The Mossgas refinery produces high-quality liquid fuels and associated products from natural gas and condensate found off South Africa's southern Cape coast. This feedstock is piped from a production platform situated 80 km offshore that in turn controls a labyrinth of sub-sea wells around the platform.
Mossgas has made use of computer-based simulation for operator training and control system testing and verification from very early on in its design and commissioning phases back in the early 1990s. This article discusses the purpose, current status and use of these tools by Mossgas.
It is the author's view that the role and importance of these tools will increase as the overall plant changes and expands to meet new feedstock, market, and environmental challenges. Process simulation is undoubtedly one of many innovative schemes used by Mossgas to continuously empower its workforce with the technology and skills to be successful.
The offshore production platform relies on two major and distinct systems for efficient and safe operation:
* Control: the Foxboro I/A Series DCS that is used to monitor and control production.
* Safety: the GE-Fanuc PLC-based emergency shutdown and fire and gas detection systems (ESD/FSG) that autonomously ensure the safety of both equipment and personnel in the potentially hazardous hydrocarbon environment.
The simulation facilities
BeST is responsible for the provision of computer-based simulation facilities for both the DCS and ESD/F&G systems that include simulations of the total dynamic process from the sub-sea gas reservoirs to the onshore reception vessels. Two separate simulators have been implemented as part of a land based support facility used for the training of process operators and for software maintenance. Both systems continue to evolve so as to 'replicate' the actual offshore systems.
The DCS simulator
This consists of a number of Foxboro I/A operator stations configured with the exact mimics, alarms, trends, etc (ie all aspects of the human interface), and a single Windows NT-based simulation station. The simulation station is connected to the Foxboro I/A stations via Nodebus (the Foxboro control-system bus). The software running on the simulation station is described below, and is capable of all the features expected of a training simulator.
The dynamic simulation of the platform processes and important utilities
The offshore production process separates the vapour, liquid and aqueous portions of the natural gas received from the wellheads. The simulator, based on SysCAD technology, provides an infrastructure for the inclusion of standard and user-defined models into the solution algorithm. Use is made of both rigorous and semi-rigorous dynamic models for multicomponent, multiphase equilibria encountered in physical separation and heat transfer unit operations.
The major equipment simulated includes glycol absorbers, heat exchangers, condensers, flash vessels, hydro-cyclones, pumps, valves and utility packages.
The emulation of the Foxboro I/A Series control system - IA Engine
To enable the DCS simulator to accurately reflect operation of the actual system, and to make maintenance simple and cost-effective, a software emulation strategy is used. The locally developed IA Engine is able to read the binary configuration data used by the actual Foxboro I/A Series implementation of over 30 control processors, and more than 200 intelligent fieldbus devices, and executes the associated control and monitoring algorithms in realtime. The IA Engine also 'listens' for operator station requests for control information and responds as if it was the control processors. Alarms and event information is also broadcast to the operator stations.
The emulation of field devices and ESD
The emulation of field devices and emergency shutdown algorithms is shared between SysCAD and the IA Engine. In the first implementation of the system colour photographs were taken of the panels of the various packages and these were then digitally scanned. National Instruments' LabView was used to create realistic computer-based replicas of the panels for operator control via a mouse. In recent times, a VBA-powered Excel spreadsheet is used.
Benefits included:
* Operators are equipped with a high degree of confidence, skill and competence borne out of 'realistic' training.
* Safe, cost-effective and flexible training is undertaken in isolation from the actual physical process ensuring no danger to equipment or people.
* Operators are introduced to plant start-up, normal process operation, normal process upsets, normal shutdown, and abnormal situations (equipment failure and emergency shutdowns). This exposure is difficult to accomplish on the actual system.
* Operators are introduced to DCS changes prior to implementation offshore. They are able to play an active role in evaluating changes and ensuring that all mimics and control operations promote simpler, efficient and less ambiguous operation.
* Assessment and accreditation of operator competence is facilitated.
* Rigorous pre-installation testing reduces costly 'errors' during hot commissioning and increases confidence that changes will perform as expected.
The ESD (emergency shutdown) simulator
In late 1999, early 2000 BeST was involved in the complete rewrite of this simulation package resulting in the development of the current ESD simulator that runs under Windows NT and accesses and stores information in an Access 2000 database. The package was developed in response to functional requirements specified by the Mossgas personnel responsible for the software maintenance and testing of the offshore ESD/F&G. The current system, replacing two separate legacy systems, serves two major functions:
* Maintenance, display and reporting of the ESD/F&G cause and effects. Queries and reports may now be invoked via the company intranet.
* Live testing and recording of cause and effect events on the onshore GE Fanuc test rig.
How it works
The ESD simulator has three main components:
* The cause & effect (C&E) and I/O database (also includes a log of test results).
* The user interface.
* The runtime communications engine.
The ESD simulator is connected to the GE Fanuc Genius Bus and monitors all inter- PLC communications. When the user interface is used to provide a command to change a value (eg a temperature) such that a trip-point is exceeded, a PLC will issue a request to open a valve, or stop a pump. The runtime communication engine 'detects' this request and writes PLC I/O such that the desired feedbacks occur. The entire process is logged and can be compared with the desired C&E logic to determine whether the PLC logic is operating correctly. The testing can be fully automated by creating test scripts in either VBScript or JavaScript.
The user interface also supports the generation of C&E drawings by using Excel and VBA. This is a highly regulated test procedure - and the results log is accepted by Lloyds as a basis for them issuing the production platform with a certificate allowing it to bring gas on-board and receive insurance backing.
Intranet use during everyday operations
As the data is stored in a standard database, easy queries and reporting are supported. When coupled with Intranet Information Server (IIS), distributed browser-based access is supported. The Mossgas intranet allows support personnel to view and query the 'latest' C&E information easily from almost anywhere and at any time instead of relying on hardcopy documentation that may or may not be up-to-date.
Onshore refinery: the methane reforming unit
BeST is currently implementing an operator training system for the onshore refinery. It will cover the methane reformer unit and again uses SysCAD for the process simulation, while both the Hartmann & Braun DCS and Allen-Bradley-based ESD systems are emulated in software. The simulator development is on schedule and will be commissioned in September 2001.
BeST SA
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