Joisy:
It is common practice to use a cone down bottom in petroleum storage tanks to aid in the sumping of water and settled solids. Of course this depends upon the type of product you are storing, etc. I have worked in the design of aircraft fuel storage tanks and they are almost cone down with a central well and dedicated sump piping for daily water and settled solids removal. I don't know about the Shell DEP referenced above, but in my experience API 650 is very commonly used as a design guide and appropriately focussed professional associations are consulted with respect to industry norms, etc. For the aircraft fuel, there is the Joint Inspection Group, which issues guidelines and conducts independent audits for aircraft fuel facilities. In this case, they are considered our "bible" though their guidelines are not followed in all parts of the world.