Repair of Steel Pipes

The repair of steel pipes can be necessary because of the following defects  

The standard repair procedure of a damaged pipe can be summarized as 

In order to achieve the correct dimensioning of FRP, reinforcement is necessary beforehand in order to condnct an appropriate analysis of the materials. 

Ideally, a battery of tests should be conducted using weights as a percentage of the lower bound failure load for the given material. Creep testing should also be conducted over several different loading time periods. 

Lap shear testing should be conducted to ensure that an adequate bond exists between the pipe and the wrap. For composite repair methods that are not monolithic, these tests should also include composite-composite test samples as well as the composite-steel test coupons. The composite-composite sample is used to assess the bond strength between the layers, while the composite-steel samples are used to determine the lap shear strength at the interface between the pipe material and the composite. 

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Figure 8.27 Illustration of the ITtP patching for fatigue crack repair of steel pipe.

Studies on internal repair of steel pipe rehabilitations... 

Carbon and steel are dissimilar metals and when they come in contact, galvanic corrosion can ensue. The best way to prevent this is to provide a dielectric barrier between the carbon fibers and steel pipe. While the epoxy resin in the FRP does cover all the fibers, the thin layer of resin is not considered by many to be a proper long-term insulating shield to prevent this contact. The most widely accepted practice in the industry is to apply a layer of glass fabric as the first layer for any repairs in steel pipes (Figure 3.1). If the glass fabric is not intended to provide any strength for the pipe. 

A limited number of stodies have been conducted on the internal repair of steel pipelines using composite material systems. As a consequence, only a few industries have used composite technologies for internal repair. In their report, Bruce et al. (2006) indicated that internal repair would have the best economics for underwater repair locations as it reduces out-of-service time and does not require divers and habitats. Such economics arise because the majority of the gas transmission line companies in the United States consider the ability of the pipeline to remain in service during internal repair to be very important. Moreover, they would considCT internal repair, even if the pipeline needs to be out of service (no flow), only if the pipeline remains pressurised and the line can still be inspected by a pipe inspection gauge after repair. However, these companies have indicated that they would consider performing a repair from inside the pipe once a proven and accepted internal repair system becomes available. Thus, the selection of the most appropriate technique of renovation, particularly on the internal repair of high-pressure and deep water pipeline applications, is a critical and ongoing issue. 

Carbon fibre has a modulus of elastieity almost similar to that of steel, thus has greater potential for internal repair of steel pipelines. Bruee et al. (2006) tested a section of 508-mm diameter and 6.35-mm-thick API 5L X52 pipe with a simulated corrosion defect. A defect was introduced to the pipe to represent a 25% reduetion in burst strength. The CF fabric used for the internal repair had a modulus of 241 GPa. 

Investigators determined that a carbon steel section of 6-inch (150 mm) line was installed in an area in which specifications required corrosion-resistant 5-percent chromium—1/2-percent molybdenum (generally called 5-chrome) alloy piping. In a process plant like a fluid coker, the materials used are a mixture of carbon steel and other steel alloys. Apparently, the welder and the maintenance crew, who previously repaired this piping some time ago, were not aware of the piping specifications. Apparently, they did not realize that if a material like carbon steel was installed in an area requiring 5-chrome alloy piping, erosion or corrosion could cause failure. 

A piping repair job and a quick return of the piping to chlorine service provided a surprise. The team was not fully aware of the hazards. It should have been planned better to allow the normal but necessary cooling of the steel pipeline before introducing chlorine. 

A combination of composite materials in conjunction with steel was introduced into the pipeline industry in the 1980s, when the application of composites overlaying steel sleeve became a standard for pipeline repairs. One of the most notable applications is that resulting from the collaborative efforts of Emon and the Gas Researeh Institute (GRl) in 1991, which resulted in installation of SSLP in their system. After eight years in service, the SSLP was inspected, and was verified to be in the same condition as in its initial state. Later on, TransCanada Pipelines Inc. also ventured into the use of SSLP pipes in several projects in 1998, 2001 and 2002 (Stephen, 2005). 

Although the code recommends taking out of service defective sections of the pipeline, it accepts that this is not always possible. It does, therefore, allow for certain methods of repair while the pipeline is in service, such as the use of mechanically split sleeves, hot tapping, encirclement of welds, etc. The engineer should, however, be aware that these methods may not apply to rubber, polyurethane, or HDPE-lined steel pipes as they would damage the internal lining by heat. 

The international and nationaT standards specify requirements and methods of test for liquid epoxy paints and internal coatings of such paints in steel pipes and fittings for the conveyance of non-corrosive gas. They also deal with the application of the paint. Other paints or paint systems are not excluded, provided they comply with the requirements given in the standards. The coating consists of one layer, which is normally shop-applied on blast-cleaned steel by airless spray or other suitable spraying techniques. The applied and cured paint film must be smooth to give the desired reduction in friction. Brush application is only used for small repair jobs. 

Pipe repair options include (1) external repairs, such as circumferential posttensioning, replacement, external steel or CFRP bands, or encasement of the distressed pipe, or (2) internal repairs, such as installing a steel liner or slip-lining with fiberglass or steel pipes, or lining with hand lay-up CFRP. Selecting the best option depends on access, acceptable duration of construction, impact on operation, the expected cost of repair, and reliability of the repaired pipeline. 

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