LNG Cargo Handling Systems and Their Operations

LNG cargo systems are critical components of the global energy infrastructure, ensuring the safe and efficient transport of liquefied natural gas. These systems involve complex processes, from the initial liquefaction of natural gas to its final delivery to end-users. Implementing safe operational procedures is paramount to mitigate risks and ensure the integrity of the LNG supply chain. This includes stringent protocols for handling, storage and transportation, designed to prevent accidents and environmental hazards.

Our comprehensive guide to safe operational procedures for LNG cargo systems provides detailed insights into best practices and regulatory requirements. It covers everything from pre-loading inspections and loading/unloading processes to continuous monitoring and emergency response plans. By adhering to these protocols, operators can enhance safety, improve efficiency and maintain compliance with international standards, ultimately contributing to a more reliable and secure LNG supply chain.

Cargo Handling System (Liquid and Vapor)

The cargo piping system is illustrated in a simplified drawing showing only two tanks and the main features of the system for a membrane ship, see Figure 1.

Figure 2 is a photograph of the cargo piping on the deck of a membrane type LNGC.

Figures 3 and 4 show the cargo piping and deck arrangement of a SPB new concept design.

It is evident that the two systems are quite similar and that there are no substantial differences of cargo systems depending on the type of Cargo containment system of gas vesselcargo containment system.

General Features of Cargo Systems

Liquid cargo is loaded and discharged by the two crossover lines at midships and is delivered to and from each cargo tank liquid dome through the main liquid line, which lays fore and aft along the deck. Each crossover line at midships separates into two loading/discharging connections, port and starboard, making a total of four loading/discharge connections on each side of the ship.

The cargo tank vapor domes are maintained in communication with each other by the vapor main running fore and aft along the trunk deck. The vapor main has a cross connection at the midship manifold for use in regulating tank pressures when loading and discharging.

During loading operation, the vapor main and crossover are used to return the displaced gas from the tanks back to the shore installation. This is obtained using the high duty compressors. When discharging, the vapor main is used in conjunction with either the vapor crossover, or a vaporizer, to supply gas to the tanks to replace the outgoing liquid cargo.

The stripping/spray line can be connected to the liquid crossover lines and can be used to drain or to cool down each cargo tank, and also to spray during discharging if the return vapor is insufficient. The spray line on each tank consists of spray assemblies just below the tank top, designed in such a way as to distribute the liquid through several spray nozzles so arranged to achieve a better cool-down rate.

The vapor main connects the gas domes to each other and permits venting any excess boil-off gas to atmosphere through vent mast riser No.1. Note that the practice of venting to the atmosphere, even though it is not harmful, is now restricted in most of the ports/countries and therefore venting to the atmosphere should be considered as an emergency practice, when all other system to dispose the boil-off are not available. The vapor main also directs the boil-off cargo vapor to the engine room for gas burning, using the low duty compressors and boil-off gas heaters.

The inert gas and dry-air system, located in the engine room, is used to supply inert gas or dry-air to the cargo tanks through a piping line, which is connected to the main cargo system through a double nonreturn valve to avoid gas returning to the engine room.

Cargo piping is of welded fabrication to the maximum extent practicable to construction practice to reduce the possibility of leakage from joints. Flanged connections are electrically bonded by means of straps provided between flanges to ensure that differences in potential, due to static electricity between cargo and other deck piping, tanks, valves and other equipment are neutralized.

Both liquid and vapor systems have been designed in such a way that expansion and contraction is absorbed within the piping configuration, by means of expansion loops and bellows on the liquid and vapor piping.

Fixed and sliding pipe supports and guides are provided to ensure that pipe stresses are kept within acceptable limits.

All sections of liquid piping that can be isolated, and thus possibly trapping liquid between closed valves, are provided with safety valves that relieve excess pressure to the nearest vapor dome. This is a safety measure, although normal working practice is to allow any remaining liquid to warm up and boil off before closing any such valves.

