Shell Repairs to Aboveground Atmospheric Storage Tanks Using Lap-Welded
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
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
- All repair material must meet the requirements of the applicable
construction standard and API-653.
- Lap patches cannot be used on any shell course whose original
construction thickness is greater than 1/2 in. (12.5 mm), and cannot
be used to replace doorsheets or shell plates. The 1/2 in. (12.5 mm)
thickness limit relates to potential brittle fracture concerns. There
has never been a documented case of a brittle fracture in plates less
than 1/2 in. (12.5 mm) thick. Therefore, limiting patch plate
thickness to 1/2 in. (12.5 mm) minimizes the brittle fracture risk
This maximum thickness limit effectively limits the use of lap patches
to small diameter tanks, or the upper courses of large diameter tanks,
where the shell plate thickness is not over 1/2 in. (12.5 mm).
- The lap patch must be at least 3/16 in. (4.8 mm) thick, but no
thicker than the smaller of 1/2 in. (12.5 mm) or the thickness of the
plate to which it is being welded. Therefore, if the design
calculations conclude that a lap patch plate must be over 1/2 in.
(12.5 mm) thick, a butt-welded insert patch must be used instead.
- The shape of the lap patch may be circular, oblong, square, or
rectangular. All corners must be rounded to a 2 in. (50 mm) minimum
radius (except at the bottom-to-shell joint). This reduces local
stress concentrations and facilitates welding.
- Minimum spacing limits between the lap patch welds and shell seams
are specified. The size and placement of the lap patch might require
adjustment to respect these limits.
- A lap patch may extend to and intersect with the external
bottom-to-shell joint at a 90° angle. Acceptable attachment details
are specified in API-653.
- Shell openings and their reinforcement shall not be positioned
within a lapped patch shell repair. The main concern here is the
additional loads that could be imposed on the lap patch fillet welds.
- The area on the shell where the lap patch welds are to be made
must first be ultrasonically examined for plate defects and remaining
thickness. The plate size must be changed (probably increased) as
needed to weld to sound shell plate material.
- Lap patches shall not be lapped on lap-welded shell seams, riveted
shell seams, other lap patch repairs, distorted areas, or unrepaired
cracks or defects.
- Minimum and maximum lap patch size limits are specified (see
API-653 for details). The patch plate must be formed to the shell
curvature in all cases.
The following Table highlights additional requirements that must be met
when lapped patch repairs are used for particular situations.