For your tank (28M diameter x 18M high) it is unlikely that you need anchors unless you are in a high wind or high seismic area, or if you have an internal pressure or are subject to flooding. If these are internal or external floating roof tanks then pressure is not an issue so you are just looking at wind and seismic and maybe buoyancy. The diameter to height ratio is such that I doubt wind would require anchors (but this is easy to check). You need to find the seismic factors for your location and then the check for anchors is fairly easy also. API 650 is fairly easy to follow and there are lots of experienced tank engineers out there that could consult with you if needed. Anchoring a tank for buoyancy reasons is not in API 650 and typically not needed because API 650 tanks are inside dike areas and the API 650 foundation design usually keeps them high and dry. Howeever, it can be a concern and should be reviewed. The best for you would be to form an understanding of API 650 by running the numbers yourself, perhaps putting the equations into MathCad, BlockPad or Excel.
