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TECHNICAL FIELD 
     The invention relates to deep-water drilling in petroleum engineering and, in particular, to a subsea wellhead pressure indicating and automatic adjusting device for deep-water dual-gradient drilling. 
     BACKGROUND 
     The development of the petroleum industry in China has been oriented to seas and oceans gradually. Especially, the efforts have been strengthened continuously in exploration and development of deep-water oil and gas fields in the South China Sea region. Offshore well drilling, especially deep-water well drilling, is confronted with a much harsher operating environment than the onshore well drilling. A prominent problem is the narrow “safe density window” incurred by the existence of seawater. To realize safe deep-water well drilling, the pressure in the wellbore must be controlled within the safe density window. Therefore, to make the pressure distribution in the wellbore closer to the two environmental pressure distributions (seawater pressure distribution above the mud line, and formation pressure distribution below the mud line) confronted in deep-water well drilling, new techniques such as dual-gradient offshore well drilling have been developed gradually, and have exhibited their obvious advantages in applications. 
     The key point in the wellbore pressure control in deep-water dual-gradient drilling is to keep the pressure distribution in the marine riser above the subsea wellhead to be equal to seawater hydrostatic pressure distribution and keep the pressure distribution in the wellbore below the subsea wellhead to be equal to formation pressure distribution, especially and, in particular, to ensure that the subsea wellhead pressure is equal to the hydrostatic pressure of seawater at the seabed. To implement deep-water dual-gradient drilling, special devices, such as rotary flow divider, etc., are installed at the subsea wellhead, so as to divide the wellbore into two parts: the upper part above the subsea wellhead is the marine riser part, and the lower part below the subsea wellhead is the wellbore part. As a result, spatial division is realized. However, the hydraulic system of the wellbore is also isolated, i.e., the marine riser part above the subsea wellhead is filled with seawater, while the wellbore part below the subsea wellhead is filled with drilling fluid. In this case, the pressure in the upper part and the pressure in the lower part of the cyclone separator are not equal to each other, which means the subsea wellhead pressure below the cyclone separator is not equal to the hydrostatic pressure of seawater at the seabed. Consequently, the detection and control of the internal pressure in the wellbore part are more difficult, and the accuracy of pressure control in deep-water dual-gradient drilling is decreased. 
     In addition, owing to the fact the seawater depth varies with time during drilling, the hydrostatic pressure of seawater at the seabed also varies with time. The monitoring of the pressure at the seabed is difficult owing to the existence of seawater, further affecting the accuracy of wellbore pressure control in dual-gradient drilling. Hence, it is necessary to design a device that can monitor the pressure at the seabed, keep the subsea wellhead pressure equal to the hydrostatic pressure of seawater at the seabed in real time and provide parameters for smooth operation of the subsea mud pump, with reference to the environmental characteristics of the deep-water well drilling. 
     SUMMARY 
     To meet the requirement for safe drilling in exploration and development of deep-water oil and gas reservoirs, the present invention provides a subsea wellhead pressure indicating and automatic adjusting device for deep-water dual-gradient drilling, which can keep the subsea wellhead pressure to be constantly equal to the hydrostatic pressure of seawater at the seabed, and can indicate the pressure at a subsea wellhead in real time to provide a reference for the operators. 
     To solve the above-mentioned technical problem, the present invention provides a subsea wellhead pressure indicating and automatic adjusting device for deep-water dual-gradient drilling, comprising: a subsea drilling fluid manifold system and a subsea wellhead pressure indicating and automatic adjusting system, wherein the subsea drilling fluid manifold system provides a flow channel for the drilling fluid returning through the wellbore so that the drilling fluid can smoothly return to a drilling platform, and provides a basis for the subsea wellhead pressure indicating and automatic adjusting system; and wherein the subsea wellhead pressure indicating and automatic adjusting system can display and adjust the subsea wellhead pressure in real time, and keep the subsea wellhead pressure to be constantly equal to the hydrostatic pressure of the seawater at the seabed, ensuring successful implementation of the dual-gradient drilling. 
     Compared with the prior art, the present invention has the following beneficial effects:
         (1) the subsea wellhead pressure is kept to be constantly equal to the hydrostatic pressure of seawater at the seabed in real time;   (2) the device is simple in structure and easy to implement, eliminating the need for additional monitoring from the drilling platform or the need for a control device or control operations;   (3) the device improves the accuracy of pressure control in the wellbore below the rotating blowout preventer for dual-gradient drilling, facilitating safe and efficient drilling.       

