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Over the last decade, numerous liquefaction trains have been commissioned to meet the increasing global demand for Liquefied Natural Gas (LNG). The LNG composition delivered from various terminals differs based on the characteristics of the gas field and the extraction processes used. While LNG typically contains a high proportion of methane, it may also include smaller quantities of heavier gases such as ethane, propane, butane, pentane, and, in some cases, nitrogen (N₂).

Although nitrogen content in LNG is generally below 0,5%, recent cases have shown levels exceeding 1%, with projections reaching up to 1,6% at certain terminals. This elevated nitrogen content in LNG presents several operational challenges:

  • Lower storage temperature requirements: LNG with high nitrogen content can reach temperatures as low as -165°C compared to the standard -163°C, necessitating an update to the LNG Carrier’s Certificate of Fitness.
  • Higher nitrogen vaporisation: Since nitrogen has a lower boiling point than methane, it vaporises more easily during the laden voyage, representing up to 30% of the total boil-off gas (BOG). As nitrogen is non-combustible, this reduces the energy content of BOG potentially making it unsuitable for engine combustion and resulting in higher fuel consumption.
  • Tank pressure management: the additional BOG generated by nitrogen vaporisation must be properly managed by Cargo Handling System (CHS) to avoid tank pressure increase.

Industry Support: Ensuring Safe and Efficient High-N₂ LNG Transport

GTT has prepared a comprehensive technical study demonstrating that both Mark III and NO96 technologies can safely operate at a temperature of -165°C. This study was reviewed by the four major classification societies in 2024, providing a solid technical foundation for LNG carriers operating under these conditions.

As the loading limit depends on the vessel design and the LNG composition, GTT has also worked with ship-owners to update the loading limit table calling at terminals supplying high N₂ LNG.

To ensure safe and efficient management of tank pressure during laden voyages, GTT has conducted dedicated BOG Management studies with the following objectives:

  • Analysis of the evolution of LNG composition and the amount & composition of BOG along the laden voyage.
  • Assessment of the required quantity of forced BOG top-up to meet the engine’s minimum Lower Heating Value (LHV).
  • Estimation of the excess gas and verifying that it can be handled by the Gas Combustion Unit (GCU).
  • Proposition for alternative BOG Management scenarii in order to ensure the onboard systems (compressors, subcooler, reliquefaction units, etc.) can accommodate the operational requirements.

These studies support the up-dating of the vessel’s Certificate of Fitness.

Tailored Solutions for Ship-owners and Operators

In addition, GTT provides tailored analyses to assist clients in adapting their fleet and operations to high nitrogen LNG cargoes:

  • Fleet analysis: Grouping vessels based on design caracteristics (containment system, capacity, CHS, and engine type) to evaluate suitability for high-N₂ LNG.
  • Tank pressure rise analysis: Evaluating pressure increases in LNG tanks when BOG cannot be fully managed by onboard gas supply system (BOG compressors) or consumers (engines, GCU, reliquefaction unit, auxiliary systems).
  • Voyage Data Analysis: Leveraging real operational data to optimise future voyages involving increased nitrogen content.
  • Operational guidance: Developping recommendations for cool down and loading procedures to better manage pressure rise at the start of the laden voyages.
  • Support for terminal and newbuild projects: GTT also assists the design of optimised CHS architectures for terminals handling high-N₂ LNG and advises on retrofit solutions to improve BOG management for existing vessels carrying High-N₂ cargoes.

Through these initiatives and a solid return of experience, GTT offers both standardised and customised solutions to help the LNG industry navigate the challenges associated with high nitrogen content LNG cargoes. By leveraging its expertise in LNG containment and BOG management, GTT ensures safe, efficient, and regulatory-compliant operations for ship-owners, operators, and terminal stakeholders.

 

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