Maksym Kulitsa, Independent FSRU Operations Consultant, Ukraine, and David Wood, DWA Energy Limited, UK, detail the advantages of keeping boil-off gas well balanced.
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oday’s tendency in LNG marine transport is to minimise fuel spent and to maximise cargo delivered to discharge ports with minimum environmental impact. New technologies have made significant advances in reducing engine fuel consumption. The introduction of dual-fuel diesel electric (DFDE), M-type electronically controlled gas injection (ME-GI), and other fuel-efficient marine engines on LNG carriers, as well as improved LNG cargo tank designs have contributed to this improvement. Total voyage consumption of LNG is not only dependent on engine efficiency; other voyage operations have an influence. Reducing the LNG heel required for tank cool down prior to arrival at a loading port reduces voyage LNG consumption. During cooling down at sea, excess boil-off gas (BOG) is consumed in the gas combustion unit (GCU) on DFDE or steam dump (SD) on steam vessels. Such consumption is wasteful and leads to atmospheric emissions. Minimising excess BOG therefore has commercial and environmental benefits. The mandatory Ship Energy Efficiency Management Plan (SEEMP) (MEPC, 2016), EU MRV (EU Monitoring, Reporting and Verification of CO2; mandatory since 1 January 2018) and IMO DCS (IMO Data Collection System on fuel consumption; mandatory since 1 January 2019)1,2 all focus on fuel and emissions reductions. The SEEMP provides incentives to adopt more efficient LNG carrier operational practices in parallel with developing more efficient engine and ship designs. Traditional procedures applied to cool down LNG carrier tanks prior to arrival at discharge ports are typically not best practice, because less LNG could be consumed for tank cool down. In some situations, liquid marine fuels are consumed to preserve sufficient LNG heel for tank cool down (e.g. during
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