A natural gas hydrate core grinding device, system and method

Abstract

This invention provides a natural gas hydrate core grinding device, system, and method. The grinding device includes: a housing; a controlled cooling platform disposed within the housing, having an internal cooling channel for the flow of liquid cooling medium; a core clamp disposed on the controlled cooling platform, capable of exchanging heat with the platform to cool the core sample it holds; and a grinding disc assembly located above the core clamp, including a first motor and a grinding disc connected to the output of the first motor, the grinding disc capable of grinding the upper surface of the core sample held in the core clamp. Based on the technical solution of this invention, the temperature of the grinding area can be acquired and controlled in real time, preventing core deformation due to excessive temperature and preventing the decomposition of natural gas hydrate in the core due to excessive temperature during grinding. Furthermore, the grinding process is fully automatic, requiring no manual intervention, thereby improving grinding efficiency and grinding quality.

Description

Technical Field

[0001] This invention relates to the field of grinding equipment technology, and particularly to a grinding device, system and method for grinding natural gas hydrate core samples. Background Technology

[0002] Natural gas hydrates are ice-like crystalline substances formed by natural gas and water under low temperature and high pressure conditions, distributed in deep-sea sediments or permafrost on land. Natural gas hydrates are a vast, clean, unconventional energy resource and one of my country's strategic resources. Hydrate sample analysis, simply put, involves using various testing methods to observe and analyze the phase composition and microstructure of hydrate core samples. These methods include reflected light microscopy, polarized light microscopy, confocal laser Raman spectroscopy, scanning electron microscopy, X-ray diffraction (XRD), and infrared absorption spectroscopy. All of these methods require sufficient surface smoothness and cleanliness of the sample; therefore, before analyzing natural gas hydrate cores, a grinding mill is needed to grind the hydrate cores into flat, thin sections.

[0003] Natural gas hydrate cores require a low-temperature environment for stable storage. Traditional rock sample grinding mills, lacking temperature control on their worktables, cause decomposition of the natural gas hydrate samples during grinding. Furthermore, their structure is unsuitable for residual hydrate cores, making it impossible to grind them into thin sections. Manual grinding also has several drawbacks, such as slow speed and inefficiency, difficulty in ensuring the flatness and thickness uniformity of the sample sections, and poor grinding quality. In addition, manual grinding requires operation in a liquid nitrogen bath, with an ultra-low temperature environment of approximately -190°C posing significant safety hazards. Summary of the Invention

[0004] To address the problem that existing core grinding methods and machines cannot meet the requirements for hydrate core grinding, this application proposes a natural gas hydrate core grinding device, system, and method.

[0005] In a first aspect, the present invention provides a natural gas hydrate core grinding device, comprising:

[0006] Box;

[0007] A cooling control platform is installed inside the box, and its interior has a cooling channel for the flow of liquid refrigerant.

[0008] A core clamp is mounted on the cooling platform and is capable of exchanging heat with the cooling platform to cool the core sample it holds.

[0009] The grinding disc assembly, located above the core clamp, includes a first motor and a grinding disc connected to the output end of the first motor. The grinding disc is capable of grinding the upper surface of the core sample clamped in the core clamp.

[0010] In one implementation, it further includes:

[0011] A clamp drive assembly, disposed below the controlled cooling table, includes a second motor and a rotating shaft connected to the output end of the second motor. The rotating shaft passes through the controlled cooling table and connects to the bottom of the core clamp. In this embodiment, the clamp drive assembly can drive the core clamp to rotate, thereby increasing the relative rotational speed between the core clamp and the grinding disc, thus improving the grinding efficiency.

[0012] In one embodiment, the grinding wheel assembly further includes:

[0013] The stepper telescopic shaft has one end connected to the output end of the first motor via a coupling, and the other end detachably connected to the grinding disc. The coupling enables the stepper telescopic shaft to move axially relative to the first motor.

[0014] The stepping mechanism, in conjunction with the stepping telescopic shaft, can control the axial extension of the stepping telescopic shaft to adjust the feed rate of the grinding disc.

