John T. Blair
Novacorp International Consulting Inc.
Calgary
Design and installation of a pipeline automation system in China by Novacorp International Consulting Inc., Calgary, illustrates the experience of a North American company in that country.
This first of two articles on the recently commissioned project sets out details of the pipeline and aspects of its operation faced by automation-design engineers. The concluding article focuses on the procurement of materials and on the commissioning and management of the system.
Automation of Shengli Oil Pipeline Co.'s 153 mile, 28 in. hot oil pipeline from Dongying to Huangdao (Dong-Huang) in Shandong Province, China, consisted of a dual redundant configuration, host scada (supervisory control and data acquisition) system and a programmable logic controller (PLC) based station control for three pumping stations, one terminal station, and one meter station.
Despite lengthy delays on the original timetable for the project, the Shengli Oil Pipeline Co. (SOPC) now operates a fully automated liquid pipeline with capability for central control in Weifang.
During the final year of the project the Chinese Pipeline Bureau (PLB) commissioned a central scada system for all the pipelines in the country. The system center is located at the PLB headquarters in Langfang, Hebei, near Beijing. The first link was to the Dong-Huang pipeline's scada system.
The PLB has pipeline design personnel and was ultimately responsible for the pipeline design.
THE PROJECT
The Dong-Huang pipeline, east of Beijing, runs from Dongying, in the Shengli oil field, to the port of Huangdao on the Yellow Sea (Fig. 1).
The project was to ensure a supply of hot, waxy crude oil to the seaport of Huangdao for overseas sales. It also resulted in several features never before realized in China: tight line operation of the complete pipeline, fully automated pump station operation, and central control by way of scada.
Novacorp was to provide conceptual and preliminary engineering for the pipeline, station mechanical, electrical and automation, and then to complete the detailed design and vendor liaison for station automation and a centrally operated scada system for control of the pipeline.
There was also a requirement for technology transfer from the North American design groups to the Chinese design groups.
Understanding automation of the pipeline requires a brief description of the pipeline and the associated pumping process.
The line's design throughput is 2,710 cu m/hr (409,000 b/d) with a station discharge maximum pressure of 6,470 kPa (938 psi). The waxy crude oil has a pour point of 27 C.
The new pipeline and associated pumping stations replaced the existing Dong-Huang pipeline, an old pipeline operating on an "open" pump-to-tank principle.
The pipeline facilities consist of an inlet receipt station, two in-line, pressure boost and line-heating stations, a meter station for a lateral pipeline feeding a refinery, and a terminal station at the sea port of Huangdao.
Main features at each station are shown in the accompanying box.
A typical booster station is shown in the flow diagram, Fig. 2.
SPECIAL CONSIDERATIONS
Although most of the conceptual design work was consistent with normal liquid pipeline and pump station design, there were a few special considerations.
Because the oil had to be maintained hotter than the pour point of 27 C. (81 F.), line heaters, heat tracing of the pipe in the pump stations, and a continuous flow of oil in the pipeline were requirements of the design.
A provision was made for providing reverse flow of oil from Huangdao to Dongying to permit flow of oil in the pipeline if the tanks at Huangdao were full and no ships were available to take delivery of the product.
The heating affected the viscosity and subsequently the pressure drop of the oil in the pipeline. Effects of increased pumping power vs. fuel for heating the oil to reduce the line losses had to be considered.
Hydraulic modeling was used to predict the effects of surge and line pack. Pressure control, surge control, and surge relief methods were reviewed.
The surge studies and subsequent field testing are discussed later.
Although all the stations were located in well populated areas and were staffed 24 hr/day, the PLB required the design to be based on the booster stations being unstaffed.
The objective was to gain experience so that the design principles learned could be applied to future stations in remote areas.
As stated previously, it was necessary to keep the oil flowing continuously in the pipeline. Because adequate flow could be established with only the pumps at Dongying in operation, that station was designed with the following contingency features:
- Standby pumping
- Redundant high-voltage power feeds from two separate transmission lines using separate transformers and 6 kv buses complete with independent protective relaying and switching
- Dual, redundant programmable logic controller (PLC) architecture
- Dual, redundant power supplies for the PLCs
- Uninterruptible power supply for all automation and telecommunication
- Dual, redundant microwave, telecommunication channels.
