Preventing accidents when clearing onshore pipelines
The dangers of the pig and blowing the line
The procedures used to decontaminate onshore pipelines depend on the facilities available at each terminal and the type of cargo being loaded. It usually involves either line blowing or rigging (see fact box below). Both methods involve transporting large amounts of liquid or gas through pipelines. Due to the enormous volume being transferred, it is extremely important that this flow in the manifold is controlled using a manifold valve. Furthermore, good communication during the entire process is key to preventing accidents.
Most accidents resulting in structural damage to ship tanks result from either too much gas being supplied, too much cargo being forced into the tank, or the flow velocity being too high.
What is the puff and pig line?
Planning and responsibilities
All shipping operations must be carefully planned and documented well in advance. Details of the plans should be discussed with all personnel, both on board and at the station. The master must ensure that all participants are properly trained and fully aware of the challenges associated with pipeline disinfection operations. A pre-shipment meeting between the personnel responsible for the operation from the ship and the terminal should confirm all important interface parameters. These include:
Risks you should be aware of
At the beginning of loading, and at each watch or shift change, the responsible employee and the station representative must ensure that the loading control communication system is understood by the personnel assigned to their duties during the loading process. During operation there must be continuous and direct communication between the station and the vessel until operation is completed and all valves are closed.
In order to reduce the risk of overpressure:
- Avoid using tanks approximately 98% loaded as receiving tanks for line disinfection. Include a margin of safety when estimating the required size of receiving tanks to account for potential inaccuracies in the plant’s declared “pig quantities.”
- Consider including provisions for a reserve cargo tank which should be ready to open should the risk of overfilling arise.
- Keep manifold valves closed during idle periods. This will prevent accidental overpressure due to a shore fault.
- Ensure that the steam return line to shore is open during operation (when available).
- Monitor manifold pressure closely and throttle the main manifold valve as required. At the beginning of the saponification process, the valve must be opened to a minimum to control the flow and volume of liquid and gas propelled into the nearly full cargo tank. Gard strongly recommends that only trained and experienced personnel be assigned tasks related to the operation of multiple vessel valves.
- Monitor the available amount of free space in the cargo tank and the pressure in the tank. If the volume and/or pressure becomes too high, close the manifold valve, and alert the terminal to stop the draining process.
- During freezing weather conditions, check tank vents (P/V valves) at regular intervals. Some products will freeze and block the ventilation holes, creating pressure.
- Close the multiple valves immediately (in agreement with the station) as soon as the pig reaches the receiver/trap to avoid pressurized propellant gas entering the loaded cargo tank.
- When loading operations are complete, ensure the manifold valve is fully closed before beginning to clear the shoreline.
- Immediately report any abnormalities or deviations from normal procedures to the station.
In summary, there are several aspects that can reduce the risk of overpressure and overfilling during shore pipeline clearing: increased awareness, training, careful advance planning with station counterparts, and last but not least, good communication between ship and station personnel.
Further reading: https://www.gard.no/web/articles?documentId=32586511