Investigation of the gas losses in transmission networks

As air pollution has become a major issue in nowadays world, reducing methane emissions from the natural gas transmission systems is an issue that definitely has to be addressed. In order to do that, there are a few solutions available, such as the replacement of steel pipes with high-density polyethylene (HDPE) pipes. The main causes of these leaks are the corrosion defects and third-party interventions. The paper presents a new methodology for technological gas loss calculation from the natural gas transmission system. In order to obtain the most accurate calculation formulas, the flow coefficients for different cases were determined by experimental measurements. The paper presents the details regarding the construction and equipment of the experimental stand, as well as a new method for calculating the volumes of gas lost due to defects of this type. Thus, the aerial and buried defects were studied and the results obtained on statistical data were verified. Using the results of the study, the average emission of CH4 per year in Romania was calculated, and it was proven to be about 30% bigger than the European average. The findings of this study can help for a better understanding of the level of the losses and the effect on the final costs for the population, as well as the negative impact on the environment, in case the transporter does not take any measures.

Analysis and Evaluation on Residual Strength of Pipelines with Internal Corrosion Defects in Seasonal Frozen Soil Region

In order to study the residual strength of buried pipelines with internal corrosion defects in seasonally frozen soil regions, we established a thermo-mechanical coupling model of a buried pipeline under differential frost heave by using the finite element elastoplastic analysis method. The material nonlinearity and geometric nonlinearity were considered as the basis of analysis. Firstly, the location of the maximum Mises equivalent stress in the inner wall of the buried non-corroded pipeline was determined. Furthermore, the residual strength of the buried pipeline with corrosion defects and the stress state of internal corrosion area in the pipeline under different defect parameters was analyzed by the orthogonal design method. Based on the data results of the finite element simulation calculation, the prediction formula of residual strength of buried pipelines with internal corrosion defects was obtained by SPSS (Statistical Product and Service Solutions) fitting. The prediction results were analyzed in comparison with the evaluation results of B31G, DNV RP-F101 and the experimental data of hydraulic blasting. The rationality of the finite element model and the accuracy of the fitting formula were verified. The results show that the effect degree of main factors on residual strength was in order of corrosion depth, corrosion length, and corrosion width. when the corrosion length exceeds 600 mm, which affects the influence degree of residual strength will gradually decrease. the prediction error of the fitting formula is small and the distribution is uniform, it can meet the prediction requirements of failure pressure of buried pipelines with internal corrosion defects in seasonally frozen soil regions. This method may provide some useful theoretical reference for the simulation real-time monitoring and safety analysis in the pipeline operation stage.

Interaction Assessment of Diagonally-Spaced Identical Corrosion Defects and Fully-Aligned Unequal-Sized Corrosion Defects Under Combined Loads

In this study, the interaction effects of closely-spaced corrosion defects on the burst capacity of oil and gas pipelines under combined internal pressure and longitudinal compression are investigated by using parametric three-dimensional elasto-plastic finite element analyses. Full-scale burst tests reported in the literature are used to validate the finite element model. It is observed that the interaction effects of diagonally-spaced defects on the burst capacity is strongly related to the overlapping portion of the defect width or circumferential spacing between the two defects. The analysis results indicate that the strongest interaction between diagonally-spaced defects under combined loads occurs if the defects have zero circumferential separation. The interaction weakens as the defects are more and more overlapped or separated circumferentially. It is also observed that the interaction effect associated with longitudinally- or circumferentially-aligned, unequal-sized corrosion defects is negligible under the internal pressure only or combined loads.

Analytical Model and Numerical Analysis of Composite Wrap System Applied to Steel Pipeline

Composite overwraps are a cost-effective repair technology, appropriate for corrosion defects, dents, and gouges for both onshore and offshore steel pipelines. The main benefit of polymer-based sleeves is safe installation without taking the pipeline out of service. This paper presents a new calculation procedure proposed in the form of an algorithm for the sizing of composite repairs of corroded pipelines when the sleeve is applied at zero internal pressure. The main objective of the presented methodology is determination of the effective thickness of the composite repair without its overestimation or underestimation. The authors used a non-linear finite element method with constitutive models allowing analysis of the steel, putty, and composite structures. The validation of the results of numerical computations compared to the experimental ones showed an appropriate agreement. The numerical calculations were applied to compare the analytical results in relation to those obtained by the standards ASME PCC-2 or ISO/TS 24817. The comparison showed that the proposed solution confirmed its effectiveness in reducing the thickness of the sleeve significantly, thus, showing that the current industrial standards provide a considerably excessive composite wrap around the steel pipe corroded area, which leads to an unnecessary increase in the repair costs.