IPC ABSTRACT

Transcription

1 International Pipeline Conference Volume I ASME 1998 IPC REPAIRING PIPE DEFECTS (CRACKING, ARC BURNS, CORROSION, DENTS) WITHOUT OPERATIONAL OUTAGES USING THE PETROSLEEVE1 COMPRESSION SLEEVE REPAIR TECHNIQUE Robert J. Smyth, P.Eng. Petro-Line Upgrading Services Ltd Avenue, Nisku Industrial Park Nisku, Alberta, T9E 7Y1 Canada Phone: (403) Fax. (403) Website: ABSTRACT Non-serviceable defects, such as cracking, dents, corrosion, or mill defects, are found in operating pipebnes. Past methods to repair these defects have included removing the defect, welding a pressure containing sleeve, or, for corrosion, installing either a mechanically tightened or fibreglass reinforcing sleeve. These methods involved potential system shut downs, potential loss of throughput, and in some cases welding to the carrier pipe. The PETROSLEEVE compression sleeve repair system consists of installing steel plate material that is shaped and sized to encircle the carrier pipe. The repair is completed while the carrier pipe continues to operate at the pipeline s operational pressure. Following installation, the pipe and sleeve act as one unit, expanding and contracting with pressure changes. Depending on the installation parameters, the repair sleeve can be installed so that compressive stresses are maintained in the pipe for all pressure operating ranges. This repair method does not require any welding to the operating pipeline. It can be installed when the carrier pipe is at maximum operating pressure. 1 Patents Pending Copyright 1998 by ASME

2 BACKGROUND This compression sleeve repair technique (Figure 1.0) was designed as a non-invasive system to provide a means to permanently repair various anomalies that are found on operating pipelines. Figure Installed Compressive Sleeve sleeve, a mechanical reinforcing sleeve or a fiberglass reinforcement sleeve. These methods could require system shutdowns, welding to the pipe, and pressure reduction. To date, this repair system has permanently repaired: > Corrosion > Stress Corrosion Cracking > Long seam indications > Arc burns > Dents With the advent of internal inspection tools, anomalies can be identified and located on operating pipelines. After inspection of the anomaly, an engineering assessment is made to determine whether the anomaly is fit for service. If the anomaly is not fit for service, repairs must be undertaken. The historical repair methods have been to cut out the defect, or to install either a pressure containment In many cases, the pipe replacement option either is not practical, or very expensive. In Alberta, there are many single line systems where the pipeline operators do not look favourably upon system shutdowns. Shutdowns may represent service interruption to upstream producers, who may lose their oil nominations. Also shutdowns cause difficulty to downstream users who may be required to obtain product from other suppliers, loss of natural gas if the line needs to be blown down, and disruption to down stream natural gas users. Welding to the earner pipe has been known to cause several problems. In Alberta, ultrasonic examination of repair welds on operating pipelines has found cracking in the weld area. In other instances, repair welds have been known to fracture, causing environmental damage and system interruption.

3 When repair welding is undertaken (Figure 2.0), the welding process must be undertaken with care to prevent subsequent weld cracking, failure, or cutout. amount of stress reduction in the carrier pipe deemed necessary for repair of an anomaly, and to mitigate any future defect propagation. Following completion of a repair, the steel repair sleeve and carrier pipe act as a single structural member. Several factors influence the degree of the designed stress reduction in the carrier pipe including: Sleeve Thickness Figure Typical Stopple Installation of Welding to the Carrier Pipe The compression sleeve repair system was developed to overcome these difficulties. It was realized that a repair system was required that could be used as a repair for anomalies on pressurized pipe such as crack defects, was compatible with the pipe material, and would remain serviceable irrespective of pipeline pressure changes. As well, when undertaking an anomaly inspection program, anomalies not fit for service would be identified. Upon finding norvserviceable anomalies, a repair must be undertaken. Using this method, a repair could be completed immediately as apposed to organizing a cutout. Following sleeve installation, work at the excavation site could be completed, with the surface restored to its original condition. No interruption in pipeline service would be required. ENGINEERING The selection of the repair sleeve thickness provides the initial basis for the determination of the percentage of stress reduction in the carrier pipe. For example, if the sleeve material is the same thickness as the carrier pipe, the effective wall thickness is doubled. Pressure forces in the pipe would be shared equally between the pipe and the sleeve, reducing the stress in the earner pipe by 50%. Interference Fit The interference fit acts as the primary mechanism by which stresses in the carrier pipe are transferred from the carrier pipe to the compression sleeve. As the level of interference is increased, the tensile stress in the carrier pipe material is reduced. Depending on the desired design repair stress conditions, an interference fit repair installation can be performed where the carrier pipe material remains in a compressive state throughout all pipeline operating pressures. Material With this repair system, a material that matches the Young s Modulus of Elasticity of the pipe material, as well as its expected physical life, is used to reinforce the anomaly (i.e. steel on steel). Following installation and coating, the repaired system then can be viewed as having the same physical characteristics as the original pipe. The installation of the compression sleeve repair system was designed to specify and obtain the

