vehicles by Bhattacharyya. These two books, are part of the Wiley-Intersience series (like Beryeaux) and one builds on the other.

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### Eugene MakJunior Member

Hi Guys, thanks a lot for your help so far. That’s quite amazing experience being able to draw on knowledge across the globe.

We have run quite a few tests in a home-made wind tunnel with the bottom part dropped out and having a rain water tub underneath. The definite conclusion is “the closer anchor attached to the surface, the less angle of a heel”.
https://dl.dropboxusercontent.com/u/41354924/March 2014/13.50, Test 3.MOV

Hi Eugene - If you publish a drawing with all the required geometry and weights etc I'll consider running it as a FE model for your info. I can establish its stability condition if this is what you want. If you have a thrust on the turbine this can be included as well. by PETERENG

Yes please. Attached is “Buoy Schematics”. That’s was our initial shot. One thing we would change is a padeye would have to move up, as close to the surface as possible. I will send the thrust value as a side force in N shortly.

I am not that strong at maths and softwares. My personal preference is hands-on model testing and observations. The typical fisherman buoy (attached) has to stay upright all the time so it can be detected from afar and has to be constructed on a shoe lace budget. That’s pretty much about us...

You state that you may use synthetic cable for the anchors. Chains are used as a damper. If you use synthetic that is close to neutral bouyancy how do you pretension the system and maintain tension in the system (due to tide?)? by PETERENG

Why do I have to pretension the system? I am not worried about it “going for a walk” in the large envelope, the only thing I am concerned is reducing a pitch/ heel. On the other hand I am going to test this prototype in Baltic, where the tide virtually non-existent.

Thanks a lot
Eugene

#### Attached Files:

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• ###### Buoy Schematics.pdf
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### TANSLSenior Member

I think that some, including myself, did not understand the nature of the buoy which you spoke and, therefore, what we have said does not apply to this case.
Cheers

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### jehardimanSenior Member

If that's the size it is, there is already a 60cm model out there.

http://www.wpi.edu/Pubs/E-project/A.../unrestricted/MQP_Final_Report_03_09_2011.pdf

FWIW, that much free surface in the water ballast is going to be a problem. You need to calculate a max/min draft based on expected water density changes and ballast for that making the water ballast tank (if any) very small with minimial free surface.

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