JP7828838B2 - Liquefied gas carrier - Google Patents

Description

This disclosure relates to a liquefied gas carrier.

Liquefied gas carriers are equipped with storage tanks for storing liquefied gas. Within these storage tanks, the liquefied gas naturally vaporizes, generating boil-off gas. This boil-off gas can be effectively utilized as energy for the propulsion system. For example, a propulsion system has been devised that generates steam by burning the boil-off gas in a boiler, and then uses this steam to rotate a propeller (see Patent Document 1 below).

In such propulsion systems, a preheater may be installed within the boiler system. A preheater is a device that facilitates heat exchange between the exhaust gas discharged from the boiler and the air supplied to the boiler. Using this preheater allows for the recovery of heat from the exhaust gas, thereby improving the efficiency of the boiler system.

Japanese Patent Publication No. 2017-109738

Incidentally, under certain conditions, such as high ambient temperatures, boil-off gas may be generated in excess. Since releasing boil-off gas overboard is regulated, the question arises as to how to handle the excess boil-off gas. Therefore, this disclosure aims to provide a liquefied gas carrier capable of appropriately handling excess boil-off gas.

A liquefied gas carrier according to one aspect of this disclosure comprises a hull propelled by rotating a propeller, a storage tank for storing liquefied gas, a boiler system for generating steam, a steam turbine for converting the energy of the steam generated by the boiler system into power and transmitting the converted power to the propeller, and a control device. The boiler system includes a boiler that mixes boil-off gas generated in the storage tank with air and burns it, generating steam from the heat of combustion, a preheater for warming the air supplied to the boiler with the heat of exhaust gas discharged from the boiler, and at least one of an air bypass channel that supplies air to the boiler by bypassing the preheater, and an exhaust gas bypass channel that discharges the exhaust gas overboard by bypassing the preheater. The control device increases at least one of the amount of air bypassed through the air bypass channel and the amount of exhaust gas bypassed through the exhaust gas bypass channel when the actual speed of the hull is greater than a preset target speed.

This configuration allows for the proper disposal of excess boil-off gas.

Figure 1 is a schematic diagram of a liquefied gas carrier. Figure 2 is a flowchart of the detour amount adjustment program.

(Overall structure)
The liquefied gas carrier 100 according to the embodiment will be described below. First, the overall configuration of the liquefied gas carrier 100 according to the embodiment will be described. Figure 1 is a schematic diagram of the liquefied gas carrier 100. As shown in Figure 1, the liquefied gas carrier 100 comprises a hull 10, a storage tank 20, a boiler system 30, a steam turbine 40, and a control device 50. These components will be described in order below.

The hull 10 is equipped with a propeller 11, and the hull 10 is propelled by rotating the propeller 11. The hull 10 includes a control panel 12 and a speedometer 13. The control panel 12 is a device for the crew to perform various operations and allows for various settings to be made. In this embodiment, the control panel 12 allows the speed (target speed) of the hull 10 to be set arbitrarily. The speedometer 13 is a measuring instrument that measures the actual speed (actual speed) of the hull 10.

The storage tank 20 is a tank for storing liquefied gas. The liquefied gas carrier 100 in this embodiment is equipped with two storage tanks 20. However, the number of storage tanks 20 equipped in the liquefied gas carrier 100 is not limited. Furthermore, the shape and type of the storage tanks 20 are not limited. The liquefied gas stored in the storage tanks 20 may be, for example, liquefied petroleum gas (LPG), liquefied natural gas (LNG), or liquefied hydrogen.

Within the storage tank 20, boil-off gas is naturally generated from the vaporization of liquefied gas. This boil-off gas is supplied to the boiler system 30 via the boil-off gas supply channel 21. The amount of boil-off gas naturally generated within the storage tank 20 varies depending on the ambient temperature and other conditions. Therefore, the amount of boil-off gas supplied to the boiler system 30 also fluctuates depending on these conditions.

The boiler system 30 is a system that generates steam. The liquefied gas carrier 100 in this embodiment is equipped with two boiler systems 30. However, the number of boiler systems 30 equipped in the liquefied gas carrier 100 is not limited. The two boiler systems 30 have the same structure and each includes a boiler 31, a preheater 32, an air bypass passage 33, and an exhaust gas bypass passage 34.

