Outline of cargo discharging operations in liquefied gas carriers

Home page||| LNG handling ||| LPG handling||| Other Gas products||| Fire & Safety||| Emergency response |||

Outline of cargo discharging operations in liquefied gas carriers

Preparation for discharging= Carrying and handling liquefied gas cargo onboard poses significant potential hazards including risk of injury or death,threats to environment and each person working on a gas carrier and terminal ashore needs to understand the risks involved, obtain the necessary training and take all the needed precautions.

Cargo is transferred to the shore tank by using the submerged cargo pump installed at the bottom of each tank. On the other hand, the pressure of each tank is adjusted by sending, from the tank top via vapor lines, vapor supplied from the shore tank as much as the cargo quantity discharged. At some bases where return gas is not available, however, vessels should make hot vapor with compressors for the reliquefaction plant, introduce it into the liquid in the tank being discharged, and generate vapor thereby adjusting the tank pressure.

Operating Procedure

(1) Air purge and leak test :
Air is purged from the loading arms, and at the same time leaks from the loading arm connections are checked, in either of the following two methods after their connection, and it is confirmed that the oxygen concentration is less than 1%.
i) Nitrogen gas is injected at the roots of loading arms on the shore side and air is purged from manifold drain valves on the vessel and the concentration of oxygen is measured. By the injection of nitrogen gas pressure is applied on the manifold section and leaks from the connections are also checked.

ii) A drain hose of the shore side is connected to the manifold drain valve on the ship’s side and air is purged by making use of remnant pressure in the loading rm through the following route : loading arm => manifold => drain valve => drain hose => shore. The concentration of oxygen is measured on the shore by checking gas discharged from the drain hose. As the remnant pressure is used, the pressure is not so high to conduct a leak test.

iii) Air is pushed out from manifold drain valves on the ship’s side into the atmosphere by the remnant pressure in the loading arms. In this case flammable gas is discharged on to the upper deck, and due caution is necessary to remove ignition sources.

iv) By supplying inert gas into the manifold through an inert gas line and releasing gas into the atmosphere from drain valves, the vessel can purge air by herself.

(2) Cooldown of liquid lines and unloading arms: The cargo pump in the tank to be discharged and most remotely located from the cargo manifold is started, and the liquid is re-circulated with the filling valve of the open tank; and part of such cargo is sent to the manifold to cool down liquid lines gradually.

(3) Adjustment of tank pressure: Vessels should try to maintain the tank pressure at a constant value by using the return gas from the shore or vapor generated by sparging.

(4) Adjustment of trim and ballasting while discharging: Vessels should load water ballast according to plan, avoiding abnormal stresses imposed on the whole range of the hull, and paying attention to excessive trim and list.

(5) Start and stop of cargo pumps:

i) Check to see that the tank has sufficient cargo liquid. In case of the main pump: 1.5 m or more. In case of the auxiliary pump: 0.9 m or more.

ii) Keep closed the master valve (throttle valve) completely.

iii) Full open the filling valve.

iv) Open the discharge valve 1/4.

v) Start the pump.

vi) Open the discharge valve slowly and operate the pump at rated levels (electric current value, discharge pressure). Circulate cargo liquid during this process.

vii) Open the liquid master valve slowly, close the filling valve in the same manner, and start sending liquid to the shore.

viii) Adjust the pump to run at rated levels.

How to Stop cargo pump

a) Pinch down the discharge valve to 1/5.

b) Stop the pump.

c) Close relevant valves.

(6) Cargo discharge

The maximum discharging rate is the value when main pumps are operated on condition that propane and butane may be discharged at the same time.

Although the auxiliary pump is installed in each tank, it is not designed to be used together with main pumps.

The stripping operation is carried out by starting auxiliary pumps after main pumps are stopped. When starting the pump it is necessary to check to see that a sufficient liquid level (1 m or more) is secured.

As it is difficult to start the cargo pump again when it has tripped owing to a low liquid level, it is very important to pay attention to the discharge pressure and the ammeter when the level becomes low.

Related Information:

Cargo machinery room safety precautions

Toxicity and associated health hazards in liquefied gas carrier

Liquefied gas cargo handling equipment

Procedure for commissioning the cargo system

Preparation for Cargo Transfer

Procedure for discussion prior cargo transfer

Procedure for safe Cargo handling in Liquefied Gas Carriers

Safety checklist for Liquefied Gas Cargo handling

Tanker Cargo Operations Logbook

How to Connect Bonding Cable

Connection and disconnection of cargo hoses and hard arms

Procedure for loading Liquefied Gas Cargoes

Procedure for Cargo Conditioning in Liquefied Gas Carriers

Cargo Transfer between Vessels (STS Operation)

Procedure for Ballasting / De-Ballasting

Procedure for segregation of Liquefied Gas Cargoes

Procedure for Stripping Liquefied Gas Cargoes

Procedure for Changing Liquefied Gas Cargoes

Use of cargo as fuel -Cargo conditioning, reliquefaction and boil-off control for LNG carriers

Displacing with Vapour of the Next Cargo (Purging)

// Home page/// LNG handling /// LPG handling/// Sea transport /// Gas products///

Cargo work ///Fire precautions ///Health hazards ///Safety Precautions

///Emergency response ///

Copyright © Liquefied Gas Carrier.com All rights reserved.

The content published in this website are for general reference only. We have endeavoured to make the information as accurate as possible but cannot take responsibility for any errors. For latest information please visit www.imo.org . Any suggestions, please Contact us !

///Links &Resources // Terms of use/// Privacy policy///Home page///