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Offshore
Oilrig Safety Using PLC
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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.
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