The material you mention is structural steel, so the welders should be qualified under AWS D.1.
In my opinion, design, fabrication and inspection should follow AISC standards. AISC is the American Institute of Steel Construction: www.aisc.org
Giovanni S. Crisi
Sao Paulo - Brazil
I believe you would want to look at API 4F. You may want, depending on what the contractual terms are, to look at DNV standards for offshore platforms (they are free from DNV) and ABS (they may be free from ABS - I don't recall at the moment.