Hellow to all!

I must simulate a nozzle at a flat header of a horizontal vessel. In order to take into account the nozzle flexibility (since as far as I know there aren't any guidelines for flat headers of vessels, like API650 or WRC107) I assumed a flexible anchor with 10^6 stiffness. My client argued that the flexible anchor has six degrees of freedom and this is not realistic for tanks and vessels nozzles because for these nozzles only three out of the six degrees of freedom are flexible. He allegated that only axial translational and bending in-plane/out-of-plane rotational degrees of freedom should be assumed flexible and the other should be rigid.

I am not quite sure about this. Can anyone help me about how many degrees of freedom can be assumed flexible for a nozzle according to API650 or WRC107?

According to my caclulations, the translations and rotations of the nozzles were very small for the flexible anchor modeled (less than 0.3mm and 0.1 degrees). Therefore I consider that the flexible anchor is a realistic approach of a tank vessel in case that API650 and WRC107 are not valid for this vessel (for instance for not cylindrical or spherical vessels, but flat planes). Im I right?