Drilling systems used for geological exploration drilling

Abstract

A drilling system for geological exploration drilling includes: a drilling platform, a production part and a drilling fluid return system. The production part includes: a borehole; a drill pipe; a watertight subsea blowout preventer arranged at the wellhead of the borehole; and a drilling fluid volume controller arranged on the top of the blowout preventer. The watertight subsea blowout preventer is configured to allow or prevent the drilling fluid in the borehole from entering the drilling fluid volume controller based on the pressure state in the borehole. The drilling fluid return system includes: a double-layer pipe, which includes an outer layer pipe and an inner layer pipe, an annular space between the outer layer pipe and the inner layer pipe; and a terminal module. The terminal module is connected to the lower end of the double-layer pipe and includes a first pipeline arranged between the annular space and the volume controller; a second pipeline connected between the inner layer pipe and the annular space; and a third pipeline connected between the inner layer pipe and the blowout preventer. The first pipeline, the second pipeline and the third pipeline can be opened or closed based on the pressure state in the borehole.

Description

Technical Field

[0001] The present disclosure relates to a drilling system for deepwater and ultra-deepwater geological exploration drilling, and in particular to a riser-free drilling system for deep-sea or ultra-deep-sea seabed geological exploration. Background Art

[0002] Ultra-deepwater drilling usually consists of a large drilling platform and a large-diameter riser system connecting the underwater well to the drilling platform. The most advanced drilling platform is designed to drill at a water depth of 3,600 meters. The current world record for drilling depth is 3,402 meters (2016). The riser has multiple functions: the conduit is used to return the drilling fluid in the well; the rigid throttling and well-killing pipeline connects the drilling platform and the underwater blowout preventer for well control.

[0003] The main difference between riser-free and riser drilling is that conventional drilling uses a riser. The drilling fluid pump installed on the floating drilling ship pumps the drilling fluid required for drilling into the well through the drill pipe, and the drilling fluid returns to the floating drilling ship through the riser, and its power is provided by the same drilling fluid pump.

[0004] Currently, riserless drilling technology has been developed to a water depth of 1,500 meters and is used for surface drilling before the installation of a blowout preventer. Existing subsea drilling fluid pump systems have limitations. In an existing riserless drilling technology, a rotating control head is installed on the top of the subsea blowout preventer to isolate the seawater from the pressure of the subsea blowout preventer and the drilling fluid in the well, creating a closed riserless drilling fluid circulation loop. The operating water depth of the pump system, which is still under testing, can reach about 2,000 meters, which has obvious limitations in deeper waters. The hydrostatic pressure of the drilling fluid in the long pipeline has a great impact on the safety control stability in the well. The well control pipeline of the riserless drilling system operating in shallow water areas usually uses an independent pipeline to run through the open water between the drilling ship and the blowout preventer. In ultra-deepwater drilling operations, such high-pressure well control pipelines are neither economical nor practical in terms of safe deployment and operation. In order to control the drilling fluid and hydrostatic head in the well, current floating drilling vessels based on this riserless drilling technology will not conduct deepwater exploration drilling without using a riser.

[0005] In another related riserless drilling technology, no swivel head control equipment is used on top of the subsea BOP. A floating drillship pumps drilling fluid into the well, and the drilling fluid and cuttings are pumped back to the drillship by a drilling fluid pump installed on the seafloor, which is connected to the drillship through a drilling fluid return line. The limitation of performing deepwater riserless drilling is limited by the current variable pump technology. When the water depth exceeds 1500-2000m, the variable pump cannot pump a large enough volume and high specific gravity of drilling fluid back to the drillship.

[0006] Therefore, existing riser-free drilling technologies are not suitable for operations on the seabed above 2,000 meters. Summary of the invention

[0007] The purpose of the present disclosure is to overcome at least one of the above-mentioned deficiencies in the prior art.

[0008] The present disclosure provides a drilling system for geological exploration drilling, which does not include a watertight pipe. The system has no size restrictions on the drilling platform and no restrictions on the drilling water depth. The drilling water depth can far exceed 3600m, and conventional well control can be performed at the same time.

[0009] In one aspect of the present disclosure, there is provided a drilling system for geological exploration drilling, comprising: a drilling platform capable of floating and moving on the water surface; a production part, comprising: a borehole formed in a seabed formation; a drill pipe extending downward from the drilling platform into the borehole; a riserless subsea blowout preventer, which is arranged at the wellhead of the borehole and the drill pipe extends through the riserless subsea blowout preventer for drilling; and a drilling fluid volume controller, which is arranged on the top of the riserless subsea blowout preventer, wherein the riserless subsea blowout preventer is constructed to allow or prevent the drilling fluid in the borehole from entering the drilling fluid volume controller based on the pressure state in the borehole. device; and a drilling fluid return system, which is connected to the drilling platform and separated from the production part, and comprises: a double-layer pipe, which comprises an outer pipe and an inner pipe, and an annular space is formed between the outer pipe and the inner pipe; and a terminal module, which is connected to the lower end of the double-layer pipe and comprises a first pipeline arranged between the annular space and the drilling fluid volume controller; a second pipeline connected between the inner pipe and the annular space; and a third pipeline connected between the inner pipe and the watertight subsea blowout preventer, wherein the first pipeline, the second pipeline and the third pipeline can be opened or closed based on the pressure state in the borehole to achieve different functions.

[0010] According to an exemplary embodiment of the present disclosure, the terminal module includes a first part and a second part that are detachably connected together, the first part is connected to the lower end of the double-layer pipe, and the second part is connected to the drilling fluid volume controller through a first flexible pipe, and is connected to the watertight subsea blowout preventer through a second flexible pipe, wherein the first flexible pipe and the second flexible pipe are both located outside the terminal module.

[0011] According to an exemplary embodiment of the present disclosure, the first pipeline of the terminal module includes a drilling fluid transfer pump and a drilling fluid control valve arranged in the first part, and the first flexible pipe, wherein the drilling fluid control valve is connected between the drilling fluid transfer pump and the annular space of the double-layer pipe, and can be opened or closed to conduct or block the first pipeline; and wherein the drilling fluid transfer pump is constructed to draw drilling fluid from the drilling fluid volume controller through the first flexible pipe when the drilling fluid control valve is opened to conduct the first pipeline.

[0012] According to an exemplary embodiment of the present disclosure, the first pipeline of the terminal module also includes a drilling fluid pipeline pressure controller arranged in the first part, and the drilling fluid pipeline pressure controller is connected between the drilling fluid control valve and the drilling fluid transfer pump, and is constructed to maintain the pressure of the drilling fluid flowing out of the outlet of the drilling fluid transfer pump to be higher than the hydrostatic pressure of seawater, thereby avoiding seawater from being sucked into the drilling fluid return system from the drilling fluid volume controller due to the U-tube effect.

[0013] According to an exemplary embodiment of the present disclosure, the second pipeline of the terminal module includes a bypass line arranged in the first part, and the bypass line is connected between the internal space in the inner layer pipe and the annular space, wherein a bypass valve is arranged in the bypass line and can be opened or closed to conduct or block the second pipeline.

