Thermal analysis of recent crude oil transportation technologies in China

Transcription

1 th International Advanced Technologies Symposium (IATS 11), 1-1 May 11, Elazığ, Turkey Thermal analysis of recent crude oil transportation technologies in China B. Yu 1, J. J. Zhang 1 China University of Petroleum, Beijing, P.R./ China, yubobox@vip.13.com China University of Petroleum, Beijing, P.R./China, zhangjj@cup.edu.cn Abstract New transportation technologies are applied in pipeline industry in China in recent years. This paper is focused on the thermal characteristics of the pipeline system based on numerical simulation. The results can not only provide technical support for construction and operation of the crude oil pipeline, but also give some instructions to the design, construction, operation and management of other pipelines in the future. Keywords thermal analysis, pipelining technology, parallel double-pipeline system, batch-wise transportation system, intermittent operation M I. INTRODUCTION ore than % of crude oils produced in China are either waxy crude oil with high pour points or viscous heavy crude oil. The poor flow ability of the oil calls for advanced pipelining technologies to ensure safe operations. Meanwhile, with the development of economy and petroleum industry in China, the pipeline industry has a quick development in recent years, including construction, operation and maintenance. Typical examples are parallel double pipelines laid in the same trench, batched pipelining crude oils at different temperatures, and intermittent operation technologies etc. The novel operation technologies, with complicated hydraulic and thermal transients, bring new problems for pipeline schedulers. Numerical simulation on the thermal analysis can provide references for pipeline operators. II. THERMAL ANALYSIS ON PARALLEL DOUBLE-PIPELINE SYSTEM IN A TRENCH The crude oil pipeline was usually constructed independently in one trench. In recent years, a new technology of parallel-laying two pipelines in one trench comes out. Compared with the traditional technology, the new one can protect the environment and save the project investment; thus it may become more and more popular in pipeline industry. Collocating crude oil pipeline and products pipeline in the same trench is a newly developed technology and no reference or design criteria could be obtained. Since the temperature of the crude oil is a key parameter for safe transportation, the most crucial problem in the design and operation of the double pipelines laid in one trench is the thermal impact between the two pipelines. Numerical simulation method is applied to study the impact of the cold products pipeline on the hot crude oil pipeline. Hydraulic and thermal calculations of the single crude oil pipeline under the same conditions are also performed for comparison. Unstructured grids of the soil domain in Cartesian coordinate system are generated using a Delaunay triangulation method, as shown in Fig.1. In order to save computation time, the grids are fined only near the pipelines. Uniform grids along flow direction of the pipeline are employed with the grid spacing of km, which can satisfy the requirements of accuracy in engineering calculation. The calculation starts at the outlet of a pumping station in the pipeline and ends at the inlet of the next pumping station,. A second-order finite volume method is used to discretize the heat conductive equations of the soil, the wax deposition, pipeline wall and the corrosion protective coating. An implicit method is used for time advancement. The discrete equations are solved by a Gauss-Seidel method[1] Fig.1 Unstructured grids of the soil A typical operating condition is proposed to make thermal analysis of the double-pipeline system. The pipelines studied are referred to a long-distance pipeline system in China. The length and the buried depth of the double-pipeline system are km and 1.m, respectively. Thermal conductivity of the soil is 1. W/(m oc) and the soil temperature at the buried depth is 1. o C. Other parameters are shown in Table 1. The physical properties of the crude oil produced in North Xinjiang field and those of 9# gasoline are used in this study. Operation is simulated using different pipeline intervals l ranging from.m to.m and is compared with the single pipeline system. l is defined as the minimum horizontal distance between the outer 3

