What is the minimum lap requirement for double-welded joints according to API 650?

• A. Lapped at least five times the nominal thickness of the thinner plate joined
• B. Lapped exactly three times the nominal thickness of the thinner plate joined
• C. Lapped at least ten times the nominal thickness of the thinner plate joined
• D. Lapped at least five times the thickness of the thicker plate joined

Answer: (A)

The minimum lap requirement for double-welded joints according to API 650 is that the joint must be lapped at least five times the nominal thickness of the thinner plate joined. This requirement ensures structural integrity and adequate strength in the welded joint, which is crucial when storing liquids, especially oil, under various environmental conditions and pressures.

The lap area is designed to provide sufficient surface for welding, thereby reducing the risk of weld failure. By setting this minimum lap length, API 650 aims to create a robust joint that can withstand the stresses imposed during both normal operations and unforeseen circumstances.

In the context of the other choices provided, they either state incorrect lap lengths or incorrect relationships between the thicknesses of the plates being joined, which do not align with the established guidelines in API 650 standards. The focus on the thinner plate is particularly important, as it is typically the limiting factor in weld strength; hence, the requirement focuses on ensuring that the joint is sufficiently robust relative to the weaker material.