Manufacturer of Pipe Sleeve
Mettalo Piping Products Inc., Manufacture a Full Line of Full Encirclement Steel Sleeve. We Supply Full Encirclement Sleeves Worldwide, for Oil and Gas Industries.
Full-Encirclement Steel Sleeves have historically been a widely used method for general repair of defects in onshore pipelines, Because they involve welding, they are generally not applicable to the repair of defects in offshore pipelines. In the early 1970s, the American Gas Association funded a major project on the effectiveness of various repair methods with emphasis on full-encirclement sleeves. The results of this work showed that a properly fabricated steel sleeve restores the strength of a defective piece of pipe to at least 100% of SMYS. The keys to properly fabricating a sleeve are the use of a proven procedure and skilled personnel.
Most of the previous research on steel repair sleeves has addressed only their response to static pressures and lateral loads. Their response to repeated pressure cycles has not been studied in depth. Pressure-cycle fatigue tests were performed as part of an in-depth evaluation of steel compression sleeves. When they are used to repair defects in pipelines subjected to significant cyclic pressurization, steel sleeves are likely to have a finite time to failure. In the case of a reinforcing sleeve, the useful life might be controlled by the repaired defect, whereas in the case of a pressure-containing sleeve, the sleeve itself would be the critical component. Whenever a steel sleeve is to be used in cyclic service, its fatigue resistance should be evaluated. The results of the fatigue evaluation should be used to establish a suitable limit on the allowed service life before replacement is required.
The Type A sleeve is particularly attractive because it can be installed on a pipeline without welding it to the carrier pipe. Such a sleeve provides reinforcement for the defective area. It cannot contain pressure and is used only for nonleaking defects. It should be installed at a pressure level below that at which the area of the line pipe with the defect might be expected to fail.
A typical configuration and weld details for a Type A sleeve are shown in Figures 1 and 2, respectively. The Sleeve consists of Two Halves of a Cylinder of Pipe or Two Appropriately curved pieces of plate that are placed around the carrier pipe at the damaged area and after positioning, are joined by welding the side seams. As shown in Figure 2, the seams may be single-V butt welds or overlapping steel strips fillet welded to both halves may join the sleeve halves. If the side seams are to be butt welded and the sleeve halves are to be made from the same diameter pipe as the carrier pipe, then each half should actually be more than half of the circumference of the piece of pipe. Otherwise, the gap to be filled by butt-welding will be too large. With the overlapping strip concept, it is not essential that each half actually be more than half of the circumference because the gap can be easily bridged.
One major advantage of a Type A sleeve over other types of repairs is that for relatively short flaws it can function effectively without itself necessarily being a high-integrity structural member. Relatively short flaws are those whose length (L) are less than or equal to , where D is pipe diameter, t is pipe wall thickness, and all dimensions are in consistent units. For such flaws, the sleeve’s role is limited to restraining bulging of the defective area. As a result, it can be fabricated simply and requires no rigorous nondestructive inspection to ensure its effectiveness. Also, because the sleeve does not carry much hoop stress in the case of short flaws, it can fulfill its restraining role without necessarily being as thick as the carrier pipe. As a rule of thumb, the thickness of the sleeve should not be less than two-thirds of the thickness of the carrier pipe when used to repair short flaws, assuming that the sleeve is at least as strong as the carrier pipe. It is common to simply match the grade and wall thickness of the carrier pipe.
With flaws longer than , the sleeve thickness should be at least as great as that of the carrier pipe, again assuming that the sleeve is at least as strong as the carrier pipe. It is permissible to use sleeves thinner than the carrier pipe if their strength is increased by an amount sufficient to compensate for their thickness being less than that of the carrier pipe. In a similar fashion, the sleeve could be of an acceptable material with lower strength than that of the carrier pipe if its thickness is increased to compensate for the difference is strength. To assure that adequate restraint exists and that the sleeve will indeed prevent a rupture, the sleeve should be extended onto sound, full-thickness pipe at least 50 mm (2 inches) beyond the ends of the defect. The operator should design the repair to carry anticipated loads.
