46 CFR §54.25-8

1. (a)Full radiography is required for all Class I and Class I-L vessels regardless of thickness. (Refer to table 2 to § 54.01-5 for applicable requirements.)
2. (b)Class II-L vessels must be spot radiographed. The exemption provided in UW-11(c) of Section VIII of the ASME BPVC (incorporated by reference; see § 54.01-1) does not apply. (Refer to table 2 to § 54.01-5 for applicable requirements.)
3. (c)Each butt-welded joint in a Class II or III pressure vessel cargo tank must be spot radiographed, in accordance with UW-52, regardless of diameter or thickness, and each weld intersection or crossing must be radiographed for a distance of at least 10 thicknesses from the intersection.