2 V B. Yu, J. J. Zhang Table 1 Parameters of the typical calculation example Pipeline Throughput Diameter Thickness of wax Thickness of corrosion Temperature at (t/a) (mm) (mm) protective coating (mm) station outlet ( C ) Crude Products pipe walls of the crude oil pipeline and the products pipeline. Soil temperature, heat loss at the ground surface and the temperature difference between the two pipelines are analyzed based on the simulation results. The temperature difference is defined as the difference of the oil temperature at the same pipeline location between the double-pipeline system and single pipeline system[]. Soil temperature fields at the station outlet for different pipeline intervals are shown in Fig.. The soil temperature field on the left side of the crude oil pipeline is affected notably by the products pipeline in the double-pipeline system at all pipeline intervals. This is because the cold products oil enlarges the low-temperature region near the ground surface on the left side and reduces the temperature gradient, which lessens the soil heat loss to the atmosphere on the left side of the crude oil pipeline at the station outlet (a) Single crude oil pipeline V1 (b) Single products pipeline (c).m (d).m The impact of the pipeline interval of the parallel double-pipeline system in this typical example on the heat flux and oil temperature difference along the pipelines are shown in Fig.3 and Fig.. With the increase of the pipeline interval, the impact of the products pipeline gradually reduces, and the heat flux of the double-pipeline system gets close to that of single pipeline system, as shown in Fig.3. The heat loss of the crude oil pipeline is partly absorbed by the environment and partly by the products pipeline in the double-pipeline system. As can be seen from Fig., when the pipeline interval is no more than 1.m, the temperature difference of the crude oil pipeline firstly increases and then decreases, so does the temperature difference of the products pipeline. When the pipeline interval is no less than.m, the temperature difference is close to zero indicating that, the heat loss and temperature of the crude oil is almost free from the impact of the products pipeline. Meanwhile, the temperature difference of products pipeline is always above zero at all pipeline intervals. This means the products temperature in the double-pipeline system is higher than that of the single products pipeline system. The smaller the pipeline interval, the higher the products temperature rises. The maximum temperature rises are 1. o C, 3. o C,.3 o C and 13. o C, corresponding to the pipeline interval of.m,.m, 1.m and.m. The stated phenomenon, of which the temperature difference of the products pipeline firstly goes up and then decreases, means that the products oil firstly absorbs and then loses heat. Other calculation examples are proposed based on the above typical example, taking into account the variation of the key parameters, such as thermal conductivity of the soil, the buried depth, air temperature, oil temperature at the station outlet and throughputs of the pipeline. Similar results can be obtained and the maximum temperature differences are listed in Table (e).9m (f) 1.m (g).m (h).m Fig. Soil temperature field at the station outlet Heat flux ( W/m ) Distance between two pipelines (m) Single crude oil pipeline x (m) Fig.3 Heat flux of the ground surface at the station outlet 31

3 Thermal analysis of recent crude oil transportation technologies in China Table Maximum temperature differences of the crude oil ( o C) No. Calculation example Pipeline interval (m) typical example buried depth of the two pipelines reduced to 1.m thermal conductivity of the soil increased to outlet temperature reduced to o C outlet temperature increased to 7 o C throughput of the crude oil pipeline doubled soil temperature at buried depth reduced to - o C soil temperature at buried depth increased to o C Temperature difference ( C) Distance between pipeline (m) Flow direction (km) (a) Temperature difference of the crude oil In the calculations above, the axes of the double pipelines are assumed to be on the same horizontal plane. However, this is an ideal condition, which can not be guaranteed in actual engineering construction. Thermal analysis on the double-pipeline system with the variation of vertical relative position of the two pipelines is conducted[3]. Results have shown that, the products pipeline has minor impacts on the crude pipeline with the variation of vertical relative position of the two pipelines, if the pipeline interval is greater than 1.m. Then through numerical simulation, conclusions can be drawn out that, when the hot crude pipeline is collocated with the products pipeline in a trench, the crude pipeline is almost not affected by the products pipeline if the pipeline interval is larger than.m. Temperature difference( C) Distance between pipelines (m) Flow direction (km) (b) Temperature difference of the products oil Fig. Temperature difference along the pipeline As can be seen from Table, following conclusions can be drawn out. (1) When