A Comparative Analysis of Cargo Operations in FSRU and LNGC Modes

July 30, 2026 (Revised: August 7, 2026)

Author: Eyüp Serkan Doğruyol, Master Mariner

1. Introduction

The increase in global natural gas demand over the past two decades has driven significant transformations in LNG transportation. One of the most notable examples of this transformation is the emergence of dual-mode vessels. Some ships, originally built as conventional LNG carriers (LNGC), are also designed to operate as Floating Storage and Regasification Units (FSRU).

This article provides a comparative analysis of cargo operations in LNGC and FSRU modes for a dual-mode vessel equipped with a 170,000 m³ class GTT Mark III membrane tank system. The objective is to highlight the differences in operational procedures, equipment utilization, pressure and temperature management, and safety systems between the two modes.

2. LNGC Mode — Cargo Operations

LNGC mode is the configuration in which a vessel operates as a conventional LNG carrier. In this mode, the vessel loads LNG from a terminal, proceeds on a laden voyage to the destination port, and discharges the cargo. Operations are conducted in accordance with the IMO IGC Code and SIGTTO standards.

2.1 Cargo Preparation Operations

Before commencing cargo operations on an LNGC, the tanks and cargo lines must be prepared in a specific sequence. This process consists of three main stages:

Inerting: The removal of oxygen from the tank atmosphere. Nitrogen (N₂) or inert gas from the inert gas generator is typically used. Inerting continues until the oxygen level drops below 2% by volume. In GTT Mark III membrane tanks, the insulation space and interbarrier space must also be inerted separately.

Gassing-Up: The replacement of the inert atmosphere with LNG vapour. This process is critical for controlling the dew point inside the tank. Gassing-up is generally carried out in two stages: in the first stage, LNG vapour is introduced from the bottom of the tank; in the second stage, the LNG vaporiser is engaged to supply LNG vapour at approximately +20°C via the spray main connection.

Cooldown: Lowering the tank internal temperature to a level suitable for receiving LNG (between -130°C and -150°C). The cooling rate is controlled not to exceed 10°C per hour to protect the tank structural components from thermal shock. In GTT Mark III membrane systems, the temperature differential between the membrane and the insulation blocks is closely monitored during cooldown.

2.2 Loading Operation

Loading is carried out between the terminal and the vessel via the cargo manifold. LNG is transferred to the ship's tanks through cargo lines. Prior to loading, cargo lines are cooled down, the vapour return line connection is established, the Emergency Shutdown (ESD) system is tested, and the ship-to-terminal communication protocol is verified.

During loading, tank pressure, liquid level, and temperature are continuously monitored. The vapour return line allows BOG generated in the tanks to be returned to the terminal. In LNGC (Sea Going) mode, the maximum tank filling level is limited to 99% (extremely high level alarm, LAEH). In FSRU mode, this limit is reduced to 98.5%.

2.3 Laden Voyage — BOG Management

During a laden voyage, continuous natural boil-off (Boil-Off Gas — BOG) occurs due to the temperature difference between the tank interior and the external environment. Managing this BOG is one of the most critical aspects of LNGC operations.

The primary BOG management strategies employed are:

NBO as Fuel — LD Compressors to DFGEs: Natural Boil-Off (NBO) gas is compressed by the Low Duty (LD) compressors and supplied as fuel to the Dual Fuel Generator Engines (DFGEs) and auxiliary boilers in the engine room. This is the preferred method under normal steaming conditions. One LD compressor and one spray pump (for the spray cooler) are typically running to maintain a stable gas supply.

GCU Disposal: If the volume of BOG exceeds the fuel demand of the generating plant and auxiliary boilers, the excess gas is burnt in the Gas Combustion Unit (GCU). The GCU starts automatically when tank pressure rises above the GCU start limit (approximately 80 mbar in laden condition).

Forcing Vaporiser (FBO Generation): When the Natural Boil-Off is insufficient to maintain the required fuel gas supply (e.g., during low ambient temperature or high engine load), the Forcing Vaporiser is engaged to generate Forced Boil-Off (FBO) and supplement the gas supply.

Tank pressure is maintained through the Gas Management System (GMS), which controls the LD compressors, GCU, and forcing vaporiser. In LNGC mode, the GCU start limit is set at approximately 80 mbar (laden), and the tank pressure control set point is at approximately 70 mbar. The Pressure Safety Relief Valves are set at 350 mbar(g) in LNGC mode.

2.4 Discharge Operation

Discharge is the transfer of LNG from the ship's tanks to the terminal. It can be carried out in two configurations. The first is discharge with vapour return, where tank pressure is balanced through the vapour return line from the terminal. This method is preferred for large-volume discharges. The second is discharge without vapour return, which is applied when the terminal lacks a vapour return line. In this configuration, tank pressure is controlled by the vessel's own BOG management system using the LD compressors, GCU, or gas burning in DFGEs.

