There are two reasons why I would avoid placing clips on the knuckle:
* The knuckle tends to be the thinnest part of the head.
* 2:1 heads under internal pressure fail by buckling (wrinkling) at the knuckle. Buckling is a failure mode which is very susceptible to geometrical imperfections. Clips will result in some local imperfection due to the transmitted load, welding, and local stiffening effects.
On the other hand, I have seen it done, and have ordered clips on a knuckle recently. This was a case where a vessel with a 2:1 head and top platform was replaced in kind *very* rapidly. The old ladder and platform were used, so I had the same old clip locations on the knuckle to match the existing platform.
Depending on platform size, the best place for platform clips is on the shell. The load is transmitted primarily through shear with virtually no shell stress to worry about.
jt