the pipeline interval is less than 1.m, the maximum temperature difference changes significantly with the variation of each parameter. The greatest difference is o C. () When the pipeline interval is 1.m or.m, the maximum temperature difference has minor change and is no more than 1 o C. (3) When the pipeline interval is.m, there is no difference between the double-pipeline system and single pipeline system. III. THERMAL ANALYSIS OF THE PIPELINE BATCHED TRANSPORTING CRUDE OILS AT DIFFERENT TEMPERATURES With the increasing demand of crude oil in China, more crudes are imported. The imported crudes with low-pour-point (CLPP) has good flowability and can be transported without heating at the pumping stations; while the domestic crudes with high-pour-point (CHPP) should be heated to make the station inlet temperature higher above its pour point for safe operation. Then, the new technology of batch pipelining crude oils at different temperatures has been applied in the pipeline industry in recent years. It satisfies the requirements of the refinery process of transporting and storing different kinds of oil separately without building other more pipelines or modifying the facilities in the refinery. Besides, appropriate operating scenario of determining outlet temperatures for different crude oils respectively is carried out. By doing so, plenty of heat energy consumption can be saved and carbon emissions can be remarkably reduced. This technology was first applied in the Pacific Pipeline system in the U.S. in 1999, which batch transported five kinds of crude oil at different temperatures ranging from 1. o C to. o C. The long-distance crude pipelines in Northwest and Northeast China also batch transported different kinds of oil at the same temperature. Although the technology is promising and economical, the hydraulic and thermal behaviors of the batch-wise transportation system are quite complicated due to the alternate operating temperatures of different kinds of oil. Unlike the 3

4 Energy consumption (TJ) Thermal analysis of recent crude oil transportation technologies in China ordinary hot oil pipeline, there may be several segments of different oils at different temperatures in the pipeline, and the flow field inside the pipeline and the soil temperature field outside the pipeline are always in a transient state. Studies in this field have obtained several achievements based on the numerical simulation applying finite volume method and characteristics method. The characteristics of the pipeline, including oil temperature, hydraulic loss between two stations and economical scenarios, are discussed in this paper. An example of the batch-wise system transporting two kinds of oil is given to study the transient thermal characteristics. The temperature at the station inlet changes periodically, as shown in Fig.. The temperature of the CHPP gradually rises as flowing along the pipeline; the oil front loses most heat to the environment and its temperature is the lowest among the CHPP in a batch. However, the temperature of the CLPP gradually decreases as flowing ahead; and the temperature of the CLPP front is the highest among the CLPP in a batch. This is due to the periodical heat absorption and heat loss of soil induced by the alternate station-outlet temperature of the pipelined crude oil. Scenario A: CHPP is heated at the pumping station while CLPP is transported without heating. Scenario B: CLPP and CHPP are heated to the same temperature at the pumping station. Scenario C: CLPP end is preheated and CHPP is heated to a lower temperature at the pumping station. Scenario D: CHPP end is heated to a lower temperature at the pumping station. Fig. Hydraulic loss between two stations Figure 7 shows the heat energy consumption of the above four scenarios of a km-long pipeline in a month keeping the lowest in-station temperatures of CHPP higher above its pour point. As can be seen from it, scenario A costs most heat energy, following that is scenario B. 3 3 Fig. Station-inlet temperature of CLPP and CHPP The hydraulic behaviors and thermal characteristics have impacts on each other. Therefore, the hydraulic loss between the two stations also changes periodically, as shown in Fig.. When the pipeline is all occupied by the CHPP, the hydraulic loss curve gradually goes down because of the temperature rise of the ambient soil. As the CLPP enters the pipeline, the hydraulic loss is sharply reduced. When the pipeline is all occupied by the CLPP, the hydraulic loss curve gradually goes up because of the temperature drop of the ambient soil. When the CHPP pushes the CLPP ahead, the hydraulic loss sharply increases[,]. The oil temperature is a key parameter for safe operation of the hot oil pipeline. When the pipeline is operated in a batch-wise system, different kinds of oil can be transported at different temperatures, even the same kind of oil can be heated to different temperatures. The station-outlet temperatures determine the energy consumption of the heating scenarios, which can be divided into four types as follows[]. 