API 650 Figure E-1 (Ci Coefficient)
API 650 Figure E-1 (Ci Coefficient)
(OP)
I am working on a tank calculation for FRP vessels and using API 650 as a guide to develop the sloshing calculations. The graph provided in Figure E-1 (Ci coefficient) seems to limit out at 1.5 (H/D) ratio. Has anyone out there had experience in what to do when the magical 1.5 H/D ratio is exceeded?
Much obliged!
Much obliged!





RE: API 650 Figure E-1 (Ci Coefficient)
If desired, calculations can be made assuming the shell is not rigid, and in that case, Fig. E-1 can be used to calculate the fundamental period of the tank shell. I believe Figure E-1 is based on a steel tank, and is an approximation made by evaluating Veletsos' method over a range of parameters, and that method could be used directly instead of using the Ci graph (be aware that it's kind of a mess). This would normally be applicable for either very large tanks or tanks in very high seismic loading, where the extra analysis is worthwhile.
For a (presumably) small FRP tank, I would use the standard method and skip the Ci calculation altogether.
For larger tanks, the consideration of sloshing results in additional freeboard required, but a decrease in overturning moment. ASCE 7 allows you to consider small tanks as entirely rigid, which would be another approach to look into.
RE: API 650 Figure E-1 (Ci Coefficient)
Much obliged!