How should vertical joints in adjacent shell courses be positioned?

• A. Aligned directly on top of each other
• B. Offset by 5t, where t= the greater plate thickness
• C. Randomly placed without requirement
• D. Offset by 3t, where t= the thinner plate thickness

Answer: (B)

Vertical joints in adjacent shell courses must be offset to enhance the structural integrity of the tank. This offset helps to minimize the potential for stress concentrations at the joints, which can lead to premature failure. The best practice as outlined in API 650 specifies that these joints should be positioned with a certain stagger, specifically by a minimum distance of 5 times the greater plate thickness (5t). This requirement ensures that the load distribution along the vertical seams is more uniform and that any possible weaknesses are not aligned directly on top of one another.

Proper joint staggering is crucial in maintaining the overall strength and durability of the tank structure, especially under conditions such as changes in temperature or pressure, which could affect the alignment and support of the shell plates. Therefore, an offset of 5t, where t is the thickness of the thicker plate being used, ensures that the structural continuity is preserved while accommodating the mechanical performance requirements of the welded tanks in oil storage applications.