The restart process of waxy crude pipelines is an unsteady thermo-hydraulic coupling process, which mainly includes two modes of the constant flow and constant pressure in industry. However, some parameters involved in the restart process have obvious uncertainties, such as the operating parameters, physical parameters of crude oil, environmental parameters, and pipeline parameters, resulting in the traditional deterministic method that cannot scientifically describe the safety of the pipeline restart process. To do this, this study introduces the reliability-based limit state method and interference principle into the safety evaluation of waxy crude pipelines during the restart process. Considering the random fluctuation characteristics of the mentioned parameters, the restart physical process, the flow and heat transfer mathematical model, and the restart failure limit state function were established. On this basis, the failure probability during the restart process for one waxy crude pipeline under constant flow was determined. This research has realized the quantitative evaluation of restart safety of waxy crude pipelines.
When the hot oil pipeline is running at a low throughput, it easily enters into an unstable condition, which seriously threatens the safety of the hot oil pipeline operation. In this study, the unsteady heat transfer and flow mathematical models for the hot oil pipeline system were established first by comprehensively considering the uncertainty of parameters during pipeline operation, such as the operating parameters (throughput and oil temperature), physical properties of crude oil (freezing point, viscosity, and thixotropic parameters), and environmental parameters (buried deep soil temperature and soil thermal conductivity). Then, the efficient Latin hypercube sampling (LHS) stochastic numerical algorithm was applied and further developed to quantitatively describe the operation safety of hot oil pipelines with low throughput in the form of probability. On the basis of the abovementioned research, the qualitative relationship between pipeline flowrate and friction loss is obtained. Finally, taking an actual crude oil pipeline as an example, the failure probabilities of the pipeline under different operating conditions were analyzed in detail. Combined with the target safety level of pipeline operation, the minimum allowable throughput of pipelines was determined. This study revealed the flow and heat transfer law of hot oil pipelines with low throughput and determined its operation safety and reliability under different operating conditions.
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