A Simulation Study Of The Sarir Field - Tobruk Terminal Crude Oil Pipeline

Transcription

1 A Simulation Study Of The Sarir Field - Tobruk Terminal Crude Oil Pipeline Awad Shamekh*, Salah Masheiti**, and Soad Ben Soud*** *Electrical and Electronic Engineering Department, University of Benghazi awad.shamekh@uob.edu.ly **Mechanical Engineering Department, University of Benghazi salah.masheiti@uob.edu.ly ***Arab Gulf Oil Company, AGOCO Soad.bensoud@agoco.com.ly Abstract This study considers the simulation of the pipeline that transports crude oil from Sarir field in the South East of the Libyan Sahara desert to the Tobruk terminal which is located in the East of the Libyan coast. The VariSim TM pipeline simulation software is to be exploited in this work. The pipeline, and its pumping facilities, was constructed in 1969 with 34 inch diameter, 514 Km long pipework from the source to the demand. In the existing operational circumstances, the pipeline ships approximately 110,000 barrels per day with aid of one pump station placed in Sarir field. The considered transportation line is owned by the National Oil Corporation and operated by Arab Gulf Oil Company (AGOCO). In this work, a hydraulic analysis study is carried out for the investigating the pipeline. Keywords- pipelines; computer simulation; hydraulic analysis I. INTRODUCTION The oil industry is the backbone of the country s economy in Libya. Recently, Libya exports about one million, six hundred thousand barrels per day. The oil is normally carried across several terminals through different pipelines which represent the most economic and safest carriage system for mineral products. Sarir field is one of the earliest discovered basins in Libya. According to [1], the maximum flow rate of Sarir crude production is Barrel Per Day (BPD). Sarir field-tobruk terminal pipelines ships about 110,000 Barrel a day, and the remaining is transported within another line to the Ras Lanuf terminal, which is located 370 Km to the west of Benghazi. As stated, the considered pipeline was assembled 45 years ago and this suggests that it needs much work to be undertaken to ensure the safety requirements as well as to comply with the efficiency and reliability required. understanding properties of wax crude, as in the case of Sarir crude which has up to 18% of wax content [1], is a significant issue to be considered in the design of the pipeline and the other transportation facilities. Many articles are published in this field, these include [4], [5], [6], [7], and [8].This article presents a simulation study for the existing circumstances and concludes with further recommendations for keeping its performance in the range of optimum operation. The scope of this paper is to provide an initial overview of pipeline system with consideration given to the accurate simulation of its hydraulic behaviour. If sufficient simulation accuracy can be achieved when compared against the actual pipeline behaviour, a further study will be undertaken that will consider the implementation of different leak detection techniques across a range of different scenarios of operation. Therefore, this work highlights the present operating conditions of the Sarir- Tobruk pipeline. Subsequent to this introduction, the paper is classified as follows; a brief idea about the VariSimTM is introduced in Section 2, Section 3 supplies the data of the pipeline, while Section 4 presents the model results, and ultimately conclusions are given in Section 5. II. VARISIMTM PACKAGE VariSimTM is a selected simulation package written in the.net language and supports the Windows operating system. The package is developed to handle with high accuracy any component of a pipeline. The software is designed to model compressible and incompressible products. All components that may influence the hydraulic reaction of a whole system can be easily simulated with this package including pipeline segments, valves, pumps, control instruments, etc. VariSimTM consists of VariBuildTM and the VariSimTM simulator. In the simulation, VariBuildTM is typically utilized to realize a detailed physical representation of the pipeline, whereas VariSimTM simulator counterpart is exploited to run the Dynamic simulation and to obtain the results[2]. A project can be modeled in several Areas, where each area defines a sub-portion of overall configuration. Depending on the aim of the simulation, the step length (pipeline segments) of the pipework should be selected. Small steps means high accuracy at the expense of speed of execution. In this work 1 Km is selected as a step length for the main pipeline, hence there will be 514 segments and at each segment a range of hydraulic information can be provide by the simulator including flow, pressure, temperature and fluid properties such as density and viscosity. With the availability of this information it becomes convenient for the operators to monitor pressure

