Sleeve installations speed pipeline defect repair.
To reduce cost and eliminate unscheduled shut downs, pipeline operating companies have adopted "in-service" repair methods to restore overall integrity of the pipeline without taking it out of service. Interprovincial Pipe Line Co. has undertaken an aggressive approach to this "in-service" repair method by using a developed sleeving system for repairing leaking and non-leaking defects. As a result, this defect repair system (DRS) is sensitive to safety and is cost effective. It restores the physical condition of the line pipe to ensure long-term structural fitness with an adequate margin of safety under maximum operating pressure (MOP).
Pre-Stressed Sleeve Repairs
A pre-stressed sleeve offers several advantages over conventional sleeve repair installations. Pre-stressing the sleeve results in much tighter fits than in standard arrangements without external loading. Furthermore, a pre-stressed sleeve enhances the hoop stress sharing capabilities of the sleeve and reduces the stress level in the carrier pipe. Pre-stressing also offers the potential for reinforcing defects with little or no need to reduce internal pressure levels (other than for immediate safety considerations). These advantages are important since they allow the operator to do preventive maintenance.
Interprovincial Pipe Line Co. approached Pipeline Products & Services Co. (PLP) to participate in a program to develop new technology and techniques for a maintenance program that improved safety and minimized effects of line pipe mechanical damage due to external forces. The result was a method of sleeve installation that obviates the need for circumferential fillet welds since the sleeve is not welded to the pipe. This minimizes weld induced defects since using fillet welds in liquid-filled, high carbon/carbon equivalent line pipe has resulted in weldments which are susceptible to hydrogen-induced cracking.
The pre-stressed sleeve prevents initiation of fractures in a pipeline by locally reducing hoop stress level in the pipe wall. Also, it can "replace" metal loss caused by corrosion pitting and allow the pipeline to operate at maximum operating pressure at all times. It consists of two half-sleeves which are set over the line pipe and externally loaded by a full encirclement leaf-type chain and hydraulic jack arrangement called a chain clamp. The two segments of the sleeve are set over pressurized pipe and jacked by the chain clamp to a pre-determined thrust. Then the sleeve halves are joined together by longitudinal butt welding and the chain clamp removed afterwards.
When fitted onto an existing line, hoop stress due to operating pressure is reduced in the damaged area and bulging is prevented. The pre-stressed sleeve also provides positive failure control in gas pipelines that are subject to brittle fracture propagation by bringing line pipe hoop stress during installation into compressive stress. Internal pressure subsequently relieves this compression stress to zero or slightly above zero but well less than the threshold required to support fractures in the brittle mode.
Structural Reinforcement Sleeve
A structural reinforcement sleeve consists of two non-fillet welded collars (one on each side of the defect) and a full encirclement sleeve welded on top of these collars. The annular space between the pipe and sleeve is filled with a hardenable, non-shrinking epoxy.
This sleeve is used as an alternative to the pre-stressed sleeve and is particularly useful where excess ovality or pipe curvature results in a gap in excess of 1/4-inch between the end of a sleeve and the line pipe. Prior to installation, the structural reinforcement sleeve is tapped with two 1/2-inch. NPT holes located at the top. They serve as a vent and inlet for pouring the hardenable epoxy. Sleeve ends are sealed with silicone or similar sealing material before filling with epoxy to prevent leakage. Epoxy is poured into the annular space with a funnel until sufficient epoxy forms at the vent hole. Then both holes are plugged but not welded.
Pressure Vessel Sleeves
Three different pressure vessel sleeves can be used for repairing certain defects. They can be used in combination with the pre-stressed sleeve or for independent repairs. The "sleeve-on-sleeve" or "repair of repair" method is used to strengthen a pipeline in cases where cracking may occur in the initial sleeve welds or, if cracking has already occurred, to prevent leakage when the cracks have grown through the pipe wall.
In a Type 1 "repair of repair" sleeve, line pressure is contained in the case of through wall leakage. The required pressure carrying capacity of this repair must equal or exceed the pressure level corresponding to 100 percent of the specified minimum yield strength (SMYS) of the pipe. This is a requirement of most pipeline codes including CSA Z183, Clause 10.16.4.2; Z184 Clause 10.10.2.3; and ASME B31.4, Paragraph 451.6.2 (c)(6). In this method, shoes or collars are longitudinally butt welded on the carrier pipe at a short distance (6 inches minimum) from the ends of the original sleeves. Only the side of each collar facing the sleeve is welded to the carrier pipe so that if hydrogen-induced toe cracks form, they will be under the outer sleeve. The outer sleeve then is fillet welded at both ends to the collar and original sleeve. If the original sleeve is short, it is better to use a longer outer sleeve over top to avoid the two circumferential welds to the original sleeve. The collar-to-pipe and collar-to-sleeve welds must not leak and the resulting annulus does not require an epoxy filler.
A Type 2 sleeve is similar to the structural reinforcement sleeve described above except that collars are fillet welded to the pipe on one end so that they are contained within the sleeve. Two-inch weldolets are used for filling inlet and vent. The annulus is filled with epoxy and the plugs must be seal welded to maintain the pressure containment of the completed assembly.
A sleeve-on-sleeve, or Type 3 sleeve, is comprised of two main components. The first is a conventional close fitting split sleeve or leaking defect repair sleeve which has been longitudinally butt welded and circumferentially fillet welded to the carrier pipe. This provides the pressure retaining capability of the assembly. The second component is an over-sleeve welded to collars identical to the structural reinforcement sleeve. This reduces or eliminates bending stresses in the area of the fillet weld. When the length of the split sleeve exceeds the nominal diameter, the over-sleeve can he cut circumferentially and fillet welded to the split sleeve with a 2-inch overlap. Annular spaces on both sides of the sleeve can be filled with epoxy after having sealed the sleeve ends with silicone or a similar sealant.
Application Limits And Specifics
The following application limits and specifics must be observed in order to assure a repair which will maintain full integrity of the pipeline:
1. Minimum sleeve length is 12-inches. Maximum length is limited only by what can be installed on a practical basis, but is generally limited to 10-feet, the maximum recommended length for unsupported pipe.
2. Ends of sleeves must not he placed within 2-inches of a circumferential butt weld.
3. Spacing between sleeves must be a minimum distance of 6-inches to avoid overlap of heat affected zones of companion sleeves. If sleeves must be placed closer together, it is preferable to extend the sleeve length and make a longer longitudinal weld rather than two extra circumferential fillet welds.
4. Prior to sleeve installation, coatings shall be removed and the line pipe cleaned. Dirt, rust, and mill scale must be removed. Any weld beads within the area to be sleeved must be ground flush with the line pipe surface. Surface defects must be filled with hardenable epoxy putty and smoothed off to restore pipe contour.
5. Ultrasonic testing for wall thickness variations and laminations must be done on the mainline pipe.
6. The two collar halves must be fitted closely to the pipe to achieve the best possible welding gap for longitudinal and end welds.
7. Mainline pipe excavated below pipe bottom must be fully supported.
8. All welds made on a pressure vessel sleeve must he non-destructively inspected. Welds on a structural reinforcement sleeve do not require nondestructive inspection.
9. Before epoxy is poured into the annulus of Type 2 and 3 sleeves, the ends of the repair must be sealed with silicone or other suitable sealant. This ensures that the epoxy will not drain out of the annular space if a leak path exists
10. Completed repairs must be protected against corrosion by using appropriate coating materials and procedures.
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|Author:||Friedrich, J.; Smith, J.|
|Publication:||Pipeline & Gas Journal|
|Date:||Dec 1, 1995|
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