CN111155931A

CN111155931A - Drilling system for geological exploration drilling

Info

Publication number: CN111155931A
Application number: CN201811322161.7A
Authority: CN
Inventors: 斯迈·格莱迪奇; 沈越
Original assignee: Paige Underwater Technology Guangzhou Co ltd
Current assignee: Paige Underwater Technology Guangzhou Co ltd
Priority date: 2018-11-07
Filing date: 2018-11-07
Publication date: 2020-05-15

Abstract

A drilling system for geological exploration drilling comprising: a drilling platform, a production section and a drilling fluid return system. The mining section includes: drilling; a drill stem; a riser-less subsea blowout preventer disposed at a wellhead of a borehole; and a drilling fluid accumulation controller disposed at the top of the blowout preventer. The riser-less subsea blowout preventer is configured to allow or prevent drilling fluid in the borehole from entering the drilling fluid volume controller based on a pressure condition in the borehole. The drilling fluid return system includes: a double tube comprising an outer tube and an inner tube, an annular space between the outer tube and the inner tube; and a terminal module. The terminal module is connected to the lower end of the double-layer pipe and comprises a first pipeline arranged between the annular space and the volume controller; a second line connected between the inner pipe and the annular space; and a third pipeline connected between the inner tubular and the blowout preventer. The first, second and third lines can be opened or closed based on the pressure conditions in the borehole.

Description

Drilling system for geological exploration drilling

Technical Field

The present disclosure relates to a drilling system for drilling deep and ultra-deep water geological exploration, and in particular to a riser-less drilling system for performing deep or ultra-deep sea seafloor geological exploration.

Background

Ultra-deep water drilling typically consists of a large drilling platform and a large bore riser system connecting the subsea well to the drilling platform. The current state of the art drilling platform designs are capable of drilling at water depths of up to 3600 meters, with current worldwide records of drilling water depths being 3402m (2016). The riser has multiple functions: the conduit is used for returning drilling fluid in the well; rigid choke and kill lines connect the drilling platform and the subsea blowout preventer for well control.

The main difference between non-riser and riser drilling is that conventional drilling uses risers. The drilling fluid pump installed on the floating drilling vessel pumps the drilling fluid required for drilling into the well through the drill pipe, and the drilling fluid returns to the floating drilling vessel through the marine riser, and the power of the drilling fluid pump is provided by the same drilling fluid pump.

Riser-free drilling techniques have been developed up to 1500 meters of water depth and are applied to surface drilling prior to installing blowout preventers. Existing underwater drilling fluid pump systems have limitations. In an existing non-riser drilling technology, a rotary control head is mounted on the top of an underwater blowout preventer to isolate the pressure of sea water at the bottom of the sea, the pressure of the underwater blowout preventer and drilling fluid in a well, a closed non-riser drilling fluid circulation loop is created, and the operation depth of a pump system under test can reach about 2000 meters, so that the pump system has obvious limitation in use in deeper water areas. The overlong hydrostatic pressure of the drilling fluid in the pipeline has great influence on the safety control stability in the well. A riser-less drilling system operating in shallow water typically operates with its well control lines running through open water between the drilling vessel and the blowout preventers, typically using separate lines. In ultra-deep water drilling operations, such high pressure well control pipelines are neither economical nor practical to deploy and operate safely. Current floating drilling vessels based on this non-riser drilling technology do not drill deep water exploration without the use of risers in order to control drilling fluids and hydrostatic heads in the well.

In another related non-riser drilling technique, no rotating head control equipment is used on top of the subsea blowout preventer. The floating drilling vessel pumps drilling fluid into the well and drilling fluid and cuttings are pumped back to the drilling vessel by a drilling fluid pump mounted on the seabed, which pump is connected to the drilling vessel by a drilling fluid return line. The limitation of performing deepwater non-riser drilling is limited by current variable displacement pump technology. When the water depth exceeds 1500-.

Therefore, none of the existing riser-less drilling techniques are suitable for operations on the sea bed above 2000 meters.

Disclosure of Invention

The present disclosure aims to overcome at least one of the above-mentioned drawbacks of the prior art.

The present disclosure provides a drilling system for geological exploration drilling, which does not contain a riser, which has no size limitation on the drilling platform and no limitation on the drilling water depth, and which can far exceed 3600m, while being able to perform conventional well control.