All major valves, such as the midships manifold (port and starboard) valves, also called ESDS (Emergency Shut Down System) manifold valves, individual tank loading and discharge valves and the BOG (Boil-off gas) valve to the engine room are remotely power operated from the cargo console, so that all normal cargo operations can be carried out from the Centralized Administration and Control Centre (CACC).

When an ESDS is activated, the manifold valves close, thus discontinuing loading or unloading operations.

A non-return valve is fitted at the discharge flange of each cargo pump. A small hole (about 5 mm) is drilled in the valve disc to allow the tank discharge lines to drain down and be gas freed. Non-return valves are also fitted at the discharge flange of the compressors. The spray/stripping and emergency cargo pump discharge lines have non-return valves located directly after the hydraulically operated discharge valves.

A small diameter spray nozzle is also fitted at the top of each cargo pump discharge line inside the tank to cool down the pump tower legs in order to maintain a cold temperature throughout the complete discharge.

Figure 5 shows a Essential Steps for Preparing LNG Tanks for Cargo Loading on LNG VesselsLNGC unloading cargo.

The figure shows how the ship is connected to the terminal loading arms.

Liquid Lines

The system is made by butt-welded stainless steel pipeline connecting each cargo tank to the loading/discharge manifolds at the ship-side through a common header. At each tank liquid dome a manifold connects the loading and discharge lines from the tank to allow for the loading and discharge of cargo.

The tank discharge lines from the cargo pumps, the loading line, the emergency pump well and spray line are connected to this manifold on the liquid dome or vapor dome.

In general, tanks N.2 and N.3 have the facility to fill the discharge line prior to starting the cargo pumps in order to prevent pressure surge.

Blank flanges and sample points are fitted in the liquid line so as to facilitate inerting and aeration of the system during refit.

All sections of the liquid line outside the cargo tanks are insulated with rigid polyurethane foam covered with a molded GRP cover to act as a tough water and vapor tight barrier.

Vapor Lines

The system is made of flanged stainless steel pipeline connecting each of the four cargo tanks by means to the shipside vapor manifold, the compressor house and the forward vent mast with a common line. The line to the compressor house allows the gas vapor to be used for:

• sending vapor ashore during cargo loading by means of the high duty compressors in order to control pressure in the cargo tanks;
• sending the boil-off gas to the engine room via the low duty compressors and heater for use as fuel in the boilers during ballast/loaded voyages;
• vaporizing the gas to purge-dry Independent Cargo Tanksthe cargo tanks during the repair periods.

The line to the forward vent acts as a safety valve to all tanks and is used to control the tank pressure during normal operations. Blank flanges and sample points are fitted in the vapor line to facilitate inerting and aeration of system during refit. All sections of the vapor line outside the cargo tanks are insulated with rigid polyurethane foam, covered with a molded GRP cover to act as a tough water and vapor tight barrier.

Spray Line

The system is made of a butt welded cryogenic stainless steel pipeline (typical diameter 2”) connecting the stripping/spray pump in each of the four cargo tanks to the spray main line and supplies liquid gas to the following piping systems/equipment:

• spray rails in each tank, to be used for tank cool-down and gas generation;
• main liquid line, to be used for cooling down lines prior to cargo operations;
• to discharge into liquid lines for cargo tanks N.2 and N.3 priming lines and so preventing line surge when starting main cargo pumps;
• vaporizers for gas generation to compressors and heaters.

Blank flanges and sample points are fitted in the spray line to facilitate the inerting and aeration of the system during refit. All sections of the spray line outside the cargo tanks are insulated with rigid polyurethane foam covered with a molded GRP cover to act as a tough water and vapor tight barrier.

Cargo Operations

All the operations relative to handling cargo typical of a LNG ship are shown on figures below.

Cargo Tank Venting and Relief Systems

Cargo Tank Relief and Vacuum Valves

Each cargo tank is fitted with two pressure/vacuum relief valves as required by the IGC Code. Each primary and secondary interbarrier space is protected by two pressure relief valves per cargo tank.

