Chris Giles
SD-Scicon Ltd.
Milton Keynes, U.K.
Ian Neilson
British Pipeline Agency Ltd.
Hemel Hempstead, U.K.
Design of the supervisory control and data acquisition (scada) system recently installed on the United Kingdom Oil Pipelines Ltd. (UKOP) multiproduct pipeline system provides a high level of automation and system availability.
UKOP is trustee and agent for BP Oil (U.K.) Ltd., Chevron International Oil Company Ltd., Fina plc, Mobil Oil Co. Ltd., Shell U.K. Oil, and Texaco Ltd.
The project to replace the existing control system was managed by British Pipeline Agency Ltd. (BPA) which designs and operates liquid petroleum pipelines for several third-party clients, including UKOP.
The scada system was designed and built by SD-Scicon, a U.K.-based subsidiary of EDS, which supplies systems integration services for the oil and petrochemical industries.
HYBRID SYSTEM
The UKOP pipeline system (Fig. 1) consists of two pipelines. One originates from the Stanlow refinery on the river Mersey and delivers to terminals at Uttoxeter, Kingsbury (near Birmingham), Northampton, and Buncefield. The second draws product from refineries and tank farms in the Thames Haven area and delivers to terminals at Buncefield, Northampton, and Kingsbury.
The system, which consists of some 470 km (292 miles) of 6-14 in. pipeline, ships eight grades of white oil for several shipping companies and transports approximately 2,000 parcels of fuel per year; individual parcels may be split between several different delivery destinations.
Before this project, the pipeline was controlled by a hybrid 1960s and 1970s control system which had evolved as the pipeline system expanded. It was nearing the end of its useful life, primarily through hardware obsolescence.
It had also become apparent that the system would be unable to satisfy new requirements expected to arise during the 1990s:
- BPA would need improved process monitoring and leak-detection facilities, to be able to maintain safety and environmental performance.
- Limitations of the existing control system meant that the pipeline system had to be controlled from three sites. Significant cost savings could be realized if it could be controlled from a single, centralized control system.
Providing a range of automation facilities would be necessary if the existing control center personnel at Kingsbury were to be able safely to operate the increased length of pipeline and number of sites which would come under their control.
- The pipeline system plays a critical role in the distribution strategies of the shipping companies and operates close to its maximum capacity. Shutdowns caused by control system malfunctions or failures were therefore becoming even more unacceptable.
- Increasing implementation of new computer systems for planning and product accounting within the shipping companies was causing demands for more process-related data, at increasingly frequent intervals, to be placed on the pipeline operations staff.
It was clear that these demands would grow further and that they could only be met if such data were available in a manageable and accessible form.
It was clear that these and other as yet unforeseen requirements could only be satisfied by installation of a new and flexible integrated control system. This new system would replace the existing computer systems and the manual control panels but would retain the comprehensive instrumentation which had already been installed.
PROJECT STRATEGY
BPA set up a five-man project team, with a broad range of expertise, to specify and to manage the procurement and installation of the new control system. The task was split into two parts: the scada system, with its large software content, and the more hardware-biased local panels.
A contract for the latter was let after a normal tendering cycle, while the scada software-system procurement was handled somewhat differently.
The team's first major task was to produce a detailed requirements specification for the scada system. End users of the system were involved in this task.
An extensive contractor prequalification exercise. This exercise concentrated on software companies which could offer a core scada product and which had experience of multiproduct pipelines.
As a result of this process, contracts were let to two companies to produce detailed functional specifications for the system, together with fixed price tenders for its implementation.
A proposal by SD-Scicon was then selected based on cost-effectiveness and technical suitability. Subsequently, BPA's team relocated to SD-Scicon's offices for the duration of the project.
SYSTEM IMPLEMENTATION
BPA defined the structure for the control system (Fig. 2) to consist of a central supervisory system, located at Kingsbury, and six location systems, installed at the major plant sites along the pipeline.
The location systems are connected directly to the local panels at their respective sites, while the satellite sites are connected into the nearest local panel by simple point-to-point telemetry hardware.
