JP2016124388A - Liquefied gas carrying vessel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquefied gas carrying vessel capable of setting a gas pressure of the downstream side of a compressor in a required range even when the load of an engine increases rapidly.SOLUTION: An LNG (liquefied natural gas) carrying vessel comprises: a tank for storing a liquefied gas; a compressor having variable discharge amount; an engine capable of using a gas as a fuel; a vaporized gas line guiding to the compressor, a gas in which the liquefied gas vaporizes naturally in the tank; a compressed gas supply line guiding the gas compressed by the compressor to the engine; a controller controlling the discharge amount of the compressor; and an input unit for switching a control mode of the controller to a steady control mode and a nonsteady control mode. The controller controls the discharge amount of the compressor when in the steady control mode so that the gas pressure of the compressed gas supply line becomes a first set pressure, and controls the discharge amount of the compressor when in the nonsteady control mode so that the gas pressure of the above compressed gas supply line becomes a second set pressure higher than the first one.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a liquefied gas carrier ship equipped with an engine that can use gas vaporized from liquefied gas as fuel.

  A liquefied natural gas (hereinafter referred to as “LNG”) carrier ship, which is one of the liquefied gas carrier ships, navigates with LNG contained in a tank. However, in a tank mounted on the LNG carrier ship, heat is input from the outside. A boil-off gas in which LNG is naturally vaporized is generated. Since the tank of the LNG carrier is large, a large amount of boil-off gas is generated. For this reason, an LNG carrier equipped with a gas engine that uses boil-off gas as fuel is known.

  For example, Patent Document 1 discloses an LNG carrier equipped with a gas supply system that compresses boil-off gas introduced from a tank during navigation with a compressor and supplies the compressed gas as fuel gas to an engine. The gas supply system mounted on the LNG carrier has a circulation line that branches from the downstream side of the compressor and joins the upstream side of the compressor, and a pressure regulating valve provided in the circulation line. In order to keep the gas pressure on the downstream side of the compressor within the required gas pressure range of the engine, the opening degree of the pressure regulating valve in the circulation line is controlled.

JP 2013-210148 A

  By the way, as a method for adjusting the gas pressure on the downstream side of the compressor, instead of controlling the opening degree of the pressure regulating valve provided in the circulation line, the rotational speed of the compressor is controlled and discharged from the compressor. It is also possible to adjust the amount of gas. However, since it is difficult to change the discharge amount of the compressor instantaneously, when the engine load suddenly increases, the pressure on the downstream side of the compressor may be out of the required gas pressure range of the engine.

  Therefore, an object of the present invention is to provide a liquefied gas carrier ship that keeps the gas pressure on the downstream side of the compressor within a required range even when the engine load suddenly increases.

  In order to solve the above problems, an LNG carrier according to the present invention includes a tank for storing liquefied gas, a compressor having a variable discharge amount, an engine that can use gas as fuel, and the tank in the tank. A vaporized gas line that guides the gas that is naturally vaporized by the liquefied gas to the compressor, a compressed gas supply line that guides the gas compressed by the compressor to the engine, and a control device that controls the discharge amount of the compressor; An input device for switching a control mode of the control device between a steady control mode and a non-steady control mode, and the control device is configured to supply the compressed gas supply line when the control mode is in the steady control mode. The discharge amount of the compressor is controlled so that the gas pressure of the compressor becomes the first set pressure, and when the control mode is the unsteady control mode, the gas pressure of the compressed gas supply line is controlled. Pressure controls the discharge amount of the compressor to be higher second set pressure than the first set pressure.

  According to the above configuration, when it is determined that the engine load can increase rapidly in future navigation, the control mode is switched from the steady control mode to the unsteady control mode, and the gas pressure in the compressed gas supply line is changed. The target pressure is changed to a second set pressure that is higher than the first set pressure. Since the gas pressure in the compressed gas supply line is maintained at a high level in this way, even if the engine load suddenly increases and the gas pressure in the compressed gas supply line rapidly decreases, the gas in the compressed gas supply line It can prevent falling below the lower limit of the required range of pressure.

