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Detection, Characterization and microstructural Investigation of Hydrogen Induced Cracking in a case study of oil and gas industry: Finger Type Slug Catcher

By Mohsen Asadipour (Department of Mechanical Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran)
Co-authors: J. Kadkhodapour (Department of Mechanical Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran)
S. Schmauder (Institute for Materials Testing, Materials Science and Strength of Materials, University of Stuttgart, Stuttgart, Germany)
S.M.H. Sharifi (Department of Marine Engineering, Petroleum University Of Technology, Mazandran, Iran)
A.P. Anaraki (Department of Mechanical Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran)
M. Moalemi (e Kavan Parto Company, Tehran, Iran)

Hydrogen Induced Cracking (HIC) is recognized as the most important damage mode for Pipeline and component steels serving in the sour environment. In this phenomenon, Hydrogen atoms produced due to surface corrosion of the steel diffuse into it through at various micro-structural features, such as metallurgical defects, non-metallic inclusions and large precipitates. When a critical amount of hydrogen is accumulated in such defects, HIC cracks initiate and propagate. In this study hydrogen induced cracking phenomenon in a real case study of oil and gas industry (finger type Slug Catcher) is investigated. This unit consists of several long pieces of pipe ('fingers'), which together form the buffer volume to separate the largest slugs expected from the upstream system. Because of the corrosive nature of the input fluid and spent ten years from the operational life of it, Occurrence of damages such as pitting corrosion, hydrogen induced cracking and stress corrosion cracking caused by hydrogen sulphides are probable. In order to remaining life assessment of finger type Slug Catcher, which is one of the important equipment in the oil and gas industry, various fracture theories can be used. The parameters in these theories are extracted by numerical simulation of crack or multiple cracks in a pipe, while taking into account loading and actual environmental and operational conditions. Due to the multidisciplinary nature of HIC, before the above analysis, the accurate microstructural Investigation of HIC phenomenon in the Slug Catcher is inevitable. In the first part of this study, advanced ultrasonic techniques, namely Phased Array (PA) and Time of Flight Diffraction (TOFD) are used in order to provide the accurate detection and characterization of defects in body of Slug Catcher. In fact HIC is a challenging type of defect from ultrasonic point of view mainly because of its geometrical configuration that includes both a planar component, favorable to zero degree longitudinal waves detection, but also a crack-wise through thickness component that requires an inspection with angled waves. Phased Array (PA) represents a sensible step forward compared to traditional UT techniques for this application since it allows using multiple techniques and angles in the same scan while registering the inspection. In the second part of this study, the effect of microstructure and inclusions on hydrogen induced cracking phenomenon are investigated. For this reason it was necessary that to remove samples from critical locations of component that detected by non-destructive testing. The main objectives of this section are to investigate of HIC crack nucleation and propagation sites and also evaluate of microstructural phases and grain boundary character distribution in crack growth path. To achieve these purposes, scanning electron microscope (SEM) and Energy dispersive spectroscopic (EDS) techniques were used to characterize the actual samples. In fact by using of these methods, HIC cracks at the cross section of tested samples were observed and also types of microstructural phases in cracking region were identified. Furthermore another aspects of HIC such as the chemical composition analyses of microstructure, the types of inclusions and precipitates serve as crack propagation sites, grain size and shape and grain boundary character were determined. The expected results are the inclusions and precipitates play an important role in HIC phenomenon as crack nucleation and propagation sites and they have been distributed randomly through the cross section of tested steel samples. However, the concentration of them is higher at the center of cross section than other areas. In fact HIC cracks initiate and propagate through the center of thickness where center segregation of elements has occurred. It is also observed that HIC cracks are initiated from several special types of inclusions and precipitates. Finally a finite element model for the numerical simulation of HIC in steel pipelines exposed to hydrogen sulphide (H2S) are presented. This model is able to simulate the pressure build-up mechanism related to the recombination of atomic hydrogen into hydrogen gas within the crack cavity. In fact, PH and partial pressure of H2S are two boundary conditions. Furthermore, in this simulation, is attempted the relation between the crack growth rate and different parameters such as the initial crack dimension and shapes and fracture toughness is also evaluated.

Ⓒ Photos:Toerisme Leuven