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According to the NEB Report of the Inquiry (Order No. MH-2-95), six of the Canadian stress corrosion cracking (SCC) failures to that time had been leaks resulting from circumferentially oriented cracks. As a result, the NEB made Recommendation 6-9 “that CEPA develop procedures for the detection and

mitigation of circumferential SCC and include them in future versions of the Recommended Practices Manual.” CEPA prepared and issued a report in

December 1997 [1], reviewing the state of knowledge concerning circumferential SCC and its mitigation. The report was later presented at the International Pipeline Conference [2] in Calgary.

This chapter of the SCC Recommended Practices is based in large part on that report.

12.1 Scope

This chapter of the CEPA Stress Corrosion Cracking Recommended Practices deals with the issue of circumferentially oriented stress corrosion cracking (C- SCC), which are cracks that deviate in direction from the longitudinal axis of the pipeline.

The chapter constitutes an integral part of the Recommended Practices (RP). As such, there will be minimal duplication of material in this chapter and it will reference only specific sections of the RP that are particular to circumferential stress corrosion cracking. The RP, including this chapter, are intended to be considered as a whole, and users are cautioned to avoid the use of individual chapters without regard for the entire RP.

Circumferentially oriented SCC (C-SCC) is a subset of transgranular SCC. In the case of C-SCC the principle stress acting on the crack is a bending stress, which is typically acting in a longitudinal direction. This bending stress is a different stress then the normal circumferential hoop stress generated from the internal pipe pressure. The bending stress may be due to differential settlement of the backfill beneath the pipe, geotechnical ground movement or possibly external loads other than soil.

12.2 Failure Mechanisms 12.2.1 Failure resulting in a leak

All but two of the C-SCC failures to date have been leaks. The cracks in the leak cases were either oriented circumferentially (perpendicular to the path of maximum axial stress) or displayed short circumferential cracks that “stepped” in a helical pattern following the path of the tape disbondment or tenting (Figure 12.2 and Figure 12.3).

Two forms of tape disbondment were associated with the C-SCC: the tape helix and wrinkles. The tape helix is essentially an area of tape tenting (Figure 12.2).

3 - 5 mm

Helical Tent

Tape Wraps Tape Overlap

Tape Pipe

As the tape was applied, successive wraps overlapped the previous wrap. As illustrated, a narrow tented area would be created at the edge of the previous wrap under the successive wrap. Normally this gap is dry. However, where damage to the coating occurs, water can gain entry.

Two patterns of tape wrinkles have been observed: longitudinal and circumferential. Longitudinal wrinkles are presumed to be a result of soil settlement in the trench. Relative movement between the pipe and the soil stretches the tape, causing it to ‘bunch up’ or wrinkle toward the bottom of the pipe. Circumferential wrinkles are seen on slopes and likely occur when coating adhesion fails in response to relative axial displacement between the pipe and the soil.

The C-SCC has been found to follow circumferential wrinkles or helical tenting. The cracking that follows the helical tenting starts as multiple smaller circumferential cracks located in the tented area that coalesce and form a larger crack that follows the tenting direction.

Figure 12.3: Cracks coalescing to follow the tape wrap tenting

12.2.2 Failure resulting in a rupture

There have been two known instances where C-SCC resulted in a rupture. One rupture occurred in 2001 on a liquid pipeline in Brazil and was a guillotine rupture. The other known rupture was in 2005 on a gas pipeline in Northern

Alberta. The Alberta failure is to date the only known case of C-SCC that involved spiral weld pipe.

The potential for rupture appears to increase when the change in angle between the initial crack direction and the preferred path of rupture is small, as can be the case for spirally welded pipe. In the case of spiral weld pipe the preferred path of rupture is parallel to the spiral weld (ie. parallel to the rolling direction of the skelp).

Figure 12.4: Ruptured pipe area showing crack initiation angle is close to the spiral weld angle

12.3 Field Program Development

Industry experience indicates that C-SCC has much the same growth factors as transgranular SCC (Figure 12.5), aside from the source of the principle stress. Therefore, as in the case of longitudinal SCC, a combination of known parameters related to susceptibility, can be used to develop a consistent process to assess and prioritize pipeline segments. In the case of C-SCC, the parameters related to SCC susceptibility are augmented by parameters that give rise to axial loading or bending of the pipe. The first step in this approach is to compile and review available operational, environmental and geotechnical data for each pipeline segment.

Geotechnical Loading Susceptible

Material

Cracking

Environment TensileStress

Hoop Axial

Circumferential SCC SCC

Figure 12.5: Circumferential SCC as a Subset of Pipeline SCC

12.3.1 Review of C-SCC Susceptible Conditions

The following conditions have been associated with the occurrence of C-SCC.

• Pipe - leaks occurred on lines constructed over a narrow range of years and involved ERW and DSAW long seam welded pipe. No pattern was observed in the pipe steel grades involved (290 to 448 MPa) or in terms of pipe diameters, which ranged from 168 to 914 mm.

• Pipe - a rupture occurred on one gas line constructed with spiral welded pipe. The pipe steel grade was 414 MPa, and the pipe diameter was 457 mm.

• Pipe - a rupture occurred on one liquid line constructed with longseam welded pipe. The pipe steel grade was 414 MPa, and the pipe diameter was 323.8 mm.

• Pipe condition - dents resulting from differential settlement at rocks or pipe wrinkles may be indicative of high axial stresses.

• Coating type - all service related incidents on CEPA-member systems have been associated with polyethylene tape. The two other known incidents occurred under a heat shrink sleeve and on damaged coal tar enamel that was over-coated with polyethylene tape.

• Coating condition - significant coating damage may not be a prerequisite for C-SCC as many of the incidents involved cracking that followed the helix of the tape overlap, and other minimal coating damage. Longitudinal and circumferential wrinkles indicate soil stress, and circumferential wrinkles are caused by relative axial displacement between the pipe and the soil.

• Operating stress level - may have a minimal impact on C-SCC as the hoop stress may contribute only a small portion to the axial stress.