[0015] In one embodiment, a gaseous refrigerant nozzle is further included. The gaseous refrigerant nozzle is disposed on one side of the core clamp and faces the core clamp, and is connected to a gaseous refrigerant source via a pipeline. This embodiment allows gaseous refrigerant to be sprayed into the core clamp through the gaseous refrigerant nozzle, combining with liquid refrigerant to improve the cooling effect.

[0016] In one embodiment, the refrigeration channel has a medium inlet and a medium outlet at both ends, and the medium inlet and the medium outlet are respectively connected to a liquid refrigeration medium storage tank and a medium pump through pipelines.

[0017] In one embodiment, the media pump has a media vaporization device inside, and the media pump can serve as a source of gaseous cooling medium for outputting gaseous cooling medium to the core clamp. By adding a media vaporization device to the media pump, the media pump can simultaneously serve as the input power for both liquid and gaseous media, facilitating simultaneous control of the flow rates of both media, reducing the need for power components, and lowering the overall cost of the device.

[0018] In one embodiment, a temperature sensor is installed inside the cooling station, and the temperature sensor is a thermistor.

[0019] In one embodiment, the cooling platform is further equipped with an electric heating wire. In this embodiment, the electric heating wire is used in conjunction with the cooling medium to adjust the final cooling temperature and simultaneously control the temperature stability of the temperature control zone.

[0020] Secondly, the present invention provides a natural gas hydrate core grinding system, comprising the aforementioned grinding apparatus; and

[0021] The grinding disc control assembly includes a controller, which is capable of inputting control commands to control the operating parameters of the grinding components in the grinding disc device;

[0022] The cooling control component includes a temperature controller electrically connected to a temperature sensor in the grinding device, which can adjust the temperature of the core sample in the grinding device by controlling the cooling components in the grinding device.

[0023] Thirdly, this invention proposes a method for grinding natural gas hydrate cores into discs, applied to the aforementioned grinding system, comprising the following steps:

[0024] S1: The core sample to be ground is held in the core clamp;

[0025] S2: Set the grinding parameters through the controller of the grinding control component, start the cooling of the core sample through the temperature controller of the cooling control component, and then start the grinding.

[0026] S3: The temperature controller controls the flow rate of the cooling medium and the heating of the heating element according to the temperature change during the grinding process, so as to keep the cooling temperature of the core sample in the optimal temperature range.

[0027] S4: The grinding is complete. The ground core sample is removed from the core clamp.

[0028] The above-mentioned technical features can be combined in various suitable ways or replaced by equivalent technical features, as long as the purpose of the present invention can be achieved.

[0029] The natural gas hydrate core grinding device, system, and method provided by this invention have at least the following advantages compared with the prior art:

[0030] This invention discloses a natural gas hydrate core grinding device, system, and method that can acquire and control the temperature of the grinding area in real time to match the optimal temperature during the grinding process, preventing core deformation due to excessive temperature and ensuring the flatness of the grinding disc. Simultaneously, it prevents the decomposition of natural gas hydrate in the core due to excessive temperature during grinding. Furthermore, the grinding process and operation of the device are fully automated, requiring no manual intervention. It offers high grinding efficiency and precise control over disc thickness, thickness uniformity, and surface flatness. Attached Figure Description

[0031] The invention will now be described in more detail with reference to embodiments and the accompanying drawings.

[0032] Figure 1 A schematic diagram of the grinding device of the present invention is shown.

[0033] In the accompanying drawings, the same parts use the same reference numerals. The drawings are not to scale.

[0034] Figure label:

[0035] 10-Box body, 20-Cooling control platform, 201-Temperature signal line, 21-Medium inlet, 211-Liquid refrigerant storage tank, 22-Medium outlet, 221-Medium pump, 30-Core clamp, 40-Grinding disc assembly, 41-First motor, 42-Stepper telescopic shaft, 43-Grinding disc, 50-Clamp drive assembly, 51-Rotating shaft, 52-Second motor, 60-Gaseous refrigerant nozzle, 70-Controller, 80-Temperature controller. Detailed Implementation

[0036] The invention will now be further described with reference to the accompanying drawings.

[0037] Example 1

[0038] As shown in the attached figure. Figure 1 As shown, an embodiment of the present invention provides a natural gas hydrate core grinding disc device, comprising:

[0039] Box 10;

[0040] The cooling control platform 20 is located in the housing 10 and has a cooling channel for the flow of liquid refrigerant.

