Offshore structure connection query

I am currently working on the offshore project (a floating production system) and also new to this type of project. I was assign to do a connection design of one of the module structure. In our go-by drawings on the connection of major pipe column and vertical pipe bracing seating on top of a plate girder, the plate girder flanges (top & bottom) and web at that portion of connectivity were thickened (some call it insert plate).

I try to use the buckling of plates principle in the book of Blodgett (which is similar to Roarks books) but the thickness of the plate girder is sufficient enough to carry the loads.

It seems that API-RP2A (correct me if I'm wrong) don't have provision on this type of connection.

Does anybody knows any other criteria on how to size the plate thickness of the thicken portion of the connection?

Thanks in advance.