  The LNG carrier is a return line that branches from the liquefied compressed gas supply line and returns a part or all of the gas in the compressed gas supply line to the tank. A return line for introducing gas to the phase; and a return valve provided in the return line, the opening degree of which is controlled by the control device, wherein the control mode is set to the unsteady control mode. And when the gas pressure in the compressed gas supply line exceeds a third set pressure lower than the upper limit value of the engine gas pressure request range and higher than the second set pressure, the opening of the return valve is increased. You may let them. According to this configuration, even when the engine load suddenly decreases and the gas pressure in the compressed gas supply line suddenly increases, the opening of the return valve is increased and the gas in the compressed gas supply line is increased. It is possible to prevent exceeding the upper limit of the required pressure range.

  In the LNG carrier, a gas header having a larger flow path cross-sectional area than other portions may be provided in the middle of the compressed gas supply line. Variations in gas pressure in the compressed gas supply line can be mitigated by the volume of the gas header.

  In the LNG carrier, a generator driven by the engine, a propulsion unit driven by electric power generated by the generator, and a power distribution control unit for supplying electric power generated by the engine and the generator to the propulsion unit And may be provided.

  According to the present invention, it is possible to provide a liquefied gas carrier ship that keeps the gas pressure on the downstream side of the compressor within the required range even when the engine load suddenly increases.

It is a schematic block diagram of the propulsion system of the LNG carrier which concerns on one Embodiment. It is a schematic block diagram of the gas supply system of the liquefied gas carrier ship which concerns on one Embodiment. It is a block diagram which shows the structure of the control system of the gas supply system of FIG. It is a flowchart which shows the flow of control of the gas supply system of FIG. It is a graph which shows an example of a time-dependent change of the gas pressure of a compressed gas supply line.

  Hereinafter, an LNG carrier which is a liquefied gas carrier according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a schematic configuration diagram of a propulsion system 90 of an LNG carrier 1 according to an embodiment of the present invention. The LNG carrier 1 employs a DFD (double fuel diesel) electric propulsion system as the propulsion system 90. However, the LNG carrier 1 to which the present invention is applied is not limited to the present embodiment, and may be provided with a gas-fired engine that can use gas as fuel. The liquefied gas is not limited to LNG, and any liquefied gas may be used as long as it can be used with engine fuel gas.

  The propulsion system 90 of the LNG carrier 1 includes a power generation unit 91, a propulsion unit 93 driven by the power generated by the power generation unit 91, and a power distribution control unit 92 provided in a power supply system from the power generation unit 91 to the propulsion unit 93. And.

  The power generation unit 91 includes a plurality of sets of power generation engines 2 and a generator 912. The mechanical energy generated by the engine 2 is taken out as electric power by the generator 912.

  The propulsion unit 93 is provided on the power transmission path of the propulsion unit 933 from the propulsion motor 931, the propulsion unit 933 driven by the output of the propulsion motor 931, and at least one propulsion motor 931. A reduction gear 932 and the like are included. The power distribution control unit 92 includes a power distribution panel 921 that distributes the power from the power generation unit 91, an inverter 922 that controls the output (ie, rotation speed) rotation speed of the propulsion motor 931, and the like. However, the rotational speed of the propulsion motor 931 may be constant, and the propulsive force may be adjusted by changing the pitch by employing a variable pitch propeller.

  For example, the number of revolutions of the propulsion motor 931 is determined by the operation of the telegraph lever in the shipyard. The propulsion motor 931 consumes electric power according to the rotational speed. On the other hand, the required gas amount of the engine 2 is determined by the electric power consumed by the propulsion motor 931, the governor is controlled by a command from the switchboard 921, and gas corresponding to the required gas amount is supplied to the engine 2.

