The use of pig launcher and receiver stations for pipeline cleaning, inspecting and maintenance is a method seen worldwide. The main benefit of this type of maintenance is that is can be carried out without interrupting the process in operation. The drawback is that it is also a high-risk event.

The vessel is equipped with a closure door to load or offload a pig. By leading the process through the launcher station, the pig will be launched and pushed through the pipeline to be received at the other end in the receiver station. Receiving or launching the pig involves a number of sequential valve operations in combination with opening and closing the closure door. One mistake in this operation could easily cause severe injury, even death to the operator.

Before opening the closure safely, there are several basic safety criteria that need to be met:

• The vessel must be drained and depressurized.
• The vessel must be isolated from the process pipeline by closing the Kicker lines and the main valves.
• In case of a closed drain systems and/or vent headers the vessel must also be isolated by closing the drain and vent valves.
• The vessel needs to be free from toxic gasses like H2S.
• At the same time the vessel must remain isolated when the door is open
• When the launching or receiving procedure starts the closure should be properly closed.

Throughout the years many accidents happened all over the world during this dangerous maintenance operation. Even though some larger valves are equipped with a MOV the process cannot be fully automated since there is manual labour involved in loading and offloading the pig. According to statistics, “70% of the reported incidents in the oil and gas industry worldwide are attributable by human error”. People can be trained to follow procedures and informed about potential danger but a mistake cannot be excluded.

Conclusion
Operating a pig launcher / receiver is a high-risk event with people involved and at the same time safe operation depends on these people. Mechanical interlocks take out the human factor of possible mistakes and guide the operator through a safe pre-determined sequence.
Several international companies recognized the potential danger and included or highly recommend interlocks in their design specifications.