4 Figure Installed Compressive Sleeve INSTALLATION PROCEDURE The installation procedure consists of two major components - the assembly and clamping of the sleeve halves around the carrier pipe, and the heating of the sleeve prior to commencing welding as shown in Figure 3.0. During this cooling process, the sleeve continues to shrink onto the pipe, creating the designed interference fit. The forces generated as a result of the interference fit cause the steel repair sleeve to be put into tension, the pipe material to be put into compression. The compression sleeve repair now acts as a single structural member. The installation process requires that the surface of the pipe, including the anomaly, be cleaned to remove all foreign materials (N.A.C.E. 2 commercial finish). Following trial placement of the sleeve around the pipe to confirm the fit, a 100% solids epoxy filler is placed on the pipe surface. The two sleeve halves are then assembled around the pipe with specially designed mechanical jack devices used to hold the sleeve in place. The temperature of the sleeve is then raised to the specified installation temperature, and the fillet welds are completed. Following completion of the welding, the sleeve cools to the temperature of the pipe. Figure Taking Interference Measurements

5 During installation, measurements are taken, as shown in Figure 4.0, so that the actual sleeve interference fit is confirmed with the engineering requirements. By applying simple mechanics of materials principles, the stresses in the carrier pipe material can be determined for various internal pipe pressures. Removal With this repair system, the compression sleeve can easily be removed to re-inspect any anomaly. Following inspection and analysis, the same sleeve material can be reinstalled. TESTING The compression sleeve repair system has been tested to verify engineering theory that the repair method can be installed with an interference fit while the pipeline is operating at its maximum operating pressure (MOP). The testing has included destructive and non-destructive test methods. made from 508mm pipe containing manufactured cracks. Strain measurements were taken during the sleeve installation phase, the pressure cycling phase, and the pressure to failure phase. Once strain gauges were installed on the inside and outside of the two vessels, as shown in Figure 5.0, they were filled with water. Steel sleeves were then installed over the cracks. As an example, Vessel #2 had two sleeves installed over two manufactured cracks. The first sleeve was installed when the vessel pressure was 0 kpa. The second sleeve was installed when the vessel pressure was 2750 kpa, or 30% Specified Minimum Yield Strength (SMYS). All strain gauges were zeroed prior to vessel pressurizing and sleeve installation. The average of the four internal strain gauges under the sleeve and under the non-sleeved pipe material are represented in Figure 6.0. This figure illustrates the strain gauge readings taken during the sleeve installation process with the internal pressure being varied between zero and 2750 kpa. Summary of testing: > Determined interference fit using strain gauges > Determined interference fit using mechanical testing methods > Determined the effective epoxy shear strength > Confirmed interference fit during sleeve installations > Determined the operating stresses in the carrier pipe after sleeve installation > Tested the heat transfer characteristics of the sleeve-carrier pipe structural member > Tested the effective use of a sleeve over a through-wall anomaly to determine the sleeves ability to prevent rupture > Tested the sleeves ability to arrest crack growth Determined Interference Fit Using Strain Gauges This testing involved installing strain gauges on the interior and exterior surfaces of two test assemblies Figure Strain Gauges Installed on Sleeves and Vessels During the installation process, at approximately 1200 kpa, it was decided for safety reasons to place and clamp the sleeve onto the pipe, so that the manufactured crack was covered. The slight decrease in strain readings at this point illustrates the effect of the clamping forces on the tensile strain in the carrier pipe (Figure 6.0).

6 Strains Beeardcil D o rio n P E T R O S L E E V I installatian Installation Procsss & Pressure PETROSLEEVE Figure Installed Compressive Sleeve At 2750 kpa, the fifth strain gauge located directly under the manufactured crack indicated a strain representing 75% of SMYS. Considering this apparent stress level, It was decided to cease increasing the internal pressure and to install the sleeve. Following sleeve installation and cooling, the average of the internal strain gauges under the steel sleeve indicated a change from +300 microstrain (Prior to Sleeve Installation) to microstrain (post sleeve installation). This represented an overall change of 700 microstrain while the pipe vessel remained at 2750 kpa. The strain condition in the pipe body under the sleeve had changed from being in tension to being in compression. At the same time, the average of the internal strain gauges not affected by the sleeve installation continued to read +300 microstrain. With the release of the internal pressure to zero, the interior strain gauges under the installed sleeve continued to increase in negative readings, finally averaging -600 micro strain. At the same time, the other internal strain gauges not affected by the sleeve installation returned to approximately zero. Determined Interference Fit Using Mechanical Testing Methods This testing was undertaken to confirm that an interference fit between the sleeve and the pipe was being achieved. The testing involved installing two sleeves on a 273mm pipe vessel pressured to 8270 kpa, or 61% of SMYS.

