9.3.2(b) Not exposed to weather. Vent pipes not exposed to the weather need not comply with the height and wall thickness required by 4-6-4/9.3.2(a). However, vent pipes passing through fuel oil or ballast tanks are to have wall thicknessess not less than that indicated in column D of 4-6-2/Table 4. Other vent pipes are to meet thickness requirements of column C of the same table.