    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the subsea wellhead pressure indicating and automatic adjusting device for deep-water dual-gradient drilling; 
         FIG. 2  is a schematic diagram of a deep-water dual-gradient drilling apparatus with the subsea wellhead pressure indicating and automatic adjusting device mounted on it. 
     
    
    
     The reference signs in the figures are explained as follows:  100 —subsea wellhead pressure indicating and automatic adjusting device for deep-water dual-gradient drilling;  101 —first tee joint;  102 —first flat gate valve;  103 —second flat gate valve;  104 —first pressure gauge;  105 —second tee joint;  106 —third tee joint;  107 —hydraulic throttle valve;  108 —check valve;  110 —second pressure gauge;  109 —subsea mud pump;  201 —rock cuttings filter;  202 —subsea pressure controller;  2021 —subsea pressure controller casing;  2022 —lower slider;  2023 —spring;  2024 —piston;  2025 —upper slider;  2026 —subsea wellhead pressure indicator;  2027 —pressure chamber;  203 —pressure booster;  301 —floating drilling platform;  302 —marine riser;  303 —drilling stein;  304 —subsea wellhead device;  305 —drill bit;  306 —drilling fluid discharge pipeline;  307 —drilling fluid return pipeline 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     As shown in  FIG. 1 , the subsea wellhead pressure indicating and automatic adjusting device for deep-water dual-gradient drilling  100  comprises: a subsea drilling fluid manifold system and a subsea wellhead pressure indicating and automatic adjusting system, the subsea drilling fluid manifold system provides a flow channel for the drilling fluid returning through the wellbore so that the drilling fluid can smoothly return to a drilling platform, and provides a basis for the subsea wellhead pressure indicating and automatic adjusting system; the subsea wellhead pressure indicating and automatic adjusting system can display and adjust the subsea wellhead pressure in real time, and keep the subsea wellhead pressure to be constantly equal to the hydrostatic pressure of the seawater at the seabed, ensuring successful implementation of dual-gradient drilling. 
     The subsea drilling fluid manifold system comprises: a first tee joint  101 , a first flat gate valve  102 , a second flat gate valve  103 , a first pressure gauge  104 , a second tee joint  105 , a third tee joint  106 , a hydraulic throttle valve  107 , a check valve  108 , a subsea mud pump  109 , and a second pressure gauge  110 , wherein, an inlet of the first tee joint  101  is connected to a subsea wellhead device  304  through a drilling fluid discharge pipeline  306 , a first outlet of the first tee joint  101  is connected to an inlet of the first flat gate valve  102  through a pipeline, an outlet of the first flat gate valve  102  is connected to an inlet of the second tee joint  105  through a pipeline on which the first pressure gauge  104  is arranged, and a first outlet of the second tee joint  105  is connected to a first inlet of the third tee joint  106  through a pipeline; a second outlet of the first tee joint  101  is connected to an inlet of the second flat gate valve  103  through a pipeline, and an outlet of the second flat gate valve  103  is connected to a second inlet of the third tee joint  106  through a pipeline; an outlet of the third tee joint  106  is connect to a fluid inlet of the hydraulic throttle valve  107  through a pipeline, a fluid outlet of the hydraulic throttle valve  107  is connected to an inlet of the check valve  108  through a pipeline, an outlet of the check valve  108  is connected to an inlet of the subsea mud pump  109  through a pipeline on which the second pressure gauge  110  is arranged, and an outlet of the subsea mud pump  109  is connected to a floating drilling platform  301  through a drilling fluid return pipeline  307 ; the first tee joint  101  implements a branch from the drilling fluid discharge pipeline  306 ; the first flat gate valve  102  controls whether the pipeline on which it is mounted is a flowing pipeline, i.e., the pipeline is a flowing pipeline when the first flat gate valve  102  is in open state, and is a non-flowing pipeline when the first flat gate valve  102  is in closed state, the first flat gate valve  102  may be opened/closed by means of an underwater robot; the first pressure gauge  104  displays the subsea wellhead pressure, which may be observed by means of the underwater robot; the second tee joint  105  separates an incoming pipeline into two outgoing pipelines, while the third tee joint  106  merges two incoming pipelines into an outgoing pipeline; the second flat gate valve  103  controls whether the pipeline on which it is mounted is a flowing pipeline, i.e., the pipeline is a flowing pipeline when the second flat gate valve  103  is in open state, and is a non-flowing pipeline when the second flat gate valve  103  is in closed state; the second flat gate valve  103  may be opened/closed by means of the underwater robot; the hydraulic throttle valve  107  is provided with a hydraulic control unit, and the opening of the hydraulic throttle valve  107  is controlled by the subsea wellhead pressure indicating and automatic adjusting system, so that the drilling fluid flow and the throttle pressure drop can be controlled by adjusting the opening of the hydraulic throttle valve  107 ; the check valve  108  presents backflow of the drilling fluid; the second pressure gauge  110  displays the inlet pressure of the pump, which can be observed by the underwater robot; the subsea mud pump  109  provides power to the returning drilling fluid, so that the drilling fluid can return through the drilling fluid return pipeline  307  to the floating drilling platform  301 . 
