A Comparative Analysis of Cargo Operations in FSRU and LNGC Modes

July 30, 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 four main stages:

Inerting: The removal of oxygen from the tank atmosphere. Nitrogen (N₂) or flue gas from an 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 regas boilers (or HD compressors) are engaged to establish circulation.

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 (liquid line cooldown)
The vapour return line connection is established
The Emergency Shutdown (ESD) system is tested
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. The maximum tank filling level is limited 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.

Three primary BOG management strategies are employed:

Gas Burning: BOG is used as fuel in the regas boilers in the engine room. This is the preferred method under normal steaming conditions.
Forced Gas Burning: When the amount of BOG exceeds boiler demand, the Forcing Vaporizer is engaged to burn the excess BOG.
Free-Flow to Regas Boilers: BOG is directed to the regas boilers through natural flow. This method reduces compressor usage, improving energy efficiency.

Tank pressure is maintained within the 50-70 mbar range through BOG management. If pressure rises, the Master Flow Control (MFC) valve regulates the flow.

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:

Discharge with Vapour Return: Tank pressure is balanced through the vapour return line from the terminal. This method is preferred for large-volume discharges.
Discharge without Vapour Return: Applied when the terminal lacks a vapour return line. Tank pressure is controlled by the vessel's own BOG management system.

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. This process includes:

Tank Heating: Vaporizing the remaining LNG to raise the tank temperature to ambient levels. Two methods are used: gas burning (combusting BOG in the boilers) or ventilation (releasing to atmosphere via the vent line).
Gas Freeing: Reducing the flammable gas concentration in the tank to below 1% of the Lower Explosive Limit (LEL).
Aerating: Introducing fresh air into the tank 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-shore transfer, regasifies it in the regasification plant, and sends high-pressure natural gas ashore.

3.1 Transition to FSRU Mode

The transition from LNGC mode to FSRU mode requires significant system configuration changes:

Cargo pumps and lines are redirected to the regasification circuit
The regasification plant (suction drum, booster pump, vaporizer, trim heater) is brought online
The high-pressure gas manifold is prepared for operation
The Minimum Send-Out (MSO) system is activated
The BOG management strategy is reconfigured, differing from LNGC mode

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:

Both the liquid and vapour manifolds of the FSRU are utilized
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

3.3 Regasification System

The regasification system consists of a series of equipment that heats LNG from -162°C to natural gas temperature and delivers it as high-pressure gas:

Suction Drum: An intermediate tank where LNG from the cargo tanks is collected. LNG from the regas feed pumps accumulates in the suction drum and supplies the booster pumps at a stable pressure.
LNG Booster Pump: Raises the pressure of LNG from the suction drum to the vaporizer inlet pressure. It typically operates at an 80-100 bar discharge pressure.
LNG Vaporizer: The primary equipment where LNG is heated and converted to the gas phase. In GTT Mark III systems, vaporizers are typically of the open rack (seawater circulation) or closed-loop glycol-water (shell and tube) type.
Natural Gas Trim Heater: The final heating stage where the gas temperature at the vaporizer outlet is precisely adjusted to the grid send-out temperature.
Heating Water System: The auxiliary system that supplies the hot water/glycol mixture to the vaporizer and trim heater.

3.4 Minimum Send-Out and BOG Management

BOG management in FSRU mode differs significantly from LNGC mode:

Minimum Send-Out (MSO): During periods of low grid demand, the regasification plant operates at minimum capacity. MSO is the lowest gas send-out level at which the system can continue operating without shutdown.
BOG Compressors: Excess BOG is managed using HD (High Pressure) and LD (Low Pressure) compressors. Unlike LNGC mode, a portion of the BOG can be recirculated back to the regasification circuit.
Tank Pressure Control: In FSRU mode, tank pressure is maintained within a narrower band compared to LNGC mode. Pressure control is managed automatically by the Gas Management System (GMS).

3.5 High-Pressure Gas Send-Out

Natural gas from the regasification plant is sent ashore via the High-Pressure (HP) Gas Manifold. This system features:

Gas pressure: 50-80 bar (depending on grid requirements)
Gas temperature: 0°C to +10°C
Gas flow: Continuous and uninterrupted
Gas chromatograph: Continuous quality monitoring
Gas metering: Commercial measurement and billing

4. Comparative Analysis

4.1 Cargo Handling Equipment Utilization Differences

EquipmentLNGC ModeFSRU ModeCargo pumpsUsed for dischargeRegasification feedSpray pumpsCooling, tank heatingTank pressure controlHD/GC CompressorBOG managementBOG management + regas circuitLD CompressorBOG transferBOG transferVaporizerPassive (Forcing Vaporizer)Active (main regas)Suction DrumNot usedPrimary equipmentHP ManifoldNot usedGas send-out

4.2 Pressure and Temperature Control Strategies

LNGC Mode:

Tank pressure: 50-70 mbar (wide band)
Temperature control: Passive (insulation + BOG management)
Pressure relief: Pilot Operated Safety Relief Valve (POSRV)

FSRU Mode:

Tank pressure: 30-50 mbar (narrow band, GMS controlled)
Temperature control: Active (regasification circuit)
Additional protection: Over-Pressure Valve (OPV) and Auxiliary Setter Unit (ASU)

The reason tank pressure is maintained within a narrower band in FSRU mode is that the regasification system requires a continuous and stable supply pressure.

4.3 BOG Management Differences

ParameterLNGC ModeFSRU ModeBOG sourceTank heat ingressTank heat ingress + regas returnBOG utilizationFuel (boiler/generator)Fuel + recirculation to regas circuitCompressor duty cycleIntermittentContinuousForcing VaporizerAs neededMinimum Send-Out support

4.4 Safety Systems and ESD Philosophy

In both modes, the Emergency Shutdown (ESD) system is configured in three levels:

ESD Level 1: Total shutdown (all cargo and regas operations stopped)
ESD Level 2: Partial shutdown (specific system isolated)
ESD Level 3: Local shutdown (single equipment stopped)

In FSRU mode, the ESD system includes additional protections beyond those in LNGC mode:

Gas detection system (for HP gas leaks)
Flame detectors (in the regas unit area)
Emergency Shut-Down Valves (ESDV)
Pressure Safety Valves (PSV) at each stage

4.5 Operational Flexibility and Advantages

The greatest advantage of a dual-mode vessel is operational flexibility:

LNGC Mode Advantages:

Full compatibility with conventional LNG trade
Access to all terminals worldwide
Well-established and standard operating procedures

FSRU Mode Advantages:

No coastal terminal infrastructure required
Faster commissioning (shorter lead time compared to conventional FSRUs)
Ability to switch modes based on seasonal demand
Investment cost optimization (one vessel, two roles)

Common Challenges:

System reconfiguration during mode transitions
Personnel training (competency in both modes)
Maintenance planning optimized for two modes
Classification society and flag state approval processes

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. 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, and field experience.