ABSTRACT The presence of H2S in hydrocarbon carrying pipelines requires significant precautions, regarding material specification to be considered. This paper aims to report how homogeneity of microstructure can seriously affect the reliability and longevity of pipelines services. The purpose of this article is to focus on failure resulted from hardness and microstructure inhomogeneity of a sour service pipeline.
INTRODUCTION Micro alloyed pipeline steels are commonly used in the petroleum industry for transport applications [1].But, the presence of H2S in well fluids imposes severe restrictions on the use of tubular goods due to sulfide stress cracking phenomenon [2]. Sulfide stress cracking (SSC) of high strength, low alloy steel is a phenomenon of particular concern to the oil and gas industry [3]. Producing wet sour gas presents some of the toughest technical challenges in the oil and gas business. Hydrogen sulfide, being very toxic to people and wild life and polluting to the environment, is also very corrosive [4]. Frequently, different components of oil wells are in contact with acid brines containing hydrogen sulfide. If stress is applied to such components, catastrophic failures can occur. It has been established that SSC is a particular case of hydrogen embitterment [3]. SSC in pipelines can occur from two sources: internally, from transporting wet, sour products or from water containing sulfate-reducing bacteria (SRB); and externally, from SRB in soil or water in contact with the pipe. Internal SSC is far more common than external, which is rare [5]. In regard to the importance of tubular goods, the greatest importance has been placed on controlling SSC susceptibility through metallurgical factors such as hardness, microstructure, chemistry, tensile strength and yield strength [6]. Strength and microstructure have a strong influence on the susceptibility to SSC. The higher the strength of a material, the higher is its susceptibility to SSC [1, 3]. The influence of microstructure is less clear, probably because a full characterization of all the components has not always been carried out [3]. Recent researches [1] show that steel microstructure (Ferrite, Pearlite, Bainite, Martensite, etc.) upon the strength of the steel, could play a more important role on the cracking susceptibility and propagation of cracks, but this is still under discussion. Furthermore the most common test for controlling the microstructure variations is Hardness test. Because of the nondestructive nature, low price and ease of hardness checking, it is used by both users and manufacturers for quality control of alloys, in accordance with the requirements of NACE MR0175 / ISO15156. But in sour system, the possibility of failures is increased if relying only on the results of hardness tests [7, 8]. More detailed metallurgy and experience should also be considered. This paper presents the results and conclusions of analyses of the tasks performed to identify the influence of microstructure on susceptibility to SSC.
FIELD HISTORY The tubular investigated in this study is a pipe (API 5L, X52) with a cracking failure that was observed on it, only after two weeks being in service at 1200 psi.