Depends on where you need the hole, how big a diameter the hole is, what is the pressure-temperature-metallurgy of the welded surfaces, what are the service conditions and what the equipment is to be used for under what regulation (Codes).
AhmedDeiaa:
Drilling at a weld shouldn’t normally be a significant problem, but it does depend on the weld joint detail and exactly what you are trying to do. Furthermore, differences in hardness’ around the weld can cause the drill to wander, and can be hard on machine tools.
If you are drilling through a weld, you are reducing the capacity of the connection as weld is being removed. I would not advise doing it without checking with the EOR first. Another concern would be the creation of stress risers where the weld terminates at the hole.
For structures subject to cyclic loading and if this welded joint sees stress reversals, design of the weld is based a category which has no discontinuity. As a rule, in such a case I would stay away from doing so, even though the hole is drilled and circular.