[0014] According to an exemplary embodiment of the present disclosure, the third pipeline of the terminal module includes a kill line control valve and a kill line in the module, and the second flexible pipe, which are arranged in the first part; the kill line control valve is connected between the inner layer pipe and the kill line in the module, and can be opened or closed to conduct or block the third pipeline, and the kill line in the module is connected to the second flexible pipe. When the third pipeline is conducted, the kill fluid can be supplied from the drilling platform to the borehole, or the drilling fluid can be returned from the borehole to the drilling platform.

[0015] According to an exemplary embodiment of the present disclosure, the third pipeline of the terminal module also includes a well-killing pipeline pressure controller arranged in the first part, and the well-killing pipeline pressure controller is arranged on the well-killing pipeline in the module, and is constructed to control the pressure and volume of the well-killing fluid pumped into the borehole from the drilling platform through the watertight subsea blowout preventer, and the pressure and volume of the drilling fluid returned from the borehole to the drilling platform.

[0016] According to an exemplary embodiment of the present disclosure, the first part and the second part are detachably connected together by an emergency disconnection system; the emergency disconnection system is constructed to enable the first part and the second part to be separated in severe weather that is not suitable for operations or when the drilling platform needs to leave the drilling site urgently; the second part is provided with a buoyancy block; and the buoyancy block is constructed to keep the emergency disconnection system in a horizontal state after the emergency disconnection is performed so that the first part can be reconnected to the second part.

[0017] According to an exemplary embodiment of the present disclosure, the drilling platform is provided with: a gas injection system for supplying high-pressure gas into the inner layer pipe; a well-killing fluid supply and choke system for supplying well-killing fluid into the inner layer pipe; and a drilling fluid separator for separating gas from the high-pressure gas-drilling fluid mixture.

[0018] According to an exemplary embodiment of the present disclosure, the drilling system also includes a selector and a pressure control manifold, which is connected to the top end of the double-layer pipe and is connected to the gas injection system, the well killing fluid supply and choke system and the drilling fluid separator, and the selector and the pressure control manifold are constructed so that the double-layer pipe can be selectively connected to the gas injection system, the well killing fluid supply and choke system or the drilling fluid separator.

[0019] According to an exemplary embodiment of the present disclosure, when the pressure in the borehole is normal, the riserless subsea blowout preventer is operated to allow the low-pressure drilling fluid in the borehole to be collected in the drilling fluid volume controller;

[0020] In the first case of borehole overpressure out of control, the riserless subsea blowout preventer is operated to seal the borehole so that the drilling fluid in the borehole cannot enter the drilling fluid volume controller, and the drill pipe is not cut off; and in the second case of borehole overpressure out of control, the riserless subsea blowout preventer is operated to: on the basis of sealing the borehole, also cut off the drill pipe; wherein the pressure in the borehole in the second case of borehole overpressure out of control is higher than the pressure in the borehole in the first case of borehole overpressure out of control, and the pressure in the borehole is lower than the pressure in the first case of borehole overpressure out of control when the pressure in the borehole is normal.

[0021] According to an exemplary embodiment of the present disclosure, the watertight pipeless underwater blowout preventer includes: a clamping sealing gate, which is arranged close to the drilling fluid volume controller; a shear gate, which is arranged close to the borehole; and a gate valve module, which is arranged between the clamping sealing gate and the shear gate; wherein the watertight pipeless underwater blowout preventer is constructed as follows: when the pressure in the borehole is normal, the clamping sealing gate and the shear gate are not actuated, and the low-pressure drilling fluid in the borehole is allowed to be collected in the drilling fluid volume controller; in a first case of borehole overpressure out of control, the clamping sealing gate is actuated to hold or clamp the drill pipe to completely seal the borehole so that the drilling fluid in the borehole cannot enter the drilling fluid volume controller, and after the borehole is completely sealed, in a second case of borehole overpressure out of control where the pressure in the borehole further increases, the shear gate is also actuated to shear the drill pipe, wherein the lower end of the cut drill pipe is retained in the gate valve module.

[0022] According to an exemplary embodiment of the present disclosure, when the pressure in the borehole is normal, the double-layer pipe is connected to the gas injection system, the first pipeline and the second pipeline are in a conducting state, and the third pipeline is in a blocked state; the low-pressure drilling fluid collected in the drilling fluid volume controller is transported to the annular space of the double-layer pipe through the first pipeline; and the high-pressure gas injection system injects high-pressure gas into the terminal module through the inner pipe of the double-layer pipe, and enters the annular space through this circulation, so that the high-pressure gas is mixed with the low-pressure drilling fluid, thereby recovering the low-pressure drilling fluid to the drilling platform.

[0023] According to an exemplary embodiment of the present disclosure, in the first case of borehole overpressure out of control, the double-layer pipe is connected to the drilling fluid separator, the first pipeline and the second pipeline are in a blocked state, and the third pipeline is in a conducting state; high-pressure drilling fluid is injected into the borehole through the drill pipe, and the drilling fluid in the borehole is transported to the drilling fluid separator through the watertight subsea blowout preventer, the third pipeline and the inner pipe of the double-layer pipe.

[0024] According to an exemplary embodiment of the present disclosure, in the second case of overpressure out of control in the borehole, the double-layer pipe is connected to the well-killing fluid supply and flow-blocking system, the first pipeline and the second pipeline are in a blocked state, and the third pipeline is in a conducting state; the well-killing fluid supply and flow-blocking system injects high-pressure well-killing fluid through the inner pipe of the double-layer pipe, and the high-pressure well-killing fluid is injected into the watertight subsea blowout preventer through the third pipeline, and then injected into the borehole to restore hydrostatic control in the borehole and continue drilling.

[0025] According to an exemplary embodiment of the present disclosure, the drilling fluid volume controller includes an open low-pressure drilling fluid containing space connected to seawater, wherein the low-pressure drilling fluid and the seawater are separated at a contact surface; wherein the rotation speed of the drilling fluid transfer pump is controlled based on a liquid level sensor that measures the liquid level of the contact surface, so that seawater will not be sucked into the first pipeline.

[0026] According to an exemplary embodiment of the present disclosure, the watertight pipeless subsea blowout preventer includes a primary control system and a secondary control system, wherein the primary control system controls the fastening sealing gate, the shear gate and the gate valve module based on a sound signal from the drilling platform, and the secondary control system controls the fastening sealing gate, the shear gate and the gate valve module through a secondary control panel that can be operated by an underwater robot.

[0027] According to an exemplary embodiment of the present disclosure, the watertight subsea blowout preventer also includes: a blowout preventer control fluid accumulator for storing and supplying driving fluid required by the primary control system and the secondary control system, wherein the blowout preventer control fluid accumulator includes an accumulator replenishing line, which is constructed to: replenish the driving fluid to the blowout preventer control fluid accumulator when necessary; wherein the accumulator replenishing line is connected to the terminal module to receive the driving fluid from the terminal module.

[0028] According to an exemplary embodiment of the present disclosure, the drilling system further includes: a drilling template, which is installed at the wellhead of the borehole; the drilling fluid volume controller and the drilling fluid return system are constructed as: before installing the watertight subsea blowout preventer, it serves as a closed-loop drilling fluid return system for drilling operations and cementing operations in the surface section of the borehole; after the watertight subsea blowout preventer is installed on the drilling template and the drilling fluid volume controller is installed on the watertight subsea blowout preventer, watertight drilling and drilling fluid return operations are performed on the remaining sections of the borehole.