The boiler 31 is a device that generates steam by mixing boil-off gas with air and burning it, using the heat from the combustion. Boil-off gas is supplied to the boiler 31 via the boil-off gas supply channel 21, and air (outside air) is supplied via the air supply channel 35. The exhaust gas generated by the combustion of the boil-off gas is discharged from the boiler 31 to the outside of the ship via the exhaust gas discharge channel 36. Furthermore, water tubes 37 are located inside the boiler 31. Water is supplied to the water tubes 37, and as the supplied water passes through the water tubes 37, it is heated by the heat generated by the combustion of the boil-off gas and turns into steam.

The preheater 32 is a device that heats the air supplied to the boiler 31 using the heat from the exhaust gas discharged from the boiler 31. The preheater 32 is located on the air supply channel 35 and the exhaust gas discharge channel 36. As the exhaust gas flowing through the exhaust gas discharge channel 36 passes through the preheater 32, it supplies heat to the air flowing through the air supply channel 35. This recovers the heat from the exhaust gas, improving the efficiency of the boiler system 30.

The air bypass passage 33 is a passage that supplies air to the boiler 31, bypassing the preheater 32. The air bypass passage 33 is positioned to connect the portion of the air supply passage 35 upstream of the preheater 32 with the portion downstream of the preheater 32. An air bypass flow rate adjustment damper 38 is provided in the air bypass passage 33. By changing the opening of this air bypass flow rate adjustment damper 38, the flow rate of air supplied to the boiler 31, bypassing the preheater 32 (hereinafter referred to as "air bypass flow rate") can be adjusted.

The exhaust gas bypass passage 34 is a passage that bypasses the preheater 32 and discharges the exhaust gas generated by the boiler 31 overboard. The exhaust gas bypass passage 34 is positioned to connect the upstream and downstream portions of the exhaust gas discharge passage 36. The exhaust gas bypass passage 34 is equipped with an exhaust gas bypass volume adjustment damper 39. By changing the opening of this exhaust gas bypass volume adjustment damper 39, the flow rate of exhaust gas bypassing the preheater 32 and discharging it overboard (hereinafter referred to as "exhaust gas bypass volume") can be adjusted.

The steam turbine 40 is a device that converts the energy of the steam generated in the boiler system 30 into power and transmits the converted power to the propeller 11. Steam is supplied to the steam turbine 40 from the boilers 31 of each boiler system 30 via the steam supply channel 41. The torque of the propeller 11 fluctuates according to the energy of the steam supplied to the steam turbine 40, and consequently, the speed of the ship's hull 10 fluctuates.

The control device 50 is a device that controls the various equipment installed in the liquefied gas carrier 100. The control device 50 includes a processor, volatile memory, non-volatile memory, and an I/O interface. The non-volatile memory of the control device 50 stores the bypass amount adjustment program (described later) and various data, and the processor performs calculations using the volatile memory based on each program.

The control device 50 in this embodiment is electrically connected to the control panel 12 and the speedometer 13, and can acquire the target speed and actual speed of the hull 10 based on signals received from these devices. Furthermore, the control device 50 is electrically connected to the air bypass amount adjustment damper 38 and the exhaust gas bypass amount adjustment damper 39, and can adjust the air bypass amount and exhaust gas bypass amount by transmitting control signals to these devices.

The functions of the elements disclosed herein can be performed using circuits or processing circuits, including general-purpose processors, dedicated processors, integrated circuits, ASICs (Application Specific Integrated Circuits), conventional circuits, and/or combinations thereof, configured or programmed to perform the disclosed functions. A processor is considered a processing circuit or circuit because it includes transistors and other circuits. In this disclosure, a circuit, unit, or means is hardware that performs the enumerated functions, or hardware programmed to perform the enumerated functions. The hardware may be hardware disclosed herein, or other known hardware programmed or configured to perform the enumerated functions. If the hardware is a processor, which is considered a type of circuit, the circuit, means, or unit is a combination of hardware and software, and the software is used to configure the hardware and/or the processor.

(Detour adjustment program)
Next, the bypass amount adjustment program will be explained. The bypass amount adjustment program is a program that adjusts the amount of air bypassed and the amount of exhaust gas bypassed, and is executed by the control device 50. Here, we will explain using one of the two boiler systems 30 as the control target. However, in reality, the control device 50 controls both boiler systems 30. Figure 2 is a flowchart of the bypass amount adjustment program. Also, at the start of execution of the bypass amount adjustment program, the amount of air bypassed and the amount of exhaust gas bypassed are assumed to be greater than zero. In other words, it is assumed that a certain amount of air and exhaust gas are flowing through the air bypass passage 33 and the exhaust gas bypass passage 34, respectively.