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 surface of the water; a mining section comprising: a borehole formed in a subsea formation; a drill pipe extending downwardly from the drilling platform into the borehole; a riser-less subsea blowout preventer disposed at a wellhead of the borehole and through which the drill pipe extends for drilling; and a drilling fluid volume controller disposed on top of the riser-less subsea blowout preventer, wherein the riser-less subsea blowout preventer is configured to allow or prevent drilling fluid in the borehole from entering the drilling fluid volume controller based on a pressure condition in the borehole; and a drilling fluid return system connected to the drilling platform and spaced apart from the production section, comprising: a double-layer tube including an outer layer tube and an inner layer tube, an annular space being formed between the outer layer tube and the inner layer tube; and a termination module connected to a lower end of the double tube and comprising a first line disposed between the annular space and the drilling fluid volume controller; a second line connected between the inner pipe and the annular space; and a third pipeline connected between the inner tubular and the riser-less subsea blowout preventer, wherein the first, second, and third pipelines can be opened or closed based on pressure conditions in the borehole to achieve different functions.

According to an exemplary embodiment of the disclosure, the terminal module comprises a first part and a second part detachably connected together, the first part being connected to the lower end of the double pipe and the second part being connected to the drilling fluid accumulation controller by a first flexible pipe and to the riser-less subsea blowout preventer by a second flexible pipe, wherein the first flexible pipe and the second flexible pipe are both located outside the terminal module.

According to an exemplary embodiment of the disclosure, the first line of the termination module comprises a drilling fluid transfer pump and a drilling fluid control valve arranged in the first section, and the first flexible conduit, wherein the drilling fluid control valve is connected between the drilling fluid transfer pump and the annular space of the double tube, which can be opened or closed to conduct or block the first line; and wherein the drilling fluid transfer pump is configured to draw drilling fluid from the drilling fluid volume controller through the first flexible conduit in a state in which the drilling fluid control valve is opened to conduct the first line.

According to an exemplary embodiment of the disclosure, the first line of the terminal module further comprises a drilling fluid line pressure controller disposed in the first section, the drilling fluid line pressure controller being connected between the drilling fluid control valve and the drilling fluid transfer pump and being configured to maintain a pressure of the drilling fluid flowing out of the outlet of the drilling fluid transfer pump higher than a seawater hydrostatic pressure, thereby avoiding a suction of seawater from the drilling fluid volume controller into the drilling fluid return system due to a U-tube effect.

According to an exemplary embodiment of the disclosure, the second line of the termination module comprises a bypass line arranged in the first section, which bypass line is connected between an inner space in the inner tube and the annular space, wherein a bypass valve is arranged in the bypass line and which can be opened or closed to conduct or block the second line.

According to an exemplary embodiment of the present disclosure, the third line of the termination module comprises a kill line control valve and a modular kill line disposed in the first section, and the second flexible conduit; the kill line control valve is connected between the inner tubular and the modular kill line, which can be opened or closed to open or block the third line, and the modular kill line is connected to the second flexible conduit. With the third line conductive, either kill fluid can be supplied from the drilling rig to the borehole or drilling fluid can be returned from the borehole to the drilling rig.

According to an exemplary embodiment of the disclosure, the third line of the terminal module further comprises a kill line pressure controller disposed in the first portion, the kill line pressure controller disposed on a modular kill line and configured to control the pressure and volume of kill fluid pumped from the drilling rig through the riser-less subsea blowout preventer into the borehole, and the pressure and volume of drilling fluid returned from the borehole to the drilling rig.

According to an exemplary embodiment of the present disclosure, the first portion and the second portion are detachably connected together by an emergency disconnect system; the emergency disconnect system is configured to: enabling separation of the first and second portions in the event of inclement weather that is not suitable for operation or the drilling platform needing emergency departure from the drilling site; the second part is provided with a buoyancy block; and the buoyancy block is configured to: after the emergency disconnect is performed, the emergency disconnect system will be kept in a horizontal state so that the first part can be reconnected to the second part.

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 tube; a control fluid supply and choke system for supplying control fluid into the inner tubular; and a drilling fluid separator for separating gas from the high pressure gas-drilling fluid mixture.

According to an exemplary embodiment of the present disclosure, the drilling system further comprises a selector and pressure control manifold connected to a top end of the double pipe and connected to the gas injection system, the control fluid supply and choke system and the drilling fluid separator, the selector and pressure control manifold being configured such that the double pipe is selectively connectable to the gas injection system, the control fluid supply and choke system or the drilling fluid separator.

