Couple things OSHA is gonna check and flag them for just from info provided:
1) The tank should have been at least partially drained if added heat from welding was induced, to prevent over expansion
2) As mentioned: the carpenter most likely had no proper training or certification to be welding at all. Least not the right job description
3) Heavily doubtful that air monitoring was being conducted. Namely, there should have been at a minimum a 4pt Air Check ( LEL: combustible gas, H2S Hydrogen Sulfide, O2 and Carbon Monoxide).
4) An daily onsite safety Inspector's report. If it wasnt conducted, they would be liable. If it was conducted, then it should have shown any potential hazards like excessive pressure or gas build up.
This sadly, was entirely preventable if everyone followed the letter of the law. Unfortunately, a man is dead and his kids are without a dad , while another worker is critically injured. All because someone, somewhere cut corners and skipped protocols