Opening in API tank wall for equipment access

[hts]

A contractor wants to be able to cut a door sheet opening in an existing out of service domed roof API tank to get equipment inside to scrape out sludge, etc. What reinforcing needs to be installed around the proposed opening to make tank safe (opening to be 12 foot square in a 126' diameter x 57' high tank). Are there any guidelines, etc. Thanks.

 

[jte]

hts-

One approach commonly used on for example vessel skirt access holes is simply area replacement. If the cross section (looking down) of the cutout is say 24" x 1" thk then add a collar on the hole which supplies 12 sq.in. on each side. Simplistic but logical and effective: You are looking to maintain the moment of inertia of the original section. By replacing the removed area the A*D^2 term remains the same. This, of course, is applicable when the cutout is not in such a small diameter vessel such that the centroid of the cutout is significantly larger than the centroid of the collar (reinforcement). I ran into this issue with a small diameter vessel which needed a part of its shell replaced.

For a tank, a similar approach should work. One additional issue for tanks which have a relatively larger radius:thickness ratio would be local buckling effects. All in all, doorsheet cutouts on large vessels and tanks are done relatively often, and I've never heard of a failure...

jt