[0041] The core clamp 30 is set on the cooling table 20 and can exchange heat with the cooling table 20 to cool the core sample it holds.

[0042] The grinding disc assembly 40 is located above the core clamp 30. It includes a first motor 41 and a grinding disc 43 connected to the output end of the first motor 41. The grinding disc 43 is capable of grinding the upper surface of the core sample clamped in the core clamp 30.

[0043] Specifically, the grinding device in this embodiment mainly includes a clamping part, a temperature control part, and a grinding part.

[0044] The clamping component mainly includes a core clamp 30, which is used to hold the core sample to be ground. Furthermore, the core clamp 30 includes multiple radially arranged adjusting screws. These adjusting screws are threaded into the clamp body. By rotating the adjusting screws, they can be moved radially to adjust the size of the clamping area of ​​the core clamp 30, thereby accommodating core samples of different sizes and shapes.

[0045] The temperature control section mainly includes a cooling table 20. The core clamp 30 is placed on the cooling table 20 and cooled by the cooling table 20 to maintain the temperature of the core sample on the core clamp 30 within a suitable temperature range during the grinding process. This prevents the core sample from decomposing due to excessive temperature and reduces uneven thickness of the core sample caused by thermal deformation, further improving the grinding and polishing quality. In addition, the cooling table 20 also has a temperature regulation function. The cooling table 20 is equipped with a temperature sensor and an electric heating wire (not shown in the attached figure). The temperature sensor is connected to an external temperature signal line 201 to acquire and transmit the temperature data of the cooling table 20 in real time. When the temperature is higher than the suitable temperature, the cooling effect is enhanced; when the temperature is lower than the suitable temperature, the temperature is increased by the electric heating wire, thereby matching the temperature to the target temperature range.

[0046] It should be noted that the cooling method of the controlled cooling platform 20 can be adapted to specific circumstances, such as using semiconductor refrigeration or conventional refrigerant refrigeration. In this embodiment, considering the cooling efficiency and cooling capacity, conventional refrigerant refrigeration is used. A cooling channel for the flow of liquid refrigerant is set inside the controlled cooling platform 20. In principle, the cooling channel can also be used with gaseous refrigerant, but the cooling effect of gaseous refrigerant is worse than that of liquid refrigerant. The two ends of the cooling channel have a medium inlet 21 and a medium outlet 22, respectively. The medium inlet 21 and the medium outlet 22 are connected to a liquid refrigerant storage tank 211 and a medium pump 221 through pipelines, respectively. The medium pump 221 draws the liquid medium from the liquid refrigerant storage tank 211 by creating a local negative pressure. During the process of the liquid medium being output from the liquid refrigerant storage tank 211 to the medium pump 221, it flows through the cooling channel inside the controlled cooling platform 20 to realize the cooling function of the controlled cooling platform 20. During the grinding process, the temperature of the core sample will change due to frictional heat. The corresponding cooling temperature can be achieved by adjusting the cooling effect of the cooling stage 20, which can be achieved by controlling the output power of the medium pump 221 to adjust the flow rate of the liquid cooling medium passing through the cooling channel.

[0047] Furthermore, the temperature control section may also include a gaseous refrigerant nozzle 60. The gaseous refrigerant nozzle 60 is connected to a gaseous refrigerant source via a pipeline. The gaseous refrigerant nozzle 60 can spray gaseous refrigerant into the core clamp 30 to further improve the cooling effect based on the liquid refrigerant. Based on the aforementioned media pump 221, the media pump 221 can be modified into a gaseous refrigerant source, i.e., a media vaporization device (not shown in the attached diagram) can be installed inside the media pump 221. After the liquid medium enters the media pump 221, it is vaporized into a gaseous medium by the media vaporization device and then sprayed out through the gaseous refrigerant nozzle 60 for cooling. Controlling the output power of the media pump 221 can simultaneously control the flow rates of both the liquid and gaseous media, achieving accurate control of the cooling temperature.