According to an exemplary embodiment of the present disclosure, the riser-less subsea blowout preventer is operated to allow low pressure drilling fluid in the borehole to be collected in the drilling fluid volume controller in the event that pressure in the borehole is normal;

in a first instance of a runaway borehole overpressure, the riser-less subsea blowout preventer is operated to seal the borehole such that drilling fluid in the borehole cannot enter the drilling fluid volume controller and the drill pipe is not severed; and in a second instance of drilling overpressure runaway, the riser-less subsea blowout preventer is operated to: on the basis of sealing the drill hole, cutting off the drill rod; wherein the pressure in the borehole in the second case of uncontrolled borehole overpressure is higher than the pressure in the borehole in the first case of uncontrolled borehole overpressure, and the pressure in the borehole in the case of normal pressure in the borehole is lower than the pressure in the borehole in the first case of uncontrolled borehole overpressure.

According to an exemplary embodiment of the present disclosure, the riser-less subsea blowout preventer comprises: a clasping seal ram disposed proximate to the drilling fluid volume controller; a shear ram disposed proximate to the borehole; and a gate valve module disposed between the clinch seal gate and the shear gate; wherein the marine riser-free subsea blowout preventer is configured to: under normal pressure in the borehole, deactivating the fastened seal rams and the shear rams, allowing low pressure drilling fluid in the borehole to be collected in the drilling fluid volume controller; in a first case of uncontrolled drilling overpressure, actuating the buckling sealing rams hugs or buckles the drill pipe to completely seal the borehole such that drilling fluid in the borehole cannot enter the drilling fluid accumulator, and after completely sealing the borehole, in a second case of uncontrolled drilling overpressure with further increase in pressure in the borehole, also actuating the shear rams to shear the drill pipe, wherein the lower end of the severed drill pipe is retained in the gate valve module.

According to an exemplary embodiment of the present disclosure, the double pipe is connected to the gas injection system under normal pressure in the borehole, the first and second lines are in a conducting state, and the third line is in a blocking state; low pressure drilling fluid collected in the drilling fluid volume controller is conveyed through the first line into the annular space of the double pipe; and the high pressure gas injection system injects high pressure gas into the terminal module through the inner pipe of the double pipe and circulates the high pressure gas into the annular space, so that the high pressure gas is mixed with low pressure drilling fluid, thereby recovering the low pressure drilling fluid to the drilling platform.

According to an exemplary embodiment of the disclosure, in a first situation of uncontrolled borehole overpressure, the double pipe is connected to the drilling fluid separator, the first and second lines are in a blocked state, and the third line is in a conducting state; high pressure kill fluid is injected into the borehole through the drill pipe, and drilling fluid in the borehole is conveyed to the drilling fluid separator through the riser-less subsea blowout preventer, the third pipeline, and the inner pipe of the double pipe.

According to an exemplary embodiment of the present disclosure, in a second situation of uncontrolled drilling overpressure, the double pipe is connected to the control fluid supply and choke system, the first and second lines are in a blocked state, and the third line is in a conducting state; the control fluid supply and choke system injects high pressure control fluid through the inner pipe of the double pipe, which is injected through the third line into the riser-less subsea blowout preventer and then into the borehole to restore hydrostatic control within the borehole and continue drilling.

According to an exemplary embodiment of the present disclosure, the drilling fluid volume controller comprises a low pressure drilling fluid containment space that is open and in communication with seawater, wherein the low pressure drilling fluid is demarcated from seawater at an interface; wherein the rotational speed of the drilling fluid transfer pump is controlled based on a level sensor measuring the level of the contact surface such that seawater is not drawn into the first pipeline.

According to an exemplary embodiment of the present disclosure, the riser-less subsea blowout preventer comprises a primary control system and a secondary control system, the primary control system controlling the fastened seal rams, the shear rams, and the ram valve modules based on sound signals from the drilling platform, and the secondary control system controlling the fastened seal rams, the shear rams, and the ram valve modules through a secondary control panel operable by a subsea robot.

According to an exemplary embodiment of the present disclosure, the riser-less subsea blowout preventer further comprises: a blowout preventer control fluid accumulator for storing and supplying drive fluid required by the primary and secondary control systems, wherein the blowout preventer control fluid accumulator comprises an accumulator makeup line configured to: for supplementing a drive fluid to the blowout preventer control fluid accumulator when needed; wherein the accumulator makeup line is connected to the terminal module to receive drive fluid from the terminal module.

According to an exemplary embodiment of the present disclosure, the drilling system further comprises: a drilling template mounted at a wellhead of the borehole; the drilling fluid volume controller and the drilling fluid return system are configured to: before installing the marine riser-free subsea blowout preventer, as a closed-loop drilling fluid return system for drilling and cementing operations of the surface wellbore section; subsequent to installing the riser-less subsea blowout preventer on the drilling template and installing the drilling fluid volume controller above the riser-less subsea blowout preventer, conducting riser-less drilling and drilling fluid return operations for a remaining section of the borehole.