[0029] According to an exemplary embodiment of the present disclosure, the first part further includes a pressure and volume regulator configured to control the volume and pressure of the high-pressure gas injected into the low-pressure drilling fluid in the annular space so that the low-pressure drilling fluid can be lifted to the drilling platform.

[0030] According to an exemplary embodiment of the present disclosure, the bypass pipeline is also constructed to allow the drilling fluid from the drilling platform to circulate between the drilling platform and the inner layer pipe, wherein, if an air injection failure occurs in the gas injection system, drilling cuttings of the air-drilling fluid mixture are deposited in the annular space, and by closing the wellbore control valve and the drilling fluid control valve and opening the bypass valve, the drilling fluid can be injected from the inner layer pipe and flow into the annular space to flush away the drilling cuttings deposits, thereby achieving the circulation of the drilling fluid.

[0031] In various embodiments according to the present disclosure, the drilling system includes a riserless underwater blowout preventer, a specially made terminal module, a drilling fluid volume controller and a double-layer pipe. The drilling system according to the present disclosure can adapt to a more variable deep-sea environment without including a rigid riser, and through the drilling system of the present disclosure, deep-sea drilling fluid recovery operations can be performed without using a variable pump disposed on the seabed, and appropriate well control operations can be performed when emergencies occur. The drilling system of the present disclosure can adaptively adjust the operation of the riserless underwater blowout preventer and the terminal module according to the pressure conditions in the borehole, so that the two work together to achieve multiple functions to adapt to the deep-sea environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The present disclosure may be fully understood by considering the following detailed description of various embodiments of the present disclosure in conjunction with the accompanying drawings, in which:

[0033] Figure 1 A schematic diagram of a drilling system according to an exemplary embodiment of the present disclosure is shown;

[0034] Figure 2 It shows that Figure 1 Schematic diagram of a drilling fluid volume controller and a riserless subsea blowout preventer of a drilling system shown;

[0035] Figure 3 It shows that Figure 1 A schematic diagram of a double-layer pipe included in a drilling fluid return system of a drilling system shown;

[0036] Figure 4 It shows that Figure 1 A schematic diagram of a terminal module of a drilling system is shown;

[0037] Figure 5 It shows that Figure 1 A schematic diagram of the connection between the terminal module and the double-layer pipe of the drilling system shown;

[0038] Figure 6 It shows that Figure 1The schematic diagram of the connection between the terminal module and the double-layer pipe of the drilling system shown is used to achieve the first function;

[0039] Figure 7 Shown as Figure 1 A schematic diagram of the connection between the terminal module and the double-layer pipe of the drilling system shown, which is used to achieve the second function; and

[0040] Figure 8 Shown as Figure 1 A schematic diagram of the connection between the terminal module and the double-layer pipe of the drilling system is shown, which is used to achieve the third function. DETAILED DESCRIPTION

[0041] The technical solution of the present disclosure is further specifically described below through examples and in conjunction with the accompanying drawings. In the specification, the same or similar figure numbers indicate the same or similar parts. The following description of the embodiments of the present disclosure with reference to the accompanying drawings is intended to explain the overall inventive concept of the present disclosure and should not be construed as a limitation of the present disclosure.

[0042] In addition, in the detailed description below, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of the embodiments disclosed herein. However, it is apparent that one or more embodiments may be implemented without these specific details. In other cases, well-known structures and devices are embodied in a schematic manner to simplify the accompanying drawings.

[0043] Figure 1 FIG. 1 shows a schematic diagram of a drilling system according to an exemplary embodiment of the present disclosure. Figure 1 As shown, a drilling system for geological exploration drilling includes: a drilling platform 17, which can float and move on the water surface 20; a production part 4, which is used to produce formation fluid samples; and a drilling fluid return system 1, which is connected to the drilling platform 17 and separated from the production part 4. Specifically, as Figure 1 As shown, the drilling system will be deployed from the floating drilling platform into two deployment centers, and the drilling center 62 is used to lower the production part 4. The drilling fluid return center 63 is offset from the drilling center 62 and is used to deploy and operate the drilling fluid return system 1. There is enough distance between the two centers, and the system can avoid mutual interference between the production part 4 and the drilling fluid return system 1 in the case of deep water and strong ocean currents.

[0044] In the illustrated embodiment, as Figure 1As shown, the production section 4 includes: a borehole 40 formed in the seabed formation 13; a drill pipe 42 extending downward from the drilling platform 17 into the borehole 40 to perform a series of production operations, such as delivering drilling fluid and, under necessary conditions, delivering well-killing fluid; a riserless subsea blowout preventer 15, which is disposed at the wellhead of the borehole 40 and through which the drill pipe 42 extends to perform drilling; and a drilling fluid volume controller 14, which is disposed on the top of the riserless subsea blowout preventer 15. The riserless subsea blowout preventer 15 is constructed to allow or prevent the drilling fluid in the borehole 40 from entering the drilling fluid volume controller 14 based on the pressure state in the borehole 40, which will be described in detail below. Figure 1 As shown, the drilling fluid return system 1 includes: a double-layer pipe 2, which includes an outer layer pipe 65 and an inner layer pipe 3, and an annular space 5 is formed between the outer layer pipe 65 and the inner layer pipe 3 (such as Figure 2 As shown). The inner pipe 3 and the outer pipe 65 of the double-layer pipe 2 can be concentric or non-concentric, and there is no difference in the functional realization of the two. The drilling fluid return system 1 also includes a terminal module 9, which is connected to the lower end of the double-layer pipe 2. The terminal module 9 includes a first pipeline 6 arranged between the annular space 5 and the drilling fluid volume controller 14; a second pipeline 7 connected between the inner pipe 3 and the annular space 5; and a third pipeline 8 connected between the inner pipe 3 and the subsea blowout preventer 15 without a watertight riser. The first pipeline 6, the second pipeline 7 and the third pipeline 8 can be opened or closed based on the pressure state in the borehole 40 to realize different functions, and these functions will be described in detail below.

[0045] In the drilling system according to the present disclosure, the operation of the riserless subsea blowout preventer 15 and the terminal module 9 can be adaptively adjusted according to the pressure conditions in the borehole 40, so that the two can work together to achieve multiple functions. Under normal mining conditions, there will not be too much liquid in the borehole 40, and the pressure is kept low. The low-pressure drilling fluid in the borehole 40 can be smoothly recovered through the cooperation of the riserless subsea blowout preventer 15, the terminal module 9, the double-layer pipe 2, etc. However, when there is too much liquid and even gas, the pressure in the borehole 40 increases sharply, and it is necessary to adjust the riserless subsea blowout preventer 15 and the terminal module 9 to perform reasonable well control. The drilling system according to the present disclosure can adapt to a more variable deep-sea environment without including a rigid riser, and through the drilling system of the present disclosure, it is possible to perform deep-sea drilling fluid recovery operations without using a variable pump set on the seabed, and it is possible to perform appropriate well control operations when emergencies occur.