Pre-cooling of cargo lines and shore arms before discharge is a critical step. Cooling is performed first with the spray pump, then with the main cargo pump.

2.5 Tank Heating, Gas Freeing and Aerating

During maintenance or dry-docking periods, tanks must be heated, gas-freed, and aerated. Tank heating, or warming-up, involves vaporizing the remaining LNG to raise the tank temperature to ambient levels. The LD compressors and forcing vaporiser are used to circulate warm gas through the tanks. Gas freeing reduces the hydrocarbon gas concentration in the tank atmosphere to a level safe for entry, typically below 1% of the Lower Explosive Limit (LEL), using the inert gas system. Aerating introduces fresh air into the tank using the inert gas system in dry-air mode to raise the oxygen level to 21% by volume.

3. FSRU Mode — Regasification and Gas Send-Out

FSRU mode is the configuration in which the vessel operates as a floating storage and regasification terminal. In this mode, the vessel receives LNG via ship-to-ship transfer, regasifies it in the regasification plant, and sends high-pressure natural gas ashore.

3.1 Transition to FSRU Mode

The transition from LNGC (Sea Going) mode to FSRU mode requires significant system configuration changes. Cargo pumps and lines are redirected to the regasification circuit. The regasification plant, including the suction drum, booster pumps, vaporisers, and NG heaters, is brought online. The high-pressure gas manifold is prepared for operation. The Pressure Safety Relief Valves are adjusted from 350 mbar(g) to 700 mbar(g) by fitting auxiliary setters to each pair of cargo tank relief valves. The mode selection switch on the Critical Alarm and Action Panel (CAAP) in the CCR is switched from "SEA GOING (LNGC)" to "FSRU" mode. Finally, the BOG management strategy is reconfigured to include the suction drum recondenser.

3.2 LNG Loading (STS Transfer)

In FSRU mode, LNG is transferred via Ship-to-Ship (STS) operation from a supplying LNGC. This operation differs from terminal loading in LNGC mode in several ways. Both the liquid and vapour manifolds of the FSRU are utilized simultaneously. The FSRU's high-pressure (HP) manifold is reserved for gas send-out. The mooring arrangement for STS operations is specially planned based on wind and current conditions. The ESD system is coordinated between both the LNGC and the FSRU. Due to the higher tank pressure on the FSRU, loading can be performed with vapour being free-flowed to the LNGC supply vessel through the vapour main.

3.3 Regasification System

The regasification system consists of a series of equipment that heats LNG from approximately -160°C to natural gas temperature and delivers it as high-pressure gas. The system is designed for a send-out rate of 50–1,000 mmscfd at a pressure range of 90–100 bar and a temperature of 5°C (based on 18°C sea water temperature). The plant consists of four identical vaporiser trains.

The Suction Drum, also called the Recondenser, is an intermediate vessel where LNG from the cargo tanks is collected via the regas feed pumps at approximately 4.0 bar(g). It acts as a buffer for the booster pumps and as a recondenser for Boil-Off Gas (BOG). Sub-cooled LNG enters at the top and flows downwards, co-current against the BOG, providing a large surface area for re-condensing. The mass flow rate relation between BOG and required LNG is approximately 1 to 15.

The LNG Booster Pumps raise the pressure of LNG from the suction drum to the vaporiser inlet pressure. Large booster pumps, one per train, are dedicated to each train, and two small booster pumps are used for minimum send-out rates of 50–100 mmscfd. The discharge pressure is within the 90–100 bar range.

The LNG Vaporiser is the primary equipment where LNG is heated and converted to the gas phase. It is of the BEU horizontal (shell and tube) type, using a glycol-water intermediate loop as the heating medium. Sea water heats the glycol water in a closed loop, preventing sea water from freezing under upset conditions. LNG is heated from approximately -160°C to approximately -30°C in the vaporiser.

The Natural Gas (NG) Heater is the final heating stage downstream of the LNG vaporiser, where the gas temperature is raised to the send-out condition. The exported natural gas temperature ranges from 5°C at 18°C sea water to 15°C at 32°C sea water. An alarm is generated if the send-out gas temperature falls to 3.5°C, and an ESD trip is activated at 3°C.

The Glycol Water Heating System is the auxiliary system that supplies the glycol-water mixture to the vaporisers and NG heaters. It consists of four glycol water heaters, four glycol water pumps, a glycol water expansion tank, and a storage tank, all located in the forward pump room. Four sea water pumps provide the primary heat source.

3.4 Minimum Send-Out and BOG Management

BOG management in FSRU mode differs significantly from LNGC mode. During periods of low grid demand, the regasification plant operates at minimum capacity (50–100 mmscfd). This is called Minimum Send-Out (MSO). At these rates, one of the two small LNG booster pumps is used for energy saving, without running the large booster pumps.