According to an exemplary embodiment of the disclosure, the first section further comprises a pressure and volume regulator configured to control the volume and pressure of high pressure gas within the low pressure drilling fluid injected into the annular space so that the low pressure drilling fluid can be lifted to the drilling platform.

According to an exemplary embodiment of the disclosure, the bypass line is further configured to circulate drilling fluid from the drilling platform between the drilling platform and the inner tubular, wherein drilling cuttings of an air-drilling fluid mixture are deposited in the annular space if an air injection failure occurs in the gas injection system, and the circulation flow of the drilling fluid is achieved by flushing away drilling cuttings deposits by closing the kill line control valve and the drilling fluid control valve and opening the bypass valve to enable drilling fluid to be injected from the inner tubular and flow into the annular space.

In various embodiments according to the present disclosure, the drilling system includes a riser-less subsea blowout preventer, a tailored terminal module, a drilling fluid volume controller, and a double pipe. The drilling system according to the present disclosure can adapt to a more variable deep sea environment without including a rigid riser, and by the drilling system of the present disclosure, it is possible to perform a deep sea drilling fluid recovery operation without using a variable displacement pump provided at the sea floor, and it is possible to perform a suitable well control operation when an emergency occurs. The drilling system of the present disclosure is capable of adaptively adjusting the operation of the marine riser-less blowout preventer and the terminal module according to the pressure conditions in the borehole such that the two cooperate to achieve multiple functions to adapt to the deep sea environment.

Drawings

The disclosure may be more completely understood in consideration of the following detailed description of various embodiments of the disclosure in connection with the accompanying drawings, in which:

FIG. 1 shows a schematic view of a drilling system according to an exemplary embodiment of the present disclosure;

FIG. 2 shows a schematic view of a drilling fluid volume controller and a riser-less subsea blowout preventer of the drilling system shown in FIG. 1;

FIG. 3 shows a schematic view of a double tube included in the drilling fluid return system of the drilling system shown in FIG. 1;

FIG. 4 shows a schematic diagram of a terminal module of the drilling system shown in FIG. 1;

FIG. 5 shows a schematic diagram of the connection of a termination module and a double-walled pipe of the drilling system as shown in FIG. 1;

FIG. 6 shows a schematic diagram of the connection of a terminal module and a double pipe of the drilling system as shown in FIG. 1 for performing a first function;

FIG. 7 shows a schematic diagram of the connection of a terminal module and a double-walled pipe of the drilling system as shown in FIG. 1 for performing a second function; and

fig. 8 shows a schematic view of the connection of a terminal module and a double pipe of the drilling system as shown in fig. 1, for performing a third function.

Detailed Description

The technical solution of the present disclosure is further specifically described below by way of examples and with reference to the accompanying drawings. In the specification, the same or similar reference numerals denote the same or similar components. The following description of the embodiments of the present disclosure with reference to the accompanying drawings is intended to explain the general inventive concept of the present disclosure and should not be construed as limiting the present disclosure.

Furthermore, in the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the embodiments disclosed herein. It may be evident, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are shown in schematic form in order to simplify the drawing.

FIG. 1 shows a schematic view of a drilling system according to an exemplary embodiment of the present disclosure. As shown in fig. 1, a drilling system for geological exploration drilling includes: a drilling platform 17 that is capable of floating and moving on the water surface 20; a production section 4 for producing a formation fluid sample; and a drilling fluid return system 1 connected to the drilling platform 17 and spaced from the production section 4. Specifically, as shown in fig. 1, the drilling system will be deployed from the floating vessel in two deployment centres, the drilling centre 62 being used to lower the production section 4. The drilling fluid return center 63 is offset from the drilling center 62 for deployment and operation of the drilling fluid return system 1. The distance between the two centers is sufficient to avoid interference between the production section 4 and the drilling fluid return system 1 in deep water and strong ocean currents.

In the illustrated embodiment, as shown in fig. 1, the mining section 4 includes: a borehole 40 formed in the subsea formation 13; a drill pipe 42 extending from the rig 17 down into the borehole 40 for a series of production operations, such as the delivery of drilling fluid and, if necessary, of kill fluid; a riser-less subsea blowout preventer 15 disposed at a wellhead of the borehole 40 and through which the drill pipe 42 extends for drilling; and a drilling fluid volume controller 14 disposed on top of the riser-less subsea blowout preventer 15. The non-riser 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 pressure conditions in the borehole 40, as will be described in detail below. As shown in fig. 1, the drilling fluid return system 1 includes: the double tube 2, which includes an outer tube 65 and an inner tube 3, forms an annular space 5 (shown in fig. 2) between the outer tube 65 and the inner tube 3. The inner tube 3 and the outer tube 65 of the double tube 2 may be concentric or non-concentric, and there is no difference in the functional implementation between the two. The drilling fluid return system 1 further comprises a termination module 9 connected to the lower end of the double pipe 2. The termination module 9 comprises a first line 6 arranged between the annular space 5 and the drilling fluid volume controller 14; a second line 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 riser-less subsea blowout preventer 15. The first, second and

third lines

6, 7, 8 can be opened or closed based on the pressure conditions in the borehole 40 to achieve different functions, which will be described in detail below.