As shown in Figure 2, when the bypass amount adjustment program is started, the control device 50 first determines whether or not other boiler systems 30 (boiler systems 30 different from the boiler system 30 being controlled) are stopped (step S1). The stopping of a boiler system 30 includes both intentional and unintentional stopping. Based on the combustion temperature inside the boiler 31, the exhaust gas temperature discharged from the boiler 31, the temperature of the steam supplied from the boiler 31 to the steam turbine 40, etc., it is possible to determine whether or not a boiler system 30 is stopped.

In step S1, if it is determined that another boiler system 30 is stopped (NO in step S1), the control device 50 reduces the air bypass amount and the exhaust gas bypass amount (hereinafter collectively referred to simply as "bypass amount") to adjust the bypass amount to a predetermined minimum value (step S2). This allows for greater recovery of heat from the exhaust gas discharged from the boiler 31, improving the efficiency of the boiler system 30. The minimum value may be zero or greater than zero.

If the other boiler system 30 is stopped, steam will be generated only by the controlled boiler system 30, resulting in a decrease in the overall energy of the steam supplied to the steam turbine 40. Therefore, in this embodiment, the amount of bypassing in the controlled boiler system 30 is reduced in such cases to improve its efficiency and suppress the decrease in the energy of the steam supplied to the steam turbine 40.

In contrast, if it is determined in step S1 that the other boiler system 30 is not stopped (YES in step S1), the process proceeds to step S3. In step S3, the control device 50 acquires the target speed and actual speed of the hull 10. The control device 50 can acquire the target speed from the control panel 12 and the actual speed from the speedometer 13. The target speed is pre-set before the detour adjustment program is executed.

Next, the control device 50 compares the target speed obtained in step S3 with the actual speed to determine whether the actual speed is greater than the target speed (step S4). If it determines that the actual speed is greater than the target speed (YES in step S4), the control device 50 increases the detour amount. This adjusts the efficiency of the boiler system 30 and suppresses the increase in actual speed.

If the amount of boil-off gas generated increases due to factors such as a rise in ambient temperature, the amount of boil-off gas supplied to the boiler system 30 will also increase accordingly. As a result, the energy of the steam supplied to the steam turbine 40 will also increase, causing the actual speed of the hull 10 to exceed the target speed. In this case, reducing the boil-off gas supply would be a possible way to suppress the actual speed of the hull 10, but this would result in excess boil-off gas. Therefore, in this embodiment, the excess boil-off gas is effectively handled by increasing the bypass amount and adjusting the efficiency of the boiler system 30 (by increasing the boil-off gas consumption).

On the other hand, in step S4, if it is determined that the actual speed is not greater than the target speed, that is, that the actual speed is less than or equal to the target speed (NO in step S4), the control device 50 reduces or maintains the detour amount (step S6). Specifically, if it is determined that the actual speed is less than the target speed, the control device 50 reduces the detour amount. As a result, the efficiency of the boiler system 30 improves, and the actual speed increases. If the actual speed still falls below the target speed even after reducing the detour amount, the liquefied gas in the storage tank 20 may be forcibly vaporized to generate boil-off gas, and the amount of boil-off gas supplied to the boiler system 30 may be increased. On the other hand, if it is determined that the actual speed is equal to the target speed, the control device 50 maintains the detour amount without changing it. After going through steps S1, S5, and S6 described above, the process returns to step S1 and each step is repeated.

(Variant)
The liquefied gas carrier 100 according to this embodiment has been described above, but the configuration of the liquefied gas carrier 100 is not limited to that described above. For example, the boiler system 30 described above includes both an air bypass passage 33 and an exhaust gas bypass passage 34, but it may include only one of these passages. In this case, in the bypass amount adjustment program, the control device 50 only needs to adjust one of the air bypass amount and the exhaust gas bypass amount. Also, even if the boiler system 30 includes both an air bypass passage 33 and an exhaust gas bypass passage 34, the control device 50 may adjust only one of the air bypass amount and the exhaust gas bypass amount. Specifically, instead of decreasing both the air bypass amount and the exhaust gas bypass amount, only one of them may be decreased. Also, instead of increasing both the air bypass amount and the exhaust gas bypass amount, only one of them may be increased.

Furthermore, the liquefied gas carrier 100 may be equipped with a condenser that returns a portion of the steam generated by the boiler system 30 back into water instead of supplying it to the steam turbine 40. In this case, when the actual speed of the hull 10 exceeds the target speed, the detour amount may be increased, and the amount of steam returned to water by the condenser may also be increased. By increasing the amount of steam returned to water in this way, the energy of the steam supplied to the steam turbine 40 (steam supply amount) decreases, and the actual speed of the hull 10 can be further reduced.

