Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor

Zhou, Wenxing

Abstract

The internal pressure is the most important operational load for oil and gas pipelines. The maximum operating pressure (MOP) is the maximum pressure the pipeline is qualified to be operated according to a given standard. In deterministic fitness-for-service (FFS) assessment of in-service pipelines containing flaws such as corrosion defects and cracks, the remaining pressure containment capacity of the pipeline is evaluated and compared with MOP multiplied by a factor of safety to determine if immediate rehabilitation actions for the pipeline are necessary. However, the actual internal pressure of an in-service pipeline is however uncertain and fluctuates with time. Due to the significant difference in the compressibility of liquid and gas, the pressure fluctuation in liquid pipelines. This thesis characterizes the statistics for the internal pressure of oil pipeline and assesses the reliability performance based on the pressure variables.

In this study it is characterized the internal pressure (discharge and suction) probabilistic properties of a major crude oil transmission pipeline including its distribution of arbitrary-point-in-time and maxima pressure, auto-correlation, power spectral density and pressure range from rain flow counting. The conclusions provide information for reliability analysis considered the pressure to be a stationary stochastic process and it gives suggestions for fatigue analysis.

It is also investigated the reliability performance for corroding pipelines considering the pressure statistics obtained in the first study and compared with different pressure assumptions. This study provides a method to consider the internal pressure to be a stochastic process and gives evidence that a certain level of conservativeness is observed if the internal pressure is considered as a stochastic process instead of a random variable suggested in present literature.

Summary for Lay Audience

Pipeline system often faces with many kinds of threats. Therefore, a mitigate procedure to reduce the incident rate is the fitness-for-service assessment. Within this assessment, the internal pressure is a major aspect of the assessment. This assessment considers uncertainties of all the aspects that influence the failure of pipeline. Therefore, the uncertainty of the pressure is one of the key interests in this assessment. The statistical characterization of the internal pressure is often assumed to follow certain distribution in papers and code, however, the internal pressure for oil pipelines is much less sourced. The present study has collected internal pressure data from one pump station of an in-service oil pipeline and characterized basic features including not only the distribution, mean and standard deviation but also time-dependent features such as the correlation between pressures with different time lags. The statistics provide the researchers more options to incorporate internal pressure for oil pipeline specifically into future studies, for instance as time-independent or time-dependent in the reliability analyses, or fatigue crack failure assessment. The present study further investigates how different assumptions about the uncertainty in the internal affect the reliability analysis results for corroding pipelines.

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