JP2019157463A - Gas production system and gas production method - Google Patents

Abstract

To stably produce gas in producing the gas by decomposing gas hydrate.SOLUTION: A gas production system comprises a long riser pipe having a tip to be buried in the ground. The riser pipe has a hole opening on the tip so as to capture, into liquid in the pipe, a gas-liquid mixture split from gas hydrate using a decompression method. The system comprises: a gas-liquid separation device for separating bubbles from the liquid into which the gas-liquid mixture is captured; a pump that in order to discharge from the riser pipe the liquid from which gas is separated by the gas-liquid separation device, sucks up the liquid while keeping pump rotation speed at an adjustment value; and a control device for controlling the pump rotation speed. The control device adjusts the pump rotation speed on the basis of measured pressure measured at the tip and a measured current amount of current flowing for rotating the pump.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a gas system and a gas production method for producing gas by decomposing gas hydrate in the ground.

In recent years, natural gas hydrate has attracted attention as a natural gas resource. Natural gas emits less carbon dioxide when combusted than oil and coal, and natural gas hydrate consisting of natural gas and water is a promising resource in terms of suppressing global warming.
Natural gas hydrate is an inclusion compound having a crystal structure in which methane molecules are surrounded by water molecules in a cage shape. Natural gas hydrate exists in a solid state in a low-temperature and high-pressure environment, and stably exists in the surface layer of the deep sea bottom and the subsurface of the sea bottom satisfying such an environment.

Conventionally, as a method for extracting natural gas from natural gas hydrate present in the seabed, a decompression method for decomposing natural gas hydrate by applying a reduced pressure to the high water pressure applied to the natural gas hydrate is known. (For example, Patent Document 1).
In the depressurization method, concretely, using a pipe (riser pipe) that carries natural gas from the seabed to the sea, the sea level in the pipe is discharged to lower the liquid level, and the pressure of the seawater in the riser pipe is reduced to the natural gas. It acts on the formation (hydrate layer) in the seabed containing hydrate and decomposes it. Natural gas produced by decomposition of natural gas hydrate is taken into seawater in the riser pipe as a mixed phase flow (gas-liquid mixture) mixed with liquid. Seawater that has taken in the multiphase flow is separated into natural gas and seawater (gas-liquid separation) in the riser pipe, and each is discharged to the sea.

  In order to increase the production amount of natural gas using the decompression method, it is important to maintain the pressure acting on the natural gas hydrate within the range of the pressure (decomposition pressure) at which the natural gas hydrate decomposes. For this reason, in the depressurization method, the seawater is discharged so that the liquid level in the riser pipe is adjusted to a height position corresponding to the decomposition pressure. In other words, in the depressurization method, while measuring the pressure (bottom pressure) at the tip of the riser pipe with the intake port for taking in the multiphase flow and monitoring the bottom pressure, the liquid in the riser pipe is changed according to the fluctuation in the bottom pressure. The height position of the surface is adjusted. Such adjustment of the liquid level is performed by controlling the pump rotation speed of the pump based on the pressure (bottom pressure) at the tip of the riser pipe to control the discharge amount of seawater.

JP 2010-261252 A

As described above, in the decompression method, the pressure that acts on the natural gas hydrate is controlled so that the pressure at the tip of the riser pipe (bottom bottom pressure) becomes the set pressure, but is introduced into the pump. In the liquid such as seawater, gas-liquid separation performed by the gas-liquid separator is not sufficient, and bubbles may be mixed. When such bubbles enter the pump, the discharge capacity of the liquid by the pump varies depending on the bubbles that have entered, so that the discharge amount of liquid such as seawater is not constant and is likely to vary. In addition, since the size and amount of bubbles that enter the pump also vary, the liquid discharge amount is more likely to vary. As a result, the pressure (bottom pressure) at the tip of the riser pipe fluctuates, and as a result, the pressure acting on the natural gas hydrate also fluctuates, so that the degree of decomposition of the natural gas hydrate is likely to change.
Thus, it is not preferable that the production of natural gas becomes unstable due to the influence of the pressure (bottom pressure) at the tip of the riser pipe that fluctuates due to insufficient gas-liquid separation in the gas-liquid separator. .

  Therefore, the present invention suppresses bubbles that enter the pump, which affects fluctuations in the liquid discharge capacity of the pump, or decomposes bubbles (bubbles) when producing gas by decomposing a gas hydrate such as natural gas hydrate. It is an object of the present invention to provide a gas production system and a gas production method capable of stably producing gas by stabilizing fluctuations in the size and amount of the gas.

One embodiment of the present invention is a system for producing gas by decomposing underground gas hydrate. The system
A long tube having a tip configured to be buried in the ground, the gas hydride being external to the tube using pressure generated by the liquid in the tube extending upward from the tip By reducing the pressure acting on the rate, a gas-liquid mixture containing bubbles generated by decomposing from the gas hydrate is taken into the liquid in the tube, and is provided at the tip portion and outside the tube. A riser tube with an open hole;
A pressure gauge that is provided at the tip of the riser tube and measures the pressure at the tip of the riser tube;
A gas-liquid separation device provided in the riser pipe and separating bubbles from the liquid that has taken in the gas-liquid mixture;
A gas generation line including a gas generation pipe for taking out the gas separated by the gas-liquid separator as a gas to be produced from the riser pipe;
A pump for sucking up the liquid while maintaining the pump rotation number at an adjusted value in order to discharge the liquid from which the gas has been separated by the gas-liquid separator from the riser pipe;
An ammeter that measures the amount of current supplied to maintain the pump speed of the pump;
And a control device that adjusts the adjustment value of the pump rotation speed based on a measured pressure measured by the pressure gauge and a measured current amount measured by the ammeter.