Excess BOG is managed using the Low Duty (LD) compressors. In FSRU mode, one or more LD compressors supply BOG via the after-coolers to the regas plant suction drum recondenser, where the BOG is recondensed back into LNG. Two LD compressors can be selected for recondenser duty in FSRU mode.

When the natural BOG is insufficient to maintain the desired tank pressure during high regas demand, the forcing vaporiser is used to provide additional BOG. The High Duty (HD) compressors are used for compressing LNG vapour for return to the supplying LNGC during STS loading operations, and for tank purging and warming-up.

Tank pressure control in FSRU mode is managed by the Gas Management System (GMS) through the recondenser, DFGEs, and GCU. The GCU start limit is set at approximately 330 mbar in FSRU mode, and the Pressure Safety Relief Valves are set at 700 mbar(g) with auxiliary setters fitted.

3.5 High-Pressure Gas Send-Out

Natural gas from the regasification plant is sent ashore via the High-Pressure (HP) Gas Manifold. The gas pressure is maintained at 90–100 bar, regulated by back-pressure control valves. The gas temperature ranges from +5°C to +15°C depending on sea water temperature, with an ESD trip activated at 3°C. The flow is continuous and uninterrupted, ranging from 50 to 1,000 mmscfd. A High Integrity Pressure Protection System (HIPPS) protects the export line, tripping at 90 bar(g). A gas chromatograph provides continuous quality monitoring, and custody transfer metering handles commercial measurement and billing.

4. Comparative Analysis

4.1 Cargo Handling Equipment Utilization Differences

The equipment used in LNGC and FSRU modes differs significantly in both function and application. Cargo pumps are used for discharge to the terminal in LNGC mode, but serve as regasification feed to the suction drum in FSRU mode. Spray pumps are used for spray cooling, cooldown, and tank warming in LNGC mode, while in FSRU mode they supply the mist separator and forcing vaporiser.

The LD compressors handle BOG to the DFGEs and GCU as fuel in LNGC mode. In FSRU mode, they additionally supply the suction drum recondenser for BOG recondensing. HD compressors are used for vapour return to the terminal during loading in LNGC mode, and for vapour return to the supplying LNGC during STS loading in FSRU mode.

The forcing vaporiser generates Forced Boil-Off when Natural Boil-Off is insufficient, in both modes. In LNGC mode this is to supplement DFGE fuel, while in FSRU mode it supports high regas demand. The LNG vaporiser is not used in LNGC mode but is active in FSRU mode with four trains of shell and tube type using glycol water. The suction drum and recondenser are not used in LNGC mode but serve as primary equipment for BOG recondensing and booster pump buffering in FSRU mode. The HP manifold is not used in LNGC mode but handles gas send-out to shore at 90-100 bar in FSRU mode. Finally, the relief valve setting is 350 mbar(g) in LNGC mode, increased to 700 mbar(g) with auxiliary setters in FSRU mode.

4.2 Pressure and Temperature Control Strategies

In LNGC mode, the tank pressure control set point is approximately 70 mbar in laden condition, with the GCU start limit set at approximately 80 mbar. The PAH (high alarm) is set at 300 mbar and the PAHH (high-high alarm) at 320 mbar. The Pressure Safety Relief Valve setting is 350 mbar(g). Temperature control is passive, relying on insulation and BOG management via LD compressors. The maximum filling level (LAEH) is 99%.

In FSRU mode, the GCU start limit is set at 330 mbar, with the PAH at 540 mbar and the PAHH at 670 mbar. The Pressure Safety Relief Valve setting is 700 mbar(g) with auxiliary setters fitted. Temperature control is active, using the regasification circuit with glycol-water heating. The maximum filling level (LAEH) is 98.5%.

The higher pressure relief valve setting in FSRU mode (700 mbar compared to 350 mbar in LNGC mode) accommodates the elevated operating pressures associated with the regasification and gas send-out process. The auxiliary setters are physically fitted to each pair of cargo tank relief valves, and the IAS is adjusted via the CAAP mode selection switch.

4.3 BOG Management Differences

BOG management differs between the two modes in several key aspects. In both modes, the BOG source is the same: natural boil-off from tank heat ingress. However, the primary handling method differs. In LNGC mode, LD compressors send BOG to the DFGEs as fuel gas or to the GCU. In FSRU mode, LD compressors additionally send BOG to the suction drum recondenser.

BOG recondensing is not available in LNGC mode but is a key feature of FSRU mode, where BOG is recondensed in the suction drum at a 1 to 15 LNG-to-BOG mass ratio. The forcing vaporiser supplements NBO when insufficient for DFGE fuel in LNGC mode, and when insufficient for regas demand in FSRU mode. The GCU start limit is approximately 80 mbar in laden LNGC mode, compared to approximately 330 mbar in FSRU mode. The HD compressor handles vapour return to the terminal during loading in LNGC mode, and vapour return to the supplying LNGC during STS loading in FSRU mode.