[0046] In an exemplary embodiment according to the present disclosure, Figure 1As shown, the drilling platform 17 is provided with: a gas injection system 19 for supplying high-pressure gas into the inner layer pipe 3; a well-killing fluid supply and flow blocking system 58 for supplying well-killing fluid into the inner layer pipe 3; and a drilling fluid separator 18 for separating gas from the high-pressure gas-drilling fluid mixture. The functions of these components or systems will be described in detail below. In addition, after separating the gas, the drilling fluid is further transported to the drilling fluid cleaning system 54, as shown in FIG. Figure 1 As shown, the drill cuttings carried in the drilling fluid are removed.

[0047] In the case where the pressure in the borehole is normal, that is, there is no excessive liquid or gas in the borehole 40, the riserless subsea blowout preventer 15 is operated to allow the low-pressure drilling fluid in the borehole 40 to be collected in the drilling fluid volume controller 14. In the first case where the borehole overpressure is out of control, that is, there is more liquid or gas, but the amount of liquid or gas has not exceeded a threshold, the riserless subsea blowout preventer 15 is operated to seal the borehole 40 so that the drilling fluid in the borehole 40 cannot enter the drilling fluid volume controller 14. In the second case where the borehole overpressure is out of control, that is, there is too much liquid or gas, and their amount exceeds a threshold, making the pressure in the borehole 40 particularly high, the riserless subsea blowout preventer 15 is operated to: on the basis of sealing the borehole 40, also cut off the drill pipe 42. Therefore, in the second case of borehole overpressure out of control, the pressure in the borehole is higher than the pressure in the first case of borehole overpressure out of control, and when the pressure in the borehole is normal, the pressure in the borehole is lower than the pressure in the first case of borehole overpressure out of control.

[0048] When the pressure in the borehole is normal, Figure 6 As shown, the double-layer pipe 2 is connected to the gas injection system 19, the first pipeline 6 and the second pipeline 7 are in a conducting state, and the third pipeline 8 is in a blocked state. How the pipelines 6, 7 and 8 are conducted and blocked will be described in detail below. In this case, the drilling fluid produced in the borehole 40 is in a low-pressure state, and the subsea blowout preventer 15 without a watertight riser is operated to allow the low-pressure drilling fluid in the borehole 40 to be collected in the drilling fluid volume controller 14, so that the low-pressure drilling fluid collected in the drilling fluid volume controller 14 is transported to the annular space 5 of the double-layer pipe 2 through the first pipeline 6.

[0049] In addition, if Figure 6As shown, the high-pressure gas injection system 19 injects high-pressure gas into the terminal module 9 through the inner tube 3 of the double-layer pipe 2, and enters the annular space 5 through this circulation, so that the high-pressure gas is mixed with the low-pressure drilling fluid, thereby recovering the low-pressure drilling fluid to the drilling platform 17. In this case, the double-layer pipe 2 and the terminal module 9 realize the first function: that is, injecting high-pressure gas into the annular space to realize the recovery of low-pressure drilling fluid. In this case, the high-pressure liquid required for drilling is transported from the drilling platform 17 to the borehole 40 via the drill pipe 42, so that operations are carried out in the borehole 40, and drilling fluid that may contain fuels such as oil is produced. Therefore, in the present disclosure, through the synergistic effect of the double-layer pipe 2, the terminal module 9, the watertight subsea blowout preventer 15, etc., the recovery of drilling fluid can be realized without using a traditional watertight subsea pump for drilling.

[0050] In the first case of uncontrolled borehole overpressure, such as Figure 7 As shown, the double-layer pipe 2 is connected to the drilling fluid separator 18, the first pipeline 6 and the second pipeline 7 are in a blocked state, and the third pipeline 8 is in a conducting state. In this case, the subsea blowout preventer 15 without a riser is only operated to seal the borehole 40 so that the drilling fluid in the borehole 40 cannot enter the drilling fluid volume controller 14, but the drill pipe 42 has not been cut off. In this case, since the drill pipe 42 has not been cut off, the high-pressure killing fluid is injected into the borehole 40 through the drill pipe 42. In addition, since the borehole 40 is sealed to be isolated from the external seawater, the drilling fluid in the borehole 40 accumulates and the pressure increases. The higher-pressure drilling fluid in the borehole 40 is transported to the drilling fluid separator 18 through the subsea blowout preventer 15 without a riser, the third pipeline 8 and the inner pipe 3 of the double-layer pipe 2 with the assistance of its own pressure and the high-pressure killing fluid introduced into the drill pipe 42. In this case, the double-layer pipe 2 and the terminal module 9 realize the second function, namely, returning the drilling fluid to the drilling platform 17. Since the drilling fluid generated at this time has a certain pressure, the drilling fluid can be delivered to the drilling platform 17 without the help of the high-pressure gas injection system 19. After the pressure control in the borehole is restored, the riserless subsea blowout preventer 15 can be further operated to allow the low-pressure drilling fluid to be collected in the drilling fluid volume controller 14 by operating the tightening sealing gate 51 and the shear gate 50 described below, and the operation performed under the condition of normal pressure in the borehole can be restored.

[0051] In the second case of uncontrolled borehole overpressure, Figure 8As shown, the double-layer pipe 2 is connected to the well-killing fluid supply and flow-blocking system 58, the first pipeline 6 and the second pipeline 7 are in a blocked state, and the third pipeline 8 is in a conducting state. In this case, the pressure in the borehole 40 is particularly high, and the watertight pipeless subsea blowout preventer 15 is operated to cut off the drill pipe 42 on the basis of sealing the borehole 40. The high-pressure well-killing fluid cannot be pumped into the borehole 40 through the drill pipe 42. At this time, the well-killing fluid supply and flow-blocking system 58 injects the high-pressure well-killing fluid into the borehole 40 through the inner pipe 3 of the double-layer pipe 2. The high-pressure well-killing fluid is injected into the watertight pipeless subsea blowout preventer 15 through the third pipeline 8, and then injected into the borehole 40 to restore the hydrostatic control in the borehole 40 and continue drilling, that is, to press any excess liquid or gas back into the formation to resume the operation. In this case, the double-layer pipe 2 and the terminal module 9 realize the third function: that is, injecting high-pressure well-killing fluid into the borehole 40 to restore pressure control. After pressure control is restored, the drill pipe 42 is salvaged and repaired, and the riserless subsea blowout preventer 15 can be further operated to allow low-pressure drilling fluid to be collected in the drilling fluid volume controller 14 by operating the tightening sealing gate 51 and the shear gate 50 described below, thereby resuming normal drilling operations and continuing to return the drilling fluid to the drilling platform 17 through the drilling fluid return system 1.

[0052] In an exemplary embodiment according to the present disclosure, Figure 1 and 2 As shown, the riserless subsea blowout preventer 15 includes: a fastening sealing gate 51, which is arranged close to the drilling fluid volume controller 14; a shear gate 50, which is arranged close to the borehole 40; and a gate valve module 16 which is arranged between the fastening sealing gate 51 and the shear gate 50. The riserless subsea blowout preventer 15 is constructed as follows: when the pressure in the borehole is normal, the clamping sealing gate 51 and the shear gate 50 are not actuated, and the low-pressure drilling fluid in the borehole 40 is allowed to be collected in the drilling fluid volume controller 14; in the first case of borehole overpressure out of control, the clamping sealing gate 51 is actuated to hold or clamp the drill pipe 42 to completely seal the borehole 40 so that the drilling fluid in the borehole 40 cannot enter the drilling fluid volume controller 14; and after the borehole 40 is completely sealed, in the second case of borehole overpressure out of control where the pressure in the borehole further increases, the shear gate 50 is actuated to shear the drill pipe 42, wherein the lower end of the cut drill pipe 42 is retained in the gate valve module 16 for subsequent salvage and recovery operations.