[0048] The grinding disc assembly mainly includes a grinding disc assembly 40. The grinding disc 43 within the assembly is connected to the output of a first motor 41 and is located directly above the core clamp 30. The first motor 41 provides power for the rotation of the grinding disc 43. When grinding the core sample, the grinding surface of the grinding disc 43 contacts the upper surface of the core sample, and the first motor 41 drives the grinding disc 43 to rotate, thus grinding the core sample. The grinding disc 43 is detachably connected to the first motor 41, allowing for the replacement of different grinding discs 43 according to different grinding requirements.

[0049] During the grinding process, as grinding progresses, the feed rate of the grinding disc 43 needs to increase to match the continuous decrease in the thickness of the core sample. Therefore, the grinding disc 43 needs to have a moving function in the direction of approaching or moving away from the core clamp 30. The moving function of the grinding disc 43 can be that the first motor 41 is fixed and the grinding disc 43 moves itself; or the first motor 41 and the grinding disc 43 can move as a whole. In this embodiment, the first motor 41 is fixed and the grinding disc 43 moves itself. Therefore, the grinding disc assembly 40 also includes:

[0050] The stepper telescopic shaft 42 is connected at one end to the output end of the first motor 41 via a coupling, and at the other end is detachably connected to the grinding disc 43. The coupling enables the stepper telescopic shaft 42 to move axially relative to the first motor 41.

[0051] The stepping mechanism, in conjunction with the stepping telescopic shaft 42, can control the axial extension of the stepping telescopic shaft 42 to adjust the feed amount of the grinding disc 43.

[0052] Specifically, the stepping telescopic shaft 42 is fixedly connected to the grinding disc 43, and the movement of the grinding disc 43 relative to the first motor 41 is converted into the movement of the stepping telescopic shaft 42 relative to the motor. Since the range of variation in the feed amount of the grinding disc 43 (i.e., the reduction in thickness of the core sample after grinding) is small and within a certain range, the stepping telescopic shaft 42 can be connected to the output end of the first motor 41 shaft via a coupling. The coupling adopts a structure that enables relative movement between the output end of the stepping telescopic shaft 42 and the first motor 41 shaft, such as a spline coupling. The stepping mechanism (not shown in the attached figure) is used to control the feed of the stepping telescopic shaft 42. It is fixedly connected to the stepping telescopic shaft 42 axially and movably engaged with it circumferentially, for example, by using bearings to connect the two. The stepping mechanism itself has a controllable movement function to control the feed of the stepping telescopic shaft 42. The specific structure of the stepping mechanism can be selected according to the situation; it can adopt a highly integrated method such as a cylinder, or other structures with controllable telescopic movement functions.

[0053] In this embodiment, since the suitable grinding temperature for natural gas hydrate is around -190°C, nitrogen is used as the cooling medium, i.e., liquid nitrogen in liquid form and nitrogen gas in gaseous form. The grinding apparatus of this embodiment can also be applied to grinding other samples requiring temperature control, such as ice samples and porous ice-containing rock samples, etc., and therefore, other cooling media can be used to match the required target temperature.

[0054] In this embodiment, liquid nitrogen is used as the refrigerant. To prevent personnel injury from liquid nitrogen leakage, all functional components of the grinding device are housed within the enclosure 10, and the enclosure 10 is sealed to a certain extent to prevent liquid nitrogen from leaking to the outside of the grinding device. Furthermore, part of the enclosure wall of the enclosure 10 is made of transparent material to facilitate observation of the grinding process inside the enclosure 10.

[0055] The grinding device in this embodiment can acquire and control the temperature of the grinding area in real time to match the corresponding temperature of natural gas hydrate, thus preventing the natural gas hydrate from decomposing due to excessive temperature during the grinding process. Furthermore, the grinding process and operation of the device are fully automated, requiring no manual intervention. It boasts high grinding efficiency and can achieve precise control over the grinding thickness, thickness uniformity, and surface flatness.

[0056] Example 2

[0057] As shown in the attached figure. Figure 1 As shown, an embodiment of the present invention provides a natural gas hydrate core grinding disc device, comprising:

[0058] Box 10;

[0059] The cooling control platform 20 is located in the housing 10 and has a cooling channel for the flow of liquid refrigerant.