Other automation contingency features were installed at the booster stations. A secondary, small PLC that monitored the critical station process parameters was considered beneficial in case the main PLC failed.
When the main controller failed, all station pumping was stopped because the station could no longer be operated, unmanned, safely.
With the small, secondary PLC still operating, the control center operator could monitor the line pressure and temperature at the pump station, as well as the status of such critical components as the number of line heaters on line. The line heaters had their own independent PLCs and could therefore remain operational even when the main station controller failed.
Further, because the failure of one line heater PLC would only cause the trip of one line heater, the ability to heat the oil was not lost.
The station pressure controls were implemented in a standalone dedicated loop controller to allow manual operation of the pump station with the PLC out of service.
In addition, the tuning and operator interface to the standalone controller were intuitive, considered a benefit for the first time user of such automation. This decision proved to be of significant benefit during commissioning and initial operation when the PLC was unavailable for use due to commissioning problems.
The preliminary design produced the design concept manual, pipeline hydraulic design manual, P&IDs, equipment specifications for mechanical, electrical and instrumentation equipment, station control-system specification, and the scada system specification.
SPECIAL STUDIES
Two special studies were required to augment the preliminary design.
An analysis of the pump driver type was necessary to determine whether variable-speed drives would provide more economical operation than series piped, fixed-speed pumps with pressure control provided via a discharge control valve and staged operation of the pumps.
Also, an analysis of the effect of hydraulic, liquid surge was warranted because the normal pipeline operating pressure was to be designed to within 10% of the maximum allowable operating pressure (MAOP).
If the surge analysis indicated a potential surge in excess of the MAOP, surge-relief devices or surge-control methods would have to be incorporated into the design.
SURGE CONTROL
The evaluation of variable speed pumping vs. fixed speed pumping for pipeline applications is not unique but must be done to ensure that the equipment selection provides the best, long-term economics to the operating company.
The Dong-Huang pipeline study favored the series, fixed speed pump configuration. For a particular flow rate, a combination of pumps at each station could be run without any station discharge throttling.
The pumping could be set to ensure no pump discharge throttling took place and the pipeline outlet was controlled under constant pressure control by the inlet valve at Huangdao.
With delivery at the Huangdao terminal buffered by the storage available, constant flow control at the pipeline inlet to Huangdao was unnecessary.
The MAOP of the pipe used in the pipeline outside the boundaries of the pump stations was 7,120 kPa (1,032 psi).
With the design operating pressure at the station discharge for each of the three pumping stations on the pipeline being 6,470 kPa, it was considered necessary to predict whether surge waves caused by the instantaneous tripping of a pump station or erroneous closing of a valve at a critical location could cause the pressure at any point in the pipeline to exceed the MAOP.
Pipeline Hydraulics Engineers (PHE), Houston, a subsidiary of Novacorp, provided simulation techniques using computer modeling to investigate the effects of surge. The simulation used DREM simulation software running on a Prime 9955 computer. The software was adapted for the application on the project by PHE.
The model was carefully built from data available from the preliminary design and field data provided by the client. Unfortunately such effects as ground temperature and ground thermal conductivity could only be estimated because no practical information was available.
In time the model was refined with actual pump curve and valve data from the applicable manufacturers.
The pressure control scheme for suction and discharge control was simulated in detail in order to determine initial field settings for the pressure controllers during commissioning.
Also, with the planned use of the simulation for operator training (discussed in the conclusion to this article), it was considered important to the operation of the pipeline to be able to demonstrate the effects of pressure controller tuning.
After preliminary runs were made to determine the more influential actions on surge, a list of nine cases, each run under six operating conditions, was established in order to focus on the most critical operating situations.
The most critical case was found to be the instantaneous trip of all the pumps at the Changyi station. Not only did this create the largest hydraulic surge, but it also was considered to be likely if there were a loss of the electrical power supply from the local power utility.
When all the cases were run, there was no indication that the surge pressure exceeded the MAOP of the pipeline. In the case of a blocked valve operation at Huangdao, however, without any corrective operator action, the line pack could build to a pressure in excess of the MAOP.
