Hemispherical heads used on high pressure Hx's are often attached to a "lip" forged or machined to the tubesheet. This is for, among other things, so that the head attachment weld and any necessary PWHT, since it is done after the tube expansion and/or welding is finished, will not sensitize the outer rows of tube joints.
It is also to properly transmit the loads of the head at pressure to the tubesheet. There is HEI and EPRI information about the radius of the transition of the lip to the tubesheet itself.
Quality manufacturers of high pressure Hx's use from 4-1/2 to 9 inches for this lip dimension. (And yes, I have seen HP heaters without any lip, fact stranger than fiction. Poor outer rows of tubes.)
So, wouldn't the "lip" mentioned above qualify for the cylindrical attachment that you are questioning regarding the tubesheet analysis, assuming we are not talking about the skirt end.
I don't know about this regarding LP Hx's using hemi head channels.
rmw