Grade X65 and X70 Heavy Wall (up to 37mm in Thick) UOE Linepipe for Sour Service

Abstract

Based on the increasing demand for natural gas to be extracted under severe environmental conditions, requirements for thicker sour service pipe steels with high strength (X65 grades or more) are increasing. In order to achieve both a resistance to hydrogen induced cracking (HIC) and better mechanical properties, it is important to obtain a homogeneous microstructure. For this purpose, we manufactured API X65 and X70 heavy wall (up to 37mm in thick) UOE linepipe for sour service utilizing an advanced thermo mechanical control process (TMCP) employing the theoretical maximum cooling rate with water (the ‘ultimate cooling rate’) and homogeneous temperature distribution in accelerated cooling. For applications in deeper water, higher pipe thickness to diameter ratio (t/D) is required. However, during pipe forming, these high ratio pipes suffer higher plastic strain in the vicinity of the surface. This plastic strain causes increasing surface hardness and HIC resistance may deteriorate. It is important to evaluate the effect on strain systematically based on the difference of microstructure morphologies. Therefore, the effect of bending strain on HIC properties was investigated through a simulated laboratory bending test with a strain of up to 8.9%. It was found that homogeneous bainitic microstructure can prevent HIC even under a higher bending strain. On the other hand, crack area ratio (CAR) was almost doubled when 5% strain was applied near the surface area in the material in which inclusion morphologies were not optimized.

Effect of Cu Addition in Pipeline Steels on Prevention of Hydrogen Permeation in Mildly Sour Environments

“…Since an accident involving the rupture of a sour gas pipeline in the 1970s, 1,2 many studies have investigated the HIC mechanism in order to improve sour resistance. At present, X65 grade linepipe is widely utilized, and X70 and X80 high grade linepipes [3][4][5][6] have been developing for sour service based on various approaches, including optimization of the chemical composition of the steel, reduction of center segregation and nonmetallic inclusions in slabs in the continuous casting process, and microstructure control by the thermo-mechanical control process (TMCP). 7,8 Laboratory evaluation methods for material resistance to HIC have been standardized in NACE TM 0284 9 and EFC 16.…”