Liquefied Gas Carrier

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Cargo handling and monitoring equipments for Liquefied Gas carriers

Liquefied gas carriers fitted with cargo, booster, ballast and stripping pumps, lines, eductors and their associated instrumentation and controls should be in good order and evidence of regular testing should be kept. Instrumentation, valves and pipe work should be clearly marked to indicate their service and where applicable the compartment to which they relate.

Equipment used for Cargo Operations, varies with the type of vessel and its age. Some of the equipment which may be found on board is listed below. Other items of equipment will be found on board and the instruction manuals for the equipment and Ship Specific Operating Manuals should be consulted for details.
  1. Inert Gas and Dry Air Generator
  2. Nitrogen Generator
  3. Cargo / Spray pumps
  4. Compressor ( High & Low Duty)
  5. Cargo Heaters (High and Low Duty)
  6. LNG Vaporizer
  7. Forcing Vaporizer
  8. Mist Separator
  9. Vacuum Pumps
  10. Fixed Gas detection Systems
  11. Emergency shutdown system (ESD) and Cargo Tank Protection System
  12. Ship Shore Link
  13. Relief Systems
  14. Cargo Tank Gauging Systems

Inert gas, dry air & Nitrogen generator

Many LNG vessels are equipped with an inert gas generator which may also be used to produce dry air. The inert gas and/or dry air is used for the inerting and gas freeing of cargo tanks, cargo pipes and void spaces when required prior to and after a refit or inspection period.

Details of Inert gas, dry air & Nitrogen generator

Cargo & spray pumps

LNG ships are typically fitted with submerged, electric, centrifugal cargo pumps. The motor windings are cooled by the pumped LNG which also serves to lubricate and cool the pump and motor bearings. As the LNG serves as both lubricant and coolant, it is critically important that the pumps are never allowed to run dry, even for short periods.

In addition to main cargo pumps, each tank will also be served by a spray pump. This pump is of limited capacity, typically around 50m3/hr, and will be used for the following:
  1. To cool down the liquid header prior to discharging.
  2. To cool the cargo tank during a ballast voyage prior to arrival at the loading terminal by discharging LNG to the spray nozzles in the tanks.
  3. In exceptional circumstances, to pump LNG from the tanks to the vapourisers when forced vaporisation of LNG to the boilers is required.
  4. To enable the tanks to be stripped as dry as possible for reasons such as tank entry.

In the case of total cargo pump failure, provision is made for Moss ships to discharge under pressure.

On LNG vessels where cargo pumps are 440V supply, insulation test are to be carried out before arrival in both the loading port and discharge ports. Also during insulation test, air temperature and humidity must be recorded.

The insulation tests will establish that all pumps are operational and to allow time for the preparation of emergency pumping arrangements should it be necessary.

On newer LNG vessels, the insulation test should be done before pump removal and after pump’s re-installation in the tank during DD period and it is not necessary to take readings monthly. However if cargo tanks are in a gas free condition and pumps have not been operated for some considerable time, readings should be taken before the pumps are brought into operation again. The preferred time may be when LNG is being loaded.

Reference should be made to on board documentation for procedures for starting, stopping and operating cargo and spray pumps, together with the specific arrangements for rigging emergency cargo pumps.

Compressor ( High & low duty)

High duty (HD) compressors are installed in the compressor room on deck and are routinely used for compressing the LNG vapour for return to shore during cargo tank initial cool down, cargo loading, tank purging and to circulate heated cargo vapour through the tanks during warming up. Low duty (LD) compressors are installed in the compressor room on deck and are routinely used for compressing the LNG vapour produced by natural boil-off to a sufficient pressure to be used in the boilers as fuel.

The HD and LD compressors are normally driven by electric motors or steam, installed in an electric motor room segregated from the compressor room by a gas tight bulkhead. The drive shafts penetrate the bulkhead with a gas tight shaft seal.

Cargo heater ( High & low duty)
Steam heated cargo heaters are provided for the following functions:
  1. Heating the LNG vapour, delivered by the HD compressors, to the specified temperature for warming up the cargo tanks before gas freeing.
  2. Heating the boil-off gas, delivered by the LD compressors, or by free-flow, prior to supplying it to the boilers or venting to atmosphere.
The heaters are typically heat exchangers of the shell and tube type. The number of plugged tubes in cargo condensers, heaters or vaporizers should not exceed 25%.

LNG Vaporizer
The LNG vaporiser is a shell and tube type heat exchanger that is used for vaporising LNG liquid for the following operations: Forcing vaporizer
The forcing vaporiser is used for vaporising LNG liquid to provide gas for burning in the boilers to supplement the natural boil-off. The LNG is supplied by a stripping/spray pump. LNG flow is controlled by an automatic inlet feed valve which receives its signal from the Boiler Gas Management System.