     The subsea wellhead pressure indicating and automatic adjusting system comprises: a rock cuttings filter  201 , a subsea pressure controller  202 , and a pressure booster  203 , wherein, an inlet of the rock cuttings filter  201  is connected to a second outlet of the second tee joint  105  through a pipeline, the rock cuttings filter  201  has a built-in filter screen, the mesh size of which is smaller than the particle size of the solid particles handled by a solid particle handler for deep-water dual-gradient drilling, so that the solid particles in the drilling fluid are filtered, to ensure no large-size solid particle will enter the cavity of the subsea wellhead pressure indicating and automatic adjusting system; 
     The subsea pressure controller  202  comprises a subsea pressure controller casing  2021 , a lower slider  2022 , a spring  2023 , a piston  2024 , an upper slider  2025 , a subsea wellhead pressure indicator  2026 , and a pressure chamber  2027 , wherein, the upper end and the lower end of the subsea pressure controller casing  2021  have the same outer diameter, which is smaller than the outer diameter of the middle part of the subsea pressure controller casing  2021 ; the upper end and the lower end are arranged with a hole respectively for the upper slider  2025  and the lower slider  2022  to slide up and down, wherein the hole for the lower slider  2022  to slide forms the inlet of the subsea pressure controller  202 ; the inlet of the subsea pressure controller  202  is connected to the outlet of the rock cuttings filter  201 ; the inner diameter of the middle part is larger than the outer diameter of the upper end and of the lower end, the subsea pressure controller casing  2021  forms a main cavity of the subsea wellhead pressure indicating and adjusting device, the piston  2024  is arranged in the main cavity, and the main cavity and the piston  2024  are dynamically sealed; the piston  2024  separates the main cavity into an upper part and a lower part, to realize pressure isolation and ensure the upper part and the lower part do not interfere with each other; the upper part is filled with seawater, while the lower part is filled with drilling fluid; when the pressure in the upper part and the pressure in the lower part are not equal to each other, the piston  2024  can move up or down under the differential pressure, and thereby the subsea wellhead pressure indicator  2026  moves up or down and the pressure in the pressure chamber  2027  is adjusted; the lower slider  2022  is fixed at the center of the bottom surface of the piston  2024 , the upper slider  2025  is fixed at the center of the top surface of the piston  2024 , the lower slider  2022  and the upper slider  2025  ensure that the piston  2024  can move up and down only; the spring  2023  is fitted over the lower slider  2022 , the top surface of the spring  2023  is at the bottom surface of the piston  2024 , and the bottom surface of the spring  2023  is at the bottom end surface of the main cavity of the subsea wellhead pressure indicating and adjusting device; the spring  2023  provides a buffer for the up-down movement of the piston  2024  and provides a preset force to compensate for the impact of the pressure in the pressure chamber  2027  on the equilibrium position of the piston  2024 ; the main cavity is arranged with an opening in its top part, which communicates with the external seawater and thereby ensures the upper cavity above the piston  2024  is filled with seawater and the pressure in the upper cavity is the hydrostatic pressure of seawater at the seabed; the subsea wellhead pressure indicator  2026  is fixed to the top surface of the piston  2024 , protrudes through the opening arranged in the top part of the main cavity, has scale marks on it, and moves with the piston  2024 , to indicate the position of the piston in the cavity of the subsea wellhead pressure indicating and adjusting device and thereby indicate the level of difference between the upper part above the piston and the lower part below the piston the indication can be observed by the underwater robot; the pressure chamber  2027  is a cylindrical shell with a sealed top end, the bottom end of the pressure chamber  2027  is fixed to the central part of the top surface of the subsea pressure controller casing  2021 , the upper slider  2025  extends upwards into the pressure chamber, the piston is fixed to the top end of the upper slider  2025  to seal an upper space of the pressure chamber  2027 , the upper space is filled with gas, while the lower space is not sealed, but communicates with the external seawater; an outlet of the subsea pressure controller  2027  is connected to an inlet of the pressure booster  203  through a hydraulic pipeline, and an outlet of the pressure booster  203  is connected to an inlet of a hydraulic control unit of the hydraulic throttle valve  107  through a hydraulic pipeline; the pressure booster  203  boosts the pressure to provide enough power to adjust the opening of the hydraulic throttle valve  107 . 
