Although I worry very little from a liability standpoint, either for myself or for the Forum, I do worry that some people will take what we say as authoritative and this could naturally prove problemmatic.
Each of us has sufficient experience, education and resources to probably adequately answer almost any question posed however when design matters come into play, which according to direcion taken could potentially have catastrophic results, we should be very cautious about offering any advice except the most general.
khodabandeh, you say the contractor has design authority (EPC) Engineering, Procurement, Construction then I guess my question is where exactly do you fall in this arrangement?
If you are a welding inspector then my advice is to seek that design approval of the material deviation has been granted (with the approval of the Owner), secondly look to see if the welding procedures have been properly qualified. If both of these turn out to be good then continue your inspections, if not, document on a nonconformance report.
On the other hand, if you are a representative of the owner, you should already know the answers to these questions. Please clarify.