Cargo tank relief valves are fitted at the liquid domes of each tank and vent to their associated vent mast riser. These valves are pilot operated (PORV type) to provide the accuracy and sensitivity required for the relatively low pressures at which they must operate. A cargo tank pressure sensing device relays the pressure directly to the pilot operating valve, in this manner, accurate operation at low pressures prevailing inside the tank are assured.

Initially, the cargo relief valves are set by the manufacturer for the requirements on the ship. If overhaul of the valves by ship’s staff is carried out, the valves must be checked and reset to the original settings.

It is extremely important that the vent mast is drained of any accumulation of water. The purpose of this is to ensure that the relief valves operate at their correct settings, which would otherwise be altered if any water was to accumulate in the vent mast and flows onto the valve assembly, where it may freeze and renders the valve inoperable.

Insulation and Interbarrier Spaces Relief Valves

The insulation and interbarrier spaces are protected by four pressure oprated relief valves per cargo tank.

Both liquid dome and vapor dome have one relief valve connected to the interbarrier space that surrounds them.

A gas detection line is led out from below each of the valves to the gas monitoring system to give a constant indication of the atmosphere inside the interbarrier spaces.

Best Practices for Gas Tank Installation and Cargo Tank InsulationThe insulation space relief valve vapor outlet is led to a separate vent line, which runs up alongside the associated vent mast. This is in order to prevent any counter pressure or back flow from the main vent mast should the cargo tank relief valves lift, or from the nitrogen purge fire and smoothering system.

It is extremely important the vent line is drained of any accumulation of water, as indicated in the previous paragraph relative to cargo tank relief and vacuum valves.

The interbarrier space relief valves vent directly to deck, via a downward facing tail pipe. It is not necessary for these to be led to a mast riser as the likelihood of there being LNG vapor in the insulation space is very remote.

The insulation and interbarrier space valves are set up initially by the manufacturer for the requirements on the ship. If overhaul of the valves is carried out, the valves must be checked and reset to the original settings.

Figure 6 shows the scheme of the relief system.

Line Relief Valves

Each section of the cargo pipe lines that can be isolated by two valves has an overpressure relief valve fitted. The cargo manifold relief lines and the cargo machinery space relief lines release back to some liquid domes (generally N.2 and N.3 respectively).

Vent Line

During normal operations the pressure in the tanks is controlled by the use of the boil-off gas in the boilers as fuel, or controlled via the forward vent mast and the common vapor line (in emergency).

Each cargo tank is also fitted with an independent means of venting. This comprises of two lines protruding from the tank top into dedicated pilot operated relief valve. From here the gas passes through a main line into a vent stack where it is vented to atmosphere.

All vent stacks are protected by a nitrogen purge fire smothering system. Sample points are fitted in the vent line to facilitate the inerting and aeration of the system during refit.

Sections of the vent line outside the cargo tanks are insulated with rigid polyurethane foam covered with a molded GRP cover to act as a tough water and vapor tight barrier.

Emergency Vent Line (One Tank Operation)

The system is made of a flanged pipeline, which can be connected to the vapor line and the forward riser for use when “One Tank Operation” is required.

The use of this line enables a single tank to be isolated and repair work to be carried out without having to warm up and inert the whole vessel.

Connection to each individual tank is made by means of a portable flexible hose between blank flanges situated at each vapor dome on the vapor and emergency vent lines.

During single tank operations it is possible to connect the inert gas/dry air plant to the forward vent mast line is by means of a portable elbow bend.

In the unlikely event of a cargo spill into the ballast tanks, it is possible to connect the inert gas/dry air plant to the ballast system through the emergency vent line using flexible hoses and to purge the ballast tank. Blank flanges and sample points are fitted in the emergency vent line, to facilitate the inerting and aeration of the system during refit.