[0053] In an exemplary embodiment according to the present disclosure, Figure 1As shown, the drilling platform 17 further includes a selector and pressure control manifold 59, which is connected to the top of the double-layer pipe 2 and is connected to the gas injection system 19, the well-killing fluid supply and flow-blocking system 58 and the drilling fluid separator 18, so that the double-layer pipe 2 can be selectively connected to the gas injection system 19, the well-killing fluid supply and flow-blocking system 58 or the drilling fluid separator 18. Through the operation of the selector and the pressure control manifold 59, the double-layer pipe 2 can be switched between the above three functions.

[0054] In an exemplary embodiment according to the present disclosure, Figure 2 As shown, the riserless subsea blowout preventer 15 includes a primary control system 56 and a secondary control system 55. The primary control system 56 controls the buckling seal gate 51, the shear gate 50, and the gate valve module 16 based on the sound signal from the drilling platform 17. The secondary control system 55 controls the buckling seal gate 51, the shear gate 50, and the gate valve module 16 through a secondary control panel 551 that can be operated by the underwater robot. Through two-stage control, the reliability of the control of the riserless subsea blowout preventer 15 can be increased.

[0055] In addition, if Figure 2 As shown, the subsea blowout preventer 15 without a riser further includes a blowout preventer control fluid accumulator 57, which is used to store and supply the driving fluid required by the primary control system 56 and the secondary control system 55. Figure 2 As shown, the BOP control fluid accumulator 57 includes an accumulator replenishment line 64, which is configured to replenish the BOP control fluid accumulator 57 with driving fluid when necessary; wherein the accumulator replenishment line 64 is connected to the terminal module 9 to receive the driving fluid from the terminal module 9. By providing the BOP control fluid accumulator 57, it can be ensured that the control of the riserless subsea BOP 15 will not be interrupted.

[0056] In an exemplary embodiment according to the present disclosure, Figure 4 , 5 , 6, 7 and 8, the terminal module 9 includes a first part 23 and a second part 24 which are detachably connected together, the first part 23 is connected to the lower end of the double-layer pipe 2, and the second part 24 is connected to the drilling fluid volume controller 14 through the first flexible pipe 10, and is connected to the subsea blowout preventer 15 without a watertight pipe through the second flexible pipe 12, wherein the first flexible pipe 10 and the second flexible pipe 12 are both located outside the terminal module 9. In the present disclosure, the first flexible pipe 10 is also called a low-pressure drilling fluid return pipeline, which is mainly used to return low-pressure drilling fluid when the pressure in the borehole is normal. The second flexible pipe 12 is also called a high-pressure well-killing pipeline, which is mainly used in the first and second cases where the borehole overpressure is out of control.

[0057] In an exemplary embodiment according to the present disclosure, Figure 2 As shown, the terminal module 9 is connected to the lower end of the double-layer pipe 2, and a flexible joint 48 is used to absorb the stress between the double-layer pipe 2 for drilling fluid return and the terminal module 9. Figure 1 As shown, the terminal module 9 is suspended in the sea at the bottom of the floating drilling platform 17 moving in the sea. The drilling fluid return pipeline 1 also includes a control and power umbilical cable 60 suspended from the operation area of ​​the universal base plate 45 and fixedly clamped on the pipeline. The control and power umbilical cable 60 is used to provide power and control signals for the functional operation of the terminal module 9.

[0058] In an exemplary embodiment according to the present disclosure, Figure 2 As shown, the riserless subsea blowout preventer 15 further includes an upper kill valve inlet 53, which is connected between the second flexible pipe 12 and the shear gate 50; and a lower kill valve inlet 52, which is connected between the second flexible pipe 12 and the buckle sealing gate 51, and a buckle connector 44, one end of which is connected to both the upper kill valve inlet 53 and the lower kill valve inlet 52. The second flexible pipe 12 is inserted into the other end of the buckle connector 44 opposite to the one end by the underwater robot, thereby being connected to both the upper kill valve inlet 53 and the lower kill valve inlet 52. By using two kill valve inlets, the kill fluid can enter the borehole 40 more smoothly.

[0059] In the present disclosure, a flexible pipe 10 is connected to a riser-less underwater blowout preventer 15 (such as a subsea blowout preventer 15) by an underwater robot. Figure 2 The other end of the flexible pipe 10 is mounted on the second part 24 of the terminal module 9. In addition, a second flexible pipe 12 is connected between the riserless underwater blowout preventer 15 and the second part 24 of the terminal module. One end of the second flexible pipe 12 is inserted into the buckle connector 44 (as shown in FIG. 1 ) located on the riserless underwater blowout preventer 15 by the underwater robot. Figure 2 ). The other end of the second flexible pipe 12 is mounted on the second part 24 of the terminal module. The heave and movement of the floating drilling vessel 17 will have an impact on the drilling system below, but in the present disclosure, since the first flexible pipe 10 and the second flexible pipe 20 are both configured as flexible pipes, the bending and stretching of these flexible pipes 10 and 20 can reliably eliminate these impacts, which is better than the case of using rigid pipes.

[0060] In an exemplary embodiment according to the present disclosure, Figure 4 , 5, 6, 7 and 8, the first part 23 and the second part 24 are detachably connected together by an emergency disconnect system 25. It is very necessary to set up the emergency disconnect system 25, because severe weather that is not suitable for operations often occurs in the sea, such as submarine earthquakes, volcanoes, typhoons and violent ocean currents, and the drilling platform 17 sometimes needs to leave the drilling site urgently. The emergency disconnect system 25 is constructed to enable the first part 23 and the second part 24 to be separated in these severe weather conditions or when the drilling platform 17 needs to be evacuated urgently. In the event of an emergency disconnection, although the drilling fluid return system 1 and the first part 23 of the terminal module leave the borehole 40 together with the floating drilling platform 17, the second part 24 of the terminal module and the watertight subsea blowout preventer 15 remain connected, thereby facilitating the subsequent positioning and reconnection functions of the second part 24. In an exemplary embodiment according to the present disclosure, as Figure 4 As shown, the second part 24 is provided with a buoyancy block 46 , which is configured to keep the emergency disconnection system 25 in a horizontal state after the emergency disconnection is performed so that the first part 23 can be reconnected to the second part 24 more easily.

[0061] In an exemplary embodiment according to the present disclosure, Figure 5 As shown, the first pipeline 6 of the terminal module 9 includes a drilling fluid transfer pump 28 and a drilling fluid control valve 29 disposed in the first part 23, and a first flexible pipe 10, which is also called a low-pressure drilling fluid return pipeline. The drilling fluid control valve 29 is connected between the drilling fluid transfer pump 28 and the annular space 5 of the double-layer pipe 2, and can be opened or closed to conduct or block the first pipeline 6. Optionally, as Figure 5 As shown, a drilling fluid input end 36 is provided on the double-layer pipe 2, and one end of the first pipeline 6 is therefore connected to the drilling fluid input end 36, so that the drilling fluid can be input into the annular space 5. The drilling fluid transfer pump 28 is installed between the drilling fluid control valve 29 and the first flexible pipe 10, and is configured to suck drilling fluid from the drilling fluid volume controller 14 through the first flexible pipe 10 when the drilling fluid control valve 29 is opened to conduct the first pipeline 6.

