Drilling for oil is one of the greatest technological breakthroughs in recent decades, once oil or gas is discovered, a production platform will replace the offshore drilling rig, which is assembled at the location using heavy lift cranes situated on a barge. These production platforms come in a variety of sizes, shapes, and types, which depend on the dimensions of field, the depth of the water and how far the oil and/or gas field is from shore. Usually the platforms are typically located in 150 to 350 feet of water, up to 10 miles offshore. These platforms are generally made of steel and fixed in the seabed with steel "post". These platforms harbor the entire well and can house a maximum of 80 workers. In addition platforms have concrete structure large enough to store oil, with gravity keeping them situated on the seabed.
Weather is a major factor in platform operation. When high sea and hurricanes approach a platform the crew must evacuate to prevent injury or loss of life. Thus safety is an important factor as far as an oilrig is concerned. Oil and gas production must continue on an uninterrupted manner to ensure profitability. To further this end, labor-intensive, repetitive task such as opening and closing valves, monitoring levels, and shut down of platform to prevent equipment damage are handed over to programmable logic controllers (PLC's).
To know about the control action of PLC the knowledge of its hard ware, its operation and ladder diagram is important.
2. THE PROGRAMMABLE LOGIC CONTROLLER (PLC)
A PLC is a special form of microprocessor-based controller that uses a programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting, and arithmetic in order to control machines and processes. When the PLC is programmed, the controller then monitors the inputs and outputs according to this program by turning on\off its output.
PLC's have the great advantage that the same basic controllers can be used with the wide range of control systems. To modify a control system and the rules that are to be used, all that is necessary is to change the program. There is no need to rewire. PLC's are similar to computers but whereas computers are optimized for calculation and display task, PLC's are optimized for control tasks and industrial environment.
The primary reason for using PLC's is to eliminating the large cost involved in replacing the complicated relay based machine control systems. Such a control system is called a relay sequencer or relay logic panel.
In earlier days of industrial control process, discrete state control where provided by physical relays to put together a circuit that satisfies the requirement of a ladder diagram. The ladder diagram technique of describing discrete state control systems originated from relay logic system that is why the diagram contains so many relay related terms and symbols. The ladder diagram is still used because it has evolved into an efficient method of defining the event sequences required in a discrete state control system. It is important to realize that, relay control each rung of the ladder is evaluated simultaneously and continuously, because the switches and relays are all hard wired to ac power. If any switch in ladder change state the consequence is immediate this is not true in the case of computer based programmable controllers.
The main reason for eliminating relays where due to its increase in cost involved in replacing the complicated relay based machine control systems. Since relays are mechanical devices they had fewer lifetimes. Troubleshooting is also difficult when so many relays where used. When a relay panel has been wired to implement a ladder diagram for a special purpose, if the event sequence is to be changed it is necessary to rewire all, or part of the panel.