The Type A sleeve has some minor disadvantages. It is not useful for circumferentially oriented defects because it has no effect on the longitudinal stress in the carrier pipe. Secondly, it cannot be used to repair leaking defects. Thirdly, it creates a potential crevice in the form of an annular space between it and the carrier pipe that may be difficult to protect from corrosion. However, there have been no reported service failures caused by this potential problem. Because of this potential problem, however, some companies use full-encirclement sleeves as Type A sleeves, but weld the ends to the pipe to prevent further corrosion, thus making them essentially Type B sleeves.
To be effective, the Type A sleeve should reinforce the defective area, restraining it from bulging radially as much as possible. First and foremost, the sleeve should be installed with a minimal gap between the sleeve and the carrier pipe in the area of the anomaly. Forming and/or positioning the sleeve so that it firmly contacts the carrier pipe, especially at the area of the defect, can assure that the gap is minimized. One or more of the following actions (discussed separately in this sub-section) can further enhance the effectiveness of a Type A sleeve:
Pressure reduction is essential if the defect being repaired is at or near its predicted failure pressure at the start of the repair operation. If the pressure is not reduced in such a case, the repaired defect could begin leaking after the sleeve is installed. In contrast, when the pressure is reduced, the radial bulging at the defect location is reduced and can be prevented from recurring during re-pressurization by using a tightly fitting sleeve. Research results(19)(20) indicate that a pressure reduction of 33-1/3% from the predicted failure pressure was adequate for the application of Type A sleeves.
Typically, a pressure reduction is not necessary if it can be shown that the defect is not at or near its predicted failure pressure. For example, if calculations based on the size of the defect show that its predicted failure pressure is 33-1/3% or more above the current pressure, a Type A sleeve can effectively repair it without a reduction in pressure.
Reinforcing sleeves usually do not share much of the hoop stress that is acting on the carrier pipe unless special application techniques are used. Even if the sleeve fits perfectly and has 100%-efficient side seams, it will at most carry one-half of the hoop stress recovered after a pressure reduction if its wall thickness is that same as that of the carrier pipe. The optimum amounts of stress sharing produced by a snugly fitting sleeve for various amounts of pressure reduction are illustrated.
ta = actual wall thickness of carrier pipe
ts = wall thickness of steel sleeve
So = initial hoop stress in carrier pipe
Sr = reduced hoop stress in carrier pipe after installation of steel sleeve
Pr = reduced pressure at time sleeve is applied
Ph = highest pressure previously experienced by the carrier pipe after defect was present.
SMYSc = specified minimum yield strength of carrier pipe.
The two halves of a sleeve can be forced to conform to the carrier pipe and their sides can be drawn together appropriately for welding by mechanical means such as those shown in Figure 5. These can consist of chains and jacks (Figure 5b) or special preloading devices (Figure 5c). Lugs can be pre-installed on each half (Figure 5a). At the option of the installer, the lugs can be cut off after installation or left in place. Cutting them off facilitates coating the sleeve, an important consideration. A third option is the special chain-clamp device shown in Figure 5c. The hydraulic actuator that accompanies this latter device can be used to produce a significant preload in the sleeve. A significant preload can enhance the effectiveness of a Type A sleeve in the same manner as a pressure reduction in the carrier pipe. However, preload should not be substituted for pressure reduction in cases where a reduction of pressure is necessary for safety prior to the start of repair operations.
Hardenable fillers, such as epoxy or polyester compounds, are frequently used to ensure that no gaps exist between the sleeve and the carrier pipe. These compounds are typically mixed and trowelled into depressions in the carrier pipe, such as dents and pits. After the mixture hardens, the filler is shaped using files or other similar tools until the outside diameter of the pipe is restored. Another alternative is described below. Before the mixture hardens, the sleeve halves are placed around the pipe, and mechanical means, such as those described above, are used to squeeze the excess filler material. By the time the side seams of the sleeve have been welded, the filler mixture has usually solidified and load transfer between the sleeve and the carrier pipe is assured at all defect locations. Tests performed on pipe sections with filled gouges and dents(19)(20) showed that such fillers are very effective.
