API-653 Now Permits Lap-Welded Patch Plates for Shell Repairs

By Vince Carucci

Addendum 1 to the Second Edition of API-653, Tank Inspection, Repair, Alteration, and Reconstruction, was issued in December 1996. Prior to this revision, the use of lap-welded patch plates to repair shell plates that have become too thin was not permissible. The only acceptable methods to repair thinned areas of the shell were to use weld overlay (for very small areas) or to cut out the thinned plates and replace them with a new plate that is butt-welded into the shell using complete penetration and complete fusion. Addendum 1 now permits the use of lap-welded patch plates provided this is specific by the Owner and requirements specified in API-653 are met.

The use of lap-welded patch plates for shell repair is frowned upon for shell repair is frowned upon in general for several seasons:

While these are the main reasons why the use of lap-welded patch plates in frowned upon, they certainly can be overcome if lap-welded patches are taken seriously. Addendum 1 of API-653 recognizes this by now allowing the use of lap-welded patches with restrictions. The following paragraphs highlight several of these restrictions/requirements.

General Requirements

Several requirements must be met in all cases.

Additional Requirements For Particular Repair Situations

Repair Situation Additional Requirements
Closure of holes cause by removing shell openings or thinned areas • Continuous fillet welds inside and outside
• Minimum Hole diameter and corner radius specified
• Completely remove nozzle and reinforcement pad
• Plate thickness calculated using E≤0.7
• Full-thickness fillet welds
• Shell overlap limits specified

Reinforce thinned areas of shell without removal

• Plate thickness calculated using E≤0.35
• Full –thickness fillet weld
• Maximum repair plate thickness limits specified
• No credit for thickness of remaining shell
Repair small leaks or minimize potential for leaks at isolated or widely scattered pits • Existing shell thickness meets minimum thickness requirements • Plate thickness calculated using E≤0.35
• Full-thickness fillet weld
• Maximum repair plate thickness limits specified
• Minimum repair plate thickness ≥3/16 in.
• Cannon be used if corrosion of fillet welds is a concern if the stored liquid leaks between the shell and repair plate
• Shell plate thickness under repair plate to be evaluated during future inspection

Summary

Addendum 1 of API-653 permits the use of lap-welded patch plates in a tank shell provided specified requirements are met. In no case can a lap-welded patch be used on a shell plate thicker than ½ in. While allowing lap-welded patch plates is a reasonable relaxation of prior requirements, the ½ in. maximum shell thickness limit restricts its use to relatively small diameter tanks or the uppermost courses of larger diameter tanks. It might be worthwhile to review the shell thickness limit considering the fracture toughness of the shell plate, the minimum design metal temperature, and shell plate thickness. It is believed that such a review could result in additional situations where the use of lap-welded patch plates in technically acceptable.