4.4 Safety Systems and ESD Philosophy

The Emergency Shutdown (ESD) system is a Kongsberg Maritime K-Chief ESDS system with dedicated safety software and hardware. The ESD system performs shutdown under two modes of vessel operations: Sea Going (LNGC) mode and FSRU mode. In each ESD level, certain inputs (causes) and outputs (effects) are activated or inhibited depending on the operating mode.

The ESD system is structured in a hierarchical shutdown architecture, from the highest to the lowest level. The highest level is the Abandon Vessel Shutdown (AVSD), which performs a total shutdown of all systems with time-delayed controlled shutdown to allow muster and evacuation. It is strictly manual activation by the Master. Systems remaining active after AVSD include emergency lights with 30 minutes of UPS capacity, the PA/GA system, and radio communication.

Below AVSD is the Emergency Shutdown High (ESDH), followed by the Emergency Shutdown Low (ESDL), which is triggered by confirmed gas or fire detection from fusible plugs and Fire Detection Systems in cargo compressor rooms, motor rooms, DF engine spaces, auxiliary boiler spaces, GCU spaces, and cargo tank domes. The Total Process Shutdown (TPSD) level shuts down all process systems and can be activated from ESDL or manually.

The Process Shutdown system is divided into several levels. PSD 4.1 protects the LNG storage and loading system against cargo tank overfilling and over or under-pressure, closing cargo manifold ESDVs and tripping spray pumps. PSD 4.2 isolates the BOG and fuel gas system, closing master gas valves to engines and regas boilers. PSD 4.3 isolates the regasification plant and HP gas export system, and is active only in FSRU mode while inhibited in LNGC mode. PSD 4.3.1 through 4.3.4 provide individual regas train shutdown based on vaporiser temperatures, blowdown valve status, and equipment trips.

Each ESD level activates all subordinate levels in a cascade. The ESD system is powered from two UPS supplies with a minimum 30-minute capacity after loss of power. In FSRU mode, the PSD 4.3 level (regas plant and HP export) is activated, providing shutdown protection for the suction drum, booster pumps, vaporisers, glycol water system, and HP gas manifold. This level is inhibited in LNGC mode. The ESD system thus uses the same hardware and software architecture in both modes, with mode-specific cause-and-effect matrix entries activated or inhibited depending on the operating mode.

Fire and gas detection inputs include confirmed gas detection in cargo compressor rooms, motor rooms, DF engine spaces, auxiliary boiler spaces, GCU spaces, and glycol water system areas. Confirmed fire detection is provided via fusible plugs at cargo tank domes and cargo manifolds, and Fire Detection Systems (FDS) in DF engine spaces and GVU rooms. Emergency Shutdown Valves (ESDV) are installed at cargo manifolds and the HP gas manifold. A High Integrity Pressure Protection System (HIPPS) protects the HP gas export line, tripping at 90 bar(g).

4.5 Operational Flexibility and Advantages

The greatest advantage of a dual-mode vessel is operational flexibility. LNGC mode offers full compatibility with conventional LNG trade, access to all terminals worldwide, and well-established standard operating procedures. FSRU mode provides the advantage of requiring no coastal terminal infrastructure, faster commissioning with a shorter lead time compared to conventional FSRUs, the ability to switch modes based on seasonal demand, and investment cost optimization by having one vessel serve two roles.

Common challenges include system reconfiguration during mode transitions, particularly for relief valve auxiliary setters and IAS mode switching. Personnel training requires competency in both modes. Maintenance planning must be optimized for two modes of operation, and classification society and flag state approval processes must be managed for both configurations.

5. Conclusion

Dual-mode LNG vessels represent an innovative solution developed to adapt to the changing dynamics of the global energy market. As discussed in this article, the operational differences between LNGC and FSRU modes manifest across many areas, from equipment utilization to pressure management, and from BOG strategies to safety systems.

While LNGC mode offers standardized procedures based on over half a century of accumulated experience, FSRU mode requires a more dynamic operational approach driven by the need for continuous and uninterrupted gas send-out. The transition between modes involves not only reconfiguring cargo and regas systems but also adjusting safety system parameters such as relief valve settings and ESD cause-and-effect matrices. Conducting successful operations in both modes demands comprehensive technical knowledge, meticulous planning, and an experienced crew.

The growing need for energy supply security, particularly in recent years, has increased demand for FSRUs, making dual-mode vessels increasingly attractive. In the future, the ability of these vessels to operate as both LNGCs and FSRUs will emerge as a significant competitive advantage in the energy market.

This article is based on SIGTTO publications, the IMO IGC Code, GTT Mark III membrane tank system documentation, vessel operating manuals, and field experience.