In a drilling system according to the present disclosure, the operation of the riser-less 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 cooperate to perform multiple functions. Under normal production conditions, excessive liquid does not appear in the borehole 40, the pressure is kept low, and low-pressure drilling fluid in the borehole 40 can be smoothly recovered through the cooperation of the marine riser-free underwater blowout preventer 15, the terminal module 9, the double-layer pipe 2 and the like. However, in the presence of too much liquid and even gas, the pressure in the borehole 40 increases dramatically, requiring adjustments to the non-riser subsea blowout preventer 15 and the terminal module 9 to perform reasonable downhole control. The drilling system according to the present disclosure can adapt to a more variable deep sea environment without including a rigid riser, and by the drilling system of the present disclosure, it is possible to perform a deep sea drilling fluid recovery operation without using a variable displacement pump provided at the sea floor, and it is possible to perform a suitable well control operation when an emergency occurs.

In an exemplary embodiment according to the present disclosure, as shown in fig. 1, a drilling platform 17 is provided with: a gas injection system 19 for supplying a high-pressure gas into the inner-layer tube 3; a control fluid supply and choke system 58 for supplying control fluid into the inner tubular 3; and a drilling fluid separator 18 for separating gas from the high pressure gas-drilling fluid mixture, the function of which components or systems will be described in detail below. In addition, after separation of the gas, the drilling fluid is further conveyed to a drilling fluid cleaning system 54, as shown in FIG. 1, to remove entrained cuttings from the drilling fluid.

In the event that the pressure in the borehole is normal, i.e., there is no excess fluid or no gas present in the borehole 40, the riser-less 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 a runaway borehole overpressure, i.e., a condition where more fluid or gas is present, but the amount of fluid or gas has not yet exceeded a threshold, the riser-less subsea blowout preventer 15 is operated to seal the borehole 40 so that drilling fluid in the borehole 40 cannot enter the drilling fluid volume controller 14. In the second case of a runaway borehole overpressure, i.e. excessive liquid or gas is present and their amount exceeds a threshold value, so that the pressure in the borehole 40 is particularly high, the non-riser subsea blowout preventer 15 is operated as: upon sealing the bore 40, the drill rod 42 is also severed. The pressure in the borehole in the second case of an uncontrolled borehole overpressure is thus higher than the pressure in the borehole in the first case of an uncontrolled borehole overpressure, and the pressure in the borehole in the case of a normal borehole pressure is lower than the pressure in the first case of an uncontrolled borehole overpressure.

In the case where the pressure in the borehole is normal, as shown in fig. 6, the double pipe 2 is connected to the gas injection system 19, the first line 6 and the second line 7 are in a conducting state, and the third line 8 is in a blocking state. How the

lines

6, 7 and 8 are brought into and out of communication will be described in detail below. In this case, the drilling fluid produced in the borehole 40 is at a low pressure and the riser-less 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, whereby the low pressure drilling fluid collected in the drilling fluid volume controller 14 is conveyed via the first line 6 into the annular space 5 of the double pipe 2.

Further, as shown in fig. 6, a high pressure gas injection system 19 injects high pressure gas into the terminal module 9 through the inner pipe 3 of the double pipe 2 and circulates therethrough into the annular space 5 so that the high pressure gas mixes with the low pressure drilling fluid, thereby recovering the low pressure drilling fluid to the drilling platform 17. In this case, the double tube 2 and the end module 9 fulfill a first function: i.e. high pressure gas is injected into the annular space to effect recovery of the low pressure drilling fluid. In this case, the high pressure fluid required for drilling is conveyed from the rig 17 into the borehole 40 via the drill pipe 42, so that work is performed in the borehole 40, producing drilling fluid which may contain fuel such as oil. Thus, in the present disclosure, recovery of drilling fluid is also possible without the need to use a conventional riser-less drilling subsea pump, through the synergistic effects of the double pipe 2, the termination module 9, the riser-less subsea blowout preventer 15, and the like.