Interbarrier and Insulation Spaces Pressurization and Inerting System

This is the only system, which is particular of membrane LNGC. This system does not exist for independent tank LNGC, as they do not have any membrane and interbarrier spaces to inert. The main features of the system are shown in a simplified drawing showing only two tanks and the main features of the system on Figure 7.

General Plant Description

Nitrogen produced by generators and stored in a pressurized buffer tank is supplied to the pressurization headers through make-up regulating valves. From the headers, branches are led to Interbarrier Space Protection: Pressurization, Inertization and Scaffolding Techniquesthe interbarrier and insulation spaces of each tank.

Excess nitrogen is vented through regulating relief valves to the nitrogen vent mast on each tank from the interbarrier space and to deck from the insulation space.

Both insulation and interbarrier spaces of each tank are provided with pressure relief valves, which open at a pressure above atmospheric sensed in each space of 3,0 kPa for the interbarrier space and 3,5 kPa for the insulation space. A manual bypass with a globe valve is provided for local venting and sweeping of a space if required.

The nitrogen production plant is maintained in an automatic mode. In general there are two packages, being each one of the two able to maintain the pressure in the buffer tank owing to the small demands placed upon the system. When a high nitrogen demand is detected, the second package will start automatically.

Insulation and Interbarrier Spaces

The inlet and outlet control valves for both spaces at each cargo tank are operated under split range control by the output of the reverse acting pressure controller for that space. Thus, when the pressure in that space falls below the desired value, the inlet valve opens and the outlet valve remains shut.

When the pressure in the space rises above the desired value, the outlet valve opens and the inlet value remains shut.

The barrier space header control valve reacts to the demand on the system and maintains the header pressure at 50 kPa. A flow meter upstream of the valve gives an indication of the current demand on the nitrogen. Pressure switches on the nitrogen buffer tank control the cut-in/cut-out of the compressors via control panel. Under normal operation, one compressor is selected as run, with the second compressor on automatic standby cut-in.

High/low and differential pressure alarms are fitted to the pressure control systems for each interbarrier space.

Nitrogen Production System

Nitrogen generators (in general two), installed in the engine room, produce gaseous nitrogen, which is used for the inerting and maintaining pressure of the interbarrier and insulation spaces; as seal gas for the high duty and low duty compressors, fire extinguishing in the vent mast risers and for purging of various parts of the cargo piping.

The two high capacity units (90 m³/h each is a typical value for ships ranging between 135 000 and 150 000 m³ capacity), are able to produce the nitrogen, which is mainly required for the topping up of the barrier insulation spaces during loading, cooldown and other services, like vent mast fire extinguishing and compressor sealing.

Figure 8 indicates a typical scheme of nitrogen production system.

The operating principle is based on the hollow fiber membranes through, which compressed air flows and is separated into oxygen and nitrogen. The oxygen is vented to the atmosphere via the engine funnel and the nitrogen stored in a buffer tank ready for use.

Oxygen Analyzer

An oxygen analyzer, after the membrane, monitors the oxygen content and, if out of range redirects the flow to the funnel. In general, the oxygen content is limited in the range of 1 %, however although GTT recommends the oxygen does not exceed 3 %, limit values up to 5 % might still be considered acceptable.

System Operation

Figure above shows the complete pressurization diagram for a typical cargo containment system. Figures 9 through 12 the same system where the above described operations are shown by color code.

Pipes indicated in black in the diagrams are to be considered void.

Inert Gas and Dry Air System

Main Features of the System

The inert gas/dry air plant, installed in the engine room, produces dry air or inert gas, which is used for the tank and piping treatments prior and after a dry-docking or an inspection period. The plant is operated locally or from the CACC, with mimic used to monitor the system.

The operating principle is based on the combustion of a low sulfur content fuel and the cleaning and drying of the exhaust gases.

A typical inert gas plant includes an inert gas generator, a scrubbing tower unit, two centrifugal fans, an effluent water seal, a fuel injection unit, an intermediate dryer unit (refrigeration type), a final dryer unit (adsorption type) and instrumentation/control system.

