Oliver,
I think you should not blame the welder if he was not the pipe fitter or job supervisor.
Sorry not easy to imagine the actual condition of your case and I also don't have a good designer's background.
If you don't want to engage a designer to fight your case, then resort to do some drama and try your luck by asking your Client to give you a proposal for method of repair work. This surely no matter what is the outcome will not be rejected by your Client.
You explain that your aim is to help them to complete the erection work on time and run the line for them to start generating revenues. This kind of approach is not a good thing to know but this normally happens in construction
.
If you have no guts to do the above approach, then ask your marketing sales people to approach your client. Anyway, your Client is partly to blame for engaging your company. You will not learn this approach in any Code book, Std or Recommeded Practices. So please don't quote my name
.