Abstract: Based on research into the complexity of seawater treatment processes on oil platforms and the harsh environment of offshore oil production, and leveraging the advantages of DCS systems, this paper studies and implements a DCS system for water treatment on oil platforms using Rockwell RSView32 configuration software. This system achieves decentralized control of various sub-processes in the platform's water treatment process and centralized management of collected information. This system represents an important application of DCS technology in offshore oil production. Its long-term operation has demonstrated its stability and reliability, improved offshore oil production efficiency, and ensured the safety of offshore personnel.
Keywords: DCS ; oil platform; RSView32; seawater treatment; process monitoring; data management
Abstract: On the base of research on the complexities of platform seawater treatment process and the difficulties of petroleum production on the sea, this paper integrates Distributed Control System (DCS) technology to construct the DCS of Platform Seawater Treatment (PSTDCS) with the help of the configuration software-Rockwell RSView32. The system, as one important application of DCS technology in petroleum production on the sea, realizes to control platform seawater treatment process and manage all collected information data. Its application enhances the productivity of petroleum production and insures worker's personal safety on the sea.
Key words: DCS; Platform; RSView32; Seawater Treatment; Process Monitoring and Control; Data Management
I. Introduction
Distributed Control Systems (DCS) emerged in the mid-1970s. They are a product of the integration of computer technology, control technology, communication technology, and CRT display technology (collectively known as the "four Cs"). Centered on a microprocessor, they organically combine microcomputers, industrial control computers, data communication systems, display and operation devices, process channels, and analog instruments into a modular system. This modular approach has created conditions for the comprehensive automation of large-scale engineering systems. Therefore, PLC-based DCS has become the mainstream of industrial automation systems. In offshore platform oil production, due to the harsh operating conditions and environment, and with the continuous expansion of offshore oil extraction, the production and management of offshore platform oil production increasingly demands rapid, accurate, and comprehensive monitoring of the operating status of production equipment. Therefore, distributed control has been increasingly widely applied in offshore oil extraction.
II. Platform Water Treatment DCS System Architecture
2.1 Platform Water Treatment Process
Seawater is pumped to a coarse filter, where over 98% of suspended solids larger than 100μm are removed. Then, coagulants and flocculants are added in proportion, and the seawater enters the vortex neutralization and reaction zone of a pressure inclined tube settling tank. The vortex action thoroughly mixes and reacts, causing tiny particles to flocculate and agglomerate into larger particles, while insoluble compounds undergo chemical reactions to form precipitates. These precipitates then enter the inclined tube settling zone for gravity separation, achieving water purification. After settling, the seawater enters a fine filter, where suspended solids are intercepted by the dual-media filter layer. The filtered seawater is then discharged to a supergravity deoxygenation device for partial deoxygenation, reducing the oxygen content. The deoxygenated seawater then enters a first-stage gas-water separator via a U-tube, separating free natural gas carried in the seawater and releasing it to a flare. Finally, it enters a second-stage gas-water separator to separate dissolved natural gas from the seawater, protecting the injection tank from natural gas impact. Seawater, after gas-liquid separation, is injected into the injection tank, then pressurized by the injection pump and transported to each well platform via subsea injection pipelines, ultimately injecting water into the injection wells. The process flow of platform water treatment is shown in Figure 1.
Figure 1. Flowchart of seawater treatment process on offshore platforms
2.2 Platform Water Treatment DCS System Architecture
To address the complexity of water treatment processes, a DCS system architecture based on a three-tiered composite topology (bus/bus logic ring/bus) is adopted. The top layer is the information management layer, which manages the platform's water treatment monitoring data; the bottom layer is the remote I/O link, enabling on-site control; and the middle layer is the DH+ network, enabling real-time monitoring of each water treatment process. The network topology is shown in Figure 2.
Figure 2 DCS System Topology Diagram
The monitoring computer communicates in real-time with microprocessor-equipped PLCs, data acquisition units, and other components via a DH+ network to store, retrieve, and modify data distributed across the industrial control site. The monitoring computer can not only promptly grasp the overall status of the entire water treatment process but also directly provide setpoints for each control loop. It also sends control signals directly to the control loops via the PLC and dynamically displays various screens of the entire water treatment process using CRT display technology, monitoring the entire platform's seawater treatment process. The management computer uses an MS SQL Server 2000 relational database to manage the water treatment process data. It obtains all online information about the water treatment process from the monitoring computer through an Ethernet information management network; therefore, in addition to direct online control, it also has the same monitoring functions as the monitoring computer.
The system employs two PLC expansion racks, PLC1 and PLC2, composed of AB's SLC500 programmable controllers, each with nine SLC500 modules. PLC2 is used for process control of the fine filter backwashing process, controlling a total of 147 variables. PLC1 is mainly used for process monitoring of sub-processes in the water injection process, including seawater lifting, ultragravity deoxygenation, chemical dosing, U-tube gas-water separation, and water injection pump units. The monitored parameters include temperature, pressure, liquid level, flow rate, seawater turbidity, seawater oxygen content, and the opening degree of regulating valves, totaling 143 parameters.