  In the propulsion system 90 configured as described above, the engine 2 according to the present embodiment is a dual fuel type four-cycle diesel engine that produces oil and gas. Therefore, the fuel supply system to the engine 2 includes a fuel gas supply system 96 that supplies fuel gas, and a fuel oil supply system 97 that supplies fuel oil such as heavy oil stored in the fuel oil tank 95. Yes. In FIG. 1, the fuel gas supply system 96 is indicated by a broken line arrow, and the fuel oil supply system 97 is indicated by a solid line arrow. The fuel gas supply system 96 includes a boil-off gas (hereinafter referred to as “NBOG”) in which the LNG in the LNG transfer tank 3 is naturally vaporized and / or a boil-off gas in which the LNG in the tank 3 is forcibly vaporized (hereinafter referred to as “FBOG”). Is configured by a gas supply system 10 that supplies the engine 2. Hereinafter, the gas supply system 10 will be described in detail.

  FIG. 2 is a schematic configuration diagram of the gas supply system 10. A gas supply system 10 shown in FIG. 2 is compressed by a tank 3 that stores LNG as a liquefied gas, a compressor 4, a vaporized gas line 41 that guides NBOG in the tank 3 to the compressor 4, and the compressor 4. And a compressed gas supply line 42 for guiding the gas to the engine 2.

  The LNG carrier 1 is provided with four large tanks 3 arranged in the captain direction. The tank 3 has a heat insulating performance capable of maintaining a cryogenic state so that LNG can be held in a liquid state of about −162 ° C. under atmospheric pressure. The tank 3 has a liquid phase part 3a on the lower side and a gas phase part 3b on the upper side through the liquid level of the accommodated LNG. A gas containing NBOG is present in the gas phase part 3b. The tank 3 is provided with a liquid level gauge 32 (see FIG. 3) for detecting the liquid level of LNG in the tank 3.

  In the LNG carrier 1, the liquid phase portion 3 a and the gas phase portion in the tank 3 are generally used when navigating from the gas producing area to the gas consuming area and when navigating from the gas consuming area to the gas producing area. The ratio with 3b is different. For example, when the LNG carrier 1 navigates from a gas producing area to a gas consuming area (that is, during LNG transportation), the tank 3 is in a state where most of the tank 3 is filled with LNG (full load: for example, tank capacity About 98.5%). When the LNG carrier 1 navigates from the gas consumption area to the gas production area, the tank 3 is brought into a state in which almost no LNG is loaded in the tank 3 (empty loading: for example, about 1.5% of the tank capacity). . During the navigation of the LNG carrier 1, the ratio between the liquid phase portion 3a and the gas phase portion 3b varies somewhat due to vaporization of LNG in the tank 3 and the like.

The vaporized gas line 41 is a pipe that guides NBOG generated in each tank 3 to the compressor 4. The vaporized gas line 41 includes a branch pipe portion extending from each of the plurality of tanks 3, a vapor header 41 b where the branch pipe portions merge, and a main pipe portion extending from the vapor header 41 b and connected to the compressor 4. At the upstream end of the branch pipe portion of the vaporized gas line 41, a gas suction port 41a is provided, and each gas suction port 41a has a boil-off gas above the tank 3 even when the tank 3 is full. It is arranged at the upper part of the tank 3 so that it can be inhaled. The vapor header 41 b of the vaporized gas line 41 is provided with a vaporized gas pressure gauge 72 that measures a gas pressure (hereinafter referred to as “tank gas pressure”) P ₂ in the tank 3. Although not shown, the vaporized gas line 41 is provided with a precooler that cools the gas flowing into the compressor 4, a mist separator that removes the liquid from the gas flowing into the compressor 4, and the like.

  The vaporized gas line 41 is connected to a downstream end of a forced vaporized gas line 51 that is a pipe connected to a main pipe portion of the vaporized gas line 41 from at least one of the plurality of tanks 3. The forced vaporization gas line 51 is provided with a pump 31 disposed at the bottom of the tank 3, a vaporizer 52, and a forced vaporization valve 53 that controls the flow rate of LNG flowing into the vaporizer 52. Connected by piping. In the forced vaporization gas line 51, the LNG in the tank 3 is pressure-fed to the vaporizer 52 by the operation of the pump 31, and FBOG obtained by forcibly vaporizing the LNG in the vaporizer 52 is sent to the compressor 4. 2 is configured to guide LNG from two of the plurality of tanks 3 to the vaporizer 52, the forced vaporization gas line 51 shown in FIG. The line 51 may be configured to guide LNG to the vaporizer 52 from one of the plurality of tanks 3 or from three or more.

