Anaerobic digester gasholder buoyancy and guidance system

[Engineer1279]

Hi all,
I have a buoyancy problem that I am stuck on. As I understand stability and buoyancy if the floating structures center of gravity is above the center of buoyancy the structure will tip over and capsize. For steel anaerobic digester covers this is a typical configuration. The average skirt length is between 6 and 8 feet making the center of gravity between 3 and 4 feet. The average ballst height is 2 to 3 feet making the center of buoyancy around 1 to 1.5 feet.

This is why there is a guidance system on these covers. The number of guides depends on the size of the cover and usually have a guidance shoe at the top and bottom of the skirt.

(1) As the cover starts to tip where is the center of rotation? Is it through the center of buoyancy (this is what I think it is) or the midpoint between buoyancy and gravity?
(2) suppose there are 9 guides evenly spaces around the cover. How do I determine which guides will hit first? A 2D cad file isn't cutting it as I can't see all the guides at once. I do have a 3D file, but can't get the constraints to cooperate as I try to rotate the cover. Also, the 3D model doesn't care if the two material physically overlap so I don't know how far to rotate.
(3) Lastly, as the cover is tipping and stopped by the guides, how much force is it applying to the guide. There are two directions that I need to look at, around the axis of rotation and perpendicular to it.

I know this has been long post but I think I described what I'm looking for. Any help would be most appreciated on this topic.

Thank you,
Engineer1279

 

[SteveWag]

Engineer1279

The last floating cover I did was 50-feet in dia., and without the ballast blocks, weighed about 40,000 pounds. The ballast blocks added an additional 250,000 pounds. Dimensions were about the same as yours. I think the center of gravity was a few inches below the top of the ballast blocks, rather than thru the horizontal center of the steel structure. The center of buoyancy was above this, making the cover stable under all conditions, assuming the safety vents were operable.

Steve

 

[bimr]

The "guides" normally consist of rollers and guides. Lack of maintenance and corrosion causes the the rollers and guides to jam, which then causes the cover to tip as the cover moves.

It does not make any sense that covers are designed to be unstable.

Do you hae design drawings from the manufacturer?