As far as I know the ASME B31 codes (and CAESAR II consequently) are developed to assess piping stresses in pipes that fall under the thin pipe theory category. Is this true?

And if this was true, what should be the limit of the D/T ratio below which the pipe is regarded as a thick pipe?

Valid use of simple beam theory places limits to the d/t ratio, both high and low. Low ratio (thin pipe) will susceptible to buckling and high ratio (thick pipe) will be susceptible to additional, through-the-wall stresses. Appendix D mentions that d/t should be less than 100 for the SIFs & k's to be valid and wall thickness calculations include special consideration when d/t is less than 6.

Thanks Dave!:)

So the limit i understood is d/t>6 to d/t<100.
Good information thanks to both of you

The d/t limits I reference are mentioned in B31 codes. Others may exist as well.