  It is preferable that the control device increases or decreases the adjustment value according to a comparison result between the measured pressure and a set pressure, and the set pressure is set according to the measured current amount.

  When the measured current amount is lower than a set lower limit value, the control device preferably changes the set pressure from a first pressure to a second pressure higher than the first pressure.

  In the case where the measured pressure is higher than the set pressure, the control device sets the first value that is set as the adjustment value when the measured current amount is lower than a set lower limit value as the measured current. The amount is preferably smaller than the second value set as the adjustment value when the amount is equal to or greater than the set lower limit value.

The control device increases the adjustment value when the measured pressure is higher than the set pressure,
When the measured pressure is equal to or lower than the set pressure, it is preferable to reduce the adjustment value.

Another aspect of the present invention is a gas production method for producing gas by decomposing underground gas hydrate. The gas production method is
Acts on the gas hydrate outside the riser pipe using the pressure of the tip part generated in the riser pipe by the liquid in the riser pipe extending upward from the front end portion, having a tip portion embedded in the ground Reducing the pressure to be applied;
A gas-liquid mixture containing bubbles generated by decomposing from the gas hydrate with a reduced pressure acting on the gas hydrate is taken into the liquid in the riser pipe from a hole opened outside the riser pipe. Steps,
Performing gas-liquid separation from the liquid that has taken in the gas-liquid mixture taken into the riser tube and taking out the gas; and
A step of sucking up the liquid after the gas-liquid separation by a pump whose pump rotation number is maintained at an adjusted value in order to discharge the liquid after the gas-liquid separation from the riser pipe;
Adjusting the adjustment value of the pump rotational speed based on a measured pressure of the pressure and a measured current amount of a current amount supplied for maintaining the pump rotational speed of the pump.

  In the adjustment of the adjustment value, it is preferable that the adjustment value is increased or decreased according to a comparison result between the measured pressure and the set pressure, and the set pressure is set according to the measured current amount.

  When the measured current amount falls below a set lower limit value, it is preferable to change the set pressure from the first pressure to a second pressure higher than the first pressure.

  In the adjustment of the adjustment value, when the measured pressure is higher than the set pressure, the first value set as the adjustment value when the measured current amount is lower than the set lower limit value is It is preferable that the measured current amount is smaller than the second value set as the adjustment value when the measured current amount is equal to or greater than the set lower limit value.

  In the adjustment of the adjustment value, it is preferable that the adjustment value is increased when the measured pressure is higher than the set pressure, and the adjustment value is decreased when the measured pressure is equal to or lower than the set pressure.

  According to the above-described gas production system and gas production method, gas production can be stably performed by suppressing bubbles that enter the pump or by stabilizing fluctuations in bubbles (the size and amount of bubbles). it can.

It is a figure showing roughly an example of a gas production system of one embodiment. It is a figure explaining an example of an internal configuration near the tip part of a riser pipe in one embodiment. It is a figure explaining an example of the change with respect to the measurement electric current amount I of the value (alpha) used for control of pump rotation speed in one Embodiment.

The gas production system and gas production method of the present invention will be described below. In the following description, natural gas hydrate is taken as an example of gas hydrate, but the gas hydrate is not limited to natural gas hydrate.
In addition, the gas production system referred to in this specification generates gas by decompressing and decomposing underground gas hydrate, and is different from a system that generates gas from gas hydrate on the surface of the seabed.

(Outline explanation of gas production system)
A gas production system (hereinafter also referred to as a system) according to an embodiment is a system that produces gas by decomposing underground gas hydrate. The system mainly includes a riser pipe, a gas-liquid separator, a gas generation line, a pump, a pressure gauge, an ammeter, and a control device.
The riser tube is a long tube having a tip portion configured to be buried in the ground. The riser tube is provided at the tip and includes a hole that opens to the outside of the tube. The hole opened to the outside is provided so that a gas-liquid mixture containing bubbles generated by decomposition from the gas hydrate is taken into the liquid in the tube. The gas hydrate is decomposed by reducing the pressure acting on the gas hydrate outside the tube using the pressure generated by the liquid in the tube extending upward from the tip of the riser tube.
The gas-liquid separation device is provided in the riser pipe and is configured to separate bubbles from the liquid that has taken in the gas-liquid mixture in the riser pipe.
The gas generation line includes a gas generation pipe that takes out a gas generated from bubbles as a gas to be produced from a riser pipe.
The pump is configured to suck up the liquid while maintaining the pump rotational speed at an adjusted value in order to discharge the liquid from which the gas has been separated by the gas-liquid separator from the riser pipe.
The ammeter measures the amount of current supplied to maintain the pump rotation speed of the pump, specifically, the amount of current that flows to the motor that rotates the pump.
The pressure gauge is provided at the tip of the riser pipe and measures the pressure (bottom pressure) at the tip.
The control device is configured to adjust the adjustment value of the pump speed based on the measured pressure measured by the pressure gauge and the measured current amount measured by the ammeter.