SOFTWARE
SD-Scicon's design for the system employs the Setcon process management software package, running on Digital Equipment Corp. (DEC) VAX computers under VMS, its operating system.
Setcon, a proprietary product of Setpoint Inc., Houston, and used in more than 350 installations worldwide, consists of a comprehensive set of scada and process-control functions which act on a real-time data base, together with a range of layered products which provide higher-level functions.
Of these, this system uses Setcon-GCS, a graphics-based operator interface running on IBM PCs, and Setcon-POSL, a sequence control language. All the pipeline-specific applications software was written by SD-Scicon in Fortran.
HARDWARE
Fig. 3 indicates that the entire system is built from standard commercial equipments fundamental design requirement.
The supervisory system consists of two MicroVAX 3500 computers, configured as a hot-standby pair, running Setcon, linked via Ethernet to four IBM PS/2 computers acting as the operator terminals.
Other peripherals include consoles, a data-entry terminal, text printers, and a color printer.
Each location system consists of a MicroVAX II computer, running Setcon, with a single operator terminal and printer, and an associated data acquisition system (DAS). Each DAS is based on an IBM industrial PC, which acquires and preprocesses analog and digital plant data obtained via the local panel and issues controls and setpoint changes.
It also handles the exchanges of data with intelligent peripherals, such as flow computers and tank-gauging systems. The plant interface is implemented with Allen-Bradley programmable-logic controllers (PLCS) and I/O modules.
The supervisory system is connected to each location system by a point-to-point communications circuit. Each circuit is implemented as an analog leased line, with automatic dial-up fallback. Supplementary dial-up circuits are also provided.
Given the volumes of data which need to be transferred and the diversity of the links which each circuit has to support, modems (9600 baud) and statistical multiplexers are employed to give four links per circuit.
SYSTEM FUNCTIONS
In addition to all the standard scada functions which are provided by Setcon, such as the man-machine interface (MMI), alarm handling, event logging, report generation, and data archiving, the system implements a comprehensive set of pipeline applications.
- Pipeline automation. The most distinctive aspect of the system is the level of automation required by BPA to enable the pipelines to be controlled from a central site with a reduced number of operators.
The operators can enter the pipeline movements schedule into the central supervisory system before the product is pumped. After a set of consistency checks have been performed, the schedule data are downloaded to the location systems.
Control sequences running on the location systems use these data, coupled with the current process values, automatically to execute the plant controls to bring about parcel changes with no further operator intervention.
The parcel-change sequences which run on each location system reflect the, type of site covered by the system. For instance, parcel changes at input sites are based on matching scheduled and measured volumes, while output parcel changes typically monitor for changes in density or color and automatically remove product interfaces.
In addition to the these local parcel handling facilities, a central mechanism is used to coordinate route changes on the pipelines. The operator is able to define the conditions at which each route change is to take place and the control sequence which is to be used to execute that change.
The automation facilities were specified by BPA to match closely the existing manual procedures and thought processes used by the operators, in order both to benefit from the accrued experience and to ease the transition process.
The facilities were implemented by SD-Scicon in a layered manner, which allowed them to be phased into use and to permit manual intervention, if appropriate, in the event of unforseen circumstances.
The centralization and automation of all pipeline controls meant that the tasks of generating and distributing all of the product movement documentation to the various shipping companies had to be automated.
- Parcel tracking. The parcel-tracking function not only provides graphical displays and reports of the positions of parcels within the pipeline system, but also monitors the actual progress of the batches against the schedule.
It then warns the operator of any potential mis-routings and of any differences between scheduled and actual movements, thereby reducing the risk of erroneous movements.
- Pipeline startup, shutdown sequencing. A complete set of control sequences for starting up and stopping each pipeline site was implemented, based on BPA's operating experience. These sequences are also linked into combined sequences which allow the entire pipeline system to be started and stopped in a coordinated manner.
All control sequences were written by SD-Scicon in Setcon-POSL, thus enabling them to be easily modified by the end-users in the future.
