YVAN,
Yes, I have done that type of work. I have welded hot tap fittings onto Natural Gas lines with product flowing. They were probably about your 3.2mm (1/8" for me). Have done same on water, jet fuel, High Octane aviation fuel, etc. These were all on existing systems doing additional runs for supply or repair.
The first object for me was to know my material. Important to know how thick and what grade before you strike an arc. Many times we would get to an underground line right as it was being uncovered and find it was not steel though that is what all the info on the lines said it was.
Second, is to check the condition of the line. If it has been underground for any time, even coated, this can be a major consideration. Rust spots, especially on natural gas, can be very hazardous.
If your lines are water (just a comparison, I know you are talking natural gas), you have fewer concerns though you might get wet if you hit a rust spot and don't handle it correctly.
Now, I'm not sure what you are trying to accomplish and what you mean by "a welding strategy". The thicknesses (or thinness) of the materials is not really a problem. These can be welded easily with SMAW, GMAW, GTAW. If in a field situation, there would obviously be other considerations. A good, certified, pipe welder is a must. Even many of them are out of their element on used pipe with product flowing through it (no offense to many who are able here on this site). There is a big difference between rusty material that stays really cool because there is water flowing through it and new pipe with no cooling agent.
I would then proceed to make sure, because of liabilities and other factors, that I had a proper WPS in place and that the welder was capable of working to that spec. Then he needs to be tested on mock ups with no product in the line. Make him do it in all kinds of awkward positions. Be unusual to be able to weld from a comfortable position all the time. How many tests do you give this type of person before you let him weld for real? Assuming you are talking about doing this work on live lines. One mistake and it can be all over for several people.
There are many ways to check the then attached fitting before hot tapping to be sure there are no leaks in the weld just applied.
Don't know how else I can, hopefully there will be others here with more info for you.
Have a Great Day, Brent