[0060] The core clamp 30 is set on the cooling table 20 and can exchange heat with the cooling table 20 to cool the core sample it holds.

[0061] The grinding disc assembly 40 is located above the core clamp 30. It includes a first motor 41 and a grinding disc 43 connected to the output end of the first motor 41. The grinding disc 43 is capable of grinding the upper surface of the core sample clamped in the core clamp 30.

[0062] The clamp drive assembly 50 is located below the cooling table 20. It includes a second motor 52 and a rotating shaft 51 connected to the output end of the second motor 52. The rotating shaft 51 passes through the cooling table 20 and is connected to the bottom of the core clamp 30.

[0063] Specifically, the grinding disc device in this embodiment further includes a clamping drive assembly 50. A rotating shaft 51 in the clamping drive assembly 50 is fixedly connected to the output end of the second motor 52, and the end of the rotating shaft 51 is fixedly connected to the bottom of the core clamp 30. In this way, the clamping drive assembly 50 can drive the core clamp 30 to rotate, thereby improving grinding efficiency. That is, the rotation direction of the second motor 52 is opposite to that of the first motor 41, resulting in the grinding disc 43 and the core clamp 30 having opposite rotational motions, thus increasing the degree of relative rotation and improving grinding efficiency. The rotational speed of the core clamp 30 driven by the second motor 52 is less than the rotational speed of the grinding disc 43 to avoid instability of the core clamp 30 structure due to excessive rotational speed. Therefore, a speed reducer needs to be further provided at the output end of the second motor 52.

[0064] The connection structure between the rotating shaft 51 of the clamp drive assembly 50 and the core clamp 30 can be such that the rotating shaft 51 passes through the controlled cooling table 20 and connects to the center of the bottom of the core clamp 30, or it can be as shown in the attached figure. Figure 1 The core clamp 30 shown has a connecting shaft extending from the center of its bottom, which passes through the controlled cooling table 20. The rotating shaft 51 is fixed relative to the connecting shaft. Regardless of the method used, the controlled cooling table 20 has a passage opening in its center for the corresponding components to pass through.

[0065] Example 3

[0066] Embodiments of the present invention provide a natural gas hydrate core grinding system, including the aforementioned grinding apparatus; and

[0067] The grinding disc control assembly includes a controller 70, which is capable of inputting control commands to control the operating parameters of the grinding components in the grinding disc device;

[0068] The cooling control component includes a temperature controller 80, which is electrically connected to a temperature sensor in the grinding device. The temperature controller 80 can adjust the temperature of the core sample in the grinding device by controlling the cooling components in the grinding device.

[0069] Specifically, the grinding disc control assembly adopts a PLC-based automatic control system controller 70, which is installed together with the first motor 41 in the control box. The control system in the controller 70 is connected to the first motor 41 of the grinding disc assembly 40, the stepper telescopic shaft 42, and the second motor 52 of the clamp drive assembly 50. It controls the operation and output power of each motor through the instructions input on its control panel, and precisely controls the motor speed, grinding disc 43 feed, and grinding thickness during the grinding process to ensure that the ground core slices have uniform thickness, good flatness, and good consistency.

[0070] The data input terminal of the temperature controller 80 is connected to the temperature sensor in the cooling platform 20 via the temperature signal line 201 to acquire temperature data in real time. Simultaneously, the signal output terminal of the temperature controller 80 is connected via signal lines to the heating device corresponding to the electric heating wire and the medium pump 221 to control the operation of the corresponding components, ensuring that the temperature at the core clamp 30 and the core sample matches the target temperature. For natural gas hydrates, the target temperature is approximately -190℃, requiring the temperature control accuracy of the temperature controller 80 to meet ±0.1℃.

[0071] Furthermore, the temperature controller includes an internal microcontroller and an external control panel, serving to regulate the temperature of the cooling platform. The microcontroller includes a data acquisition module, a media pump power control system, and a temperature control system. Its circuit operation principle is as follows: an electrical signal generated by a temperature sensor is sent to the microcontroller's data acquisition module to obtain a sampled temperature value. The microcontroller performs PID calculations, and based on the calculation results, controls the microcontroller's pulse width modulation (PWM). The PWM output is sent to the power regulation circuit to adjust the output power of the corresponding media pump and heating wire, achieving power regulation. The cooling and heating outputs are controlled by independent PID controllers.