  The opening degree of the forced vaporization valve 53 is controlled by the control device 7. By changing the opening degree of the forced vaporization valve 53, the flow rate of LNG flowing into the vaporizer 52 changes, and as a result, the amount of FBOG generated changes. A portion of the discharge amount of the pump 31 that cannot pass through the forced vaporization valve 53 is returned to the tank 3 through an unillustrated line. In the forced vaporization gas line 51 according to the present embodiment, the amount of forced vaporization gas is changed by changing the opening degree of the forced vaporization valve 53. However, the present invention is not limited to this. For example, the rotational speed of the pump 31 is changed. The amount of FBOG generated can also be changed by changing.

  The compressor 4 is a device that pumps the gas guided from the upstream side to the downstream side. The NBOG and / or FBOG sucked by the compressor 4 is compressed and discharged to the compressed gas supply line 42. The NBOG and / or FBOG thus compressed is used in the engine 2 as fuel gas. The compressor 4 according to the present embodiment is, for example, an axial flow type or centrifugal type compressor, and the discharge amount (or suction amount) is adjusted by adjusting the rotational speed of the compressor 4 (that is, the rotational speed of the motor). ) Is configured to be variable. The discharge amount (or suction amount) of the compressor 4 is controlled by the control device 7. The discharge amount of the compressor 4 may be controlled by adjusting the opening degree of the suction port while keeping the rotation speed of the compressor 4 constant.

The compressed gas supply line 42 is configured by at least one or more pipes that connect the discharge port of the compressor 4 and the inlet of the engine 2. The compressed gas supply line 42 is provided with a gas header 42a having a larger flow path cross-sectional area than other portions. The gas header 42 a has a function as a branch pipe, and the compressed gas supply line 42 branches downstream from the gas header 42 a and is connected to the plurality of engines 2. In FIG. 2, only one of the plurality of engines 2 is shown. The gas header 42 a is provided with a compressed gas pressure gauge 71 for measuring the gas pressure of the compressed gas supply line 42 (hereinafter referred to as “compressed gas pressure P ₁ ”).

A return line 61 for returning the gas in the compressed gas supply line 42 to at least one of the plurality of tanks 3 is connected to the compressed gas supply line 42. The upstream end of the return line 61 according to the present embodiment is connected to the gas header 42a. A discharge port for discharging gas into the tank 3 is formed at the downstream end 63 of the return line 61. The downstream end 63 of the return line 61 is disposed so as to release gas to the liquid phase portion 3a of the tank 3 when navigating from the gas producing area to the gas consuming area (that is, when LNG is transported). Return gas from the return line 61, so released into the liquid phase portion 3a of the tank 3 condenses, it is possible to suppress an increase in the gas pressure P ₂ in the tank 3.

The return line 61 is provided with a return valve 62 having a variable opening so that the flow path cross-sectional area of the return line 61 is variable. The return valve 62 is a pressure adjustment valve. By controlling the opening degree of the return valve 62, it is possible to adjust the compressed gas pressure P ₁ on the upstream side of the return valve 62. The return valve 62 may be a flow control valve. In addition, although the downstream part from the return valve 62 in the return line 61 shown in FIG. 2 is configured to return gas to two of the plurality of tanks 3, the return line 61 includes a plurality of tanks. It may be configured to be returned to one of the three tanks or to three or more tanks.

Further, an exhaust line 81 that guides gas to a gas combustion unit (GCU) 83 is connected to the compressed gas supply line 42. The upstream end of the exhaust line 81 according to the present embodiment is connected to the gas header 42a. The gas combustion device 83 burns the gas guided from the compressed gas supply line 42 via the exhaust line 81 and exhausts it to the outside of the LNG carrier 1. An exhaust valve 82 is provided in the exhaust line 81. The exhaust valve 82 is a pressure adjustment valve. By controlling the opening of the exhaust valve 82, it is possible to adjust the compressed gas pressure P ₁ on the upstream side of the exhaust valve 82. The exhaust valve 82 may be a flow control valve.