Each forcing vaporiser is equipped with a temperature control system to obtain a constant and stable discharge temperature for various ranges of operation. The temperature of the gas produced is adjusted by spraying a certain amount of bypassed liquid into the outlet side of the vaporiser through a temperature control valve and liquid injection nozzles.

Mist separator
The mist separator shall prevent liquid from entering the compressors. It receives natural boil off from the cargo tanks and forced boil off gas from the forcing vaporizer.

Vacuum pumps
On GT96 membrane cargo containment vessels, the vacuum pumps where fitted are used to evacuate the atmosphere within the primary and secondary spaces in the following cases:
  1. To replace air with nitrogen for inerting.
  2. To replace methane with nitrogen for gas freeing before dry docking after there has been a leakage of cargo.
  3. To test the tightness of the membranes at regular intervals or after membrane repairs
  4. When the associated tank is opened up.
  5. It also helps pull the tank membrane on to the associated supports and insulation when the cargo tank is not pressurised.

Care must be taken to ensure that the pressure within the primary space is not reduced below that in the secondary space as there is a danger of distorting the secondary barrier by lifting it off its supporting insulation. A maximum pressure difference of 3kPa should not be exceeded.

Fixed gas detection systems
There are two types of gas detection system commonly used on board LNG carriers, a sampling system and a gas detection system incorporating remote heads.
The sampling system draws gas samples from each monitored location into a central analyser located in a ‘safe’ area. Typically, samples will be drawn from cargo areas in a pre-programmed sampling sequence and will be passed through an infrared analyser. The system alarms if pre-set limits are exceeded.

Remote detector heads may also be used to monitor gas concentrations. The signal from flameproof infrared gas detectors will be passed to a central control unit having visual and audible alarm functions.

Emergency shut down ( ESD) and cargo tank protection
The emergency shutdown (ESD) system is a requirement of the IMO code for the carriage of liquefied gases in bulk and is a recommendation of SIGTTO. It is fitted to protect both the ship and terminal in the event of power loss, cryogenic or fire risks, on either the ship or in the terminal. The system will stop the flow of LNG liquid and vapour by shutting down the pumps and gas compressors as well as manifold and shipside valves, by the activation of a single control. Shut down of the cargo system can be initiated either manually or automatically if certain off-limit conditions occur.....

Details of cargo emergency shutdown procedure

Ship shore link
Linked ship/shore emergency shut down systems have been recommended by SIGGTO since the early days of LNG transportation and are now mandated by IMO. The ship and terminal emergency systems are linked via a ship-shore umbilical that carries ESD, telecommunications and data signals.

Relief systems
As required by IMO, each cargo tank is fitted with two pressure/vacuum relief valves. In addition, on membrane ships, the primary and secondary insulation spaces around each tank are protected by two pressure relief valves. On Moss Rosenberg ships, hold spaces around each tank are similarly protected.

The cargo tank relief valves vent to their associated vent mast riser. The valves are of the pilot operated relief valve type. A cargo tank pressure sensing line relays the pressure directly to the pilot operating valve. In this manner, accurate operation is assured at the low pressures prevailing inside the tank.

It is extremely important that the vent mast is checked at regular intervals and drained of any accumulation of water. This is to ensure that the relief valves operate at their correct settings which would otherwise be altered if any water were to accumulate in the vent mast and flow onto the valve assembly.

In addition to the cargo tank and hold or interbarrier space relief valves, each section of the cargo pipework that can be isolated by two valves will be fitted with an overpressure relief valve. Arrangements for safely relieving pressure in the lines to the cargo tanks will vary from ship to ship.

Cargo tank gauging systems
All gauging systems used are specifically designed for the extreme low temperatures experienced on LNG carriers.

Various systems may be fitted to a vessel dependent upon the owner’s specifications and cargo containment system. There will generally be at least two independent gauging systems fitted to each tank, in addition to low, high and high-high level alarms

FLOAT ACTUATED GAUGES – these employ a float connected by an invar tape to a tensator spring. This spring acts as a counter balance system, maintaining a constant tape tension at the float. This ensures that the float maintains the same level of immersion irrespective of the amount and weight of the tape paid out. The accuracy of this system is dependent upon tank construction and on the operating conditions, however the accuracy should remain within 1 cm.

CAPACITANCE TYPE GAUGES – these gauges operate using the variation of electrical capacitance between two probes when a liquid level changes. A coaxial sensor is installed within a tank, and is constructed of a number of individual segments, depending upon the height of the tank. As the liquid level in the tank changes, the capacitance varies.

RADAR TYPE GAUGES – these gauges operate by generating and transmitting radar waves from a generating device mounted externally on the tank. As the speed of the radar waves is known, if the time needed by the signal to reach the cargo liquid level, bounce back and be picked up by the antenna, can be measured accurately, the cargo ullage can be calculated.

ULTRASONIC TYPE GAUGES – these gauges operate in a similar manner to an echo sounder, where the time taken for a sound wave to be reflected back through the liquid is accurately measured, and then used to calculate the liquid level.