[0062] In an exemplary embodiment, Figure 1 As shown in FIG. 1 , the drilling fluid volume controller 14 includes an open low-pressure drilling fluid containing space 38 that is connected to seawater 39. The low-pressure drilling fluid and seawater 39 are separated at a contact surface 37. The drilling fluid volume controller 14 includes an open containing space 38. When the pressure in the borehole 40 is normal, the extracted drilling fluid is low-pressure drilling fluid, which is collected in the containing space 38 (such as the pressure of the drilling fluid volume controller 14) of the drilling fluid volume controller 14 through the subsea blowout preventer 15 without a watertight riser. Figure 1The low-pressure drilling fluid can be located at the bottom of the accommodation space 38, and the seawater 39 is located at the upper part of the accommodation space 38, and the two can be separated at the contact surface 37. The rotation speed of the drilling fluid transfer pump 28 is controlled based on the liquid level sensor 381 that measures the liquid level of the contact surface 37, so that the contact surface 37 is maintained within a certain range, preferably maintained at a fixed liquid level, so that the seawater 39 will not be sucked into the first pipeline 6 or the first flexible pipe 10.

[0063] The first pipeline 6 of the terminal module 9 further includes a drilling fluid pipeline pressure controller 33 disposed in the first portion 23. The drilling fluid pipeline pressure controller 33 is connected between the drilling fluid control valve 29 and the drilling fluid transfer pump 28, and is configured to maintain the pressure of the drilling fluid flowing out of the outlet of the drilling fluid transfer pump 28 higher than the hydrostatic pressure of seawater, thereby preventing seawater from being sucked into the drilling fluid return system 1 from the drilling fluid volume controller due to the U-tube effect.

[0064] Since the high pressure air supplied by the high pressure gas injection system 19 passes through the inner layer pipe 3 and the pressure and volume regulator 35 (such as Figure 5-8 3) is injected into the annular space 5, at which time the pressure of the mixed gas drilling fluid in the annular space 5 is underbalanced, that is, when the pressure of the mixed gas-drilling fluid in the annular space 5 is lower than the corresponding seawater pressure, if the drilling fluid pipeline pressure controller 33 is not provided, seawater will invade the drilling fluid return system 1 due to the U-tube effect. However, after the drilling fluid pipeline pressure controller 33 is provided, it can maintain the pressure at the outlet of the drilling fluid transfer pump 28 higher than the seawater hydrostatic pressure 39 (as shown in FIG. Figure 1 As shown), it is possible to avoid the seawater being sucked into the drilling fluid return system 1 from the drilling fluid volume controller 14 due to the U-tube effect.

[0065] In the terminal module 9, the first pipeline 6 is mainly used to transport the low-pressure drilling fluid from the borehole 40 to the annular space 5. How to transport the low-pressure drilling fluid to the drilling platform 17 requires the cooperation of the second pipeline 7. In the following, the composition of the second pipeline 7 will be described in detail. The second pipeline 7 of the terminal module 9 includes a bypass pipeline 21 arranged in the first part 23, and the bypass pipeline 21 is connected between the internal space in the inner layer pipe 3 and the annular space 5. The bypass valve 32 is arranged in the bypass pipeline 21 and can be opened or closed to conduct or block the second pipeline 7. High-pressure gas is introduced into the second pipeline 7 through the above high-pressure gas injection system 19, so that the high-pressure gas can enter the annular space 5 to mix with the low-pressure drilling fluid entering the annular space 5 through the first pipeline 6, so that the mixture is pressed back to the drilling platform 17.

[0066] In an exemplary embodiment according to the present disclosure, Figure 5As shown, the third pipeline 8 of the terminal module 9 includes a kill line control valve 31 arranged in the first part 23, a kill line 34 in the module, and a second flexible pipe 12. The kill line control valve 31 is connected between the inner layer pipe 3 and the kill line 34 in the module, and can be opened or closed to conduct or block the third pipeline 8, and the kill line 34 in the module is connected to the second flexible pipe 12. When the third pipeline 8 is conducted, the kill fluid can be supplied from the drilling platform 17 to the riserless subsea blowout preventer 15, or the drilling fluid can be returned to the drilling platform 17 from the borehole 40. For example, in the first case of overpressure out of control in the borehole, the higher pressure drilling fluid can be returned to the drilling platform 17 from the borehole 40 through the third pipeline 8. In the second case of overpressure and loss of control in the borehole, high-pressure drilling fluid can be supplied from the drilling platform 17 to the watertight subsea blowout preventer 15 through the third pipeline 8, and further supplied to the borehole 40 to pressurize excess liquid or gas into the formation, restore pressure control in the borehole and continue operations.

[0067] The third pipeline 8 of the terminal module 9 also includes a well-killing line pressure controller 27 arranged in the first part 23, and the well-killing line pressure controller 27 is arranged on the well-killing line 34 in the module. It is very necessary to set up the well-killing line pressure controller 27, which is constructed to control the pressure and volume of the well-killing fluid pumped from the drilling platform 17 into the borehole 40 through the subsea blowout preventer 15 without a watertight riser, and the pressure and volume of the drilling fluid returned from the borehole 40 to the drilling platform 17. The well-killing pipeline pressure controller 27 is an important component of the present invention, especially for use in deep water. When the pressure in the borehole 40 is out of control, such as the first and second situations of borehole overpressure out of control, the watertight subsea blowout preventer 15 will lock the drill pipe 42 or even cut off the drill pipe 42, thereby sealing the borehole 40 so that it is no longer connected to the external seawater. It is necessary to properly adjust the well-killing pipeline pressure controller 27 so that the high-pressure well-killing fluid can smoothly enter the borehole 40 under high pressure, and in another case, it can also allow the drilling fluid discharged from the borehole 40 to return to the drilling platform 17.

[0068] In an exemplary embodiment of the present disclosure, Figure 1As shown, the drilling system further includes a drilling template 41, which is installed at the wellhead of the borehole 40. The subsea riserless blowout preventer 15 is also equipped with a wellhead connector 49, which can be connected to the drilling template 41. All components of the wellhead connector 49 are installed in the frame module to supplement the riserless subsea blowout preventer 15 system of the floating drilling platform 17. The drilling fluid volume controller 14 and the drilling fluid return system 1 are constructed as: before the riserless subsea blowout preventer 15 is installed, it is used as a closed-loop drilling fluid return system for drilling and cementing operations in the surface well section of the borehole 40. After the riserless subsea blowout preventer 15 is installed on the drilling template 41 and the drilling fluid volume controller 14 is installed on the riserless subsea blowout preventer 15, the riserless drilling and drilling fluid return operations are performed for the remaining well section of the borehole 40. Through the above-mentioned segmented operation, the riser-less underwater blowout preventer 15 can be used in the case of a more complex formation environment, so as to respond to various emergencies.