One concern with respect to applying Type A sleeves is the presence of a crown or reinforcement on the seam weld of submerged-arc-welded (SAW) carrier pipe or the flash on flash-welded carrier pipe. To assure a tight-fitting sleeve, three options are available. The first option is to remove the weld crown or flash by grinding it flush to the surface of the carrier pipe. This option is acceptable if the pressure has been reduced as suggested in Section 220.127.116.11. The second option is to grind a compensating groove in one of the sleeve halves. If this second option is selected, it may be desirable or necessary (in the case of long defects) to use a sleeve that is thicker than the carrier pipe by an amount that compensates for the thickness of material removed, including any compensation needed for differences in material strength. The third option is to force the unmodified sleeve over the weld reinforcement after sufficient filler material has been deposited to fill the expected gaps. This third option is acceptable if the resulting fit-up of the sleeve halves is adequate for side-seam fabrication. With the standard method of application shown in Figure 5b, there is no risk of damaging the weld. This third option should not be used with relatively high-force methods, such as lug and bolt or chain clamp, as local bending adjacent to the seam weld reinforcement may result.
British Gas developed a variation of the filled-sleeve concept in the form of their epoxy-filled shell repair method.(25) In this case, the shell is a sleeve with a standoff distance of several millimeters from the carrier pipe. The shell is placed on the defective pipe, and bolts are used to center it. The side seams are then welded, and the gaps at the ends are sealed with trowelled filler. After these seals have hardened, epoxy is pumped into the annular space until it comes out an overflow hole at the top of the sleeve.
Once the epoxy filler has hardened, the radial bulging tendency of the defect is restrained by the epoxy in the same manner as a conventional Type A sleeve would have if it were directly in contact with the sleeve. Data have been presented(26) that show that the epoxy-filled shell also can be used to repair weakened, but not leaking, girth welds. Bonding between the epoxy and the sleeve and the epoxy and the pipe permits the transfer of longitudinal stress. If the sleeves are used for an under-water repair, an epoxy that cures properly in water should be used.
Steel compression sleeves are a special class of Type A sleeves. They are designed, fabricated, and applied so that the repaired section of the carrier pipe is maintained under compressive hoop stress during subsequent operation. This approach is attractive for repairing longitudinally oriented crack-like defects because without a tensile hoop stress there is no driving force for crack growth. This type of sleeve is not suitable for the repair of circumferential cracks or for defects in field bends. CSA Z662(10) addresses the use of steel compression sleeves.f
Steel compression sleeves involve installing two sleeve halves over the defect area and drawing them together using clamps, jacks and chains, or lugs and bolts. The sleeve halves are then welded together using conventional welding techniques. Pressure reduction during installation is normally used to induce compression in the carrier pipe. Thermal contraction of the longitudinal seam welds also promotes compression in the carrier pipe. Epoxy filler is used between the carrier pipe and sleeve to achieve the transfer of stresses. As pointed out previously, pressure reduction alone will only transfer a portion of the hoop stress from the carrier pipe to the sleeve.
Half Pipe Sleeve is a commercial product that was developed to combine pressure reduction with thermal shrinkage of the sleeve for achieving full compression in the carrier pipe. Figure 6 illustrates the installation process for Half Pipe Sleeve. Two steel sleeve halves with sidebars are installed over the defect, the sleeve halves are heated, and are initially held in place with chain clamps or hydraulic jacks. The halves are then welded together using two longitudinal sidebars. During installation, an epoxy layer is applied between the sleeve and the carrier pipe. The epoxy is used as a lubricant when the halves are placed on the carrier pipe and later acts as a filler to evenly transfer the load between the sleeve and the pipe. As with other versions of Type A sleeves, no welds are made to the carrier pipe. Thermal shrinkage of the sleeve upon cooling helps induce a compressive stress into the carrier pipe.
Several factors influence the degree of stress reduction in the carrier pipe. These include the fit of the sleeve, the pipe wall thickness and diameter, the sleeve wall thickness, the internal pressure during installation, and the installation temperature. Specially developed software can be used to determine the target sleeve installation temperature and to help confirm that the desired amount of sleeve compression has been achieved.