In the first case of a runaway borehole overpressure, as shown in figure 7, the double pipe 2 is connected to the drilling fluid separator 18, the first and

second lines

6, 7 are in a blocked state, and the third line 8 is in a conducting state. In this case, the non-riser subsea blowout preventer 15 is only operated to seal the borehole 40 so that drilling fluid in the borehole 40 cannot enter the drilling fluid volume controller 14, but the drill pipe 42 has not yet been severed. In this case, high pressure kill fluid is injected into the borehole 40 through the drill pipe 42, since the drill pipe 42 has not yet been severed. Furthermore, because the borehole 40 is sealed from the surrounding seawater, drilling fluid within the borehole 40 accumulates and thus increases in pressure. The higher pressure drilling fluid in the borehole 40 is conveyed to the drilling fluid separator 18 by its own pressure and with the aid of high pressure kill fluid introduced in the drill pipe 42 through the riser-less subsea blow out preventer 15, the third pipeline 8 and the inner pipe 3 of the double pipe 2. In this case, the double tube 2 and the end module 9 fulfill the second function, namely: the drilling fluid is returned to the drilling platform 17. Since the drilling fluid produced at this time has a certain pressure, the drilling fluid can be delivered to the drilling platform 17 without the aid of the high-pressure gas injection system 19. After pressure control in the borehole is resumed, the above-described operations performed under normal pressure in the borehole can be resumed by further operating the marine riser-less subsea blowout preventer 15 to allow low pressure drilling fluid to be collected in the drilling fluid volume controller 14 by operating the fastened seal rams 51 and shear rams 50 described below.

In a second situation of uncontrolled drilling overpressure, as shown in fig. 8, the double pipe 2 is connected to the control fluid supply and choke system 58, the first 6 and second 7 lines are in a blocked state, and the third line 8 is in a conducting state. In the event that the pressure within the borehole 40 is particularly high, the riser-less subsea blowout preventer 15 is operated to also sever the drill pipe 42 upon sealing the borehole 40. High pressure kill fluid cannot be pumped through the drillpipe 42 into the borehole 40, at which point the kill fluid supply and choke system 58 injects high pressure kill fluid into the borehole 40 through the inner pipe 3 of the double pipe 2. This high pressure kill fluid is injected through the third line 8 into the riser-less subsea blowout preventer 15 and further into the borehole 40 to restore hydrostatic control within the borehole 40 to continue drilling, i.e. to force any excess fluid or gas back into the formation to enable recovery of the operation. In this case, the double tube 2 and the terminal module 9 fulfill a third function: i.e., injecting high pressure kill fluid into the borehole 40 to restore pressure control. After pressure control is restored, the drill pipe 42 is salvaged and repaired, and normal drilling operations can be resumed by operating the below-described fastened seal rams 51 and shear rams 50 to further operate the marine riser-less blowout preventer 15 to allow low pressure drilling fluid to be collected in the drilling fluid accumulator controller 14, thereby continuing to perform the return of drilling fluid to the drilling rig 17 via the drilling fluid return system 1.

In an exemplary embodiment according to the present disclosure, as shown in fig. 1 and 2, the riser-less subsea blowout preventer 15 comprises: a fastening seal ram 51 disposed adjacent to the drilling fluid volume controller 14; a shear ram 50 disposed proximate the borehole 40; and a gate valve module 16 disposed between the snap seal gate 51 and the shear gate 50. The riser-less subsea blowout preventer 15 is configured to: under normal borehole pressure conditions, the snap-on seal rams 51 and shear rams 50 are not actuated, allowing low pressure drilling fluid in the borehole 40 to be collected in the drilling fluid volume controller 14; in a first instance of a loss of drilling overpressure, the shear seal rams 51 are actuated to grip or clasp the drill pipe 42 to fully seal the borehole 40 so that drilling fluid in the borehole 40 cannot enter the drilling fluid accumulator 14, and after fully sealing the borehole 40, in a second instance of a loss of drilling overpressure with further increase in pressure in the borehole, the shear rams 50 are actuated to shear the drill pipe 42, with the lower end of the severed drill pipe 42 retained in the gate valve module 16, for subsequent fishing recovery operations.

In an exemplary embodiment according to the present disclosure, as shown in fig. 1, the drilling platform 17 further includes a selector and pressure control manifold 59 connected to the top end of the double pipe 2 and connected to the gas injection system 19, the control fluid supply and choke system 58 and the drilling fluid separator 18, such that the double pipe 2 is selectively connected to the gas injection system 19, the control fluid supply and choke system 58 or the drilling fluid separator 18. The double tube 2 is enabled to switch between the above three functions by the operation of the selector and the pressure control manifold 59.

