https://www.bulgartransgaz.bg/en/news/saobshtenie-za-avariya...
Message for failure in PIG station
Today at 16.15 pm during preparation works for implementation of scheduled inspection on the transit gas pipeline to Greece a blast has occurred at Pigging station Stryama, close to villages of Kalekovec and Stryama.
In the accident were killed two workers, and another two are injured.
All necessary measured have been undertaken to ensure the safety in the area. There is no fire. The people of the settlement nearby are not directly treated.
On the site is the director of Bulgartransgaz Mr. Kiril Temelkov, who is in constant communication with Minister Traicho Traikov.
At the time being ongoing are clarifications for the causes of accident.
The gas flow along the transit gas pipeline has not been discontinued.

https://en.wikipedia.org/wiki/Pigging
Pigging
In pipeline transportation, pigging is the practice of using devices known as pigs or scrapers to perform various maintenance operations. This is done without stopping the flow of the product in the pipeline. These devices are known as pigs because they scrape or clean just like a normal pig.

These operations include but are not limited to cleaning and inspecting the pipeline. This is accomplished by inserting the pig into a "pig launcher" (or "launching station") — an oversized section in the pipeline, reducing to the normal diameter. The launching station is then closed and the pressure-driven flow of the product in the pipeline is used to push the pig along down the pipe until it reaches the receiving trap — the "pig catcher" (or "receiving station").

https://earthworks.org/issues/pigging_stations/
The oil and gas industry inspects and cleans pipelines by “pigging.”
The name originates from either Pipeline Inspection Gadget, or the squealing sound pigging tools make as they pass through the pipeline.
“Pigs” are made of different durable materials, and more advanced “smart pigs” are essentially robots capable of detecting leaks and corrosion.