III. Functional Implementation of the Platform Water Treatment DCS System
3.1 Implementation of System Lower-Level Control Functions
The system's lower-level PLC controls the water treatment site using RSLinx500 ladder logic programming software. It is designed to monitor the status of various equipment and process devices (such as limit switches, buttons, selector switches, pressure switches, etc.) in real time, perform logical judgments based on the monitored status, and accurately control the output devices (such as solenoid coils, valves, signal devices, etc.) on the equipment or in the process. It also implements timing and calculation functions. Furthermore, based on relay instructions, arithmetic operation instructions, and logic operation instructions, it expands to include transmission and conversion instructions, communication instructions, analog input instructions, and PID control instructions to achieve logic control and sequential control. It is also responsible for acquiring continuous quantities in the process, performing complex control algorithm calculations, and outputting the results to the actuators for fine-tuning. The specific control functions implemented are as follows:
1) Seawater Lifting: Based on the outlet turbidity of the pressure inclined tube settling tank, the frequency of sludge discharge from the pressure inclined tube settling tank and the dosage of corrosion inhibitors, coagulants, and flocculants are controlled. Using the set value of the water injection tank level as a standard, the seawater lift pump is controlled by frequency conversion speed regulation to control the water supply: when the water injection tank level is lower than the set value, the controller outputs a signal to the frequency converter to increase the speed, and the frequency converter increases the motor speed, increasing the displacement of the seawater lift pump; conversely, the displacement of the seawater lift pump is reduced. The monitored parameters include the outlet pressure, outlet flow rate, and outlet turbidity of the seawater lift pump; the post-filtration pressure, post-filtration flow rate, and post-filtration turbidity of the coarse filter; and the outlet turbidity of pressure inclined tube settling tanks #1 and #2.
2) Filtration Backwashing: Based on the turbidity at the fine filter outlet, control the number of backwashing cycles and the time intervals during the backwashing process, including water addition, filtration, drainage, air rinsing, exhaust, water rinsing, settling, and emptying. Also control the dosage of the bactericide. Monitored parameters include: backwash tank level, fine filter inlet manifold pressure, fine filter outlet discharge pressure, instrument air pressure, backwash water pressure, and fine filter turbidity.
3) Chemical Dosing: The system controls the start and stop of chemical dosing on each dosing skid based on the high and low limits of the liquid level in each dosing tank. If the liquid level in a dosing tank is lower than the low limit, the system automatically starts the dosing skid to add the chemical solution to that tank; when the liquid level in that tank reaches the high limit, the system automatically stops the dosing action of the dosing skid. The monitored parameters are: the liquid level of the corrosion inhibitor 1#, coagulation 2#, flocculation 3#, sterilization 4#, standby 5#, and standby 6# dosing tanks, and the chemical discharge rate of each dosing pump.
4) High-gravity deoxygenation: Based on the oxygen content of the seawater at the outlet of the secondary gas-liquid separator, the ratio of the air intake to the water intake of the high-gravity deoxygenator (i.e., the gas-liquid ratio) is controlled. If the oxygen content of the seawater is too high, the system automatically increases the gas-liquid ratio of the high-gravity deoxygenator. The monitored parameters are the air intake pressure, air intake flow rate, air outlet pressure, water intake pressure, and water intake flow rate of the high-gravity deoxygenator.
5) U-tube gas-water separation: Real-time monitoring and detection of the liquid level in the U-tube, the liquid level in the first-stage gas-water separator, the exhaust flow rate of the first-stage gas-water separator, the exhaust pressure of the first-stage gas-water separator, the oxygen content at the outlet of the second-stage gas-water separator, and the liquid level in the water injection tank during the gas-water separation process.
6) Water Injection Pump Unit: The water injection pump unit adopts variable frequency speed control. An AC variable frequency drive is used as the starting and speed regulation device, with a PLC as the control core. Based on meeting the water injection pressure and flow rate of the injection well, the unit automatically adjusts the speed and number of pumps in operation to achieve automatic operation. Monitored parameters include: water injection pump inlet flow rate, inlet pressure, outlet pressure; water injection pump front bearing temperature, water injection pump rear bearing temperature; motor front bearing temperature, motor rear bearing temperature, motor stator #1 platinum resistance thermometer, motor stator #2 platinum resistance thermometer, and motor stator #3 platinum resistance thermometer.
3.2 Implementation of System Upper-Level Monitoring System Functions
Figure 3 Overall interface of the oil platform water treatment monitoring system
The supervisory control and data acquisition (SCADA) system, developed using RSView32 configuration software according to the water treatment process flow, consists of six monitoring subsystems. The main system interface is shown in Figure 3. The seawater lifting subsystem monitors the relevant parameters and on-site operation of three seawater lifting pumps, three seawater coarse filters, and two pressure inclined tube settling tanks. The filtration backwashing subsystem includes five fine filters, one blower, one backwash water tank, and one backwash water pump, responsible for the automatic control of the entire backwashing process and monitoring of status parameters. The high-gravity deoxygenation subsystem dynamically simulates and displays the usage and standby status of two high-gravity deoxygenators, while simultaneously monitoring relevant parameters. The chemical dosing subsystem monitors the liquid level and flow rate of each chemical dosing tank and is responsible for accumulating and summarizing the daily chemical dosage. The U-tube gas-liquid separator monitoring subsystem monitors the liquid level in the U-tube and the gas-liquid separation status of the gas-liquid separator. The water injection pump unit subsystem monitors the usage and standby status of each water injection pump and is responsible for real-time monitoring of important parameters of the water injection pumps. Its specific functions are as follows:
1) Real-time monitoring and dynamic simulation of water treatment process; through real-time collection and recording of system data, the parameters, status, trends, etc. of all control loops and detection points in the water treatment process are displayed and simulated in a dynamic manner, realizing real-time dynamic simulation and global monitoring of the entire industrial control site.