In an exemplary embodiment according to the present disclosure, as shown in FIG. 2, the riser-less subsea blowout preventer 15 comprises a primary control system 56 and a secondary control system 55. The primary control system 56 controls the clinch seal rams 51, shear rams 50 and ram valve modules 16 based on acoustic signals from the drilling platform 17. The secondary control system 55 controls the fastening seal rams 51, shear rams 50 and ram valve modules 16 via a secondary control panel 551 operable by the underwater robot. By the two-stage control, the reliability of the control of the marine riser-less blowout preventer 15 can be increased.

Furthermore, as shown in fig. 2, the riser-less subsea blowout preventer 15 further comprises: a blowout preventer control fluid accumulator 57 for storing and supplying the drive fluid required by the primary control system 56 and the secondary control system 55. As shown in fig. 2, the blowout preventer control fluid accumulator 57 includes an accumulator makeup line 64 configured to: to supplement the blowout preventer control fluid accumulator 57 with drive fluid when needed; wherein the accumulator makeup line 64 is connected to the termination module 9 to receive the driving fluid from the termination module 9. By providing this blowout preventer control fluid accumulator 57, it can be ensured that control of the riser-less subsea blowout preventer 15 is not interrupted.

In an exemplary embodiment according to the present disclosure, as shown in fig. 4, 5, 6, 7 and 8, the termination module 9 comprises a first portion 23 and a second portion 24 detachably connected together, the first portion 23 being connected to the lower end of the double pipe 2, and the second portion 24 being connected to the drilling fluid accumulation controller 14 by a first flexible pipe 10 and to the riser-less subsea blowout preventer 15 by a second flexible pipe 12, wherein the first flexible pipe 10 and the second flexible pipe 12 are both located outside the termination module 9. In the present disclosure, the first flexible conduit 10, also referred to as a low pressure drilling fluid return line, is used primarily in situations where the pressure in the borehole is normal, for returning low pressure drilling fluid. The second flexible pipe 12, also known as a high-pressure kill line, is used primarily in the first and second cases of uncontrolled drilling overpressures.

In an exemplary embodiment according to the present disclosure, the termination module 9 is connected to the lower end of the double pipe 2, as shown in fig. 2, and a flexible joint 48 is used to absorb the stress between the double pipe 2 for drilling fluid return and the termination module 9. As shown in fig. 1, the termination module 9 is suspended in the sea at the bottom of a floating drilling platform 17 that moves in the sea. The drilling fluid return line 1 also includes a control and power umbilical 60 suspended from the operating area of the gimbaled base plate 45 and fixedly clamped to the line. The control and power umbilical 60 is used to provide power and control signals for the functional operation of the terminal module 9.

In an exemplary embodiment according to the present disclosure, as shown in FIG. 2, the riser-less subsea blowout preventer 15 further comprises an upper kill valve inlet 53 connected between the second flexible pipe 12 and the shear ram 50; and a lower kill valve inlet 52 connected to the second flexible conduit 12 and the snap seal ram 51, a bayonet connector 44, one end of the bayonet connector 44 being 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 bayonet joint 44 opposite to the one end by the underwater robot to be 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 be more smoothly introduced into the borehole 40.

In the present disclosure, one flexible pipe 10 connects a flexible pipe joint 43 to a riser-less subsea blowout preventer 15 (shown in FIG. 2) by operation of a subsea robot. The other end of the flexible conduit 10 is mounted on the second portion 24 of the termination module 9. Furthermore, a second flexible pipe 12 is connected between the riser-less subsea blowout preventer 15 and the second part 24 of the termination module, and one end of the second flexible pipe 12 is inserted by means of a subsea robot operation into a bayonet connection 44 (shown in fig. 2) located on the riser-less subsea blowout preventer 15. The other end of the second flexible conduit 12 is mounted on the second portion 24 of the termination module. Both heave and movement of the floating drilling vessel 17 will have an effect on the underlying drilling system, but in the present disclosure, since both the first flexible pipe 10 and the second flexible pipe 20 are provided as flexible pipes, bending and stretching of these

flexible pipes

10 and 20 can reliably eliminate these effects, and is therefore superior to the case where rigid pipes are used.