The connection to the cargo piping system is made through two non-return valves and a spectacle blank, which is in the normally closed position, and the connection to the cargo compressor room is made through a removable bend (not normally connected).
Figure 13 shows a typical diagram of an inert gas plant.

Working Principle

Inert gas is produced by the combustion of light diesel oil supplied by the fuel pump with air provided by blowers, in the combustion chamber of the inert gas generator. Good combustion is essential for the production of a good quality, soot free, low oxygen inert gas.

The products of the combustion are mainly carbon dioxide, water and small quantities of oxygen, carbon monoxide, sulfur oxides and hydrogen. The nitrogen content is generally unchanged during the combustion process and the inert gas produced consists mainly of 85 % nitrogen and 15 % carbon dioxide.

Initially, the hot combustion gases produced are cooled indirectly in the combustion chamber by contact with the sea water jacket. Thereafter, cooling of the gases mainly occurs in the scrubber section of the generator where the sulfur oxides are washed out. The seawater for the inert gas generator is supplied by one of the ballast pumps via ballast main isolating valve. The seawater flow into the scrubber section is adjusted by an apposite valve.

Before delivery out of the inert gas generator, water droplets and trapped moisture are separated from the inert gases by a mist separator. Further removal of water occurs in the intermediate dryer stage, where the refrigeration unit cools the gas to a temperature of about 5 °C.

The bulk of the water in the gas condenses and is drained away with the gas leaving this stage by a mist separator. In the final stage, the water is removed by an absorption process in a dual vessel desiccant dryer. The desiccant dryer units work, in sequence, on an automatic change over cycle, where the offline desiccant unit is first reactivated with hot dry air, which has gone through the reactivation dryer process.

A pressure control valve located at the outlet of the dryer unit maintains a constant pressure throughout the system, thus ensuring a stable flame at the generator.

Dew point and oxygen content of the produced inert gas are continuously monitored. The oxygen level controls the ratio of the air/fuel mixture supplied to the burner. The oxygen content must be below 1 % by volume and the dew point up to a maximum of -65 °C with a minimum of -55 °C. Both parameters are displayed locally and remotely through the integrated automation system.

For delivery of inert gas to the cargo system, two combined remote air-operated control valves operated through solenoid valves are fitted on the distribution system, i.e., the purge valve and the delivery valve.

Dry Air Production

The inert gas generator can produce dry-air instead of inert gas with the same capacity. However, for the production of dry-air:

• there is no combustion in generator;
• there is no measure of oxygen content;
• the oxygen signal is overridden when the mode selector is on dry-air production;
• after the processes of cooling and drying, and, if the dew point is correct, the dry air is supplied to the cargo system through the delivery valve (with the purge valve closed).

Inerting/Dry Air Lines

The system is comprised of a flanged line, which supplies inert gas/dry air to the cargo tanks and pipelines for inerting and drying during refit periods. The inert gas/dry air is supplied from the inert gas plant situated in the engine room.

The line is connected to the emergency vent line and the liquid line by means of portable elbow bends. By selective use of the bends and flexible hoses it is possible to inert/aerate all or a single cargo tank.

The cargo machinery room can also be flooded with inert gas/air by swinging the spectacle flange on the line leading to this space.

Air Drying and Inerting Operations

All the operations typical of a LNG ship are shown on Figures 14 through 36. In particular all operations relative to inerting the tanks are shown in this set of figures.

Cargo System Operations

The following figures 14 through 36 show all the operations, which are particular to a gas carrier. They are originated from GTT drawings relative to a 137 000 m³ capacity membrane LNG ship and have been elaborated using color codes to highlight the various operations in logical order. The pipes indicated in black in the diagrams are to be considered empty (not operating in that particular operation).

As the operations of membrane pressurization have been dealt with separately in the previous part , the content of the following diagrams may be considered valid also for Quality control of cargo handling work in LNG carriersLNG carriers with independent cargo tanks.

Author

Olga Nesvetailova

Freelancer

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