(summary)
The first item disclosed herein is a liquefied gas carrier comprising: a hull propelled by rotating a propeller; a storage tank for storing liquefied gas; a boiler system for generating steam; a steam turbine for converting the energy of the steam generated by the boiler system into power and transmitting the converted power to the propeller; and a control device, wherein the boiler system includes: a boiler that mixes boil-off gas generated in the storage tank with air and burns it to generate steam by the heat of combustion; a preheater that heats the air supplied to the boiler with the heat of the exhaust gas discharged from the boiler; an air bypass passage that supplies air to the boiler by bypassing the preheater; and at least one of an exhaust gas bypass passage that discharges the exhaust gas overboard by bypassing the preheater, wherein the control device increases at least one of the amount of air bypassed through the air bypass passage and the amount of exhaust gas bypassed through the exhaust gas bypass passage when the actual speed of the hull is greater than a preset target speed.

This configuration allows for the proper handling of excess boil-off gas by adjusting the boiler system's efficiency by increasing at least one of the air bypass rate and the exhaust gas bypass rate when excess boil-off gas is generated.

A second item disclosed herein comprises a hull propelled by rotating a propeller, a storage tank for storing liquefied gas, a plurality of boiler systems for generating steam, a steam turbine for converting the energy of the steam generated by the plurality of boiler systems into power and transmitting the converted power to the propeller, and a control device, wherein each of the plurality of boiler systems is a boiler that mixes the boil-off gas generated in the storage tank with air and burns it, generating steam from the heat of combustion, and exhaust gas discharged from the boiler A liquefied gas carrier comprising: a preheater that heats the air supplied to the boiler using heat; an air bypass channel that supplies air to the boiler by bypassing the preheater; and at least one of an exhaust gas bypass channel that discharges the exhaust gas overboard by bypassing the preheater; wherein the control device reduces at least one of the amount of air bypassed through the air bypass channel and the amount of exhaust gas bypassed through the exhaust gas bypass channel of the remaining boiler systems when some of the boiler systems stop.

This configuration allows for improved efficiency in the remaining boiler systems even if some of them shut down. This is achieved by reducing at least one of the air and exhaust gas bypass rates in the remaining boiler systems. As a result, the reduction in steam energy supplied to the steam turbine can be suppressed.

10 Hull 11 Propeller 20 Storage Tank 30 Boiler System 31 Boiler 32 Preheater 33 Air Bypass Channel 34 Exhaust Gas Bypass Channel 40 Steam Turbine 41 Steam Supply Channel 50 Control Device 100 Liquefied Gas Carrier

Claims (2)

A ship's hull that is propelled by rotating a propeller,
A storage tank for storing liquefied gas,
A boiler system that generates steam,
A steam turbine that converts the energy of the steam generated by the boiler system into power and transmits the converted power to the propeller,
A control device is provided,
The boiler system is
A boiler that mixes the boil-off gas generated in the aforementioned storage tank with air and burns it, generating steam from the heat of combustion,
A preheater that heats the air supplied to the boiler using the heat of the exhaust gas discharged from the boiler,
It includes at least one of the following: an air bypass passage that supplies air to the boiler by bypassing the preheater, and an exhaust gas bypass passage that discharges the exhaust gas overboard by bypassing the preheater.
A liquefied gas carrier wherein the control device increases at least one of the amount of air bypassed through the air bypass passage and the amount of exhaust gas bypassed through the exhaust gas bypass passage when the actual speed of the hull is greater than a preset target speed. A ship's hull that is propelled by rotating a propeller,
A storage tank for storing liquefied gas,
Multiple boiler systems that generate steam,
A steam turbine that converts the energy of the steam generated by the aforementioned multiple boiler systems into power and transmits the converted power to the propeller,
A control device is provided,
Each of the boiler systems in the aforementioned plurality of boiler systems is
A boiler that mixes the boil-off gas generated in the aforementioned storage tank with air and burns it, generating steam from the heat of combustion,
A preheater that heats the air supplied to the boiler using the heat of the exhaust gas discharged from the boiler,
The vessel comprises at least one of the following: an air bypass passage that supplies air to the boiler by bypassing the preheater, and an exhaust gas bypass passage that discharges the exhaust gas overboard by bypassing the preheater.
A liquefied gas carrier wherein, when some of the boiler systems among the plurality of boiler systems are stopped, the control device reduces at least one of the amount of air bypassed through the air bypass passage of the remaining boiler systems and the amount of exhaust gas bypassed through the exhaust gas bypass passage.