This paper presents the failure analysis of the crack and leakage accident of a crude oil pipeline under CO₂-steam flooding in the western oilfield of China. To analyze the failure behavior and cause, different testing, including nondestructive testing, chemical composition analysis, tensile property testing, metallographic analysis, and microanalysis of fracture and chloride stress corrosion cracking (SCC) testing, are applied in the present study. The obtained results showed that the pipeline under the insulation layer of high humidity, high oxygen content, and high Cl⁻ environment occurred pit corrosion, and the stress concentration area at the bottom of the corrosion pit sprouted cracks. Besides, it is demonstrated that the cracks were much branched, mostly through the crystal, and the fracture showed brittle, which is consistent with the typical characteristics of chloride SCC. Meanwhile, the insufficient Ni content of the pipeline material promoted the process of chloride SCC, and the high-temperature working conditions also aggravated the rate of chloride SCC. In addition, efficient precautions were provided to avoid fracture.

Gas reservoir-type underground gas storage (UGS) plays a critical role in China’s natural gas reserves and peak shaving, serving as an essential component of the energy security system. Its unique cyclic injection and production operations not only stabilize the natural gas supply but also impose stringent requirements on the safety and integrity of geological structures, wellbores, and surface facilities. Weaknesses in current practices can cause accidents, directly threatening energy security. Therefore, continuously improving integrity management is the key to mitigating energy risks. Currently, the integrity management of gas storage faces challenges such as an abundance of standards and the complexity of management elements, which affect both operational safety and management efficiency. To address these issues, this study systematically analyzes domestic and international standards related to gas storage and establishes a technical system based on “three-in-one” integrity management (geological structure, wellbore, and surface facilities). Key elements of integrity management are identified and optimized, and recommended execution standards for critical factors are proposed to provide a theoretical basis and decision-making support for the safe operation of gas storage. This study not only offers a reference for optimizing and implementing integrity management standards but also has significant practical implications for enhancing energy security and reducing energy risks, ensuring the smooth execution of China’s natural gas reserve and peak shaving initiatives.

T-joint welding is a key manufacturing process of large storage tanks. However, complex residual stresses are generated and have a great effect on the structural integrity of storage tanks. The high residual stress caused by welding and the discontinuous structure may result in tank cracking and failure. In this work, the residual stress distributions on the inner surface, outer surface, and thickness direction of the T-joint were investigated by using the finite element method and indentation test method. The effect of local PWHT with different heating temperatures, heating rates, and heating widths on the residual stress distribution was also discussed. Results show that the residual stress of the T-shaped joint is high due to the serious structure discontinuity, multi-layer welding, and high strength. Among all the stresses, the circumferential residual stress is the highest and most concentrated in the outer weld connected with the annular plate. The residual stress gradually decreases with the increase in the heat treatment temperature. When the heating rate is less than 106 °C/h, the residual stress gradually decreases with the decrease in the heating rate. The large thermal deformation caused by heat treatment can be simultaneously avoided by heating the inside and outside of the T-joint. The residual stress decreases with the decrease in the width of the heating zone. The residual stress can be regulated by using a smaller width in the heating zone. An optimized heat treatment scheme with a heating temperature of 700 °C, heating rate of 56 °C/h, and heating width of 200 mm was proposed, which has a good ability to control residual stresses and improve the quality of the T-joint. It also has a good application in engineering.

Diffusion reaction was a crucial route to enhance the wear resistance of Ti-6Al-4V alloys surface. In this work, the Ni/Cu/Ni composite layers were fabricated on the surface of Ti-6Al-4V alloy by electroplate craft, and then different annealing temperatures were applied to further optimize its tribological properties. The diffusion behaviors at various temperatures were systematically analyzed to reveal the physical mechanism of the enhanced tribological properties of the coatings. It was demonstrated that Cu_xTi_y and Ni_xTi_y intermetallic compounds with high hardness and strength were produced in the Ni/Cu/Ni coating, which acted as the reinforcing phases and improved the microhardness, reduced the friction coefficient, and lessened the wear rate. Specially, this effect reached the maximum when the annealing temperature was 800 °C, showing excellent wear resistance. This work revealed the relationship between annealing temperatures and tribological properties of the Ni/Cu/Ni coating, and proposed wear mechanism, aiming to improve the surface performance of Ti-6Al-4V alloy by appropriate diffusion behavior.

Girth welds are weak points in pipelines, and failures occur frequently. In a gas transmission pipeline, a girth weld experienced cracking, prompting a failure analysis using experimental methods and finite element analysis (FEA). Experimental results showed that X-ray non-destructive testing (NDT) revealed cracks, porosity, and lack of fusion in the girth weld. However, the hardness and microstructure of the material showed no abnormalities. During operation, the pipeline experienced an increase in soil cover and was subjected to ground subsidence and vehicle loads. Finite element analysis was conducted on the defective girth weld under different conditions, including varying soil cover depths, different levels of subsidence, and varying vehicle loads, to examine the pipeline’s stress response. The results indicated that the combination of soil cover, subsidence, and vehicle loads led to pipeline failure, whereas none of these factors alone was sufficient to cause girth weld failure. To prevent such failures from occurring again, the following measures are recommended: strengthen on-site welding quality control of girth welds, conduct inspections for defects in girth welds of in-service pipelines, and promptly address any defects that exceed acceptable limits.

The integrity of oil and gas pipelines is critical to energy transportation and has significant implications for national energy security. This study employs finite-element numerical simulation to investigate the impact of multiple repairs on the microstructure and mechanical integrity of X80 pipeline girth welds. The effects of varying repair iterations on the microstructure, toughness, and loading capacity of high-strength steel pipes were analyzed. The results revealed that the microstructure of the welded joint remained unchanged across different repair instances, but the toughness, optimized by welding heat input, diminished after three repairs due to grain growth from repeated thermal cycles. Specifically, the impact toughness of the welding line, fusion line, and adjacent areas decreased significantly after three repairs, with the toughness of the welding line dropping to 25 J and the fusion line dropping to 30 J. The hardness of the welded joint decreased with repairs, and the dispersion of hardness increased. The average hardness at the welding line decreased from 25 HV to 20 HV after three repairs. Residual stress in the repaired girth weld was highest in the filling welding layer, increasing with the number of repairs. The loading capacity of the girth weld significantly decreased after the first repair (by 12.1%) and continued to decrease with additional repairs (15.3% after the second repair and 16.7% after the third repair). It is concluded that X80 pipeline girth welds should be repaired no more than twice to maintain optimal structural integrity.