Thanks Ibrahim
For providing more clear explanation on the issue which I probably could not explain well even though I was trying to point out same thing.
Regarding zero rigid modelling I like to refer "COADE pipe stress seminar notes" clause 3.2 modelling techniques, simplest method / more accurate / most accurate method, page no 3-10 to 3-12.
Some good company I have worked with give clear instruction to use more accurate method ( zero rigid model from centre of pipe to bottom of pipe)above particular size of pipe.

Regards

Habib