A Study on the Optimization of BOG Handling for LNG-Fueled Ship under Various Bunkering Scenarios

Liquefied natural gas (LNG) as a marine fuel is considered as a realistic and feasible solution that complies with the stringent emissions regulation issued by International Maritime Organization (IMO). For LNG-fueled ships, the bunkering process of LNG and heavy fuel oil are completely different since the cryogenic liquid transfer generates a considerable amount of boil-off gas (BOG). In this study, the commercial software, Aspen HYSYS V10, for process design is used to investigate and analyze the optimization in the dynamic simulation on the BOG handling between the cargo tank of a bunking ship and bunker tank of a receiving ship (LNG fueled ship) under various bunkering scenarios. With respect to the modeling of the study, for the standard ship-to-ship (STS) and truck-to-ship (TTS) LNG bunkering methods, the diameter of the bunkering lines are set as 8 inch and 3 inch while that of the BOG return pipelines are set as 4 inch and 2 inch to satisfy the pressure of the receiving ship and BOG generation, respectively. The capacities of the cargo tank and fuel tank for bunkering and receiving ships are set as 4,538 m3 (70 m3) and 700 m3 (70 m3) for the STS and TTS LNG bunkering methods, respectively. The results indicated that the BOG amount with different LNG bunkering scenarios is variable. The BOG flow rate varies proportionally with the temperature difference, methane number and diameter of BOG/LNG pipe in case of temperature, methane number (MN) and pipe diameter disturbance and inversely with respect to the bunkering time limit after 20 min. in case of different bunkering time limits. Additionally, for the optimal BOG handling (STS bunkering method), it is necessary to control the bunkering time within 120 min. since additional BOG is generated when the capacity of the pump exceeds 100,000 kg/h. Meanwhile, when the diameter of the BOG line (DB) divide the diameter of the LNG line (DL) DB/DL = 0.5 is considered the best value both in the STS and TTS LNG bunkering methods, thus the tank pressure difference between bunkering and receiving ship may be reduced. It is believed that the results of the research could provide feasible assistance for STS and TTS LNG bunkering for the ports, and could give a specific guideline for the amount of the BOG generation and the standardized diameter of pipeline ratio.Contents ⅰ List of Tables ⅳ List of Figures ⅴ Abstract ⅷ Chapter 1 Introduction 1 1.1 Research Background 1 1.2 Dissertation Outline 5 Chapter 2 Overview of LNG Bunkering 7 2.1 LNG Bunkering Supply Chain 7 2.2 LNG Bunkering Method 9 2.3 LNG Tank and Bunkering Equipment 13 2.4 Literature Review 19 2.4.1 LNG Bunkering Technology and Safety 19 2.4.2 Current International Regulations, Standards, Class Rules and Guidelines 21 2.4.3 Challenges on BOG Handling 27 Chapter 3 Methodology & Process Design 30 3.1 Mathematical Model Classification 32 3.2 Dynamic Model of Main Bunkering Facilities and Equipments 38 3.2.1 LNG Tank Setup 38 3.2.2 Pipeline System Setup 45 3.2.3 Pump System Setup 49 3.3 Bunkering Process 52 3.4 Equation of State Selection & System Description 54 3.4.1 Equation of State Selection 54 3.4.2 System Description (STS & TTS Case) 56 3.5 Initial Conditions 57 3.6 Model Validation 60 Chapter 4 Results and Discussion 62 4.1 Disturbance of Temperature 62 4.2 Disturbance of Methane Number 69 4.3 Optimal BOG Generation in Different Bunkering Time Limits 74 4.4 Optimal BOG Generation in Different Pipe Diameter Ratio 86 Chapter 5 Conclusions 93 References 96Docto