Shell Repairs to Aboveground Atmospheric Storage Tanks Using Lap-Welded Patch Plates

By Vincent A. Carucci

API-653, Tank Repair and Alteration, intends that repairs and alterations made on an Aboveground Atmospheric Storage Tank (AST) result in general equivalence to API-650 requirements. Section 9 of API-653, contains specific requirements that are intended to meet this objective. After the AST has been in service for some time, thinning due to corrosion may require that sections of the shell be repaired in order for the tank to have adequate strength for its next period of operation. Such repairs are generally done by installing insert plates into the shell using full-thickness butt welds.

Before Addendum 1 issued in December 1996, API-653 did not permit the use of lap-welded shell patch plates under any circumstances. The primary concerns with using lap-welded patch plates relate to the quality and strength of the attachment fillet weld, the possible increased risk of initiating a brittle fracture at the fillet weld (especially in steels that do not meet current fracture toughness requirements), and the ease with which the patch plate is installed (which may lead some individuals to not take it as seriously as they should). Addendum 1 relaxed this general prohibition on the use of lap-welded patch plates provided that the owner specifies their use and they meet requirements stated in API-653. Despite this relaxation, the specified requirements for lap-welded patch plates will still limit their widespread use. As noted below, the maximum shell and patch plate thickness limit is a significant constraint.

Lapped patch repairs may be considered permanent repairs subject to an established inspection and maintenance program. The requirements specified in API-653 may be used to evaluate pre-existing lapped patch repairs, except that the various plate thickness limits need not be applied.

The following highlights several of the requirements for using lap-welded patch plates. This is not an all-inclusive list; so, refer to API-653 for additional details.

The following Table highlights additional requirements that must be met when lapped patch repairs are used for particular situations.