  In general, seawater or the like flows into the riser pipe from the hole opened at the tip to increase the pressure at the tip, so in order to maintain the pressure acting on the gas hydrate at a predetermined decomposition pressure, The pump speed is controlled so that the bottom pressure is set to the target pressure. However, even if the number of revolutions of the pump is controlled, the amount of liquid discharged by the pump does not become the target discharge amount but varies. For example, gas-liquid separation in the gas-liquid separator may not be performed sufficiently, and liquid containing bubbles may flow into the pump. In this case, since the bubbles flowing into the pump are compressed, the amount of liquid discharged from the pump is not constant and fluctuates even if the pump rotation speed is maintained at a set adjustment value.

  For this reason, the control device sets the adjustment value of the pump rotation speed to the measured current amount (measurement result of the bottom hole pressure) measured by the pressure gauge and the measured current amount (measured by the motor that rotates the pump). Adjust based on the amount of current). Since the pump is controlled so that the pump rotation speed is maintained at the adjusted value, fluctuations in the measured current amount are not sufficient for gas-liquid separation, and the size and amount of bubbles that enter the pump together with the liquid are reduced. Represents fluctuations. For this reason, when the measured current amount is small, it can be determined that the size and amount of bubbles entering the pump are large, that is, the gas-liquid separation by the gas-liquid separation device is not sufficiently performed.

As described above, the measured current amount can be used as an index representing information of bubbles mixed in the liquid entering the pump. Therefore, when the size and amount of bubbles mixed in the liquid introduced into the pump are larger than necessary, that is, when the measured current amount falls below the set lower limit value, the liquid flow in the gas-liquid separation device Since it is fast, it is determined that the bubbles cannot sufficiently float on the liquid surface, and the rotation speed of the pump is lowered to reduce the suction speed of the liquid into the pump (liquid flow speed). Thereby, in the gas-liquid separator, the bubbles are separated from the liquid flow, and the effect of gas-liquid separation can be exhibited more greatly. Therefore, in addition to the measurement pressure (measurement result of bottom hole pressure) measured by the pressure gauge, the control device sets the adjustment value of the pump rotation speed so that gas-liquid separation can be more effectively demonstrated based on the measured current amount. Therefore, bubbles that enter the pump together with the liquid can be suppressed, or fluctuations in the bubbles (size and amount) can be stabilized. As a result, gas production can be performed stably.
Hereinafter, a specific embodiment will be described.

(Embodiment of gas production system)
FIG. 1 is a diagram schematically illustrating a system 1 according to an embodiment. FIG. 2 is a diagram for explaining an example of the internal configuration in the vicinity of the distal end portion 10a of the riser tube 10. Hereinafter, the system 1 which decomposes | disassembles the natural gas hydrate in the sea floor and produces natural gas is demonstrated to an example.

The system 1 is a system that generates and extracts natural gas generated by decomposing natural gas hydrate in the ground from a riser pipe 10 that extends into the ground from the drilling vessel 3 on the sea via the seabed. is there.
The system 1 mainly includes a riser pipe 10, a gas-liquid separator 20, a pump 23, a gas generation line 12, a gas generation line, a liquid discharge line 13, and a control device 40.

  The riser tube 10 is a long tube having a tip portion 10a configured to be buried in the ground. In the example shown in FIG. 1, the riser pipe 10 extends vertically downward from the excavation ship 3, and the tip end portion 10 a is embedded in a well 7 on the seabed. The well 7 is a hole provided by excavation. In the example shown in FIG. 1, the well 7 penetrates the upper layer 4 including the sea bottom 2 and is closed in the hydrate layer 5 located in the lower layer. The upper layer 4 is, for example, a muddy layer containing a lot of mud. The hydrate layer 5 is a layer called a sand-mud alternate layer containing a lot of mud and sand, for example. The hydrate layer 5 has a sandy layer spreading in the lateral direction in which natural gas hydrate is taken in by sand or mud. The boundary between the upper layer 4 and the hydrate layer 5 is, for example, at a position several hundred meters below the sea bottom, and the sea bottom 2 is at, for example, a position having a depth of 300 to several hundreds of meters.

The riser tube 10 includes a tube body 11 and a screen 19 (see FIG. 2).
A gas-liquid separator 20, a pump 23, a gas generation line 12, and a liquid discharge line 13 are provided in the pipe body 11.
In addition, a heater 26 is provided in the tube main body 11.

  The pipe body 11 is a member that isolates the inner space from seawater except for a hole 18a of a portion 18 that functions as a collection pipe described later. In the example shown in FIG. 1, the pipe body 11 is provided with partition walls 17 a and 17 b and a partition wall 17 c that partition the inner space vertically. A portion of the tube main body 11 extending from the partition wall 17c to the tip of the riser tube 10 is a portion 18 in which the gas-liquid mixture taken in from the hydrate layer 5 flows upward (hereinafter, this portion is also referred to as a lifting tube portion 18). In the example shown in FIG. 1, the tube diameter is smaller than that of the portion of the tube body 11 above the partition wall 17c. The lifting pipe portion 18 is located in the hydrate layer 5.

