Guangli Zhang

Impact of improving design factor over 0.72 on the safety and reliability of gas pipelines and feasibility justification

Many years experience of the operation of high stress (>72% specified minimum yield strength, SMYS) gas pipelines and statistical analysis results of pipeline incidents showed that the operating pipelines at stress levels over 72% SMYS have not presented problems in USA and Canada, and design factor does not control incidents or the safety of pipelines. Enhancing pipeline safety management level is most important for decreasing incident rate. The application history of higher design factors in the U.S and Canada was reviewed. And the effect of higher factors to the critical flaw size, puncture resistance, change of reliability with time, risk level and the arrest toughness requirements of pipeline were analyzed here. The comparison of pipeline failure rates and risk levels between two design factors (0.72 and 0.8) has shown that a change in design factor from 0.72 to 0.8 would bring little effect on failure rates and risk levels. On the basis of the analysis result, the application feasibility of design factor of 0.8 in China was discussed and the related suggestions were proposed. When an operator wishes to apply design factor 0.8 to gas pipeline, the following process is recommended: stress level of line pipe hydro test should be up to 100% SMYS, reliability and risk assessment at the design feasibility or conceptual stage should be conducted, Charpy impact energy should meet the need of pipeline crack arrest; and establish and execute risk based integrity management plan. The technology of pipeline steel metallurgy, line pipe fabrication and pipeline construction, and line pipe quality control level in China achieved tremendous progresses, and line pipe product standards and property indexes have come up to international advanced level. Furthermore, pipeline safety management has improved greatly in China. Consequently, the research for the feasibility of application of design factor of 0.8 in China has fundamental basis.

Research on the Fatigue Character of ERW Pipe Welded Seam and Fatigue Life Assessment Method

The fatigue character of electric resistance weld (ERW) seams in API X65 grade line pipe steel at the stress ratio of 0.1 and 0.6 have been investigated. Repeated loading was applied to compact tension specimen, and the fatigue crack propagation rate and threshold of X65 ERW pipe seam are tested using high-frequency fatigue testing machine. Radiographic inspection has shown that the crack caused by the cold welding is the main weld defect in the ERW pipe. Based on the failure assessment diagram (FAD) recommended in the API 579-2007 and the Miner’s linear cumulative damage model, considering the influence of stress ratio to the fatigue life, the fatigue life assessment method for the ERW pipe containing seam defects is established.Copyright

Research on the Critical Buckling Strain Criterion for Pipelines

The local buckling of pipelines is regarded as a kind of failure. The critical buckling strain is an important parameter in strain-based design of pipelines. According to the grey system theory, the factors affecting the critical buckling strain were analyzed and the correlation between these factors and the critical buckling strain was quantitatively measured based on test data. According to the dimension analysis theory, the factors affecting the critical buckling strain were analyzed and the correlation between the factors and a new critical buckling strain equation was proposed based on test data. And some advice was provided for the improvement of critical buckling strain equations. Comparing the results with data from full scale tests, these as-obtained equations could provide a higher precision.Copyright

Study on Acceptable Risk for Oil and Gas Pipelines in China

Risk assessment is basis to put pipeline integrity management in practice and the acceptable risk level is important criteria to execute risk assessment and constitute maintenance safeguard. So it is very important to establish a rational and practicable acceptable criterion and present a specific acceptable risk level. It is just for this need that the present paper gave a review of all the available research around the acceptable risk level and analyzed various domestic and overseas standards and documentation concerning how to define the acceptable risk criteria. As a result, a criterion suitable for oil and gas pipeline was presented and recommended acceptable risk level was gave.Copyright

The Analysis for Relation of Fracture Toughness and Charpy Impact Energy of High Strength Pipeline Steel

According to the linear regression analysis, a new empirical equations between the fracture toughness (KIc ) and Charpy impact energy (CVN) of X80 high strength pipeline steel, were developed on the basis of experiment sample data. And the theory of statistical test was used to test the significance of the new equation. The test result indicated that the new empirical equation has very high significance with the sample data. Comparing with the empirical equation recommended by the API 579-2007, the empirical equation developed here was more suitable for the X80 high strength pipeline steel, and can make better estimation for the fracture toughness of X80. And for the estimation of fracture toughness of X100 high strength pipeline steel, the equation developed here is applicable too.Copyright

Status of Fitness-for-Service Assessment of Defected Pipeline in China

Fitness-for-service assessment of defected pipeline is an important part of oil and gas pipeline integrity assessment and management. Fitness-for-service assessment comprises a residual strength evaluation and a remaining life prediction for a damaged pipeline. In the present paper, the objectives and methods for fitness-for-service assessment are classified scientifically, and their worldwide research status has been reviewed. Several novel fitness-for-service assessment criteria proposed in China are presented. Challenging problems and key technologies for fitness-for-service assessment of pipelines are outlined.Copyright