Most pipeline simulations are concerned primarily with the hydraulic performance of the pipeline. As a result, an idealised model of the control systems in assumed. Control devices are often assumed to operate instantaneously and perfectly. This is sufficient for a wide range of simulation tasks. However, for simulations that are intended to be used in training pipeline operators this is no longer suitable. What is required is to provide a ‘virtual pipeline’ that responds in the same way as the real pipeline would. This means that the delays, limitations and interactions of the control system need to be modelled in addition to the hydraulics of the pipeline. This paper describes how a generic control capability was added to an existing liquid simulation package. The practical lessons learned from implementing a simulation of the full control system for a pipeline training system will be discussed. The potential drawbacks of the method chosen will also be highlighted.
In a simple model of a pipeline, if you ask the simulation to close a valve or start a pump, then it does. In the real world, if an operator asks a Supervisory Control And Data Acquisition (SCADA) system to close a valve, any number of things may happen:
The valve might close as requested
The valve might not close because the pressure downstream is too low for the system to allow this.
The valve might close, and a valve on an alternative route may open
The valve might close, and several other related valves may also close.
The valve might close, and after some time various other changes occur automatically.
The overall effect on the operation of the pipeline will obviously be different in each case. Similarly, the behaviour of the real control system to a change in set point may be completely different than that exhibited by an ideal control. As a result when training an operator in the running of the pipeline, it is important that the training system can mimic this behaviour. In a real pipeline, this sort of control can be implemented in several places. It can be built directly into the SCADA control screens that the operator uses, implemented in the Programmable logic controllers (PLCs) used to directly control the pipeline objects, or some combination of the two. Training systems are required to simulate various levels of this control. In addition training systems often need the ability for a trainer to interfere with the normal operation of the pipeline or control logic in order to instruct a trainee in dealing with various faults. In a pipeline, there are also usually two main forms of control, which interact with each other.