Recommendations for LNG storage tanks and fuel gas supply system design for dual-fueled (LNG) vessels

Recommendations for LNG storage tank and fuel supply system design for dual-fueled (LNG) vessels:

Temperature Monitoring at Various Levels within LNG Storage Tanks: Install multiple temperature sensors within the tank to monitor temperature changes at different loading levels. This helps detect temperature discrepancies under various operational conditions.

Ensure Adequate BOG Handling Capacity: Design the fuel supply system to have sufficient capacity to handle both design and operational BOG. Operational BOG depends on the operating conditions of the vessel and may thus vary significantly. Neglecting operational BOG can lead to insufficient BOG handling capacity, increasing the probability of BOG dumping or venting.

Top Filling Line for Pressurized LNG Storage Tanks : Ensure the installation of a top filling line on pressurized LNG storage tanks to facilitate better fuel mixing and prevent LNG stratification. This assists in maintaining an optimal loading flow rate and helps control Boil-Off Gas (BOG) during bunkering.

Holding Time and Pressure Accumulation inside the LNG storage tanks : Ensure the LNG storage tank’s holding time meets the IGF requirements (15 days) and provides sufficient pressure accumulation capacity to cover realistic operational conditions. Consider redundancy and alternative means of pressure management to prevent unwanted emergency venting.

Run Realistic Holding Time Calculations for Different Scenarios: Understand that operating holding times can significantly differ from theoretical calculations due to LNG ageing, the liquid level in the tank, etc. Designers and vessel operators should collaboratively define conditions for different scenarios early in the design process.

Redundancy in BOG Pressure Management Equipment : Implement at least two fully independent active systems of BOG pressure control, especially for tanks with limited pressure accumulation capability, to ensure availability and reliability in managing BOG.

Free Flow BOG to Consumers: Utilize free flow BOG systems for efficient handling, especially in simpler systems or as a backup in more complex ones. Free flow of BOG means feeding consumers with excess gas by simply using the pressure difference between the tank and the consumer. Ensure the machinery operates in the full gas system flow and consider the installation of heat exchangers for the BOG temperature increase for consumption.

Emergency BOG Line: Install a dedicated emergency BOG line for specific tank types to maintain BOG consumption and control tank pressure in emergency scenarios, as required by IGF standards (paragraph

Select Appropriate Pressure Management Means: Choose active pressure management means for consumers based on their intended use, considering the entire pressure range, gas flow rate, and nitrogen content.

Crew-Operated Tank Pressure Control Valves: Install tank pressure control valves that can be controlled by the crew in emergency venting vents to prevent their uncontrolled opening/venting.

Read the IGF Code and SGMF Guidelines Carefully: Familiarize yourself with the IGF Code and the latest SGMF Guidelines on LNG Fuel Tanks – Pressure and Temperature Management Strategies for Gas Fueled Vessels.

Note: This document provides recommendations and not detailed technical specifications.

Source: IGF Code + SGMF Guidelines on LNG Fuel Tanks – Pressure and Temperature Management Strategies for Gas Fueled Vessels.