The main mechanisms of stress corrosion cracking in natural gas trunk lines

Abstract

620.193 Stress corrosion cracking in a pipe material presents a most critical hazard to natural gas trunk lines. The paper addresses the causes (mechanism) of crack formation and growth. Understanding of these issues will enable a proper implementation of methods for early identification, diagnostic, and prediction of this type of failure. A most comprehensive explanation of a joint influence of internal and external factors on the corrosion cracking can be provided by means of the probabilistic local electrochemical corrosion theory. It is based on the probabilistic nature of initiation of a local corrosion damage. A probabilistic approach is used to develop a concept of stress corrosion cracking. The factors responsible for the formation of local damage nuclei have been determined. The pipe manufacturing operations are shown to have a certain effect on the pipeline susceptibility to stress corrosion cracking. Modeling has been performed for the corrosion processes that may occur in a gas trunk line section subjected to a stress corrosion cracking hazard.Nowadays, stress corrosion cracking (SCC) in a pipe material is a most critical hazard to natural gas trunk lines [1]. This type of corrosion damage was first revealed in a trans-Canadian gas trunk line in 1965, then in Central Asian gas pipelines in the 1970s, and in the northern Russian regions in the 1990s. Recently there have been three SCC accidents on the Urengoy-Pomary-Uzhgorod trunk line in Cherkassy region (Ukraine). This gas pipeline was constructed in 1982 using 1420-mm-diameter pipes of X-70 grade. The accidents happened on the line sections close (at about 5 km) to the neighbor compressor stations (CS), were accompanied by an explosion and gas inflammation. The resulting craters were up to 50 m long, 30 m wide, and 10 m deep, with a thermal emission zone up to 750 m in radius, and fragments flying as far as 500 m.A crucial prerequisite for minimizing the SCC hazard is to know the causes (mechanism) of crack initiation and growth [1-6]. Understanding these issues will enable a proper implementation of the methods for identification, diagnostic, and prediction of this type of failure at early stages.Stress corrosion cracking is a specific type of corrosion cracking which is initiated and propagates on the outer surface of a coated and cathodically protected pipes due to the combined action of a tensile stress and a corrosive environment. A most comprehensive explanation of a joint influence of internal and external factors on the corrosion cracking is provided by the probabilistic local electrochemical corrosion theory. It is based on the probabilistic nature of initiation of local corrosion damages.We will use the probabilistic approach to develop the SCC concept. In this case, the initiation of local corrosion damages is assumed to be stochastic. A review of available SCC data for pipelines enables us to identify the external and internal factors responsible for the SCC initiation: 456 0039-2316/09/4105-0456

Fatigue and brittle fracture of carbon steel of gas and oil pipelines

“…The tired and corrosive-tired tests of specimens of the basic metal and the welding joint of the sea pipelines were carried out within a total range of the amplitude tensions Á from the durability limit to the limit of the corrosive endurance (Polyakov, Rybakov 2009). In our present work, the kinetics of deformation and basic supporting members' failure of oil and gas pipelines was studied on the basis of ground analysis of curves 'deformationÁtime (quantity of cycles)', obtained in accordance with the method developed earlier (Vianello, Maschio 2011) with the use computer registration technique under the conditions of real time with increment from 0.01 to 1.0 sec, which was selected automatically on the assumption of the change rate of measured parameter.…”

Analysis of the causes of fracture of the main gas pipeline

“…Damages to pipelines caused by stress-corrosion have expanded the boundaries not only in geographical but also in technological terms: stress-corrosion defects are revealed on connecting gas pipelines and main oil pipelines of a large diameter [8,9]. The experience of operating such structures shows that at the proper functioning of the cathode protection system, both on rectilinear as well as on spiral welded pipes with a film and rubber-bitumen insulation, in places of its defects and delaminations, propagation of SCC is possible [10]. In the problem zones of river floodplains and wetlands on those lines of main pipeline, where a large number of thin-walled double-welded pipes are laid, the activity of stresscorrosion process grows gradually.…”

Characterisation of hidden defects using the near-field ultrasonic enhancement of Lamb waves

“…Surface-breaking cracks, such as defects caused by stress-corrosion cracking (SCC), in which defect growth occurs when a material is placed under stress in a corrosive environment, are of concern in industrial pipework and chemical storage systems [1][2][3][4] . SCC defects typically have a size scale of several millimetres with a complicated branched structure, and can occur singly or in groups of defects located close together [1] .…”

“…Edwards) Ultrasonic inspection is an attractive alternative to dye penetrant inspection, however, conventional piezoelectric ultrasonic transducers require the use of couplant, and as such have a limited capacity for scanning large samples or at elevated temperatures [4,[8][9][10] . The use of non-contact ultrasonic generation and detection methods, such as laser ultrasonics [11] and electromagnetic acoustic transducers (EMATs) [12] removes the need for couplant, and thereby provides the potential to perform simple scanning inspections on components [13] .…”