(69c) Flow Assurance in China West Crude Pipeline Transporting Multiple Waxy Crudes

The China West Crude Pipeline (CWCP), commissioned in 2007, starts from Urumqi, the capital of Xinjiang Uygur Autonomous Region, and ends at Lanzhou, the capital of Gansu Province. The capacity of the 1838 km, 813/711 mm O.D. pipeline is 20 million tons annually (about 400 kBPD). Connected with the Kazakhstan-China crude pipeline and three major oilfields in Xinjiang, supplying crudes for two refineries, this pipeline acts as a strategic channel in western China for petroleum transportation.

Among the four crudes transported through this pipeline, three are waxy crudes with the pour point possibly as high as 20 ^oC. Flow assurance has been a major challenge in waxy crude pipeline. For the CWCP, flow assurance has been much more complicated as a result of the considerably variable cold flow properties of the crudes (the standard deviation of the pour point may be as high as 5 ^oC), combined with cold environment (the soil temperature at the buried depth of the pipeline may drop to 2 ^oC or even below in winter), highly variable transporting requirements of multiple crudes, complex thermal interaction between the crude line and the product line which was buried 1.2 m apart from the crude line.

Actually, more than 80% of crude production in China is waxy crude, with the most typical being the crude produced in Daqing oilfield, the largest oilfield in China, which has a waxy content of 25 wt% and a pour point as high as 35 ^oC. Currently in China, pipeline companies operate 30,000 km of crude oil pipelines, with most of them transporting single of waxy crude or blend of waxy crudes. Waxy crude rheology and flow assurance of waxy crude pipelines has been a major field of research in China. Thanks to our experiences and achievements obtained in decades of researches in waxy crude rheology and flow assurance of crude oil pipeline, we successfully tackled these problems, ensuring safe and efficient operation of this pipeline. The main technological advances are as follows.

1. Batch Transportation of Multiple Crudes Treated with PPD

Pour point depressant (PPD) modification has been successfully and widely used in China’s waxy crude pipelines since the mid-1990’s. The PPD-modification was promoted to the CWCP that transports multiple waxy crudes in batch, that is, to carry out proper PPD treatment according to the cold flow properties and the transportation task of the crudes. Experiments were firstly conducted for determination of the PPD dosage and treatment temperature, and a dosage of 20-50ppm and a treatment temperature of 55 ^oC were found proper for the crudes to be transported. More importantly, for transporting PPD-treated waxy crude through long distance pipeline, the accurate understanding of the shear and thermal effect is crucial, since the high shear imposed while flowing through pumps and low but long-term pipe flow shear may make the reduced pour point and viscosity recover more or less, and improper reheating to the treated crude may do harm to the flow improvement of the PPD. These phenomena are called the shear effect and thermal effect, respectively. We accurately obtained this shear and thermal effect by quantitatively simulating pipelining process by using the approach we originally developed that uses the entropy generation of viscous flow as the rule of similarity between experimental simulation and the industrial pipeline flow. The use of PPD modification successfully enhanced the flow assurance capability of the pipeline in winter operation. As a result, fuel consumption of this pipeline may be reduced by 83% compared with operation via heating the crudes.

2. Intermittent Operation

Because of the need for full delivery of the Tuha crude at the intermediate station at Yumen, the downstream segment of the CWCP, 792 km long, is operated normally in an intermittent state, with averagely shut down for 1 day after 5.4 days of running and 25.4 h of mean time for shutdown. The intermittent operation of waxy crude pipeline challenges flow assurance because its possibility of pipeline restart failure could be much higher than that of the continuously operated pipelines. This is mainly attributed to the less thermal storage in the soil and the resultant quicker cooling of the oil. To ensure safe operation of the intermittently operated pipeline, specifically to ensure restartability of the shutdown pipeline, it is crucial to fully understand the thermal and hydraulic characteristics of the pipeline. However, since the endlessly and irregularly varying soil temperature field which is a function of the pipeline operation history, it is very difficult to obtain accurate simulation result. To tackle problems from the intermittent operation of the CWCP, we used the PPD to improve the intrinsic safety of flow on one side, and on the other side we developed a model and corresponding algorithm for numerical simulation of the intermittently operated pipeline. Based on the understanding of the thermal and hydraulic characteristics thus obtained, and the assessment of restartability using the reliability-based limit state approach mentioned below, scenarios for intermittent operation were further optimized. Field tests were conducted to validate the reliability of this operation, and finally intermittent operation became part of the normal operations of the CWCP. Intermittent operation was also carried out in the whole pipeline from January to April 2009, when the CWCP was impacted by the global financial crisis and the throughput was reduced to one fourth of the designed value.

3. Assessment of the Restartability of Waxy Crude Pipeline with the Probabilistic Approach

Pipeline restart failure as a result of the gelation of waxy crude has been a major threat of the safe operation of waxy crude pipeline. Many influencing factors such as crude properties, operation and environmental parameters have the attribute of uncertainty. Current restart calculation methods take no consideration of this uncertainty, including the unsteady heat transfer and flow algorithm as well as the traditional force balance equation. The reliability-based limit state approach is an effective method to tackle problems with uncertainty, and this probabilistic approach has been successfully used in some fields such as aerospace industry and structural 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 (ISO 16708) in 2006. Inspired by this idea, we, for the first time, introduced this approach to assess the restartability of waxy crude pipeline so that the influence of parameter uncertainty can be taken into consideration, and the restartability can be assessed quantitatively in the way of restart failure probability. Consequently, the lowest allowable pumping temperature and the maximum allowable time for pipeline shutdown can be determined. The key problems in development of this approach include, firstly, the limit state equation that can accurately describe the complex unsteady heat transfer and unsteady flow of non-Newtonian fluid, and secondly, an efficient algorithm for reliability analysis because of the extremely heavy load of probability computation resulted from the thousands of random sampling of the stochastic variables and the consequent thousands of the numerical solution of the differential equations of unsteady flow and heat transfer.

4. Transporting Hot and Cold Crudes in Batch.

Nearly 50% crudes transported through the CWCP can be pumped without heating or any other treatment such as PPD modification in the whole year, and the remaining crudes have to be heated to 55 ^oC at the initial station in winter to achieve PPD-modification. Heat consumption may be greatly reduced if the crudes having good flowability are pumped without heating. However, this way of transporting hot and cold crudes in batch is difficult and complex because the crude temperature, pressure, and soil temperature are always in the complex alternating process, and the hydraulic and thermal conditions interact with each other. Therefore, flow assurance is much more complicated than the normal hot crude pipelines. Moreover, the structural safety of pipeline under the alternating load is also a critical concern. Via numerical simulations on this transportation process using software we developed, we well understood the hydraulic and thermal characteristics including normal and restart operation, and further optimized the pumping scenarios. The structure reliability under the conditions of batching hot and cold crudes was confirmed after the analysis on the strength, stability and fatigue life of the pipeline structure. The validity of this pumping scenario was tested in field trials. It has become one of the regular operations in the CWCP. Each year in the early winter from November and December and the early spring from March and April, the Tuha crude is transported via PPD-treatment, while the remaining crudes are transported without heating.

According to various transportation requirements and by the integrated use of these technologies, the CWCP has successfully risen to a variety of transportation challenges and well met the demands either from the oil fields or petrochemical enterprises. The pipeline has been operated safely, efficiently and flexibly since 2007.