[0072] Example 4

[0073] An embodiment of the present invention provides a method for grinding natural gas hydrate cores into discs, applied to the aforementioned grinding system, comprising the following steps:

[0074] S1: The core sample to be ground is held in the core clamp 30;

[0075] S2: Set the grinding parameters through the controller 70 of the grinding control component, start the cooling of the core sample through the temperature controller 80 of the cooling control component, and then start the grinding.

[0076] S3: The temperature controller 80 controls the flow rate of the cooling medium and the heating of the heating components according to the temperature changes during the grinding process, so as to keep the cooling temperature of the core sample in the optimal temperature range.

[0077] S4: The grinding is complete. Remove the ground core sample from the core clamp 30.

[0078] In the description of this invention, it should be understood that the terms "upper", "lower", "bottom", "top", "front", "rear", "inner", "outer", "left", "right", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0079] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A grinding disc device for natural gas hydrate core samples, characterized in that, include: Box; A cooling control platform is installed inside the enclosure, and its interior has a cooling channel for the flow of liquid refrigerant; a temperature sensor, which is a thermistor, is installed inside the cooling control platform; an electric heating wire is also installed inside the cooling control platform. A core clamp is mounted on the cooling platform and is capable of exchanging heat with the cooling platform to cool the core sample it holds. A grinding disc assembly, located above a core clamp, includes a first motor and a grinding disc connected to the output end of the first motor. The grinding disc is capable of grinding the upper surface of the core sample clamped in the core clamp. The core clamp includes multiple radially arranged adjusting screws, which are threadedly engaged with the clamp body. By rotating the adjusting screws, the screws move radially to adjust the size of the clamping area of ​​the core clamp, thereby accommodating core samples of different sizes and shapes. The grinding disc assembly also includes: A stepper telescopic shaft is connected at one end to the output end of the first motor via a coupling, and at the other end is detachably connected to the grinding disc. The coupling enables the stepper telescopic shaft to move axially relative to the first motor. A stepping mechanism, in conjunction with the stepper telescopic shaft, can control the axial extension of the stepper telescopic shaft to adjust the feed amount of the grinding disc. A clamp drive assembly is located below the controlled cooling table. It includes a second motor and a rotating shaft connected to the output end of the second motor. The rotating shaft passes through the controlled cooling table and is connected to the bottom of the core clamp. The second motor rotates in the opposite direction to the first motor, so the grinding disc and the core clamp have rotational movements in opposite directions. The rotational speed of the core clamp driven by the second motor is less than the rotational speed of the grinding disc. The refrigeration channel has a medium inlet and a medium outlet at both ends, and the medium inlet and the medium outlet are respectively connected to a liquid refrigeration medium storage tank and a medium pump through pipelines; It also includes a gaseous cooling medium nozzle, which is located on one side of the core clamp and faces the core clamp, and is connected to a gaseous cooling medium source through a pipeline; the medium pump has a medium vaporization device inside, and the medium pump can serve as a gaseous cooling medium source for outputting gaseous cooling medium to the core clamp.

2. A natural gas hydrate core grinding system, characterized in that, Includes the grinding apparatus as described in claim 1; as well as The grinding disc control assembly includes a controller, which is capable of inputting control commands to control the operating parameters of the grinding components in the grinding disc device; The cooling control component includes a temperature controller electrically connected to a temperature sensor in the grinding device, which can adjust the temperature of the core sample in the grinding device by controlling the cooling components in the grinding device.

3. A method for grinding natural gas hydrate cores, characterized in that, The grinding system described in claim 2 is further comprising the following steps: S1: The core sample to be ground is held in the core clamp; S2: Set the grinding parameters through the controller of the grinding control component, start the cooling of the core sample through the temperature controller of the cooling control component, and then start the grinding. S3: The temperature controller controls the flow rate of the cooling medium and the heating of the heating element according to the temperature change during the grinding process, so as to keep the cooling temperature of the core sample in the optimal temperature range. S4: The grinding disc is completed, and the ground core sample is removed from the core clamp.

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