Page 82 - Pipeline Rules of Thumb Handbook

Construction 69

metal is applicable only for undercuts and is subject to special spot from the environment, which may cause it to become
requirements given in footnotes of Table 1. cracked.* Hard spots at the I.D. surface should not be
Type B pressurized sleeves used on nonleaking defects can repaired with Type A sleeves since they may be susceptible
be pressurized only by using a tapping nipple. Since other to cracking in certain types of product service. Such a sleeve
means of sleeve repair are entirely adequate when used as would not prevent a leak if the resulting crack grew through
directed, the intentional pressurization of a sleeve when no the wall.
leak or near leak is present is not necessary. Selective corrosion and hydrogen stress cracking in ERW
Inside or interior defects require special consideration and FW seams are subject to low toughness fracture behav-
since they are not readily visible. Removal may be the best ior. It is recommended that they be removed or repaired with
alternative unless one can be reasonably certain of the extent pressurized Type B sleeves to relieve stress.
of the defects. Hot-tapping is not recommended because of General or widespread corrosion is that which covers too
the uncertainty of the extent of an inside or interior defect. wide an area to be repairable by means of hot-tapping or
Electric resistance welded (ERW) or flash welded (FW) deposited weld metal. Any other repair means are acceptable
seam defects include: upturned fiber imperfections; incom- if they cover the critically affected area. When internal cor-
plete fusion; penetrators; cold welds; cracks. These can only rosion is present, its extent must be reasonably well known
be repaired with pressurized sleeves and removal since the and further corrosion must be prevented. Removal may
welds are susceptible to brittle fracture or low-resistance sometimes be the only suitable choice.
ductile fracture initiation. The value of restraint of bulging Pitting corrosion may involve isolated pits or groups of pits.
from either type of nonpressurized sleeve is uncertain. Hence Generally, isolated single pits will not require repair if further
they are not recommended for these defects. Hot-tapping corrosion can be halted. ASME guidelines can be used to
also is not recommended because of the involvement of determine whether or not a corroded region requires repair.
the seam weld. It is not recommended that weld metal be Groups of interacting pits can substantially lower remaining
deposited because of possible low ductility. Required grind- strength. If strength has been lowered, the pipe should be
ing before such welding would involve creating or enlarging repaired. Any of the methods of Table 1 are suitable if the
a defect in a potentially low-toughness material. Only removal noted precautions are observed.
or use of a Type B pressurized sleeve which stress-relieves the Stress-corrosion cracks occur in clusters and often cover
defect is recommended. large areas. It is difficult to repair these cracks by hot-tapping
Other seam defects include lap welds and furnace butt or deposited weld metal. Repair methods for stress-corrosion
welds. These must be dealt with on a case by case basis. Such cracks are confined to those which can strengthen the entire
welds usually appear in older or smaller and lower pressure affected area.
pipelines where fracture resistance requirements are less Hydrogen stress cracking appears in hard spots attacked by
stringent. On the other hand, some of these older materials hydrogen emitted from bacteria external to the pipe and from
can be quite susceptible to low-toughness fracture behavior. cathodic protection. Internally the hard spots can be attacked
It is probably best to treat these materials with the same by certain types of products—especially those containing
caution as ERW and FW seams as they are being operated at hydrogen sulfide. Because flatness often occurs near hard
stress levels exceeding 40 percent of SMYS. spots, nonfilled sleeves are not recommended. Deposited
Laps, pits, seams, cracks, and rolled-in plugs should be weld metal is not recommended because grinding of a hydro-
repaired subject to these limitations: special requirements of gen stress crack in a hard—and usually brittle—spot while the
I.D. and interior defects and leaks must be observed; hot- pipe is under pressure is not safe. Interior hydrogen stress
tapping is acceptable on nonleaking, O.D. defects as long as cracking should not be repaired by a Type A sleeve since such
the entire defect is removed with the coupon; deposited weld a sleeve cannot prevent leaks if the crack grows through
metal may be used to repair pits or laps if they can be entirely the wall.
exposed by grinding. Dents and gouges and a combination gouge-in-dent result
Hard spots created in the plate by accidental quenching on from external encroachment by mechanical excavating equip-
the run-out table may become flat spots in the pipe since they ment, other kinds of equipment, or rocks. Plain dents are
do not yield when the plate is formed to pipe. Such hard spots
do not fail spontaneously unless they contain quench cracks,
or unless they undergo hydrogen stress cracking. Such spots
should be repaired if they are not cracked. Type A or B sleeves
without filler would not be acceptable since they would not * One method of protecting hard spots from hydrogen stress cracking which,
restrain flat spots from bulging. so far, has proven adequate, is that of using a concentric band of sheet
Hard spots can be remedied by several methods if they metal spaced away from the pipe by rubber seals. The annular space is
filled with coal tar to exclude ground water, and the metal band (which is
occur as outside nonleaking defects. Sleeve repair methods itself coated) shields the hard spot from hydrogen generating cathodic pro-
with filler not only provide strengthening but shield the hard tection current.