2 and flow of the fluid. Sudden leaks can be introduced to simulate the fluids hydraulic in the flow and pressure wave propagation.. In addition to the monitoring and leak detection the package can be also exploited in project optimization and sizing. More details about the product can be found in [2]. The scope of this paper is to provide an initial view regarding the considered pipeline system which will be deployed in the next coming study for implementing different leak detection techniques. Therefore, the work highlights the present operating conditions of the Sarir- Tobruk pipeline. Subsequent to these sections, the paper is classified as follows; section 3 supplies the data of the pipeline, while section 4 presents the model results, and ultimately conclusions are given in section 5. III. DATA OF THE CONSIDERED PIPELINE General Data: Pipeline Material : API5L X Gr.52 Pipeline length: 514 Km Pipeline diameters: 34 inch, 46 years old Wall thickness: inch Designed Pressure : 826 Psig. The following tables contain the information regarding elevation profile, pump specification, and crude oil properties[1]. TABLE I. ELEVATION PROFILE, ELEVATION OF THE STATIONS Total distance in KM 514 KM Location and Distance Elevation or max allowable operating head Sarir pump station KM First point at Sarir pump station FT KM FT KM FT KM( last point at Tobruk Terminal) FT TABLE II. PUMP STATION SPECIFICATIONS Type Horizontal, Centrifugal pump Model no. ZMI 375/05 Manufacture Rhurumpen Liquid Centrifugal oil Pumping temperature 140 F Capacity 6330 G.P.M Suction / discharge pressure 80 Psig/ variable 350 Psig Differential head 595 FT Break Horsepower 1080HP Speed 2975 R.P.M Nozzle size Suction 18 - discharge 16 No. Of stage 1 Shaft seal Mechanical seal Driver type Model no. Manufacturer Electrical Motor MT 7105 Parsons Peebles Rated speed 2975 R.P.M Rated horsepower 1270 HP (948 KW) Installed in 1969 motor pump 1965 In the crude oil shipping pumps in operation are five in series (model no. ZMI 375/05) followed by pressure control valve. At the present one pump is in the duty. KM 265 booster pump station consists of two in parallel and in series with another one. However, this station is not currently in operation. TABLE III. SARIR CRUDE OIL PROPERTIES I Specific 60F API Characterization factor Total Sulphur % wt 0.17 Mercuptan Sulphr ppm wt 8.0 Total nitrogen % wt 0.10 Cloud point (F) 90 Pour point C (F) +21(+70) Melting point of wax F 127 F cst F cst 11.8 Sphaltens % wt 0.30 Wax content per vol. % TABLE IV. SARIR CRUDE OIL PROPERTIES II Tempe Density API Sp.Gr. F Ib/ft3 cst viscosity ft2/sec IV. SYSTEM SIMULATION AND RESULTS As shown in Fig 1, the project is constructed based on the data provided in the last section. In this simulation, boundary1 (supply) represents Sarir field, followed by the main pump station, which contains two parallel banks each bank consists of 5 pumps connected in series. It is important to clarify that two pumps are in operation at the considered rate of production. The control valve, controlled by PID controller, is connected to the downstream of the further pump. The purpose of using the PID is to ensure the flow rate as required. The pipeline 3 signifies the main line which is utilized for shipping the oil to the Tobruk terminal and which is denoted by boundary2 (demand). In Table II, the pump curves are displayed in Fig 2. The elevation profile of the pipeline is demonstrated in Fig 3, and the CV profile of the control valve is shown in Fig 4. The procedures for obtaining the pump flow/head and flow/efficiency curves as well as CV valve profile are explained in further details in [2]. Figures 5 through 20 display the obtained results in a specific period of time as shown in each figure. These enclose the output flow of supply and demand, Fig 5 and 6 respectively. It is worth to state that the variable units can be easily converted inside the simulator. Pump pressure and volume flow rate are given in Figures 7 and 8

