Kela-2 Gas Field is the main supply for the West-East Gas Pipeline which runs across China with length of 4000 kilometers. Natural gas from Kela-2 contains CO2/H2S sour components and 10% condensation water, which makes the medium present very strong corrosivity. In avoidance of corrosion failure, DSS UNS S31803 line pipes were introduced to transmit the gas from the Gas Field to gathering center and processing factory. And it is the first time DSS pipes which is totally about 13km were widely used in oil and gas pipeline around the world. As it is known, DSS UNS S31803 pipes and traditional linepipes X80 were welded together between the gathering lines and mainline. Duplex stainless steel (DSS) UNS S31803 and X80 were welded by metal inertia gas welding (MIG) with consumable ER2209, and the joints take on good mechanical properties. The type II boundary close to the fusion boundary at the carbon steel side was observed by SEM. Obvious concentration gradient of Ni and Cr was observed in the region between the two boundaries, where the hardness was much higher. The weld metal with columnar and some polygonal ferrites take on good passivation against test solution. The heat affected zone beside the X80 base material is mostly granular bainite, some polygonal ferrites and few MA, X80 base material present obvious rolling state, the microstructure of which is the same to X80 heat affected zone, but the level of crystal size reach to 11.6. The corrosion behavior of them dosen’t have any difference, but the corrosion potential of X80 HAZ is much higher than X80 BM, about 50 mv, the corrosion rate of X80 HAZ is also much higher than X80 base material. Potential curves of different weldment regions were also studied in 3.5% NaCl solution and 3.5% NaCl solution with saturated CO2. Galvanic corrosion behavior of weld metal and X80 steel was also tested and the conclusion is: The cathode reaction is controlled by oxygen diffusion, and when area ratio S (S = Ac / Aa, Ac is the area of DSS WM, Aa is the area of X80 BM) increases, corrosion current of X80 is enhanced rapidly, which is called “gathering principle”. When the weld metal and X80 coupled, cathode has much higher polarization potential and is protected completely.
The field service conditions of a well’s collapse tubing were investigated, and the physical and chemical properties of metal materials (chemical composition analysis, mechanical property test, metallographic structure analysis of tubing materials), hydrogen sulfide stress corrosion cracking test of tubing materials, and scanning electron microscopy analysis of tubing cracks and fracture surfaces were tested in this paper. The results show that the major cause of tubing collapse is that the tubing material is super 13Cr (S13Cr). When it is used in the environment containing hydrogen sulfide, plenty of longitudinal stress corrosion cracks is generated on the inner wall, which greatly reduces the actual anti extrusion ability of the tubing. During the gas lift of the well in the later stage, the upper part of the oil pipe was blocked and the oil jacket was in series, and the gas lift pressure was transmitted into the oil jacket annulus, so that the pressure difference between the inner and outer parts of the oil pipe exceeded its actual anti extrusion capacity and collapse failure occurred.
In recent years, fiber reinforced thermoplastic pipe (FRTP) has been widely used in oil fields of China due to its excellent corrosion resistance and coiling performance. With the further expansion of application scope, some failure accidents have occurred. Based on the inspection of the wall thickness, chemical composition and physical properties of a failed FRTP in the western oilfield of China, the causes are systematically analyzed. The results show that the main cause of the sample fracture is the decrease of the strength of FRTP due to the oil and gas media constantly penetrate into the lining and reinforced layer during the long-term service. Moreover, it is found that the outer sheath is worn and penetrated to the reinforced layer which further reduces the bearing pressure performance of FRTP and finally causes the fracture failure.
According to the cracking accident of repaired tubing used in surface gas injection pipeline in a western oilfield in China, the cause of the failure tubing was analysed by means of macroscopic analysis, non-destructive testing, chemical analysis, mechanical testing, metallographic examination, macro and micro fracture analysis. Combined with the working environment of the repaired tubing, conclusions were made that the failure cause of the tubing was sulfide stress corrosion cracking (SSCC) and the crack originated from the clamp area of make -and-break test in the outside surface of the tubing. Finally, suggestions and measures for prevention congener failure were put forward.
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