The screen 19 is provided in the lifting pipe portion 18 so as to cover a hole 18 a opened to the outside of the riser pipe 10. The hole 18a is provided in the leading end portion of the riser pipe 10 that extends at the deepest position in the ground. The hole 18 a is provided in the lifting pipe portion 18 at a depth position in contact with the sandy layer in the hydrate layer 5.
The screen 19 provided in the hole 18a is a member that separates and removes sand and mud from air bubbles and water generated by decomposition of natural gas hydrate, and further from seawater. The screen 19 has a function of allowing bubbles, water, and seawater to pass therethrough but preventing sand and mud from passing therethrough. For example, the screen 19 is a sheet-like or plate-like structure having a large number of holes, and is composed of a plurality of structures having different hole sizes and shapes. Specific examples of the combination of a plurality of structures include a Johnson screen, a mesh, and a grating. Johnson screen is well known as a wire mesh structure manufactured by Johnson Screen. Grating is a member in which steel materials are assembled in a grid pattern. The Johnson screen, the mesh, and the grating are arranged so as to overlap the lifting pipe portion 18 in this order from the lifting pipe portion 18 side toward the hydrate five-layer side.

As shown in FIG. 2, the collection pipe portion 18 is provided with a plurality of holes 18 a along the depth direction for taking in the gas-liquid mixture that has passed through the screen 19. The hole 18 a passes through the wall portion of the lifting pipe portion 18 and opens to the outside of the lifting pipe portion 18. By providing the hole 18 a in the riser pipe 10, the pressure acting on the natural gas hydrate can be reduced using the bottom hole pressure, and thereby the gas-liquid mixture can be taken into the riser pipe 10.
The bottom hole pressure is a pressure generated in the tip portion 10a in the riser pipe 10 by the liquid filled in a predetermined range in the riser pipe 10 extending upward from the tip portion 10a of the riser pipe 10, specifically, The water head pressure received by the lower end of the riser pipe 10 by the liquid below the liquid level S, which will be described later, the pressure in the gas phase space above the liquid level S, and the total pressure. The lower end of the riser pipe 10 is located at the same height as the hole bottom (well bottom) of the well 7. Here, the distal end portion 10a includes a portion of the riser tube 10 where the hole 18a is provided.
The liquid in the riser pipe 10 includes water and seawater in addition to a gas-liquid mixture generated by decomposing from natural gas hydrate such as bubbles and water. The water and seawater are taken from the holes 18a from the water and seawater contained in the hydrate layer 5 and the water and seawater contained in other formations in contact with the hydrate layer 5. As will be described later, the liquid in the riser tube 10 includes those in which bubbles are taken.

  The riser pipe 10 further includes a pressure gauge 31 that measures the bottom bottom pressure, which is provided at the distal end portion 18 a of the lifting pipe portion 18, specifically the lower end of the riser pipe 10. The pressure gauge 31 is connected to the control device 40 and outputs a measurement signal of the bottom hole pressure toward the control device 40.

  As shown in FIG. 2, the separation device 20, the pump 23, and the heater 26 are provided in a space 15b of the riser pipe 10 partitioned by partition walls 17b and 17c. In the space 15b, in the example shown in FIG. 2, a gas phase space G into which natural gas separated from the liquid by the gas-liquid separator 20 flows is formed above the liquid level S of the gas-liquid mixture. The gas phase space G is preferably formed in the riser pipe 10 so as to be positioned above the sea bottom 2.

  The gas-liquid separation device 20 is a device that separates at least some of the bubbles in the gas-liquid mixture that is taken into the collection pipe portion 18 and taken into the liquid. The gas in the separated bubbles is the gas that is produced. According to one embodiment, the gas-liquid separation device 20 includes an enclosure 21 and a centrifuge 22.

  The surrounding container 21 is a cup-shaped member that surrounds the lower end of a liquid transport pipe 14 (described later) constituting the liquid discharge line 13 from the outside. A centrifuge 22 and a pump 23 are provided in the liquid transport pipe 14. In the example shown in FIG. 2, the surrounding container 21 has a cylindrical side wall 21 a disposed with a gap from the inner wall of the tube body 11, and a bottom wall 21 b that closes the lower end of the side wall 21 a. The upper end of the side wall 21 a is located above the lower end of the liquid transport pipe 14. As a result, the liquid containing the gas-liquid mixture ascends the gap between the pipe body 11 and the side wall 21a along the thin arrow shown in FIG. 2, and then the liquid descends the gap between the side wall 21a and the liquid transport pipe 14. And flows into the liquid transport pipe 14. In this process, since relatively large bubbles and gas phase flows have a high ascent rate, the liquid separates from the gas-liquid mixture and rises to the liquid surface S before the liquid flows into the liquid transport pipe 14, and the natural gas is vaporized. Released into the phase space G. In FIG. 3, the flow of natural gas is indicated by thick arrows.

  The centrifuge 22 is a device that separates, from the liquid, relatively small bubbles that remain in the liquid flowing into the liquid transport tube 14. In the example shown in FIG. 2, the centrifuge 22 includes a rotating body 22 a that is provided in the liquid transport pipe 14 and rotates around a rotation center line extending in the vertical direction. The rotating body 22a is driven by a motor 24 described later. When the liquid containing bubbles approaches the rotating body 22a, it flows along a swirling flow created by the rotation of the rotating body 22a. At this time, centrifugal force acts on the bubbles and the liquid, and the liquid has a greater specific gravity than the bubbles. Therefore, the liquid moves away from the rotation center line, and the bubbles are collected closer to the rotation center line than the liquid. At this time, the bubbles that are collected and enlarged are discharged from a hole provided in the liquid transport pipe 14 and opening to the outside of the liquid transport pipe 14. As a result, it floats on the liquid surface S and is released into the gas phase space G. On the other hand, minute bubbles that have not been released from the holes of the liquid transport pipe 14 rise in the liquid transport pipe 14 together with the liquid.

