(145a) Development of a Probabilistic Approach to Assessment of the Restartability of Waxy Crude Pipeline

Pipeline restart failure due to oil gelation after prolonged shutdown is the main threat of the safe operation of waxy crude pipeline. Many factors influencing the restartability of a waxy crude pipeline such as oil properties, operation and environmental parameters have uncertainty. Current methods for pipeline restartability assessment take no consideration of this uncertainty, including the traditional force balance equation, which is known to be over-conservative, and the numerical method based on unsteady heat transfer and thixotropic fluid flow.

Reliability-based method is an effective approach to solve this kind of uncertainty problems, which has been widely used in some other fields such as structure engineering. In the pipeline industry, ISO issued the first standard of reliability-based limit state method for the design and assessment of oil and gas pipeline structure in 2006, i.e. ISO 16708. In this study, we introduce this reliability-based approach to solve the problem of pipeline restartability with consideration of parameter uncertainty, and the restartability is reported in the way of restart failure probability.

A limit state function for restart failure is developed based on a model describing the unsteady flow and heat transfer of pipeline during the shutdown and restart process, and an algorithm is developed which can solve the probability model fast and accurately, which is necessary to break through the bottle-neck in computation of the failure probability with truly-unsteady flow and heat transfer model because of the huge computation load. And moreover, the target safety level, i.e. the maximum acceptable failure probability, for the restart of crude pipelines is proposed. Consequently, quantitative assessment of restart safety of waxy crude pipeline can be achieved in the form of restart probability, and the lowest allowable pumping temperature and the maximum allowable time for pipeline shutdown can be determined based on the reliability of pipeline restart.

Compared to the traditional methods which determine the pumping temperature of waxy crude by mainly referring to its pour point and determine the restartability of a waxy crude pipeline without taking the uncertainty of the influencing parameters into consideration, this probabilistic approach provides new and scientific methodology for safe and economic operation of waxy crude pipelines.