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Ladycroft39 I have some basic doubts on your problem. Your first note informed that only a small portion between annular ring plate and concrete base you expect rain water seepage. The recent description gives a different impression (based on your discussion with an off-shore engineers). If it is crevice attack, i expect, it cannot be large areas between concrete base and annular ring plate. To my mind, it looks it is a simple problem of choosing a good epoxy sealant which can be pressure injected in both concrete base (where there are cracks) and between the annular ring plate and cocrete base whereever you have gaps or voids. Alternatively, as Debasis had suggested, remove portions of concrete where you have large cracks or voids in concrete - pressure seal with fine pressurized sealing concrete mixture + give a tight adhering epoxy grouting sealing compound (either hand trowel type or pressure grouting type). This can be done - on-line- when tank is in service without much problem. 2. My doubt or i would like clarification - If lifting of tank (during a shut down) by balloning float type - i think your problem is one of major flooding to affect a major portion of the bottom of the tank with pitting. For this injecting an inhibitor , to my mind, is not a solution. Epoxy sealing of local areas would be a better solution. How much inhibitor you will inject and how are you sure the inhibitor injected is protecting the bottom surface already pitted. Once pitting had already started and you expect continuous rain water seepage through voids or cracks in concrete or between annular gaps from ring plate to concrete base, this means the problem is not one of crevice attack. Flooding due to rain water at tank areas has to be stopped - according to my view from the description of the problem in this discussion forum. 3. Yes it is possible to map out by UT thickness profile of the ring plate and the inside surface of bottom plates to get even an idea of how much the bottom plate is pitted, the profile area of pitting of the bottom plate (underneath surface) especially the tank has been constructed with a 120 deg gradient taper from center to edge. 4. If flooding is the cause, then study the gradient at the outer and eliminate flooding at all costs. That should solve major problem - I guess. 5. In that case, all excercise of liftign tank bottom during a major shut down and give side support etc may not arise - i guess. 6. I suggest that you study the problem more deeply to weigh all possible repair or remedial actions before considering action for temporary repair or during shut down repairs. I guess that this could be a problem of flooding than simple crevice affecting a large portion of ring plate. Crevice corrosion will not extend beyond a certain distance in such tanks to affect the entire bottom plate. The exercise of breaking open concrete and boroscope view of bottom plate may not arise . Trust this helps to weigh all options before attempting temporary repair or shut down repair of the tank bottom paltes C.V.Srinivasan Nishi Engineers Pvt Ltd India June 20, 2007 E-mail: nishi @vsnl.com >Dear All, > >Thank you for all your suggestions. I am greatly considering >every feedback and consolidating them. > >As an update, last night, I was able to have a chance to >talk to an Offshore Engineer. His idea comes in 3 ways and I >would like to share it with everybody for comments. > >1. Since the client required no lifting of the tank and no >shutdown, we are only limited to applying a corrosion >inhibitor or an epoxy at the problem area. This is only a >temporary repair. > >The client wants to determine how much water has gone in, >but with limited access, we can only break a part of the >concrete base and try to insert a boroscope. > >The amount of water and the extent of the corrosion have to >be determined to fully arrest the problem. > >2. Breaking parts of the concrete but avoiding the supports >underneath the tank and then jacking up the tank. This must >be done with a shutdown as we have to disconnect the pipes >connected to the tank. > >3. Let the corrosion grow but must have a second support >above the corroded area. > >Every 5 years, the Thais require a plant an inspection >wherein they have to put the plant or part of the plant in a >major shutdown. We can use #1 method and suggest to them to >have the tank scanned with UT from the inside to determine >the extent of the damage and repair the whole thing from >there while it is not in operation. > >By the way, the floor of the tank has a design with an angle >of 120 degrees from the center sloping down to 0 to the >annular plate. The tank has a fixed roof. > >For comments please. > >Thanks and regards, >Ladycroft39