Page 81 - Pipeline Rules of Thumb Handbook

68 Pipeline Rules of Thumb Handbook

Welding criteria permit safe and effective pipeline repair

Battelle Laboratories studies show weld sleeves and deposited weld metal can be applied without
removing defective lines from service

J. F. Kiefner, Senior Research Engineer, Battelle Houston Operations
Research conducted by the American Gas Association Deposited weld metal completely fills and eliminates
(AGA) 1–5 and others 6–8 showed that pipeline defects can be clearly visible defects such as corrosion. Access to an entire
repaired without removing the lines from service. Two repair defect is required so that weld metal can penetrate and bond
methods evaluated were full-encirclement sleeves and direct to sound metal. It may be possible to enlarge pits, laps, or
deposition of weld metal on defects.* To ensure minimum undercut by grinding a groove wide enough to permit access.
risk when using these methods, criteria for repairing pipelines Deposited weld metal can be selected when requirements to
in service have been formulated (Table 1). prevent burn-through are met. These requirements are dis-
The criteria in the table are intended as a repair guide for cussed later.
lines that operate at 40 percent of their specified minimum Removal of a damaged pipe section is another way of elim-
yield strength (SMYS) or more. The criteria may by unnec- inating a defect. The line must be taken out of service,
essarily restrictive for lines which operate at or below 40 purged, removed and replaced with a sound tie-in piece.
percent of SMYS. Removal is often a poor economic choice in terms of wasted
Note that defects in pressurized pipelines, especially product and interrupted service. Thus, removal is recom-
pneumatically pressurized lines, can cause sudden cata- mended in Table 1 with the understanding that it may not be
strophic ruptures. When the procedures are carefully practical in certain instances.
planned, risks are minimized. Nevertheless, repair errors Removal by hot-tapping takes advantage of a widely
could reduce the safety margin of these criteria. Conse- accepted method for making branch connections on live
quently, users of Table 1 are urged to exercise caution when pipelines. The coupon removed from the line must contain
performing repairs. the entire defect. Limitations of this technique will be dis-
cussed later.

Choice of repair method
Types of defects
The repair method options included in Table 1 are:
Classes of defects listed in Table 1 are:
• Type A sleeve
• Type B sleeve • Manufacturing defects—cracks, undercut, lack of fusion,
• Deposited weld metal and lack of penetration in seam welds; laps, pits, cracks,
• Removal and rolled-in slugs in the pipe body; hard spots.
• Removal by hot-tapping • Environmentally caused defects—selective corrosion
and hydrogen stress cracking in electric resistance
Type A sleeve repair consists of placing a reinforcing band welded or flash welded seams; general pipe corrosion;
concentric with the pipe while leaving the band ends un- pitting corrosion; stress corrosion cracks; hydrogen stress
sealed. The sleeve strengthens a defective area by restraining cracking in hard spots.
bulging that would otherwise occur when the weakened pipe • Defects caused by outside forces which include dents
shell is pressurized. Such a sleeve may also carry a small and gouges.
portion of the hoop stress, but in no case can it become the • Construction defects.
sole pressure containing element since its ends are not sealed.
As a result, it cannot be used to repair a leaking defect. If a Submerged arc-welded (SAW) seam defects, both straight
gap exists over the defect, filling of the gap is required. Two seam and spiral, include undercut, incomplete fusion, incom-
classes of Type A sleeve exist: no filler; with filler. plete penetration, and cracks. Repair methods recommended
Type B sleeve repair requires a pressure-tight reinforcing for these defects are shown in Table 1. No filler is required
concentric band. Its ends are sealed to the carrier pipe and it with sleeves because after the weld reinforcement is ground
can contain pressure. It can be used to repair leaking defects. flush, no gap between the pipe and sleeve should exist.
In addition, a Type B can also be used as a Type A sleeve. Removal by hot-tapping is not recommended because the tap
Three classes of Type B sleeves exist: no filler; with filler; would involve cutting through the seam, a practice considered
pressurized. unacceptable by many companies. Repair by deposited weld