As far as EC is concerned, the analysis of this type of connections (EN 1993-1-8) is based on the equivalent T stub method which as you know uses yield line analysis, takes account of prying forces, etc.
I am not sure how well this method applies to pipes - they are not quite rectangular. In addition, connection design feels better if it is supported by test data. Therefore I recommend to you an instruction (in Russian which may not be very helpful for you) that can be found in the net. The title is:
"???????????? ?? ???????, ??????????????, ???????????? ? ??????? ????????? ?????????? ???????? ???????????? ???????????"
It contains recommendations for choosing steel grade for the end plates, bolt classes, weld sizing, stiffener dimensions. There is also tabulated data for some tested types of pipe end plate connections so you can at least compare your calculations to some readily available results.
Wish you success, and please write back and let us know your results
Mike