2) Real-time acquisition and storage of monitored data: Data communication between the monitoring computer and the PLC is achieved using DH+ industrial LAN; real-time acquisition of monitored data is achieved through the internal settings of RSView32, with the foreground data scan set to 1 second and the background data scan set to 5 seconds. ADO technology and SQL Server 2000 database management technology are used to extend the functionality of the RSView32 configuration software to achieve data storage. Specifically: First, a timer 1 is set within the monitoring system with a clock cycle of 1 second, storing the instantaneous value of the monitored parameter once per second in the monitoring value storage database MonitValueDB. The MonitValueDB database only stores 30 days of monitoring values, mainly used for depicting the historical trend of the water treatment process. If the data exceeds 30 days, the data from the previous day is automatically deleted, and the currently acquired data is stored. Additionally, an EVENT event is created within the monitoring system, and a timer 2 is set, with its EVENT start time customized to 00:00, 00:20, 00:40, 01:00, 01:20, 01:40, ..., 23:00, 23:20, 23:40. When the timer expires, the EVENT event starts immediately, retrieving the instantaneous values of the industrial control parameters collected by the system within the 20 minutes prior to the EVENT event from MonitValueDB and averaging them as the valid value of the monitored parameter. All of these values are then stored in the average value database AveValueDB for report printing and historical data retrieval, serving as a basis for production decisions. The real-time acquisition and storage function of the monitored parameters is shown in Figure 4.
Figure 4. Schematic diagram of real-time acquisition and storage of monitored parameters
3) Historical trend depiction: Based on the data stored in the monitoring value storage database MonitValueDB, the historical data of each monitored parameter is dynamically displayed, and the historical operation process of the water treatment process is dynamically depicted in the form of historical curves.
4) Equipment start-up and shutdown operation and standby status identification: Based on the actual operation of the water treatment system, the start-up and standby status of 3 seawater lift pumps, 2 gravity deoxygenators and 7 water injection pumps were correctly identified, and the equipment in standby status was successfully kept from alarming.
5) Alarm Information Display and Processing: Alarm information bars are displayed on the main interface of the monitoring system and on the sub-screens of each subsystem. These bars display recently occurring alarm information, including the instrument workstation number represented by the alarm tag, the date and time of the alarm, the instantaneous value at the time of the alarm, and the high/low limit alarm status. Operators can view, back up, and print out the alarm summary table at any time. When an alarm occurs, the monitoring system will issue different alert sounds based on the severity of the alarm to remind staff to locate and inspect the fault, thus enabling troubleshooting.
The management of alarm information displayed by the system and alarm limits considered for improving water treatment processes is also extended using RSView32's embedded language VBA, as well as ADO remote data access and database management technologies. The program is similar to the real-time acquisition and periodic storage of the monitored data. Alarm information and alarm limits are stored in the AlertingDB database.
6) Data Management and Maintenance: Supported by ADO technology, database management technology and network technology, an industrial control information management system was developed using VB 6.0 software. It realizes data management and maintenance operations such as adding, deleting, modifying and querying industrial control data, and realizes the automatic generation and printing of daily, monthly and annual reports under conditional operation.
IV. Conclusion
The implementation of DCS (Distributed Control System) automation for the water treatment process on offshore platforms has successfully achieved data acquisition, remote transmission, and process control of monitored parameters, enabling truly unmanned operation. This significantly reduces the workload of staff, improves on-site management, effectively avoids waste of chemicals and equipment wear, increases water treatment efficiency, and reduces production costs. Statistical comparisons based on oilfield production yearbook data show that the use of this DCS system can save over 1.9 million yuan in annual production costs, with project economic benefits exceeding 4.7 million yuan.
References
[1] Luo Chao, Xue Fuzhen. Design and implementation of DCS real-time communication network. Computer Applications. 2001, 21-9: 49-50.
[2] Zhang Zhaolin, Sui Jingming. Design and application of DCS-based fan control system. Automation Instrumentation, 2001, 22-7: 49-51.
[3] He Bin. Overview and trend of DCS-distributed control system development. Zhejiang Paper, 2000, 1: 44-46.
[4] Hao Cheng, Chang Jisheng. Research and application of PC-based DCS, Microcomputer Information. 2005, 21, 5: 55-56.
[5] Xu Shuping, Su Xiaohui, Li Feng, Fan Huimin. Design and implementation of DCS monitoring system for cement plant. Microcomputer Information, 2005, 21-7: 78-80