1 1 A B C D Scenarios Fig.7 Heat energy consumption of different scenarios For scenario C, the heat energy consumption is different according to different proportions of the CLPP being heated and different station outlet temperatures. Similar results can be obtained in scenario D. Figure shows the relationship between the heat energy consumption and the heated proportion of the crude oil in a batch. With the increase of the proportion, the heat energy consumption firstly decreases and then goes up. The heat energy consumption is relatively low when 1%~% CLPP end is preheated. As can be seen from the results above, scenario C and D consume less heat energy among the four scenarios. Then a new heating scenario is proposed combining the advantages of 33

5 Energy consumption (TJ) Energy consumption (TJ) B. Yu, J. J. Zhang the two scenarios, i.e. preheating the CLPP end and meanwhile lower the station-outlet temperature of CHPP end. The new scenario can further reduce the energy consumption. The batch pipelining technology was trailed and applied in a long-distance crude pipeline in northwest China. Totally 39 batches of kinds of oil were transported in the 79km-long pipeline, and 3. tons of fuel oil were saved Proportion (%) (a) Scenario C 1 1 Proportion (%) (b) Scenario D Fig. Heat energy consumption of scenario C and D IV. THERMAL ANALYSIS ON INTERMITTENT OPERATION OF HOT CRUDE PIPELINE AT EXTREMELY LOW THROUGHPUT The throughput of the pipeline may decrease to an extremely low level because of the sharply decreased production of the oilfield at the late development stage. The extremely low throughput may cause great hazards to safe operation of the pipeline. Firstly, the throughput of the crude pipeline can not satisfy the minimum acceptable pump flow-rate, and the extremely low flow rate definitely makes the pipeline system in an unsteady operation state. Secondly, the total heat capacity of the oil is small when the throughput is extremely low; the slow flow velocity induces more heat loss to the environment. Even if the oil is heated at the pumping station, the station inlet temperature is still lower than its pour point, which may cause oil gelling in the pipeline. Some long-distance crude pipelines in China were confronted with the low flowrate problem at the late stage of the oilfield development. Some gathering pipelines in the oilfield or long-distance pipelines in other regions of the world, such as Rotterdam-Rhine pipeline and Meleiha-EI Hamra pipeline in Egypt, also operated at the low flowrate conditions because of the uncertainty of oil sources supply. Intermittent operation scheme, a cycle operation including normal operation, shutdown and restart processes, can be applied to this problem. When the pipeline is under intermittent operation, the soil temperature field around the pipeline determined by the operating conditions, such as operation duration, shutdown duration and the station outlet temperature etc, is always under transient state. Therefore, the heat transfer between crude oil and the environment is then enhanced due to the frequent shutdown and restart, and the temperature drop of the crude oil during shutdown is greater than that of last time. Thermal analysis on the operation is conducted based on numerical simulation. However, the three stages have impacts on each other: the final state of the previous process is the initial state of the current process. This will enlarge the accumulative error of numerical simulation. The hydraulic and thermal behaviors of the crude oil pipeline under intermittent operation are complicated. Take a long-distance crude pipeline in Northwest China operating in an extremely low throughput as an example[7]. The pipeline is 13/711 mm O.D. and 11 km long. Impacted by the uncertainty of the crude oil sources in the winter of, the throughput of this pipeline is only one fourth of the design throughput. Constrained by the pump configuration, the minimum acceptable flow rate of the pipeline is 1 m 3 /h, therefore the ratio of operation time to shutdown time is about to 1. Accordingly different intermittent operation schemes are studied, including -day/1-day, -day/-day and -day operation/3-day shutdown. Oil temperature at the station inlet and hydraulic loss between every two stations are analyzed through numerical simulation. Take the -day operation/ -day shutdown scheme as an example. Figure 9 shows the cyclic-operation-induced periodical