  The control device 7 is, for example, a computer, and includes an arithmetic processing unit such as a CPU, and a storage unit such as a ROM and a RAM (all not shown). The storage unit stores programs executed by the arithmetic processing unit, various fixed data, and the like. The arithmetic processing unit performs data transmission / reception with an external device. The arithmetic processing unit inputs detection signals from various instruments and outputs control signals to each control object. In the control device 7, processing for controlling the operation of the gas supply system 10 is performed by the arithmetic processing unit reading and executing software such as a program stored in the storage unit. Note that the control device 7 may execute each process by centralized control by a single computer, or may execute each process by distributed control by cooperation of a plurality of computers.

FIG. 3 is a block diagram showing the configuration of the control system of the gas supply system 10. In this figure, in particular, the structure for controlling the compressed gas pressure P ₁ is shown, configured in accordance with other control are omitted. As shown in FIG. 3, detection signals are input to the control device 7 from the compressed gas pressure gauge 71, the vaporized gas pressure gauge 72, and the liquid level gauge 32.

In addition, a target speed signal is input from the telegraph lever 80 to the control device 7. The telegraph lever 80 is a speed changing means for changing the speed of the LNG carrier 1 installed in the maneuvering room. The telegraph lever 80 can be operated in the front-rear direction to change its position.
Target speed according to the operation position of the telegraph lever 80 (for example, “FULL (port full speed)”, “HALF (half speed)”, “SLOW (fine speed)”, “DEAD / SLOW (very fine speed)”) And “NAVIGATION / FULL (voyage full speed)”. A target speed signal of the telegraph lever 80 according to the position of the telegraph lever 80 or a signal corresponding to the target speed signal is input directly or indirectly to the control device 7. However, the speed changing means of the LNG carrier 1 is not limited to a lever type, and may be a button type or a touch panel type.

  Further, a mode switching signal is input to the control device 7 from the mode switching switch 85. The mode changeover switch 85 is an input device for switching the control mode of the control device 7 between a steady control mode and an unsteady control mode, which will be described later.

  The control device 7 includes a compressor control unit 75 that controls the discharge amount of the compressor 4, a forced vaporization valve control unit 76 that controls the opening degree of the forced vaporization valve 53, and a return that controls the opening degree of the return valve 62. Each function part of the valve control part 77 and the exhaust valve control part 78 which controls the opening degree of the exhaust valve 82 is provided. The control device 7 outputs control signals to the compressor 4, the forced vaporization valve 53, the return valve 62, and the exhaust valve 82.

In the gas supply system 10 having the above configuration, the compressed gas pressure P ₁ is the pressure of the gas supplied to the engine 2 and needs to be within the required gas pressure range R of the engine 2. Therefore, in this embodiment, the compressed gas pressure P ₁ is controlled to be maintained at the set target pressure gas pressure required range R.

However, when the compressed gas pressure P ₁ is controlled by the control of the compressor 4, when the load of the engine 2 increases rapidly, the change in the discharge amount of the compressor 4 cannot be made in time, and the compressed gas pressure P ₁ does not reach the engine. There is a risk of deviating from the required gas pressure range R of 2. Therefore, in the LNG carrier 1 according to this embodiment, the control mode of the control device 7 is switched depending on whether or not the LNG carrier 1 is in a state where a sudden load increase of the engine 2 can occur.

  Specifically, the control mode of the control device 7 is switched to the steady control mode when it is determined that the LNG carrier 1 is not in a state where a sudden load increase of the engine 2 can occur. Further, the control mode of the control device 7 is switched to the unsteady control mode when it is determined that the LNG carrier 1 is in a state where a sudden load increase of the engine 2 can occur. This switching of the control mode is performed by the ship operator operating the mode switch 85 after determining whether or not the LNG carrier 1 is in a state where a sudden load increase of the engine 2 can occur.