Quality control procedures for Half Pipe Sleeve involve monitoring sleeve temperature during the heating process and verification of the achieved carrier pipe compression by measuring how far the two sleeve halves advance towards each other using caliper measurements. Three sets of measurements on each side of the sleeve are typically made. Nondestructive inspection of the completed welds is conducted after cooling.
The other type of steel sleeve used to make pipeline repairs is known as a Type B sleeve. The ends of a Type B sleeve are fillet welded to the carrier pipe. The installation of a Type B sleeve. Detailed discussions of the issues related to welding on an in-service pipeline are presented in Appendix A. Since its ends are attached to the carrier pipe, a Type B sleeve can be used to repair leaks and to strengthen circumferentially oriented defects. In fact, a Type B sleeve has been used in place of a girth weld to make a tie-in on a pipeline. Because a Type B sleeve may contain pressure and/or carry a substantial longitudinal stress imposed on the pipeline by lateral loads, it should be designed to carry the full pressure of the carrier pipe. Additionally, it should be carefully fabricated and inspected to ensure its integrity.
The typical configuration of a Type B sleeve is illustrated. It consists of two halves of a cylinder or pipe or two appropriately curved plates fabricated and positioned in the same manner as those of a Type A sleeve. Since the Type B sleeve is designed to contain full operating pressure, the ends are welded to the carrier pipe, and butt-welded seams are recommended. For a sleeve that will be pressurized, the overlapping strip concept of Figure 2 is not recommended because it is inherently weaker than a sound full-thickness butt weld and because it would be vulnerable to the stresses induced by direct internal pressurization. A Type B sleeve should be designed to the same standard as the carrier pipe. This usually means that the wall thickness of the sleeve will be equal to that of the carrier pipe and that the grade of the sleeve material also will be equal to that of the carrier pipe. It is acceptable to use a sleeve that is thicker or thinner than the carrier pipe and is of lesser or greater yield strength than the carrier pipe as long as the pressure-carrying capacity of the sleeve is at least equal to that of the carrier pipe. Many companies simply match the wall thickness and grade of the pipe material.
The diameter of the sleeve is slightly greater than that of the carrier pipe so it fits over the carrier pipe. Usually, this point is ignored in the sleeve design even though it causes the sleeve to be slightly under-designed when it’s made from the same material as the carrier pipe. If material is removed from the sleeve for a groove to accommodate the carrier pipe seam weld or a backing strip for the sleeve side seam welds, the thickness of the sleeve should be greater than that of carrier pipe by an amount that compensates for the material that is to be removed. If the sleeve thickness exceeds that of the carrier pipe by more than 2.4 mm (3/32 inch), it is recommended that the ends be tapered to the carrier pipe thickness on a 4 to 1 slope (four units along the sleeve for one unit of thickness). It is also recommended that the leg lengths of the fillet welds be equal to the thickness of the carrier pipe.
Type B sleeves are more likely to pose fabrication problems than Type A sleeves. Poor fit may lead to inadequate quality side seams, which in turn will degrade the pressure-carrying capacity of the sleeve. The end fillet welds are also possible sources of problems. A Type B sleeve installed over a leak that develops a leak itself is potentially as severe a problem as the defect that it was intended to repair.
Implementation of Type B sleeves requires (1) the development of adequate and appropriate welding procedures and (2) the training and qualification of personnel specifically for the purpose of fabricating such sleeves. The objectives of the procedures, training, and qualification should be to assure full-penetration side-seam butt welds and crack-free end fillet welds. Low-hydrogen consumables should be employed, and the recommended practices outline in Appendix B of API STD 1104(29) or some other recognized industry standard should be followed.
There are two primary concerns with welding onto an in-service pipeline. The first is for maintenance crew safety during repair welding, since there is a possibility of the welding causing the pipe wall to be penetrated and allowing the contents of the pipe to escape. The second concern is for the integrity of the system following repair welding, because welds made in service typically cool at an accelerated rate as a result of the ability of the flowing contents to remove heat from the pipe wall. These welds, therefore, may have hard heat-affected zone (HAZ) microstructures and may be susceptible to hydrogen cracking.