I am interpreting the OP as asking about welding onto a live natural
gas pipeline. This is sometimes done, for example to install a new
branch connection by "Hot-tap" (i.e. tapping into the line while it is
"hot" [operating]) or for repair of localized corroded areas.
There is specialized equipment associated with this. The best known
company for the associated equipment and providing this service is TD
WiIliamson (Tulsa, OK). Some experienced gas companies just buy the
equipment from TDW and do their own hot-taps.
The attachment welds are typically fillet welds connecting the special
pieces (e.g. a "hot-tap split tee fitting" for making a new branch
connection or a split sleeve for reinforcing an area with excessive
corrosion) to the outside of the existing pipe.
API 1104 actually has a special section on this type of welding. It
is "Appendix B - In-Service Welding" which provides details of how
such welding is to be qualified.
The trick for the welding is to:
a) Make sure the pipe wall thickness is great enough and the welding
parameters are "low enough" to prevent burning through to the inside
b) making sure the welding parameters are "high enough" so that the
welds are not excessively hard (because the flowing fluid on the
inside makes the welds cool much faster than normal).
This is a real balanacing act and there is actually a computer program
(VERY expensive) that allows to you to determine the two effects
listed above for various welding conditions.
1) Yes, welding onto live, operating gas pipelines can be done. It is
done "routinely" by a number of pipeline operating companies but
requires a significant amount of expertise, engineering, and welder
2) There are a number of technical articles on this subject (several
have been published in the Welding Journal). There has been a lot of
technical research into this subject.
[Yes, this is actual "hands-on" knowledge rather than just repeating
information. I have been involved in a number of hot-taps myself.]