  FIG. 4 is a flowchart showing a control flow of the gas supply system 10, and FIG. 5 is a graph showing an example of a change over time in the gas pressure of the compressed gas supply line. In the following, referring to FIG. 4 and FIG. 5, as an example, the control device in the case where the LNG carrier 1 shifts from a state where a sudden load increase of the engine 2 does not occur to a state where a sudden load increase of the engine 2 can occur. The flow of control by 7 will be described.

  In the LNG carrier 1 when it is determined that a sudden load increase of the engine 2 does not occur, the mode switching switch 85 is operated by the operator and the steady control mode is selected.

As shown in FIG. 4, the control device 7 first sets a target pressure of the compressed gas pressure _{P 1} in the first set pressure _{P s1} (step S1). Here, the first set pressure P _s1 is a value within the required gas pressure range (for example, 500 to 600 kPa) R of the engine 2, and is an upper limit value P _MAX and a lower limit value P _MIN of the required gas pressure range R of the engine. A value (for example, 560 kPa) obtained by adding a pressure loss (for example, 10 kPa) to the inlet of the engine 2 to an average value (for example, 550 kPa).

Thereafter, the control device 7 determines whether or not the control mode selected by the mode changeover switch 85 is an unsteady control mode (step S2). As described above, since the steady control mode is selected by the vessel operator, the process proceeds from step S2 to step S3 (NO in step S2). In step S3, the compressed gas pressure P ₁ controls the rotational speed of the compressor 4 so that the first set pressure P _s1. Thereafter, the process returns to step S2, and the control in step S3 is repeated at a predetermined control cycle (for example, 200 msec) while the steady control mode is selected.

Thus, as shown in t = 0 to t ₁ in FIG. 5, the stationary control mode, when the load of the engine 2 is increased, the compressed gas pressure P ₁ is a compressor 4 so that the first set pressure P _s1 The rotation speed is controlled.

When the boat operator determines that the engine load can increase rapidly in future navigation, the boat operator operates the mode switch 85 to switch the control mode from the steady control mode to the unsteady control mode. As a result, in step S2, the control device 7 determines that the unsteady control mode has been selected (YES in step S2). Thereafter, the controller 7 sets the target pressure of the compressed gas pressure _{P 1} to a second set pressure _{P s2} (step S4). Here, the second set pressure P _s2 is a value within the gas pressure request range (for example, 500 to 600 kPa) R of the engine 2, and is lower than the upper limit value P _MAX of the gas pressure request range R, and the first set pressure P _The value is higher than _s1 (for example, 580 kPa).

When the target pressure is set to the second set pressure P _s2 , as shown at t = t _{1 to} t ₂ in FIG. 5, the control device 7 changes the compressed gas pressure P ₁ from the first set pressure P _s1 to the second set pressure P _s2 . The rotational speed of the compressor 4 is increased so as to increase to the set pressure P _s2 , and the compressed gas pressure P ₁ is increased from the first set pressure P _s1 to the second set pressure P _s2 . Thereafter, as shown at t = t _{2 to} t ₃ in FIG. 5, the control device 7 controls the rotational speed of the compressor 4 so that the compressed gas pressure P ₁ is maintained at the second set pressure P _s2. (Step S5). For example, as shown at t = t _{3 to} t ₄ in FIG. 5, when the load of the engine 2 (that is, the gas consumption amount in the engine 2) increases rapidly, the compressed gas pressure P ₁ decreases rapidly. but then, the increase in the rotational speed of the compressor 4, the compressed gas pressure _{P 1} is gradually increased, as shown in _{t =} t 4 ~t ₅ in FIG. 5, it is maintained again in the second set pressure _{P s2} The

Thereafter, the control device 7, the compressed gas pressure _{P 1,} it is determined whether more than a third set pressure _{P s3} (step S6). Here, the third set pressure P _s3 is a value within the gas pressure request range (for example, 500 to 600 kPa) R of the engine 2, and is lower than the upper limit value P _MAX of the gas pressure request range R, and the second set pressure P _The value is higher than _s2 (for example, 590 kPa).