The pump 23 draws the liquid into the liquid transport pipe 14 and discharges it from the riser pipe 10.
Specifically, in order to discharge the liquid separated from the bubbles by the gas-liquid separation device 20 from the riser pipe, the pump 23 sucks up the liquid while maintaining the pump rotation number at an adjusted value. The adjustment value of the pump speed is set by a control signal from the control device 40.
The pump 23 in the example illustrated in FIG. 2 is an auger pump that is disposed in the liquid transport pipe 14 and includes a motor 24 and a screw 25 that is driven by the motor 24. The screw 25 has an axis extending in the vertical direction and blades extending spirally around the axis, and has a function of sending the liquid in the liquid transport pipe 14 upward while stirring. The motor 24 is connected to the control device 40 of the excavation ship 3. The motor 24 receives the signal output from the control device 40 and is controlled to drive at the adjusted frequency. The adjusted frequency is the same as the adjustment value of the pump rotation speed described above. The motor 24 is disposed in the liquid transport pipe 14 so as to form a gap serving as a liquid flow path in the liquid transport pipe 14. During operation of the system 1, seawater or water continues to flow into the riser pipe 10 through the opening 18a, so that the pump 23 is normally maintained in an operating state.
An ammeter 24a is provided for measuring the amount of current flowing through the motor 24 in order to maintain the pump rotation speed of the pump 23 at the adjustment value set by the control device 40. The ammeter 24 a is connected to the control device 40 and outputs information on the measured current amount to the control device 40.

  The heater 26 is a device that heats the liquid that has flowed into the space 15b. The heater 26 is connected to the control device 40. Since the decomposition reaction of the natural gas hydrate is an endothermic reaction, the temperature of the gas-liquid mixture taken into the liquid is lowered and the natural gas hydrate is regenerated. For example, the lower end of the liquid transport pipe 14 may be blocked. is there. The heater 26 continuously or intermittently heats the liquid during operation of the system 1 and suppresses the regeneration of natural gas hydrate. In addition, when it is determined that the natural gas hydrate has been regenerated, it is controlled to be driven by receiving a signal output from the control device 40, and the regenerated natural gas hydrate is heated by heating the liquid. And disassemble.

The gas generation line 12 includes a gas generation pipe 12a.
The gas generation pipe 12a takes out, from the riser pipe 10 as a natural gas to be produced, gas generated from the bubbles floating on the liquid surface S and flowing into the gas phase space G. The gas generation line 12 carries the gas in the gas phase space G to the drilling ship 3 as natural gas that produces the gas. The gas generation pipe 12 a is disposed above the liquid surface S in the pipe body 11, and the lower end of the gas generation pipe is connected to the gas phase space G.
The upper end of the gas generation pipe 12a is connected to, for example, a storage tank (not shown) provided in the excavation ship 3 or another ship, or a pipeline extending to a storage system on land. The natural gas stored in the storage tank is appropriately liquefied and transported on the sea by the drilling ship 3 or other ships.

The liquid discharge line 13 includes a liquid discharge pipe 13 a that carries the liquid separated from the natural gas in the pipe body 11 to the drilling ship 3.
In the example shown in FIG. 1, the liquid discharge pipe 13a includes a liquid transport pipe 14 that extends from the gas-liquid separator 20 to the space 15a, and a pipe 16 that branches from the pipe body 11 and extends from the space 15a to the excavation ship 3. Have. The space 15a is a space partitioned by the partition walls 17a and 17b.
The discharged liquid is collected, for example, on the excavation ship 3 and stored in a predetermined container.

The control device 40 measures the pump rotation speed of the pump 23 with the pressure gauge 31 so that the liquid level S can be formed in the gas-liquid separation device 20 and the decomposition of the natural gas hydrate can be controlled. An adjustment value is set based on the measured pressure in the unit 10a, and the drive of the motor 24 is controlled so that the adjustment value set by the pump rotation speed of the pump 23 is maintained. At this time, the control device 40 changes the adjustment value based on the measured current amount of the motor 24 measured by the ammeter 24a. That is, the control device 40 is configured to adjust the adjustment value of the pump rotational speed based on the measured pressure measured by the pressure gauge 31 and the measured current amount measured by the ammeter 24a.
The contents of control of the control device 40 will be described later. The control device 40 is configured by a computer including a CPU, a memory, and the like. The control apparatus 40 is provided in the excavation ship 3 in the example shown in FIG.

  The system 1 is assembled by, for example, loading the material to be the riser pipe 10, the pressure gauges 30 and 31, and the control device 40 on the excavation ship 3 and transporting them to a predetermined position on the sea. The well 7 is previously excavated before assembling the system 1.

  The system 1 may include a fixed or floating offshore platform instead of the excavation ship 3. In this case, it is preferable to provide a pipeline that connects the offshore platform and the land and transports natural gas from the offshore platform to the land.

