PSI - Issue 37

Jan Kec et al. / Procedia Structural Integrity 37 (2022) 598–605 Jan Kec / Structural Integrity Procedia 00 (2019) 000 – 000

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Nomenclature MAOP

maximum allowable operating pressure

D

external diameter of pipeline

t

thickness of pipeline

CS CL

Clock Spring Cold Lock

The presence of defects on the pipeline results in a reduction in the safety and reliability of the section. The most reliable way of making the pipeline safe again is to replace the entire affected section, but this entails a number of costs as the pipeline has to be completely shut down, dug out of the ground, the defective section replaced, the pipeline cleaned and put back into service. Since the cost of replacing the entire defective section is extremely high, a less expensive way to ensure the safety and reliability of the pipeline is reinforced with a sleeve. The first sleeves began to appear around the early 1970s due to the work of Kiefner at Battelle Laboratories, these were steel sleeves designed to provide structural reinforcement (Type A) and even containment (Type B) in the event that a defect leak or will eventually leak during subsequent operations (Bruce, 2015). However, the normally difficult field welding conditions meant that the full encirclement welded sleeves did not meet the quality requirements. The welds on these sleeves were often made with high deposition rates and, in addition, cellulose coatings, hence defects occurred and the welds had reduced ductility and toughness due to diffusion of atomic hydrogen into the molten pool (Otegui et al., 2001). This resulted in the occurrence of the first accidents in the 1980s. Today, it appears that in addition to the defects produced, welding into the transfer pipeline itself is highly unsuitable due to the high residual stresses that arise after welding (Alian et al., 2016). Perhaps because of this, the late 1980s saw the use of fibre-reinforced composite materials, which is still used today under the commercial name Clock Spring (CS). The CS sleeve consists of an E glass and polyester resin based composite material that is pre-molded into a multi-layered coil (Lesmana et al., 2015). Full-scale tests are very rare in the literature, apart from their financial and facility requirements, they provide the most realistic view of the behaviour of defects in service. Ma et al., 2019 verified the reliability of a type B steel sleeve on X80 pipe and concluded that it can effectively guarantee the safe operation of the pipe, but considerable care must be taken with fillet welds as these affect the reliability of the sleeve. The INTEMA group from Argentina has worked quite intensively on the various influences in the field of steel sleeves. The results of the work of Chapetti et al, 2001 have shown some operational aspects that lead to the minimization of stresses in steel sleeves and reduce the risk of failure. It is worth mention that sleeves longer than 1 m should be used, higher operating pressure during repair leads to minimization of stresses in the sleeve welds or tandem welding should be avoided. According to the work of Otegui et al., 2002, it was found that there is no significant interaction between two steel sleeves when they are at least half the pipe diameter apart. As already mentioned, the fiber-reinforced composite sleeves are not welded to the pipe, hence eliminating many of the aforementioned problems related to stress increase from welding. Sleeves of this type are recommended for blunt defects, but Law et al, 2008 compared the fatigue crack growth rate of sleeved and non sleeved pipelines. Repair of composite sleeves has been shown to retard fatigue crack growth. The stress in the pipe under the composite casing is approximately 93% of the stress on the bare pipe. Beak et al., 2006 compared different types of repairs on pipelines. For the experiment, API 5L X65 pipe with artificial defect reinforced by three types of sleeves was chosen - steel sleeve, epoxy sleeve, CS (composite rapping). All three types of sleeves achieved high burst test pressures corresponding to a safety factor greater than 2,5 with respect to maximum allowable operating pressure (MAOP). The lowest values were achieved on the CS sleeve while the highest values were achieved on the steel sleeve. The present paper extends Beak's work in that a higher diameter to thickness ratio D/t was used, resulting in lower residual thickness for equally deep defects. Among other things, the paper evaluates the residual strength of the pipes after two decades of service. 2. Experiment 2.1. Test method and procedure After the shutdown and cleaning of the line, two segments of the DN500 and DN700 pipes were cut out and replaced with new pipes. The DN500 segment (530 x 8 mm) was 6 m long and had a steel sleeve with a composite filling ,