In step S6, when the compressed gas pressure _{P 1} is determined not to exceed a third set pressure _{P s3} (NO in step S6), and the process proceeds to step S8. In step S6, as shown in t = t ₅ ~t ₆ in FIG. 5, by the load of the engine 2 is reduced, the compressed gas pressure P ₁ increases, when it is determined that exceeds the third set pressure P _s3 (YES in step S6), the controller 7 such that ,, compressed gas pressure _{P 1} becomes the second set pressure _{P s2,} increases the degree of opening of the return valve 62 (step S7), and then proceeds to step S8 .

  In step S8, the control device 7 determines whether or not the unsteady control mode has been canceled (step S8). If the unsteady control mode is not released (NO in step S8), the process returns to step S5, and steps S5 to S8 are performed until the unsteady control mode is released, that is, until the steady control mode is switched. It is repeated at a control cycle (for example, 200 msec).

Thus, as shown after t = t ₁ in FIG. 5, when the load of the engine 2 increases in the unsteady control mode, the compressor 4 is set so that the compressed gas pressure P ₁ becomes the second set pressure P _s2 . The rotation speed is controlled.

When the ship operator determines that the engine load does not increase rapidly in future navigation, the ship operator operates the mode switch 85 to switch the control mode from the unsteady control mode to the steady control mode. Accordingly, in step S8, the control device 7 determines that the steady control mode has been selected (YES in step S8). Thereafter, the control device 7, a target pressure of the compressed gas pressure _{P 1} is set to the first set pressure _{P s1} (step S9), and the process proceeds to step S2, steps S2 and S3 of the stationary mode described above is repeated.

As described above, the control device 7, when the control mode is in a steady control mode, the compressed gas pressure P ₁ controls the discharge rate of the compressor 4 so that the first set pressure P _s1, control mode when in the non-stationary control mode, the compressed gas pressure P ₁ is controlling the discharge amount of the compressor 4 so that the second set pressure P _s2 higher than the first set pressure P _s1.

According to the LNG carrier 1 having the above-described configuration, when it is determined that the load of the engine 2 can rapidly increase in future navigation, the control mode is switched from the steady control mode to the unsteady control mode, and the compressed gas is target pressure of pressure _{P 1} is changed to the second set pressure _{P s2} higher than the first set pressure _{P s1.} Thus, since the compressed gas pressure P ₁ is maintained at a higher load of the engine 2 is rapidly increased, even reduced compressed gas pressure P ₁ is suddenly a request for the compressed gas pressure P ₁ ranging R Can be prevented from falling below the lower limit value P _MIN .

  In addition, when the control mode is in the steady control mode or when the load of the engine 2 increases rapidly when the control mode is in the non-steady control mode, it is not necessary to change the opening of the return valve 62. The opening degree of the return valve 62 may be set relatively small. Thereby, the amount of gas passing through the compressor 4 can be suppressed, the useless work amount of the compressor 4 can be suppressed, the amount of gas returned from the return line 61 to the tank 3 can be reduced, and Heat input can be suppressed.

The controller 7 is located the control mode to the non-stationary control mode, and third set pressure higher than the second set pressure P _s2 lower than the upper limit value P _s1 gas pressure required range R of the compressed gas pressure P ₁ is the engine 2 When _Ps3 is exceeded, the opening degree of the return valve 62 is increased.

According to this configuration, the load of the engine 2 is rapidly decreased, even when the compressed gas pressure P ₁ increases rapidly increases the opening degree of the highly responsive return valve 62, compressed gas It can be prevented that the upper limit value P _MAX of the required range R of the pressure P ₁ is exceeded. Further, since the gas returned from the return line 61 is discharged and condensed in the liquid phase portion 3a of the tank 3, an increase in gas pressure in the tank 3 can be suppressed.

Further, a gas header 42 a having a larger flow path cross-sectional area than other parts is provided in the middle of the compressed gas supply line 4. The volume of this gas header 42a, it is possible to alleviate the fluctuation of the compressed gas pressure P _1.

  The above-described embodiment is an example in all respects, and should be considered not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  For example, in the above embodiment, the input device for switching the control mode of the control device 7 between the steady control mode and the non-steady control mode is the mode changeover switch 85, but is not limited thereto. For example, the input device for switching the control mode of the control device 7 between the steady control mode and the non-steady control mode may be speed change means (for example, the telegraph lever 80) that changes the target speed of the LNG carrier 1. Good. According to this configuration, it is possible to save time and labor for the operator to operate the input device only for switching the control mode.

