<p>In extremely cold regions, the natural gas pipelines are susceptible to the catastrophic thermal mechanical failure of brittle fracture. There is a temperature (or a temperature range), called ductile-to-brittle transition temperature (DBTT), below that, the pipeline is considered brittle enough to fracture abruptly without any large deformation. Therefore, to prevent or minimize pipelines fracture potential, predicting the minimum temperature that the metal would reach during its operation is crucial. This design parameter is referred to as Minimum Design Metal Temperature (MDMT). In this research, we have developed a new practical methodology to predict the MDMT for the buried transmission natural gas pipelines of FortisBC Inc. in the cold regions of British Columbia (BC) in Canada. A thermal-hydraulic mathematical model was proposed to simulate the heat transfer among the natural gas flowing inside the pipe, metal pipeline, surrounding soil, and ambient air. For the gas inside the pipe, a 1D compressible flow model was employed considering the convective heat transfer, Joule-Thomson effect, and heating of the gas due to wall friction. A significant attempt was also done to predict the minimum temperature of the soil exposed to the unsteady ambient temperature for the period of one year. For this purpose, some computational fluid dynamics (CFD) simulations (by ANSYS-Fluent and GeoStudio) were performed to investigate and analyze the sensitivity of the important parameters and define proper thermal boundary conditions for the soil domain. These unsteady simulations were carried out on a number of scenarios—including various soil types, phase change in soil, and hourly/daily average climate data of three different locations at BC—to calculate the soil temperature versus depth over time. Then, these predicted temperature fields were utilized to develop a reduced-order model (ROM), for predicting the minimum soil temperature over the year based on geographical location, soil depth (2 ft to 10 ft), and soil type. This ROM leads to a more practical approach to determine MDMT by replacing the computationally expensive transient numerical approach. The developed ROM was employed as a boundary condition to the pipeline's thermal-hydraulic model to interact thermally with the surroundings. The accuracy of this model has been examined and approved by comparing the results with available data in the literature and field data at FortisBC. Then, the effects of different parameters on MDMT were investigated, including pipeline gas and flow properties (e.g., gas flow rate, compression station temperature and pressure, pipeline’s length, diameter, insulating coating layer, and buried depth), soil type (e.g., sand and clay), and climate condition (e.g., hourly or daily average ambient temperature data, and wind speed). Based on the results, due to the soil thermal storage, following the quick time-varying ambient temperature becomes slower in deeper depths. The phase change phenomenon increases the minimum soil temperature and consequently MDMT, especially in the soils with high water content. Thus, in a conservative design scenario, it is possible to ignore the phase change in simulations to avoid computational complexity and for the sake of pipeline safety. In conclusion, the proposed methodology, using a reduced-order model, can replace the transient CFD simulation for practical MDMT prediction in pipelines. This approach can be applied for both purposes of designing new pipelines and monitoring in-service systems.</p>

A Practical Approach for Determining Minimum Design Metal Temperature (MDMT) of Transmission Gas Pipelines

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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