- Pipeline integrity monitoring. The comprehensive integrity monitoring functions included in both the supervisory and location systems monitor the pipeline plant and instrumentation and alert the operator to product leakage from the pipeline.
The primary monitoring technique is based on volumetric throughput balance, in which on-line pressures, volumetric flows, and temperatures are used in several short and long-term balance calculations.
The calculated volume imbalances are displayed graphically and are checked against several different alarm limits, depending on the operating condition of the pipeline. A pipeline shutdown is automatically initiated if the appropriate alarm limit is exceeded.
A similar set of flowrate-balance calculations is also performed to provide rapid indication of catastrophic leaks. Temperature-compensated pipeline pressures are also monitored, under both running and shut-in conditions, and any unexplained pressure trends are reported to the operator.
- Split of functions. The functions just described are split between the supervisory and location systems, with the basic division being that pipeline-wide functions are performed by the supervisory system and functions affecting only one site are performed by the location systems.
The basic scada functions are performed on all systems. Thus, under normal operating conditions, the supervisory system performs schedule entry and dissemination, integrity monitoring (balance calculations), parcel tracking, line start-up/shut-down and route changes, production and distribution of movement accounting documents, and data exchange with external systems.
The location systems perform parcel-change sequencing, integrity monitoring (pressure checks and feeder line balance), site start-up/shut-down and operating mode changes, and special control sequences (additive injection, reinjection, etc.).
SYSTEM OPERATION
During normal operation, when the pipeline is controlled by operators at the supervisory system, each DAS acquires plant data twice a second and passes them to its respective location system, where it is used to update the Setcon data base.
The supervisory system then receives sets of plant data from all the location systems upwards of every 3 sec to give it a complete picture of the state of the entire pipeline. The scada and custom applications software in every computer then acts on the data held in its Setcon data base, exchanging data with other computers as required.
Should a location system fail, the DAS can bypass the location computer and pass the plant data directly to the supervisory system, thus enabling the operators to continue to control the pipeline. The only degradation suffered in this case is that the automatic control facilities normally performed by the location system are not available.
If for any reason the entire supervisory system is unavailable, fallback operator terminals are provided at Kingsbury for the operators to log on to the location systems via the supplementary dial-up circuits.
This enables them to control the location systems directly.
Under these circumstances, the events and alarms detected by all the locations systems are logged on a central printer.
The final fallback facility provided for the operators is the ability to connect directly to a DAS from a remote terminal and to examine the plant inputs and issue either single controls or to execute predefined sequences of controls.
SYSTEM LASTING, INSTALLATION
Because the system had to be installed on a live pipeline, with severely restricted opportunities for site testing, it was necessary to ensure that the factory testing was as rigorous as possible.
In addition to the normal range of hardware and communications tests and software testing with test harnesses, a comprehensive set of test tools was developed to enable realistic testing of the entire system.
These tools enabled the software to be run as it would in the real world, with the test tools supplying realistic plant data to the Setcon data base and responding to controls issued by the system.
These tools greatly facilitated the factory testing and enabled it to be conducted more thoroughly than would otherwise have been possible.
Installation of the system presented several major logistical challenges. The most significant was the need to ensure minimal disruption to the throughput on the pipeline.
This could only be achieved by phasing in the installation over a 12-month period to enable BPA's movements planners to make sites available for the installation at the required times without affecting the delivery plans of the shipping companies.
The installation work was also arranged to make the maximum use of shutdowns occasioned by statutory pressure tests and routine maintenance.
Even with this planning, it was necessary to bring some sites back into operation before their new location system was fully available.
This problem was solved by BPA's developing a portable, trailer-based control facility. This consisted of a flow computer, programmable logic and loop controllers, alarm annunciator, and plant shut-down logic.
The first step at each site during the installation was to connect this control facility to the critical plant controls and protective devices which would enable the site operations to continue without prejudicing process safety or product quality.
Installation of the local panels and the location systems then proceeded, with control being progressively transferred to the new control system.
The system has been operational since late 1990.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