  Further, for example, the input device for switching the control mode of the control device 7 between the steady control mode and the non-steady control mode is the position information acquisition for acquiring the position information of the LNG carrier 1 provided in the LNG carrier 1 Means (for example, a GPS device) may be used. In this case, the position information of the LNG carrier 1 is input to the control device 7 from the position information acquisition unit. The control device 7 may switch the control mode between the steady control mode and the unsteady control mode based on the navigation plan stored in advance and the position information input from the position information acquisition means. According to this configuration, the control mode can be automatically switched regardless of the operation of the operator.

  In the above embodiment, the engine 2 has been described as a dual fuel engine. However, the present invention is not limited to this, and the engine 2 may be a gas-only engine that uses only gas fuel as fuel. The engine 2 may be a two-cycle diesel engine. Moreover, although the four tanks 3 were provided in the LNG carrier 1 which concerns on the said embodiment, the number of the tanks 3 is not limited to this, For example, one may be sufficient and several other than four It may be.

1 LNG carrier (liquefied gas carrier)
DESCRIPTION OF SYMBOLS 10 Gas supply system 2 Engine 3 Tank 3a Liquid phase part 3b Gas phase part 4 Compressor 41 Vaporized gas line 41a Gas inlet 41b Vapor header 42 Compressed gas supply line 51 Forced vaporization gas line 52 Vaporizer 53 Forced vaporization valve 61 Return line 62 Return valve 7 Control device 71 Compressed gas pressure gauge 72 Vaporized gas pressure gauge 81 Exhaust line 82 Exhaust valve 83 Gas combustion device 85 Mode selector switch (input device)
90 propulsion system 91 power generation unit 912 generator 92 distribution control unit 921 switchboard 922 inverter 93 propulsion unit 931 propulsion motor 932 speed reducer 933 propulsion device 95 fuel oil tank 96 fuel gas supply system 97 fuel oil supply system P ₁ compression gas pressure (compression Gas pressure in gas supply line)
P _s1 first set pressure P _s2 second set pressure P _s3 third set pressure P _MAX upper limit value P _MIN lower limit value R gas pressure required range

Claims (4)

A tank for storing liquefied gas;
A compressor with variable discharge rate,
An engine that can use gas as fuel,
A vaporized gas line that guides the gas in which the liquefied gas is naturally vaporized in the tank to the compressor;
A compressed gas supply line for guiding the gas compressed by the compressor to the engine;
A control device for controlling the discharge amount of the compressor;
An input device for switching the control mode of the control device between a steady control mode and a non-steady control mode;
The controller is
When the control mode is the steady control mode, the discharge amount of the compressor is controlled so that the gas pressure of the compressed gas supply line becomes the first set pressure,
When the control mode is the unsteady control mode, a liquefied gas that controls the discharge amount of the compressor so that the gas pressure of the compressed gas supply line becomes a second set pressure higher than the first set pressure. Carrier ship. A return line that branches from the compressed gas supply line and returns part or all of the gas in the compressed gas supply line to the tank, and returns the gas to the liquid phase of the tank during transport of the liquefied gas Line,
A return valve provided in the return line, the opening of which is controlled by the control device; and
The control device has a third setting in which the control mode is the unsteady control mode, and the gas pressure of the compressed gas supply line is lower than an upper limit value of a required gas pressure range of the engine and higher than the second set pressure. The liquefied gas carrier according to claim 1, wherein when the pressure is exceeded, the opening degree of the return valve is increased.   The liquefied gas carrier according to claim 1 or 2, wherein a gas header having a larger flow path cross-sectional area than other parts is provided in the middle of the compressed gas supply line. A generator driven by the engine, a propulsion unit driven by electric power generated by the generator, and a power distribution control unit that supplies the electric power generated by the engine and the generator to the propulsion unit, The liquefied gas carrier ship as described in any one of Claims 1-3.

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