All cargo measuring systems in use are highly accurate, and form part of the Custody Transfer System, which is checked and verified by an independent organisation during vessel dry docking periods. A certificate of accuracy for the system will be issued.

Generally if any ship repairs are carried out on any gauging system, it will be necessary for the gauge to be re-calibrated and a new certificate issued.

The vessel will carry out and record comparison checks of the various gauging systems in use during each cargo operation, to enable the early detection of any problems with any of the systems. Where the completion of these tests reveals any significant errors the Company is to be advised immediately with a request for attention.

Cargo piping system
Cargo piping systems comprise of the following and to be kept in order :

Details of various cargo pipe lines

Cargo pump emergency shut down system
Pump alarms and trips, level alarms, etc., where fitted, should be tested regularly to ensure that they are functioning correctly, and the results of these tests should be recorded

Cargo and ballast system valves;To be kept in order

Cargo system innage gauges;To be kept in order

Remote and local temperature and pressure sensors and gauges Each cargo tank should be provided with at least two devices for indicating cargo temperatures, one placed at the bottom of the cargo tank and the second near the top of the tank below the highest allowable liquid level. The temperature indicating devices should be marked to show the lowest temperature for which the cargo tank has been approved by the administration. The vapour space of each cargo tank should be provided with a pressure gauge which should incorporate an indicator in the cargo control position.

Liquefied natural gas ship underway

Fig:Liquefied natural gas ship on sea passage

Cargo tank high level & overflow alarms;To be kept in order

Within 5 days of the ship’s estimated time of berthing, the following checks and tests shall be carried out, and the results recorded. These records are to be made available to the terminal upon request.

(1) Deck water spray line

(2) Water curtain

(3) Gas free condition of hold space

(4) Alarm function of fixed gas detection equipment

(5) Cargo gauging system and alarm set points.

(6) Operation of the emergency shutdown system(ESD) the permitted operation period for emergency shut-down equipment of up to 30 seconds

(7) Operation of cargo system remote control valves and their position indicating systems.

(8) Confirm Cargo transfer emergency stops fully operational and date of last test.

(9) Confirm tank high level and pressure alarms operational.

(10) Confirm that remotely operated manifold valves have been operated through a complete open/closed cycle, functioning and advise valve type(ball,gate, etc)and actual closing time. The corresponding records shall be produced by the master on the ship arrival at berth. Any defects or deficiencies must be reported to the terminal as an addendum to the Pre-Arrival information notice

(11) Deep well cargo pump and booster pump mechanical seals are free of oil leaks.

Related Information:

  1. Gas measuring equipments calibration procedure

  2. Cargo piping system

  3. Connection and disconnection of cargo hoses and hard arms

  4. Volatile nature of liquefied gases

  5. How to achieve maximum drainage of liquid during discharge

  6. The hazards of liquefied gases - Cargo information and safety factors

  7. Procedures for various cargo handling equipment onboard

  8. Personal protective equipments for people working onboard gas carriers

More Info pages

Procedure for cargo planning in Liquefied Gas Carriers

Details of various cargo handling equipment onboard

Cargo piping layout for LNG carriers

Procedure for commissioning the cargo system

Preparation for Cargo Transfer

Procedure for discussion prior cargo transfer

Procedure for loading Liquefied Gas Cargoes

Procedure for Cargo Conditioning in Liquefied Gas Carriers

Cargo Transfer between Vessels (STS Operation)

Procedure for segregation of Liquefied Gas Cargoes

Procedure for Stripping Liquefied Gas Cargoes

Procedure for Changing Liquefied Gas Cargoes

Displacing with Vapour of the Next Cargo (Purging)

Procedure for Water washing after Ammonia Cargoes

Details of various cargo handling equipment onboard

Custody Transfer Measurement (CTM) System

Records of the calibration of key cargo instrumentation, including temperature and pressure gauges

The high level alarm system

Manifold arrangements

Gas analyzing equipment

Safety equipment

Decontamination showers and an eye-wash

Emergency response & contingency plans

Cargo conditioning, reliquefaction and boil-off control requirement for a liquefied gas carrier

Cargo Containment Systems in Liquefied Gas Carriers

cargo emergency shutdown requirement

damage stability guideline for liquefied gas carriers

Various Cargo handling equipments onboard

Cargo hoses connection guideline

Documents accompanying a liquid gas cargo

How LNG transferred from shore to ships cargo tanks ?

Cargo operation guideline onboard a liquefied gas carrier

Cargo piping Systems in Liquefied Gas Carriers

cargo planning requirement

cargo and pumproom safety precautions

cargo stripping guideline

Emergency response for cargo system leaks

Emergency response for cargo tank rupture

Risk of overfilling of cargo tank during loading onboard a liquefied gas carrier

Preparation for cargo transfer

cargo transfer between vessels- safety guideline

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