[0069] In an exemplary embodiment according to the present disclosure, the first part 23 further includes a pressure and volume regulator 35, which is configured to control the volume and pressure of the high-pressure gas in the low-pressure drilling fluid injected into the annular space 5 so that the low-pressure drilling fluid can be lifted to the drilling platform 17. The pressure and volume regulator 35 can perform pressure compensation when the double-layer pipe 2 and the terminal module 9 perform the first function.

[0070] In an exemplary embodiment according to the present disclosure, the bypass line 21 is also configured to allow the drilling fluid from the drilling platform 17 to circulate between the drilling platform 17 and the inner pipe 3. If the gas injection system 19 fails to inject air, drilling cuttings of the air-drilling fluid mixture will be deposited in the annular space 5. In this case, by closing the kill line control valve 31 and the drilling fluid control valve 29 and opening the bypass valve 32, the drilling fluid from the drilling platform 17 can be injected from the inner pipe 3 and flow into the annular space 5 to flush away the drilling cuttings, so as to achieve the circulation of the drilling fluid.

[0071] Those skilled in the art can understand that the embodiments described above are exemplary and can be improved by those skilled in the art. The structures described in various embodiments can be freely combined without causing conflicts in structure or principle.

[0072] Although the present disclosure is described in conjunction with the accompanying drawings, the embodiments disclosed in the accompanying drawings are intended to exemplify the preferred implementation modes of the present disclosure and should not be construed as limiting the present disclosure.

[0073] Although some embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made to these embodiments without departing from the principles and spirit of the present general inventive concept, the scope of which is defined by the claims and their equivalents.

[0074] It should be noted that the word "comprising" does not exclude other elements or steps, and the word "a" or "an" does not exclude a plurality. In addition, any element reference signs in the claims shall not be construed as limiting the scope of the present disclosure.

Claims

1. A drilling system for geological exploration drilling, comprising: A drilling platform (17) capable of floating and moving on the water surface (20); The mining part (4) comprises: a borehole (40) formed in a seafloor formation (13); a drill pipe (42) extending downwardly from the drilling platform (17) into the borehole (40); A riserless underwater blowout preventer (15) is disposed at the wellhead of the borehole (40) and the drill pipe (42) extends through the riserless underwater blowout preventer (15) for drilling; and A drilling fluid volume controller (14) is arranged on the top of the riserless underwater blowout preventer (15). wherein the riserless subsea blowout preventer (15) is configured to allow or prevent drilling fluid in the borehole (40) from entering the drilling fluid volume controller (14) based on the pressure state in the borehole (40); and A drilling fluid return system (1), which is connected to the drilling platform (17) and separated from the production section (4), comprises: A double-layer tube (2), comprising an outer tube (65) and an inner tube (3), wherein an annular space (5) is formed between the outer tube (65) and the inner tube (3); and A terminal module (9) is connected to the lower end of the double-layer pipe (2) and includes a first pipeline (6) arranged between the annular space (5) and the drilling fluid volume controller (14); a second pipeline (7) connected between the inner layer pipe (3) and the annular space (5); and a third pipeline (8) connected between the inner layer pipe (3) and the watertight subsea blowout preventer (15), wherein the first pipeline (6), the second pipeline (7) and the third pipeline (8) can be opened or closed based on the pressure state in the borehole (40) to achieve different functions.

2. The drilling system according to claim 1, wherein the terminal module (9) comprises a first part (23) and a second part (24) which are detachably connected together, wherein the first part (23) is connected to the lower end of the double-layer pipe (2), and the second part (24) is connected to the drilling fluid volume controller (14) through a first flexible pipe (10), and is connected to the riserless subsea blowout preventer (15) through a second flexible pipe (12), wherein the first flexible pipe (10) and the second flexible pipe (12) are both located outside the terminal module (9).

3. The drilling system according to claim 2, wherein: The first pipeline (6) of the terminal module (9) comprises a drilling fluid transfer pump (28) and a drilling fluid control valve (29) arranged in the first part (23), and the first flexible pipe (10), wherein The drilling fluid control valve (29) is connected between the drilling fluid transfer pump (28) and the annular space (5) of the double-layer pipe (2), and can be opened or closed to open or block the first pipeline (6); and The drilling fluid transfer pump (28) is connected between the drilling fluid control valve (29) and the first flexible pipe (10) and is configured to draw drilling fluid from the drilling fluid volume controller (14) through the first flexible pipe (10) when the drilling fluid control valve (29) is opened to conduct the first pipeline (6).

4. The drilling system according to claim 3, wherein: The first pipeline (6) of the terminal module (9) further comprises a drilling fluid pipeline pressure controller (33) arranged in the first part (23), The drilling fluid pipeline pressure controller (33) is connected between the drilling fluid control valve (29) and the drilling fluid transfer pump (28), and is configured to maintain the pressure of the drilling fluid flowing out of the outlet of the drilling fluid transfer pump (28) to be higher than the hydrostatic pressure of seawater, thereby preventing seawater from being sucked into the drilling fluid return system (1) from the drilling fluid volume controller (14) due to the U-tube effect.

5. The drilling system according to claim 3, wherein: The second pipeline (7) of the terminal module (9) comprises a bypass pipeline (21) arranged in the first part (23), the bypass pipeline (21) being connected between the internal space in the inner layer pipe (3) and the annular space (5), wherein a bypass valve (32) is arranged in the bypass pipeline (21) and can be opened or closed to conduct or block the second pipeline (7).

6. The drilling system according to claim 5, wherein: The third pipeline (8) of the terminal module (9) comprises a well-killing pipeline control valve (31) and an intra-module well-killing pipeline (34) arranged in the first part (23), and the second flexible pipeline (12); The well-killing pipeline control valve (31) is connected between the inner layer pipe (3) and the well-killing pipeline (34) in the module, and can be opened or closed to conduct or block the third pipeline (8), and the well-killing pipeline (34) in the module is connected to the second flexible pipe (12); When the third pipeline (8) is connected, the well-killing fluid can be supplied from the drilling platform (17) to the borehole (40), or the drilling fluid can be returned from the borehole (40) to the drilling platform (17).

7. The drilling system according to claim 6, wherein: The third pipeline (8) of the terminal module (9) further comprises a well-killing pipeline pressure controller (27) arranged in the first part (23), The well-killing line pressure controller (27) is arranged on the well-killing line (34) in the module, and is constructed to control the pressure and volume of the well-killing fluid pumped from the drilling platform (17) into the borehole (40) through the watertight subsea blowout preventer (15), and the pressure and volume of the drilling fluid returned from the borehole (40) to the drilling platform (17).

8. The drilling system according to claim 2, wherein: The first part (23) and the second part (24) are detachably connected together via an emergency disconnect system (25); The emergency disconnection system (25) is configured to enable the first part (23) and the second part (24) to be separated in the event of severe weather that is not suitable for operation or when the drilling platform (17) needs to leave the drilling site urgently; The second part (24) is provided with a buoyancy block (46); as well as The buoyancy block (46) is configured to maintain the emergency disconnect system (25) in a horizontal state after the emergency disconnection is performed so that the first part (23) can be reconnected to the second part (24).

9. The drilling system according to claim 1, wherein the drilling platform (17) is provided with: a gas injection system (19) for supplying high-pressure gas into the inner layer pipe (3); a well-killing fluid supply and flow blocking system (58) for supplying well-killing fluid into the inner layer pipe (3); and a drilling fluid separator (18) for separating gas from the high-pressure gas-drilling fluid mixture.