(Control of the pump speed in the control device 40)
As described above, the liquid that has passed through the gas-liquid separation device 20 is introduced into the pump 23 by the suction of the pump 23, but the gas-liquid separation in the gas-liquid separation device 20 is not sufficient and is introduced into the pump 23. Bubbles may be mixed in the liquid. Such bubbles are mixed in because the amount of bubbles mixed in the liquid is larger than that in the liquid, and some of the bubbles cannot be gas-liquid separated, or the upward flow of liquid flowing in the gas-liquid separator 20 (FIG. 2). The flow of the thick arrow) and the downward flow (the thin arrow pointing downward in the figure shown in FIG. 2) are fast, and the bubbles in the liquid follow the flow of the liquid and are caught in the downward flow of the liquid. This depends on the bubbles not being sufficiently separated.
Therefore, when bubbles that are mixed in the liquid introduced into the pump 23 are present to such an extent that the liquid discharge amount is affected, it is preferable to reduce the speed at which the pump 23 draws the liquid.

The measured pressure measured by the pressure gauge 31 is Pv, the reference pressure determined by the control device 40 is Sv0, α (> 0) is added to the reference pressure Sv0, and the measured current amount measured by the ammeter 24a Is set to I, and the lower limit value at which the degree to which the bubbles entering the pump 23 decrease the liquid discharge amount of the pump 23 can be ignored is I0. Here, α is preferably a value within the range of 5 to 20% of the reference pressure Sv0, for example.
The pressure that the control device 40 compares with the measured pressure Pv in order to increase or decrease the adjustment value of the pump rotational speed is a set pressure, and the set pressure is measured by an ammeter 24a as shown in Table 1 below. It is changed according to the current amount I. That is, the set pressure is a reference for determining control for increasing or decreasing the pump rotation speed.

  Using such a set pressure, the control device 40 controls the adjustment value of the pump speed as shown in Table 2 below. In Table 2 below, the control 1 is performed when the measured current amount I> the lower limit value I0 and the measured pressure Pv> the reference pressure Sv0, and the control 4 is the measured current amount I> the lower limit value I0 and the reference pressure Sv0 + α. It is performed when ≧ measurement pressure Pv. Therefore, the control device 40 is configured to perform the control 1 or the control 2 using the reference pressure Sv0 as a set pressure for comparing with the measured pressure Pv when the measured current amount I> the lower limit value I0. The control device 40 is configured to perform the control 3 or the control 4 by using the reference pressure Sv0 + α as a set pressure for comparing with the measured pressure Pv when the measured current amount I ≦ the lower limit value I0.

Controls 1 to 4 in Table 1 will be described below.
When the measured current amount I exceeds the lower limit value I0 and the measured pressure Pv exceeds the reference pressure Sv0, the number of bubbles entering the pump 23 is small (the gas-liquid separation is sufficiently functioning) and measurement is performed. The pressure Pv is decreased (the position of the liquid level S is decreased) to promote the decomposition of the natural gas hydrate. That is, the control device 40 performs the control 1 that increases the adjustment value of the pump rotation speed so that the pump rotation speed increases.
Further, when the current measurement amount I exceeds the lower limit value I0 and the measurement pressure Pv is equal to or lower than the reference pressure Sv0, the number of bubbles entering the pump 23 is small (the gas-liquid separation is sufficiently functioning). ) And increasing the measurement pressure Pv (increasing the position of the liquid level S) to suppress decomposition of the natural gas hydrate. That is, the control device 40 performs the control 2 that decreases the adjustment value of the pump rotation speed so that the pump rotation speed increases.

  On the other hand, when the measured current amount I is equal to or lower than the lower limit value I0 and the measured pressure Pv exceeds the reference pressure Sv0 + α, there are many bubbles entering the pump 23 (a state where gas-liquid separation is not functioning sufficiently). Then, the measurement pressure Pv is decreased (the position of the liquid surface S is decreased) to promote the decomposition of the natural gas hydrate. That is, the control device 40 performs the control 3 that increases the adjustment value of the pump rotation speed so that the pump rotation speed increases. In this case, since the flow rate at which the pump 23 sucks the liquid is increased, the gas-liquid separation effect of separating the bubbles from the liquid in the gas-liquid separation device 20 is reduced, but the natural gas hydrate is not sufficiently decomposed. Acceleration of gas hydrate decomposition is given priority over the effect of gas-liquid separation. However, if the number of revolutions of the pump is greatly increased, gas-liquid separation cannot be performed sufficiently, and the amount of liquid discharged by the pump tends to fluctuate. For this reason, the control device 40 keeps the increase amount of the pump rotation speed in the control 3 low compared to the increase amount of the pump rotation speed in the control 1. Therefore, when the measured pressure Pv is higher than the reference pressure Sv + α, which is the set pressure, the control device 40 is set as an adjustment value for the pump rotation speed when the measured current amount I falls below the set lower limit value I0. The first value (adjusted value set in the control 3) is a second value (set in the control 1) that is set as the adjusted value of the pump speed when the measured current amount I is equal to or greater than the set lower limit I0. Smaller than the adjustment value).

  When the measured current amount I is equal to or lower than the lower limit value I0 and the measured pressure Pv is equal to or lower than the reference pressure Sv0 + α, there are many bubbles entering the pump 23 (the state where gas-liquid separation is not functioning sufficiently). Then, the measurement pressure Pv is increased (the position of the liquid level S is increased) to suppress the decomposition of the natural gas hydrate. In this case, it is possible to simultaneously suppress the generation of bubbles by suppressing the bubbles entering the pump 23 and suppressing the decomposition of the natural gas hydrate.