characteristics of inlet temperature and pressure of a pumping station between November and next May. The inlet temperature decreases from November to February and increases from March to May, agreeing with the change trend of soil temperature. It should be noted that, the lowest oil temperature appears in March, although the lowest soil temperature appears in February. The reason is that, the heat capacity of the soil has a lag compared with the soil temperature change. The pressure at the station inlet also changes periodically and has a more significant fluctuation in February and March when the inlet temperature is the lowest. Similar characteristics can be found out in the intermittent operation schemes of -day operation/ 1-day shutdown and -day operation/ 3-day shutdown. Then minimum inlet temperatures of the pumping stations during the winter of different schemes are listed in Table 3. The differences between different schemes are within 1 o C. 3

6 Inlet pressure (MPa) B. Yu, J. J. Zhang Table 3 Lowest station-inlet temperature of different intermittent schemes ( o C) Inlet of pumping station PS PS3 PS PS PS PS7 PS PS9 PS1 PS11 Scheme 1 (/3) Scheme (/) Scheme 3 (/1) Before industrial application, field trials were conducted including 1 h, h, 3 h and h of pipeline shutdown during December and January 9. With extension of shutdown duration, the temperature drop of the oil increased. The maximum temperature drop reached 3.1 o C after the pipeline shut down for h, as shown in Fig.1. Meanwhile, the field data further validated the models and software development. The oil temperatures at each pumping station inlet before and after h shutdown, both the field data and simulation results, are shown in Fig. 1. The deviation of the temperature drop is within 1 o C, which verified the numerical models and software. After the successful field trials, intermittent operation of 7-day operation/ 1-day shutdown was employed in the West Crude Pipeline. In this way, the pipeline satisfactorily met the challenge of operation at the extremely low flowrate. V. CONCLUSIONS This paper is focused on the thermal characteristics of the new technologies in crude pipelining industry. Analysis on the thermal characteristics of the parallel double-pipeline system will not only directly provide technical support for construction and operation optimization of the crude oil pipeline and the products pipeline, but also give some instructions to the design, construction, operation and management of other pipelines in the future. Economic scenario is proposed based on the numerical simulation of the technology of batch pipelining crude oils at different temperatures. Industrial application has proved the feasibility and economy of this new technology. Intermittent operation can be applied to the pipeline confronted with low throughput problem. Inlet temperature ( ) Temperature Pressure Nov. Dec. Jan. Feb. Mar. Apr. May Fig.9 Thermal and hydraulic behaviors of intermittent operation pipeline Temperature drop ( o C) Field data Simulation Deviation PS PS3 PS PS PS PS7 PS PS9 PS1 PS11-1. Location Fig. 1 Temperature drop at the station inlets after h shutdown VI. ACKNOWLEDGMENT This paper is supported by the National Science Foundation of China (No.93 and No.711). REFERENCES [1] B.Yu, Y. Wang, J.J. Zhang, X. Liu, Z.W. Zhang, K. Wang. Thermal impact of the products pipeline on the crude oil pipeline laid in one ditch The effect of pipeline interval. International Journal of Heat and Mass Transfer, 7, 1(3-):1-13. [] B. Yu, Y. Wang, X. Liu, J.J. Zhang, Z.W. Zhang, K. Wang. Model studies thermal effects of liquid pipeline Collocation, Oil & Gas Journal, 1(1). pp. -,7 [3] B. Yu, X. Ling, Jinjun Zhang, et al. Study on laying technology of products pipeline along with hot crude pipeline in one ditch, Acta Petrolei Sinica, (). pp. 19-1, 7 [] K. Wang, J.J. Zhang, B. Yu, J. Zhou, J.H. Qian, D.P. Qiu. Numerical simulation on the thermal and hydraulic behaviors of batch pipelining crude oils with different inlet temperatures. Oil & Gas Science and Technology - Revue de l'ifp, (). pp. 3-, 9. [] K. Wang, J.J. Zhang, B. Yu, Numerical simulation of the temperature fields in the crude oil within pipeline and in the soil around the pipeline for sequentially transporting the crude oils of different property, Journal of Xi'an Shiyou University(Natural Science Edition), 3(), pp.3-, [] K. Wang, Numerical study on batch pipelining of crude oils with different out-station temperatures, Ph.D. dissertation, Department of Petroleum Storage and Transportation, China University of Petroleum, Beijing, China, 9. [7] X. Liu, J.J. Zhang, H.Y. Li, B.Yu, PPD treatment, intermittent operation address low waxy crude throughput rates, Oil & Gas Journal, 1(). pp. -,1 3