46 CFR §54.20-3

1. (a)Fabrication by welding must be in accordance with the provisions of this part and with part 57 of this subchapter.
2. (b)Welding subject to UW-11(a) of Section VIII of the ASME BPVC (incorporated by reference; see § 54.01-1) must be modified as described in § 54.25-8 for radiographic examination.
3. (c)A butt-welded joint with one plate edge offset, as shown in Figure UW-13.1(k) of Section VIII of the ASME BPVC, may only be used for circumferential joints of Class II and Class III pressure vessels.
4. (d)Attachment welds for nozzles and other connections must be in accordance with UW-16 of Section VIII of the ASME BPVC. When nozzles or connections are made to pressure vessels, as shown in Figure UW-16.1 (a) and (c) of the ASME BPVC, and are welded from one side only, backing strips must be used unless it can be determined visually that a full penetration weld has been achieved.
5. (e)When fabricating by welding the minimum joint requirements must be as specified under the column headed “minimum joint requirements” in table 2 to § 54.01-5 for various classes of pressure vessels.
6. (f)Joints in Class II or III pressure vessel cargo tanks must meet the following:
   1. (1)Category A and B joints must be type (1) or (2).
   2. (2)Category C and D joints must have full penetration welds extending through the entire thickness of the vessel wall or nozzle wall.