For this reason a relief valve, placed ahead of the Huangdao inlet control and block valve, was added.
RAREFACTION CONTROL
Although the study indicated there were no abnormal surge effects, the operating pressure transient did approach the MAOP of the pipeline in some cases. As a result, it was decided to implement "surge rarefaction control."
The theory of rarefaction control is that the effects of a surge pressure wave traveling upstream from a sudden, flow blockage can be offset somewhat if a negative surge wave can be initialized at the upstream pump station to travel downstream colliding with the positive surge wave.
The scheme was implemented in the simulation and was effective. The negative surge wave was created by stopping one or more pumps in the upstream station.
Not only was the surge wave reduced, but the step decrease in discharge pressure at the upstream station when the pump or pumps stopped allowed more pressure margin for the residual, positive surge wave arriving from the downstream station. Further, the ensuing line pack effects were reduced.
Rarefaction surge control was implemented in the PLC in Dongying. This controller communicated to the controllers at the other sites on a microwave telecommunications channel independent of the scada channels. This controller was used for three key reasons:
- Use of the scada host would involve too many components, thus influencing the reliability of the scheme. All data would have to be buffered in the host thus slowing the communication between stations.
With the surge wave's travelling time of approximately 1 min. between stations, the rarefaction control was required to activate within 10 sec of detection of the potential surge condition. The scan time of the surge link was 2 sec.
- The Dongying PLC was a redundant configuration and therefore more reliable than any other station controller.
- The PLCs could communicate directly with each other very efficiently and could be programmed by any of the station control engineers without requiring special programming language skills such as would be required if this feature were implemented at the host.
When a failure of a downstream station was detected, the status was transmitted to the Dongying controller. The logic in the controller then created the corrective action and sent out commands to the applicable station to stop a predetermined number of pumps.
The data from actual field tests of the pipeline surge effects followed the simulation work done previously, and the design used for rarefaction control worked to reduce the surge effects as predicted.
The field test results for station suction at Changyi and the station discharge at Shouguang may be seen in Figs. 3 and 4.
DETAILED DESIGN
PLB in its design offices in Langfang, China, conducted the civil, mechanical, electrical, and instrumentation detailed design. Either Novacorp in its Calgary offices or PLB and Shengli Oil pipeline Co. (SOPC) engineers under Novacorp's supervision in Langfang, conducted detailed design for the scada and automation.
SOPC is operator of the Dong-Huang pipeline.
Design of the station facilities by the PLB included piping, buildings, mechanical, electrical, and instrumentation and followed established practices in China.
PLB completely coordinated telecommunications design for the project and utilized an NEC microwave system from Japan.
PLB had never before done detailed design of a station automation and scada system. Novacorp provided the supervision and specialists as required to complete this part of the design successfully.
Detailed automation design included the following tasks:
- Station PLC programming from Boolean logic diagrams
- Station PLC operator interface screen development for the station operator control screens
- Station PLC report development
- Station PLC programming for serial port interface to other station "smart" devices such as the tank monitoring system, the heater PLCS, and the station pressure controller
- Scada host data base building
- Scada host operating interface screen building
- Scada host operator and maintenance report building.
Novacorp personnel prepared design-basis manuals with guidelines for all the detailed automation-design work then traveled to China to work with the PLB and Shengli engineers in the PLB offices to supervise the work. The PLB decided to use the SOPC engineers to do the detailed programming because these engineers would have to keep the system operating on site.
During work on the detailed design in the PLB offices, problems of communication with the vendors arose. It was impossible to telephone vendors from the PLB offices; the only method of communication was via Telex.
With the type of technical questions that arose, such communication medium was impractical. Therefore, the designers had to resolve the problems with their own skills and experience with very little outside assistance.
A PLC development system was shipped early to the PLB offices to facilitate programming and station operator screen design.
Unfortunately there was no similar development system for the scada system. The data base was developed on a PC using data base software, however, and then a conversion program was written to allow loading of the data base attributes to the main scada host.
This approach was very successful and permitted development of the data base prior to delivery of the scada system to site.
As the SOPC engineers proceeded with the configuration work, they became more familiar with the systems and gained confidence. Further, a sense of ownership was developed which was an excellent motivator when problems developed during commissioning.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