     As shown in  FIG. 2 , a deep-water dual-gradient drilling apparatus mounted with the subsea wellhead pressure indicating and automatic adjusting device for deep-water dual-gradient drilling described above comprises a floating drilling platform  301 , a marine riser  302 , a drilling stein  303 , a subsea wellhead device  304 , a drill bit  305 , a drilling fluid discharge pipeline  306 , and a drilling fluid return pipeline  307 . 
     The floating drilling platform  301  is at the sea level, the subsea wellhead device  304  is at the mud line, the marine riser  302  is suspended below the floating drilling platform  301 , the lower end of the marine riser  302  is connected to the subsea wellhead device  304 , the drilling stein  303  passes through the marine riser  302  and the subsea wellhead device  304 , the drill bit  305  is mounted at the bottom end of the drilling stein  303 , the subsea wellhead device  304  is connected to the inlet of the first tee joint  101  of the subsea wellhead pressure indicating and automatic adjusting device for deep-water dual-gradient drilling  100  through the drilling fluid discharge pipeline  306 , and the outlet of the subsea mud pump  109  of the subsea wellhead pressure indicating and automatic adjusting device for deep-water dual-gradient drilling  100  is connected to the floating drilling platform  301  through the drilling fluid return pipeline  307 . 
     The floating drilling platform  301  provides a working space for pressure controlled drilling operation; the marine riser  302  provides a channel for the drilling stein  303  to move up and down, the seawater in the marine riser  302  maintains pressure balance between the interior and the exterior of the marine riser  302  to prevent the marine riser  302  from being damaged under the action of seawater pressure; the drilling stein  303  lifts up and lowers down the drill bit  305 , the drill bit  305  drills in the formation; the subsea wellhead device  304  includes in a subsea wellhead, a blowout preventer unit, a solid particle handler, and a rotary flow divider, wherein a drilling fluid outlet is arranged below the rotary flow divider to split the drilling fluid from the lower wellbore, so that the drilling fluid flows to the subsea wellhead pressure indicating and automatic adjusting device  100  for deep-water dual-gradient drilling; the drilling fluid discharge pipeline  306  provides a flow channel for the drilling fluid discharged from the wellbore; the subsea wellhead pressure indicating and automatic adjusting device  100  for deep-water dual-gradient drilling adjusts the subsea wellhead pressure automatically; and the drilling fluid return pipeline  307  provides a flow channel for the drilling fluid to return to the floating drilling platform  301 . 
     During deep-water dual-gradient drilling, an annular space formed by the marine riser  302  above the subsea wellhead device  304  and the drilling stein  303  is filled with seawater; the drilling fluid from the floating drilling platform  301  reaches to the downhole drill bit  305  through the drilling stein  303 , flows out, enters the annular space formed by the drilling stein  303  and the wellbore and returns upwards carrying the rock cuttings. 
     When the returning drilling fluid reaches the mud line, it flows through a cyclone separator in the subsea wellhead device  304  into the subsea drilling fluid manifold system; the pipeline mounted with the first flat gate valve  102  is the working pipeline, while the pipeline mounted with the second flat gate valve  103  is a standby pipeline; 
     During normal deep-water dual-gradient drilling, the first flat gate valve  102  is in open state, the second flat gate valve  103  is in closed state, and the drilling fluid flows through the pipeline mounted with the first flat gate valve  102 ; in case the subsea wellhead pressure indicating and automatic adjusting system fails or in any other accidental circumstance, the first flat gate valve  102  can be closed and the second flat gate valve  103  can be opened by an underwater robot, so that the drilling fluid flows through the pipeline mounted with the second flat gate valve  103 . 
     During normal deep-water dual-gradient drilling, the first pressure gauge  104  displays subsea wellhead pressure; the returning drilling fluid flows through the pipeline mounted with the first flat gate valve  102  into the cavity below the piston  2024  of the subsea wellhead pressure indicating and automatic adjusting system, to ensure the pressure in the cavity is the subsea wellhead pressure, which is denoted as P 1 ; the seawater flows through the opening in the top part of the subsea pressure controller casing  106  into the cavity above the piston  2024  of the subsea wellhead pressure indicating and automatic adjusting system, to ensure the pressure in the cavity is the hydrostatic pressure of seawater at the seabed, which is denoted as P 2 . 
