On the Effect of Burial Depth Increase on Dynamic Response of Pipelines, Embedded in Soil Slope By Numerical Modeling

Abstract eng:
Not only do Pipelines, as routes to convey energy (gas and oil), have significant role in countries economy, but also tie intercontinental relations by economic collaborations. Therefore, governments annually allot budgets to ensure their safety and permanent operation against unpredicted events. Consequently, many researches have been performed on the task and many methods have been suggested to guarantee their function and to prevent their failure during PGDs. However, not limited to, but PGDs are principally caused by dynamic loading, particularly earthquakes, which have resulted unrestricted damages to pipelines everywhere and unfortunately, these destructions have led to devastative results when the pipes have been buried in potential landslides. Tehran, Iran's capital, which is constructed on either soil or rock slopes, are in danger of landslides and consequent gas pipe damages during earthquakes. In this regard, Sharif University of technology, have performed several dynamic shaking table tests and many numerical modellings to discover the mechanisms of damage occurrence and to suggest methods to ensure their safety. One suggestion which has been numerically investigated and is reported in this paper is burial depth change. The numerical modellings have been performed by ABAQUS program and the models have been excited with predicted earthquake. The loading had 3.7 HZ frequency, 25 cycles and 0.32 g amplitude, resembling a 975 year return period record in the area. While the slope included cemented granular materials and had 30o inclination, it had 15 m width, 13 m height and 30 m length. Also, soil nonlinearity along with damping characteristics were introduced to the program by USDFLD subroutines. Adjacent soil horizontal pressure and dynamic behavior were modeled by a function and spring-dashpot systems respectively. Besides, steel gas pipes which followed API-X42 standard had 20" diameter and 0.25" thickness and 0.5 frictional coefficient with soil. Four burial depths of 1.1 m, as the standard value in Tehran gas pipelines, and three alternatives (2.1, 3.1 and 4.1 m) were modeled and the results were investigated. Numerical calculations which included slope displacement, pipe elastic and plastic strains in horizontal and vertical modes and strains time history, showed the helpful effect of higher burial depths in decreasing pipe deformations. It is showed that increased burial depth has reduced the horizontal and vertical pipe strains up to 76 % for maximum burial depth. Besides, the resultant strains has also been introduced and the useful effect of higher burial depths has been evidenced. Since, the results have been numerically calculated, reliable geotechnical investigations and numerical modellings, before pipeline construction in real cases, are recommended to accurately determine the failure plane depth. As, placing the pipe beneath the predicted depth, where possible, has proved advantageous in decreasing pipe deformation.

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