7 One sleeve was installed with epoxy, the other without. Following installation, the internal pressure was released and 1 circular coupons from each sleeve installation were prepared for destructive testing. Using a hydraulic press (Figure 7.0 & 8.0), the line pipe ring inside the sleeve was separated from the sleeve. For the non-epoxy sleeve, a 2800 pound force was required to press the line pipe ring from inside the sleeve ring. This illustrated that the outside circumference of the pipe was greater than the internal circumference of the sleeve, an interference fit had been achieved. Determined the Effective E p o x y Shear Strength As an additional aspect of the mechanical testing of the sleeve pipe structural member as described above, the separating of the line pipe ring from inside the sleeve ring containing epoxy was performed (See Figure 7.0 and 8.0). For this case, a 40,000 pound force was required. This defined a combination of the interference fit and the shear strength of the epoxy. Figure Ring Test Coupons Confirmed Interference Fit During Sleeve Installations Over 450 compressive sleeve installations have been completed to date. During installation, interference measurements have confirmed the sleeve shrinkage. Applying simple engineering mechanics of materials principles, the stresses in the carrier pipe material have been determined for the installation internal pressure condition, and then for all other operating pressure levels. Determined Operating Stresses in the Carrier Pipe after Sleeve Installation During the testing undertaken in conjunction with the strain gauges, the two 508mm test vessels with sleeves installed were pressure cycled to measure internal strains for various internal pressure levels. Figure 9.0 illustrates the typical average strain levels under the steel sleeve and strain levels in the adjacent pipe. Figure Hydraulic Press Figure 9.0 illustrates that the pipe and installed sleeve act as a single structural member. For each incremental pressure increase, the ratio of the strain in the pipe under the sleeve in relation to the strain in the adjacent non-sleeved pipe, remains constant. This same strain ratio is represented when pipeline wall thickness changes from thin wall to thick wall.

8 The second aspect is that the transition from compressive strain to tensile strain in the steel under the sleeve occurred at about the 8300 kpa or 92% SMYS. If, for example, the maximum operation pressure for this vessel was 7200 kpa or 80% SMYS, this situation illustrated that the steel under the sleeve would remain in a compressive state throughout all operating pressure ranges. The two test vessels were then pressured to failure. In both cases, the failure pressure was in the range of 15,000 kpa or 167% SMYS. For one vessel, the non-sleeved manufactured crack ruptured, for the other vessel, massive yielding occurred in the pipe adjacent to the sleeve, prompting the test to be terminated. Figure 9.0 illustrates that at the rupture pressure level, the steel under the sleeve was experiencing a strain representing approximately 2000 kpa or 23% SMYS. Tested the Heat Transfer Characteristics of the Sleeve-Carrier Pipe Structural Member This testing was undertaken to determine the rate of heat transfer from the outside sleeve surface to the interior surface of the pipe. A sleeve was installed under very low airflow conditions. The interference measurements obtained from this test confirmed that a sleeve could be installed on a gas pipeline under low flow conditions. Tested the Effective Use of a Sleeve Over a Through-wall Anomaly to Determine the Sleeves* Ability to Prevent Rupture This testing was undertaken to determine how a sleeve installation would perform if a long through wall defect was present in the carrier pipe. Following

9 installation of a sleeve, a long through wall defect was ground into the carrier pipe. Test heads were then welded to the sleeve assembly. The vessel was hydrostatically pressured to approximately 11,000 kpa, representing 98% of SMYS. After varying and holding the pressure at these levels, no physical change or leakage of the vessel was noted. Tested the Sleeves Ability to Arrest Crack Growth Following the destructive testing of the 508 mm vessels with the strain gauge testing, the sleeves were removed from the vessels and sent to an inspection company for them to determine if crack growth had occurred after the sleeves had been installed. A post metallographic examination of the manufactured cracks confirmed that there was no evidence of crack growth beyond the fatigue crack tip on any of the sleeved samples during the pressure testing.2 INSTALLATIONS COMPLETED TO DATE Over 450 sleeves have been installed to date. They have been installed on oil, sweet gas, sour gas, and NGL pipelines operating in Alberta and British Columbia, ranging in diameter from 168 mm to 610 mm. Figure 10.0 illustrates an example of multiple compressive sleeves installed on an operating pipeline. In all cases, the pipeline remained in service, transporting product. The internal pressures during various repair sleeve installations have ranged from zero to 8260 kpa. ECONOMIC ADVANTAGES This compressive sleeve repair system provides the following significant economic advantages: > Repairs can be completed without disrupting pipeline operations or pipeline operating pressures Figure Three Installed Compressive Sleeves > No welding to the carrier pipe is required > Cutouts are avoided, eliminating down time, product loss, and other associated risks Various companies using this repair method have realized these economic advantages. In all cases, the anomalies were repaired without subjecting the pipeline system to any significant reductions in pipeline operating pressure, expensive cutouts, or downtime. 2 TCPL Presentation to the Pipeline Rehabilitation Committee Meeting