  As shown in Table 2, in the state of reference pressure Sv0 + α ≧ measured pressure Pv> reference pressure Sv0, the direction of increase / decrease in the adjustment value of the pump rotation speed varies depending on the magnitude of the measured current amount I and the lower limit value I0. This is because when the measured current amount I is lower than the lower limit I0 and a large amount of bubbles enter the pump 23 and the liquid discharge capacity of the pump 23 decreases and fluctuates, the decomposition of the natural gas hydrate is promoted. Prior to measures to generate a large amount of bubbles (measures to increase the adjustment value of the pump rotation speed), it is possible to suppress fluctuations in the discharge amount of the liquid due to the bubbles flowing into the pump 23, and the natural gas hydrate This is to take measures to stabilize the decomposition. Moreover, although the decomposition of the natural gas hydrate is suppressed by the decrease in the pump rotation speed, there is a time lag until the change in the amount of bubbles generated by the decomposition appears in the gas-liquid separator 20, so during this time lag, It is possible to effectively suppress the bubbles entering the pump 23 that responds in a short time or stabilize the fluctuation of the bubbles.

Thus, the control device 40 adjusts the adjustment value of the pump rotation speed based on the measured pressure Pv measured by the pressure gauge 31 and the measured current amount I measured by the ammeter, and therefore enters the pump 23. Bubbles can be suppressed, or fluctuations in bubbles (the size and amount of bubbles) can be stabilized, whereby gas production can be performed stably.
At this time, the control device 40 increases or decreases the adjustment value according to the comparison result between the measured pressure Pv and the set pressure, and the set pressure is changed and set according to the measured current amount I as shown in Table 1. Therefore, bubbles that enter the pump 23 can be effectively suppressed, or fluctuations in bubbles (the size and amount of bubbles) that enter the pump 23 can be stabilized more effectively.

  In the state of the reference pressure Sv0 + α ≧ measured pressure Pv> reference pressure Sv0 shown in Table 2, the control device 40 changes the set pressure Pv to the first pressure when the measured current amount I higher than the lower limit value I0 is lower than the lower limit value I0. Since the reference pressure Sv0 is changed from the reference pressure Sv0 to the reference pressure Sv0 + α, which is a second pressure higher than the first pressure, as described above, a measure for promoting the decomposition of the natural gas hydrate and generating a large amount of bubbles ( Precedence of measures to increase the adjustment value of the pump rotation speed), measures to stabilize the promotion of natural gas hydrate decomposition by suppressing the flow of bubbles into the pump 23 and fluctuation of the liquid discharge amount (Measures to reduce the adjustment value of the pump speed) can be taken.

  When the measured pressure Pv is higher than the set pressure, the control device 40 increases the adjustment value of the pump rotation speed. When the measured pressure Pv is equal to or lower than the set pressure, the control apparatus 40 decreases the adjustment value of the pump rotation speed. Can be produced stably.

  It should be noted that the value α added to the reference pressure Sv as the set pressure described above may be a constant value or a fluctuation value that varies with the measured current amount I. FIG. 3 is a diagram illustrating an example in which the value α used in the embodiment is a variation value. For example, as shown in FIG. 3, the value α may change so as to increase as the measured current amount I decreases from the lower limit value I0 to the value I1. By doing in this way, the pump rotation speed of the pump 23 can be finely controlled in accordance with the amount and size of bubbles entering the pump 23.

In such a system 1, by controlling the pump rotation speed as described below, bubbles entering the pump 23 are suppressed, or fluctuations in bubbles (the size and amount of bubbles) are stabilized, so that the gas Production can be performed stably.
That is, when cracking the underground gas hydrate to produce gas,
(1) A natural gas hydrate that has an end portion embedded in the ground and that is generated in the riser tube 10 by the liquid in the riser tube 10 that extends upward from the end portion is used to generate natural gas hydrate. Reduce the applied pressure.
(2) The gas-liquid mixture containing bubbles generated by decomposing from natural gas hydrate by the reduced pressure acting on the natural gas hydrate is introduced into the riser pipe 10 from the hole 18a opened to the outside of the riser pipe 10. Take in the liquid.
(3) Gas-liquid separation is performed by the gas-liquid separation device 20 from the liquid in which the gas-liquid mixture taken in the riser tube 10 is taken out, and gas is taken out.
(4) In order to discharge the liquid separated by the gas-liquid separator 20 from the riser pipe 10, the pump 23 keeps the pump rotational speed at an adjusted value and sucks it up.
(5) At this time, the adjustment value of the pump rotational speed of the pump 23 is adjusted based on the measured current amount I of the current amount supplied for maintaining the pressure measured by the pressure gauge 31 and the pump rotational speed of the pump 23. .

  According to one embodiment, the adjustment value of the pump rotation speed is increased or decreased according to the comparison result between the measured pressure Pv and the set pressure, and the set pressure is set according to the measured current amount I. When the measured current amount I is lower than the set lower limit I0, the set pressure is changed from the reference pressure Sv (first pressure) to the reference pressure Sv0 + α (second pressure higher than the first pressure).

