Preventing accidents when clearing onshore pipelines

Incidents related to coastal pipeline clearing operations are on the rise. Overpressure or overfilling of tanks can cause material damage, contamination and even personal injury. This article provides an overview of best practices for reducing risk and preventing losses.

Over the past few months, Gard has seen a significant increase in incidents related to onshore pipeline clearing. Causes can vary: Excessive pressure in tanks can cause physical damage, and overfilling tanks can cause spillage, contamination and personal injury. At Gard, we have seen examples of all of this – including several “near-miss” cases over the past few years.

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? The purpose of line blowing and cutting is to clear and clean the pipeline as much as possible, to ensure that it is ready for the next process. This is important to avoid mixture of goods and contamination of goods. Pipelines are cleaned either by blowing the line or by hogs. Line blowing involves forcing compressed air or nitrogen through a pipeline to purify it. The swine process involves pushing an object, often a rubber ball or cylinder known as a “pig,” through a pipeline by means of a pressurized liquid or gas.

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

Parameters to be discussed in pre-shipment meeting

The pressure in the line rises Overpressure tank Significant increase in filling rate Cargo tank overflow due to excess cargo Cargo tank overflow due to ingress of compressed gas

The stages in which the line disinfection process will be carried out The notice period required by the vessel prior to line clearance operations Motivational means to use Cargo volume in onshore pipeline The time required for the pig to travel along the line Pressures and relief capacity of the ship’s receiving tank The volume of cargo remaining in the line and the amount of lean space available in the ship’s receiving tank Capacity of steam return line to shore Modifications to the shipping operating plan as a result of pipeline clearances, including quantities available for filling Communication procedures during the entire process

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.

Recommendations

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