10. The drilling system according to claim 9, further comprising a selector and pressure control manifold (59), which is connected to the top end of the double-layer pipe (2) and is connected to the gas injection system (19), the well killing fluid supply and choke system (58) and the drilling fluid separator (18); The selector and pressure control manifold (59) is configured so that the double-layer pipe (2) can be selectively connected to the gas injection system (19), the well killing fluid supply and choke system (58) or the drilling fluid separator (18).

11. The drilling system according to claim 9, wherein: When the pressure in the borehole is normal, the riserless subsea blowout preventer (15) is operated to allow low-pressure drilling fluid in the borehole (40) to be collected in the drilling fluid volume controller (14); In the first case of uncontrolled borehole overpressure, the riserless subsea blowout preventer (15) is operated to seal the borehole (40) so that the drilling fluid in the borehole (40) cannot enter the drilling fluid volume controller (14) without cutting off the drill pipe (42); and In the second case of overpressure in the borehole out of control, the riserless subsea blowout preventer (15) is operated to: on the basis of sealing the borehole (40), also cut off the drill pipe (42); In the second case of borehole overpressure out of control, the pressure in the borehole is higher than the pressure in the first case of borehole overpressure out of control, and when the pressure in the borehole is normal, the pressure in the borehole is lower than the pressure in the first case of borehole overpressure out of control.

12. The drilling system according to claim 11, wherein: The riserless underwater blowout preventer (15) comprises: Fastening a sealing gate (51) which is arranged close to the drilling fluid volume controller (14); a shear ram (50) disposed proximate to the borehole (40); and a gate valve module (16), which is arranged between the fastening sealing gate (51) and the shear gate (50); Wherein, the riserless underwater blowout preventer (15) is constructed as follows: When the pressure in the borehole is normal, the fastening sealing gate (51) and the shear gate (50) are not actuated, and the low-pressure drilling fluid in the borehole (40) is allowed to be collected in the drilling fluid volume controller (14); In the first case of overpressure out of control in the borehole, the tightening and sealing gate (51) is actuated to tighten or tighten the drill pipe (42) to completely seal the borehole (40) so that the drilling fluid in the borehole (40) cannot enter the drilling fluid volume controller (14); and After the borehole (40) is completely sealed, in a second case of borehole overpressure runaway where the pressure in the borehole further increases, the shear gate (50) is also actuated to shear the drill pipe (42), wherein the lower end of the cut drill pipe (42) is held in the gate valve module (16).

13. The drilling system according to claim 11, wherein: When the pressure in the borehole is normal, the double-layer pipe (2) is connected to the gas injection system (19), the first pipeline (6) and the second pipeline (7) are in a conducting state, and the third pipeline (8) is in a blocked state; The low-pressure drilling fluid collected in the drilling fluid volume controller (14) is transported to the annular space (5) of the double-layer pipe (2) through the first pipeline (6); and The gas injection system (19) injects high-pressure gas into the terminal module (9) through the inner tube (3) of the double-layer tube (2), and circulates the gas into the annular space (5) so that the high-pressure gas is mixed with the low-pressure drilling fluid, thereby recovering the low-pressure drilling fluid to the drilling platform (17).

14. The drilling system according to claim 11, wherein: In the first case of overpressure out of control in the borehole, the double-layer pipe (2) is connected to the drilling fluid separator (18), the first pipeline (6) and the second pipeline (7) are in a blocked state, and the third pipeline (8) is in a conducting state; High-pressure drilling fluid is injected into the borehole (40) through the drill pipe (42), and the drilling fluid in the borehole (40) is transported to the drilling fluid separator (18) through the riser-less underwater blowout preventer (15), the third pipeline (8) and the inner layer pipe (3) of the double-layer pipe (2).

15. The drilling system according to claim 11, wherein: In the second case of overpressure out of control in the borehole, the double-layer pipe (2) is connected to the well-killing fluid supply and flow blocking system (58), the first pipeline (6) and the second pipeline (7) are in a blocked state, and the third pipeline (8) is in a conducting state; The well-killing fluid supply and choke system (58) injects high-pressure well-killing fluid through the inner layer pipe (3) of the double-layer pipe (2), and the high-pressure well-killing fluid is injected into the watertight subsea blowout preventer (15) through the third pipeline (8), and then injected into the borehole (40) to restore hydrostatic control in the borehole (40) and continue drilling.

16. The drilling system of claim 3, wherein: The drilling fluid volume controller (14) comprises an open low-pressure drilling fluid containing space (38) connected to seawater (39), wherein the low-pressure drilling fluid and the seawater (39) are separated at a contact surface (37); The rotation speed of the drilling fluid transfer pump (28) is controlled based on a liquid level sensor (381) that measures the liquid level of the contact surface (37), so that seawater (39) is not sucked into the first pipeline (6).

17. The drilling system of claim 12, wherein: The riserless underwater blowout preventer (15) comprises a primary control system (56) and a secondary control system (55). The main control system (56) controls the buckle seal gate (51), the shear gate (50) and the gate valve module (16) based on the sound signal from the drilling platform (17), and The secondary control system (55) controls the fastening sealing gate (51), the shear gate (50) and the gate valve module (16) through a secondary control panel (551) operable by an underwater robot.

18. The drilling system of claim 17, wherein: The riserless subsea blowout preventer (15) further comprises: a blowout preventer control fluid accumulator (57) for storing and supplying driving fluid required by the primary control system (56) and the secondary control system (55), wherein The blowout preventer control fluid accumulator (57) includes an accumulator replenishment line (64) configured to: replenish the blowout preventer control fluid accumulator (57) with driving fluid when necessary; The accumulator replenishment line (64) is connected to the terminal module (9) to receive the driving fluid from the terminal module (9).

19. The drilling system of claim 1, further comprising: A drilling template (41), the drilling template (41) being installed at the wellhead of the borehole (40); The drilling fluid volume controller (14) and the drilling fluid return system (1) are configured to serve as a closed-loop drilling fluid return system for drilling and cementing operations in the surface well section of the borehole (40) before installing a riserless subsea blowout preventer (15); After the riserless subsea blowout preventer (15) is installed on the drilling template (41) and the drilling fluid volume controller (14) is installed on the riserless subsea blowout preventer (15), riserless drilling and drilling fluid return operations are performed on the remaining well section of the borehole (40).

20. The drilling system of claim 2, wherein: The first part (23) also includes a pressure and volume regulator (35) configured to control the volume and pressure of the high-pressure gas in the low-pressure drilling fluid injected into the annular space (5) so that the low-pressure drilling fluid can be lifted to the drilling platform (17).

21. The drilling system of claim 6, wherein: The bypass pipeline (21) is also configured to allow drilling fluid from the drilling platform (17) to circulate between the drilling platform and the inner layer pipe (3). If an air injection failure occurs in the gas injection system (19) in the drilling platform (17) for supplying high-pressure gas to the inner pipe (3), drilling cuttings of the air-drilling fluid mixture are deposited in the annular space (5). By closing the wellbore control valve (31) and the drilling fluid control valve (29) and opening the bypass valve (32), drilling fluid can be injected from the inner pipe (3) and flow into the annular space (5), thereby flushing away the drilling cuttings and realizing the circulation of the drilling fluid.

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