3 correspondingly. Figures 10 to 15 show the potential head, volume flow rate, and pressure at two different locations of the pipeline. These are at the beginning which represents 0 Km point (segment 0) and at the end of the pipeline (514 Km, segment 514). The control valve current and target positions are illustrated in Figures 16 and 17. Similarly, Figures 18 and 19 provide the potential head and pressure of the same element. Finally, Figures 20 and 21 display the measured signal of the PID controller and Pressure-Mass flow rate profiles of the whole pipeline. Obviously, from the results that the target is to have a steady state flow rate of 110,000 barrels per day that represents the current shipping crude from the Sarir field. Figure 3. Elvation profile (m) Figure 1. A schmatic descrpition of the pipeline Figure 4. CV profile of the control valve Figure 2. Pump curves The PID controller is introduced to regulate the flow rate to comply with the required set point. Therefore, as it has been shown on Fig 20, the PID controller function is to manipulate the valve position according to the desired flow rate output. Figure 5. Boundary1Output Flow

4 Figure 6. Boundary2 Output Flow Figure 9. Pump volume flow rate at segment 0m Figure 7. Boundary 2 Operational Status Figure 10. Pipe3 PotentialHead at segment 0m Figure 8. Pump Pressure at segment 0m Figure 11. Pipe3 Potential Head at segment 514Km

5 Figure 12. Pipe3Volume Flow rate at segment 0 m Figure 15. Pipe3 Pressure at segmant 514 Km Figure 13. Pipe3Volume Flow rate at segment 514Km Figure 16. Control Valve - Plug Current Position Figure 14. Pipe3 Pressure at segmant 0 m Figure 17. Control Valve - Plug Target Position

6 Figure 18. Control Valve - Plug Potential Head of the implemented package. All the defined variables, at any period of time, are accessible in accurate quantities. The software considers all the fluid properties, such as Non-Newtonian, shear stress, shear rate, Kinematic viscosity, etc. and also the fluid principles (e.g. Darcy-Weisbach and Bernoulli s equations) to make sure that the hydraulic analysis is carried out at high standard of precision and reliability. This actually enables the package to be installed in different phases of operation which includes a standalone form, which is normally used in research, and also as real time applications that mainly constructed for real monitoring and further for leak detection purposes. It is very important to notice that the results may not reflect a good mode of operation for this shipping pipeline. This essentially will be the task of the next part of the work, where it will consider in addition to the operation optimization, it will also concentrate on the leak detection issue. Figure 19. Control Valve - Plug Pressure Figure 21. Pressure- Massflow rate profiles of the pipeline Figure 20. Measured signal of the PID controller V. CONCLUSIONS The scope of this part of the study has been developed to establish if a simulation package can accurately simulate the behaviour of the pipeline. The paper reveals the proficiency REFERENCES [1] Soad Ben Soud, Transportation of Sarir crude oil to Tobruk, Master thesis, Cranfield University, Jan [2] Varisim training manual, Simulation Software limited. [3] Computational pipeline monitoring for liquid pipelines, API 1130, second edition, November [4] Kennedy, J.L., 1993, Oil and Gas Pipeline Fundamentals, 2nd edition, PennWell Publishing Co.,Tulsa, Okla. [5] PBS&J, 1998, Chemical and Physical Properties for Crude Oil, Gasoline, Diesel, and Jet Fuel,prepared for Lower Colorado River Authority, Austin, Texas, Oct. [6] Wang, S., & Carroll, J. J. (2007, June 1). Leak Detection for Gas and Liquid Pipelines by Online Modeling. Society of Petroleum Engineers. doi: / pa. [7] Johnstone, J., & Curfew, J. (2012, August 1). Twelve Steps to Engineering Safe Onshore Oil and Gas Facilities. Society of Petroleum Engineers. doi: / pa. [8] Massey, B.; Ward-Smith, J. (2005), Mechanics of Fluids (8th ed.), Taylor & Francis, ISBN