Uncertainties in internal pressure of oil pipelines and implications for the reliability analysis
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.