     If P 1  is greater than P 2 , it indicates that the subsea wellhead pressure exceeds the hydrostatic pressure of seawater at the seabed. In this case, the pressure in the upper part above the piston  2024  and the pressure in the lower part below the piston  2024  are not equal to each other, specifically, the pressure in the upper part is lower than the pressure in the lower part, the piston  2024  moves upwards under the differential pressure and drives the subsea wellhead pressure indicator  2026  to move upwards together. As a result, the exposed length of the subsea wellhead pressure indicator  2026  increases, indicating that the subsea wellhead pressure is too high. The upper slider  2025  is also driven by the piston  2024  to move upwards to compresses the gas in the pressure chamber  2027 , so that the pressure in the pressure chamber  2027  is increased. The pressure in the pressure chamber  2027  is transferred through the hydraulic pipeline to the pressure booster  203 , whereby the hydraulic control unit of the hydraulic throttle valve  107  is actuated. As a result, the opening of the hydraulic throttle valve  107  is increased, the throttle pressure drop is decreased, the drilling fluid flow is increased, and thereby P 1  is decreased towards P 2  until they are equal to each other; 
     If P 1  is smaller than P 2 , it indicates that the subsea wellhead pressure is lower than the hydrostatic pressure of seawater at the seabed. In this case, the pressure in the upper part above the piston  2024  and the pressure in the lower part below the piston  2024  are not equal to each other, specifically, the pressure in the upper part is higher than the pressure in the lower part, the piston  2024  moves downwards under the differential pressure, and drives the subsea wellhead pressure indicator  2026  to move downwards together. As a result, the exposed length of the subsea wellhead pressure indicator  2026  decreases, indicating that the subsea wellhead pressure is too low. The upper slider  2025  is also driven by the piston  2024  to move downwards, resulting in gas expansion in the pressure chamber  2027 , so that the pressure in the pressure chamber  2027  is decreased. The pressure in the pressure chamber  2027  is transferred through the hydraulic pipeline to the pressure booster  203 , whereby the hydraulic control unit of the hydraulic throttle valve  107  is actuated. As a result, the opening of the hydraulic throttle valve  107  is decreased, the throttle pressure drop is increased, the drilling fluid flow is reduced, and thereby P 1  is increased towards P 2  until they are equal to each other; 
     If P 1  is equal to P 2 , it means that the subsea wellhead pressure is equal to the hydrostatic pressure of seawater at the seabed. In this case, the pressure in the upper part above the piston  2024  and the pressure in the lower part below the piston  2024  are equal to each other. As a result, the position of the piston  2024 , the exposed length of the subsea wellhead pressure indicator  2026 , as well as the opening of the hydraulic throttle valve  107  will not change, and the throttle pressure drop and the drilling fluid flow will be maintained; 
     The check valve  108  in the subsea drilling fluid manifold system ensures backflow of the drilling fluid will not occur; the second pressure gauge  110  displays the inlet pressure of the subsea mud pump  109 , which can be observed by the subsea robot; the returning drilling fluid returns along the drilling fluid return pipeline  307  to the floating drilling platform  301  under the action of the subsea mud pump  109 ; after necessary operations, such as solid particle removal, etc., the drilling fluid is injected into the drilling stein  303  again, and thereby the drilling fluid is circulated; thus, safe dual-gradient drilling is realized.

Summary:
The present invention relates to deep-water drilling in petroleum engineering and, in particular, to a subsea wellhead pressure indicating and automatic adjusting device for deep-water dual-gradient drilling, comprising a subsea drilling fluid manifold system and a subsea wellhead pressure indicating and automatic adjusting system, wherein the subsea drilling fluid manifold system provides a flow channel for the drilling fluid returning through the wellbore so that the drilling fluid can smoothly return to a drilling platform, and provides a basis for the subsea wellhead pressure indicating and automatic adjusting system; and wherein the subsea wellhead pressure indicating and automatic adjusting system can display and adjust the subsea wellhead pressure in real time, in order to keep the subsea wellhead pressure to be constantly equal to the hydrostatic pressure of the seawater at the seabed. The present invention makes it possible to keep the subsea wellhead pressure to be constantly equal to the hydrostatic pressure of the seawater at the seabed in an automatic and real-time way, ensuring successful implementation of the dual-gradient drilling.