  According to one embodiment, when the measured pressure Pv is higher than the set pressure, the first value set as the adjustment value of the pump rotation speed when the measured current amount I falls below the set lower limit value I0. Is smaller than the second value set as the pump rotation speed adjustment value when the measured current amount I is equal to or greater than the set lower limit value I0.

  Such control of the pump rotational speed increases the adjustment value of the pump rotational speed when the measured pressure Pv is higher than the set pressure, and decreases the adjusted value of the pump rotational speed when the measured pressure Pv is equal to or lower than the set pressure. Thus, natural gas hydrate can be decomposed stably.

  It is preferable that the control device 40 performs such control of the pump rotation speed at regular time intervals.

  The gas production system and the gas production method of the present invention have been described in detail above. However, the present invention is not limited to the above-described embodiment, and various improvements and modifications may be made without departing from the gist of the present invention. Of course.

DESCRIPTION OF SYMBOLS 1 Gas production system 2 Sea bottom 3 Drilling ship 4 Upper layer 5 Hydrate layer 7 Well 10 Riser pipe 10a Tip part 11 Pipe body 12 Gas generation line 13 Liquid discharge line 13a Liquid discharge pipe 14 Liquid transport pipe 15a Space 16 Pipe 17a, 17b, 17c Bulkhead 18 Lifting pipe portion 20 Gas-liquid separator 21 Enclosure 21a Side wall 21b Bottom wall 22 Centrifuge 23 Pump 24 Motor 24a Ammeter 25 Screw 26 Heater 31 Pressure gauge 40 Controller

Claims (10)

A system for producing gas by decomposing gas hydrate in the ground,
A long tube having a tip configured to be buried in the ground, the gas hydride being external to the tube using pressure generated by the liquid in the tube extending upward from the tip By reducing the pressure acting on the rate, a gas-liquid mixture containing bubbles generated by decomposing from the gas hydrate is taken into the liquid in the tube, and is provided at the tip portion and outside the tube. A riser tube with an open hole;
A pressure gauge that is provided at the tip of the riser tube and measures the pressure at the tip of the riser tube;
A gas-liquid separation device provided in the riser pipe and separating bubbles from the liquid that has taken in the gas-liquid mixture;
A gas generation line including a gas generation pipe for taking out the gas separated by the gas-liquid separator as a gas to be produced from the riser pipe;
A pump for sucking up the liquid while maintaining the pump rotation number at an adjusted value in order to discharge the liquid from which the gas has been separated by the gas-liquid separator from the riser pipe;
An ammeter that measures the amount of current supplied to maintain the pump speed of the pump;
A gas production system comprising: a control device that adjusts the adjustment value of the pump rotation speed based on a measured pressure measured by the pressure gauge and a measured current amount measured by the ammeter.   The gas production system according to claim 1, wherein the control device increases or decreases the adjustment value according to a comparison result between the measured pressure and a set pressure, and the set pressure is set according to the measured current amount. .   The control device changes the set pressure from a first pressure to a second pressure higher than the first pressure when the measured current amount falls below a set lower limit value. Gas production system.   In the case where the measured pressure is higher than the set pressure, the control device sets the first value that is set as the adjustment value when the measured current amount is lower than a set lower limit value as the measured current. The gas production system according to claim 2 or 3, wherein the gas production system is smaller than a second value set as the adjustment value when the amount is equal to or greater than a set lower limit value. The control device increases the adjustment value when the measured pressure is higher than the set pressure,
The gas production system according to claim 1, wherein the adjustment value is reduced when the measured pressure is equal to or lower than the set pressure. A gas production method for producing gas by decomposing gas hydrate in the ground,
Acts on the gas hydrate outside the riser pipe using the pressure of the tip part generated in the riser pipe by the liquid in the riser pipe extending upward from the front end portion, having a tip portion embedded in the ground Reducing the pressure to be applied;
A gas-liquid mixture containing bubbles generated by decomposing from the gas hydrate with a reduced pressure acting on the gas hydrate is taken into the liquid in the riser pipe from a hole opened outside the riser pipe. Steps,
Performing gas-liquid separation from the liquid that has taken in the gas-liquid mixture taken into the riser tube and taking out the gas; and
A step of sucking up the liquid after the gas-liquid separation by a pump whose pump rotation number is maintained at an adjusted value in order to discharge the liquid after the gas-liquid separation from the riser pipe;
Adjusting the adjustment value of the pump rotational speed based on the measured pressure of the pressure and the measured current amount of the current amount supplied for maintaining the pump rotational speed of the pump. Gas production method.   The adjustment of the adjustment value, the adjustment value is increased or decreased according to a comparison result between the measured pressure and a set pressure, and the set pressure is set according to the measured current amount. Gas production method.   The gas production method according to claim 7, wherein when the measured current amount is lower than a set lower limit value, the set pressure is changed from a first pressure to a second pressure higher than the first pressure.   In the adjustment of the adjustment value, when the measured pressure is higher than the set pressure, the first value set as the adjustment value when the measured current amount is lower than the set lower limit value is The gas production method according to claim 7 or 8, which is smaller than a second value set as the adjustment value when a measured current amount is equal to or greater than a set lower limit value. The adjustment of the adjustment value increases the adjustment value when the measured pressure is higher than the set pressure, and decreases the adjustment value when the measured pressure is equal to or lower than the set pressure. The gas production method according to claim 1.

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