BOG Line - A simple lever to improve energy efficiency of PRS, FRS and A-PRS
At the forefront of cryogenic system technological advancement for LNG carriers, GTT continuously improves the thermal performance and reliability of its containment systems. To enhance the overall energy efficiency of vessels, GTT also focuses on optimising boil-off gas (BOG) conditions when supplying sub-coolers, reliquefaction units or full re-liquefaction systems (FRS), partial re-liquefaction systems (PRS) or advanced partial re-liquefaction systems (A-PRS).
To support this objective, GTT has developed a BOG supply line designed to maximise the operational efficiency of the BOG reliquefaction process. Beyond enhancing system performance, this innovation also simplifies the deck piping system, including the vapour headers.
Two Implementation Approaches
GTT has identified two ways to integrate the BOG supply line:
Option 1 - A fully dedicated BOG line. A single, optimised gas line, running from the LNG tank to reliquefaction units (such as PRS, FRS or A-PRS), ensures that BOG reaches the treatment system at the lowest temperature possible. The line is specifically designed with an optimal diameter, insulation thickness and piping features to minimise heat ingress and pressure losses.
Option 2 - Insulated Gas main line with reduced diameter. In this approach, the existing gas main, typically used for maintenance operations, is adapted to also route the BOG towards the reliquefaction system in an improved manner. Gas main insulation thickness and diameter have been revisited and the impact on maintenance operations has been properly assessed. This option offers further opportunities for vapour header simplification and CAPEX reduction.
Measurable Gains in Efficiency
Both configurations result in significant efficiency improvements:
- Colder BOG at reliquefaction units, FRS/PRS/A-PRS entry: up to 12°C colder according to estimates.
- Higher liquefaction capacity: e.g. 4-7% for FRS and 6-12% for PRS.
A Cost-Effective and Rapidly Amortised Investment
From a finanical perspective, both CAPEX and OPEX considerations indicate a return on investment within just a few years. GTT is committed to working closely with shipyards to identify the most suitable options tailored to the specific needs of LNG carriers. By collaborating with ship-owners and charterers on possible operating profiles, actual benefits have been consolidated, and the system is ready to be proposed.
