Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Investigation and analysis of BOG generation during LNG ship to ship transfer operation
Fangqiu Li Jiakun Fan Hao Cheng Liang Yang Xinxin Li
CNOOC Gas &Power Group
Beijing，China

solutions to reduce BOG loss during STS transfer operation andsuggestions of BOG Re-Liquefied Installation for the selected LNG
carrierABSTRACT
The loading process of small LNG carrier in onshore LNG terminal will
be charged a processing fee, which affects the economy of the small LNG
carrier transportation. The shipping cost can be reduced by ship to shiptransfer in the Yangtze River Estuary. However, a large amount of BOG
gas would be generated because of the heat leak of the LNG carrier andthe energy input during the loading process. If it is not handled properly,
it will cause a huge amount of BOG relief which lead to LNG loss andpollution. To achieve carbon peak and neutrality goals, it is important torealize the zero BOG emission and the re-liquefied of BOG during STS
transfer operation. In this paper, the relationship between BOG
generation and operating conditions is investigated, the influence law ofvarious parameters on BOG generation of the loading process iscalculated, and the optimistic parameters of operation conditions and thesuggestion of BOG Re-Liquefied Installation are proposed for theselected LNG carrier.

PARAMETERS ON BOG GENERATION DURING STSTRANSFER
Control Factors of BOG Generation

The BOG generation during STS transfer operation is the difference
between the amount of BOG generated and the amount of BOG
consumed in the whole transfer operation. The main reasons causing
BOG generated include heat leakage of system, heat input during
operation and cabin cooling (Jinxiao and Xiukang, 2019). The BOG isconsumed mainly by the re-liquefaction of the undercooled LNG and by
burning as a fuel of the LNG carrier.

Heat leakage of SystemSince the temperature of LNG is much lower than the ambienttemperature, the LNG carrier absorbs heat from the environment duringtransportation, and then produces natural evaporation of LNG in thecabin. In the loading and unloading process, the low temperature LNG
pipeline will also produce a certain amount of heat leakage. Therefore,
the heat leakage of the system during the transfer operation can bedivided into the heat leakage of low-temperature pipeline system and theheat leakage of the LNG carrier. The heat leakage of the pipeline systemshould not be higher than 25W/m2 as required. The heat leakage of LNG
carriers is calculated according to the daily evaporation rate, and themain control factors are cabin type and cabin volume.
Heat Input during Operation(1) Heat input of LNG unloading pump. The energy provided by theLNG offloading pump is mainly used to overcome the resistance loss
along the offloading pipeline system and the level difference between thetwo carriers. The energy other than overcoming the resistance loss along
the path and the effective work done by the pump is converted into
internal energy of LNG, thus generating the evaporation of BOG.
Therefore, the heat input of the LNG offloading pump is an importantfactor for BOG generated in the transfer operation. The main control
factors of this heat input are loading rate, cabin volume and volume

KEY WORDS: LNG carrier; STS (ship-to-ship) transfer; BOGgeneration; BOG Re-Liquefied Installation.
INTRODUCTION
During LNG ship-to-ship transfer operation, the generation of BOG willlead to cabin pressure rise. If not handled in time, the superfluous BOG
could only be discharged into a special combustion device forcombustion, resulting in unnecessary loss of LNG products andenvironmental pollution, and will increase carbon emissions in theoperation of the STS transfer. Therefore, it is significant to study theamount of BOG generated during LNG STS transfer operation and put
forward corresponding control measures and recovery methods
according to the influence rule of the amount of BOG generated, so as toreduce the generation of BOG during LNG STS transfer operation(Endao, et.al, 2021).
This paper focuses on the study of BOG generation in the transfer
operation of different LNG ships, analyzes the main influencing factorsof BOG generation in the transfer operation, and proposes reasonable

831