Clearing of Cargo Lines

The lines used for cargo operations must be cleared and the pressure released in order to prevent any unwanted reaction or contamination of the cargo. Nitrogen is one of the mediums used for clearing the cargo lines, a pipeline pig might be used additionally with the chosen medium. Line clearance should be carried out after the following operations:

• Upon completion of loading.
• After any internal transfer of cargo.
• After cargo recirculation when part of the line is used.
• After a partial discharge from a tank.
• As required by MARPOL (International Convention for the Prevention of Pollution from Ships) upon completion of discharge.

Procedure

When line clearing is being carried out, the following procedures are followed:

• At all times, a person in charge will be supervising the operation.
• An agreement on the procedures to be followed is made between the terminal representative and a record of the agreement is noted in the ship-to-shore checklist.
• All personnel involved in the operation should wear appropriate PPE (Personal Protective Equipment).
• The Master will take into account the list and trim of the vessel in order to ensure an efficient cargo line-clearing operation.
• The tank into which the cargo lines are to be cleared must have enough ullage in order to prevent overflow.
• A crew member will be assigned to remain at the manifold valve to control the pressure.
• The tank vent will be kept open in order to prevent over-pressurization of the tank.
• During the line clearance procedure, the tank vapor pressure must be continuously monitored. This is done to avoid over-pressurization.

Upon completion of cargo line clearing:

• All valves will be systematically closed, starting at the manifold.
• The pressure inside the lines will be checked to ensure that none is present. This will be done by closing the drop line valves last.
• The drain-cock outboard of the manifold valve will also be opened to ensure no pressure remains in the cargo lines.
• The need for further clearance of the cargo lines can also be ascertained by knocking the lines with a suitable instrument.

Precautions

During such procedures a number of precautions should be observed:

• A set of instructions must be issued with regard to the procedures and the safety issues to be observed during the clearing of cargo lines.
• The plan of line clearing must be followed appropriately and systematically, in order to ensure its effectiveness.
• Communication must be established between all parties in the line-clearing operations.
• Personal protective equipment (PPE) must be worn by all personnel involved in the cleaning operations.
• Personnel or crew who are not involved in the operation must be kept clear of the area.
• Risk Assessments must be carried out.
• The choice of clearance medium to be used must be considered carefully as choosing the wrong medium may affect both the safety of the ship and the cargo.

Having said that, there are three mediums that can be used for the clearing of cargo lines, these are:

• Compressed Air.
• Steam.
• Nitrogen.

Nitrogen is usually used for clearing operations in order to ensure maximum safety. Compressed air tends to contain water vapour or droplets which causes a static electricity hazard, whilst steam will introduce condensed water which will also cause a static electricity hazard. Additionally, when steam is used, the heat imposed during the procedure might cause an adverse effect on the pipelines or tank coating.