Minimum Liquid Level to Mitigate Overturning...

[Swlamech]

Hello,

I am performing Tank re-rates and basic API-650/653 calculations for clients along the Gulf Coast. Almost all of the clients have a Hurricane preparedness procedure that involves filling their tanks to a recommended liquid level to provide overturning stability and additional stiffness to counteract the possibility of an elevated external pressure situation. I have developed a nice spreadsheet that takes care of most of this but I am now trying to incorporate this "Additional" liquid level calculation so I can recommend an liquid level to clients.

The second overturning condition of API-650 uses the M(f) term for fluid weight and determines the maximum liquid hold down moment based on a band of fluid a certain distance inward from the wall. I understand that this is supposed to account for the fact that the floor plate is relatively weak and acts as a membrane and after a certain point will begin to diaphragm thereby lifting the tank.

I have been instructed to just add the weight of the additional liquid level directly to the dead load and I believe this to be incorrect.

My question for you is: How can I correctly take into account additional liquid level to combat overturning ?

Thank You in advance for any input...

 

[IFRs]

Based on experience from hurricane Sandy, stability was an issue but also buoyancy when the dike was filled with water. Some tank dikes were filled and the water kept rising, floating tanks out of the diked area. To be truly prepared you may want to consider filling the tanks to prevent them from becoming boats.

 

[JStephen]

If you KNEW the tank was going to be full, you could reasonably use the same overturning resistance due to liquid as is used for seismic design. The allowance given for resistance to wind loading is assuming that minimum fill and maximum wind are less likely to occur simultaneously, and also requires a larger factor of safety when checking for this condition.

 

[Swlamech]

Thank You, I will look into the seismic overturning section. We never use appendix E, it looks like it may have what I need.

 

[Duwe6]

And don't forget buoyancy. The 'odds' of having your dike fill with stormwater is much higher than your odds of having to withstand even a Category 3 hurricane-force winds. Sandy wasn't even a Cat-1, and the storm surge was tremendous.

Also, you can 'think outside the box' and deliberately add water to your hydrocarbon tanks if the contents are not misciable with water. A tank with a water-fill up to the top of the dike, and a few feet of oil on the oil is much easier to accomplish than finding enough oil to fill that tank full enough that a dike-full of water won't float it. Or to convince your EPA-compliance guy/gal that you REALLY need to cut notches in your tank dikes to limit the stormwater fill level.

 

[Swlamech]

I am currently using some non-linear FEA models to develop a relationship between additional liquid level and usable hold-down load. I will post my results at a later time.

 

[Duwe6]

We will look forward to the results, sir/ma'am.

 

[darthsoilsguy2]

i am looking forward to those results too. earlier in my career, i had recommended minimum levels for overturning weight, and came up with the same question you posted clowerymech... i suspect this is not a bad assumption for small diameter (<15')tanks but at 40+ diameters i'm not so sure.