In one exemplary embodiment according to the present disclosure, as shown in fig. 4, 5, 6, 7 and 8, the first portion 23 and the second portion 24 are removably connected together by an emergency disconnect system 25. The emergency disconnect system 25 is very necessary because severe weather conditions that are not suitable for operation, such as ocean bottom earthquakes, volcanoes, typhoons, and violent ocean currents, etc., are often present in the sea, and the drilling platform 17 sometimes needs to be urgently removed from the drilling site. The emergency disconnect system 25 is configured to enable separation of the first portion 23 and the second portion 24 in the event of severe weather conditions or emergency evacuation of the drilling platform 17. In case of an emergency disconnection, although the drilling fluid return system 1 and the first part 23 of the terminal module leave the borehole 40 with the floating rig 17, the second part 24 of the terminal module and the non-riser subsea blowout preventer 15 remain connected, thus facilitating subsequent positioning and reconnection functions of the second part 24. In an exemplary embodiment according to the present disclosure, as shown in fig. 4, the second portion 24 is provided with a buoyancy block 46, the buoyancy block 46 being configured to: after the emergency disconnect has been performed, the emergency disconnect system 25 will be kept in a horizontal state so that the first part 23 can be reconnected to the second part 24 more easily.

In an exemplary embodiment according to the present disclosure, as shown in fig. 5, the first line 6 of the termination module 9 comprises a drilling fluid transfer pump 28 and a drilling fluid control valve 29 arranged in the first section 23, and a first flexible pipe 10, which first flexible pipe 10 is also referred to as a low pressure drilling fluid return line. A drilling fluid control valve 29 is connected between the drilling fluid transfer pump 28 and the annular space 5 of the double pipe 2, which can be opened or closed to conduct or block the first line 6. Optionally, as shown in fig. 5, a drilling fluid input 36 is provided on the double tube 2, whereby one end of the first line 6 is connected to the drilling fluid input 36, so that 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 in a state where the drilling fluid control valve 29 is opened to conduct the first line 6.

In an exemplary embodiment, as shown in FIG. 1, drilling fluid volume controller 14 includes a low pressure drilling fluid containment space 38 that is open and in communication with seawater 39, the low pressure drilling fluid being demarcated from seawater 39 at interface 37. The drilling fluid volume controller 14 includes an open receiving space 38. In the case of normal pressure in the borehole 40, the extracted drilling fluid is a low pressure drilling fluid that is collected in the receiving space 38 (shown in fig. 1) of the drilling fluid volume controller 14 by the riser-less subsea blowout preventer 15. The low pressure drilling fluid can be located at the bottom of the containment space 38 and the seawater 39 at the upper portion of the containment space 38, and both can be demarcated at the interface 37. The rotational speed of the drilling fluid transfer pump 28 is controlled on the basis of a level sensor 381 measuring the level of the contact surface 37 such that the contact surface 37 is maintained within a certain range, preferably at a fixed level, such that seawater 39 is not sucked into the first line 6 or the first flexible pipe 10.

The first line 6 of the termination module 9 also includes a drilling fluid line pressure controller 33 disposed in the first section 23. The drilling fluid line 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 above the hydrostatic pressure of seawater, thereby avoiding the suction of seawater from the drilling fluid volume controller into the drilling fluid return system 1 due to the U-tube effect.

Since the high pressure air supplied by the high pressure gas injection system 19 is injected into the annular space 5 through the inner pipe 3 and the pressure and volume regulator 35 (shown in fig. 5-8), the pressure of the mixed gas drilling fluid in the annular space 5 is under balanced, i.e. when the pressure of the mixed gas drilling fluid in the annular space 5 is lower than the corresponding seawater pressure, seawater will invade the drilling fluid return system 1 due to the U-pipe effect if the drilling fluid line pressure controller 33 is not provided. However, after the drilling fluid line pressure control 33 is set, it is able to maintain the pressure at the outlet of the fluid transfer pump 28 above the seawater hydrostatic pressure 39 (as shown in fig. 1), thus avoiding the suction of seawater from the drilling fluid controller 14 into the drilling fluid return system 1 due to the U-tube effect.

In the termination module 9 the first line 6 is primarily used for conveying low pressure drilling fluid from the borehole 40 into the annular space 5, the cooperation of the second line 7 being required how the low pressure drilling fluid is conveyed into the drilling platform 17. Hereinafter, the composition of the second pipeline 7 will be described in detail. The second line 7 of the termination module 9 comprises a bypass line 21 arranged in this first section 23, which bypass line 21 connects between the inner space in the inner tube 3 and the annular space 5. A bypass valve 32 is provided in this bypass line 21 and can be opened or closed to open or block the second line 7. High pressure gas is fed into the second line 7 through the above high pressure gas injection system 19 so that it can enter the annular space 5 to mix with the low pressure drilling fluid entering the annular space 5 through the first line 6, pressing the mixture back onto the drilling platform 17.

In an exemplary embodiment according to the present disclosure, as shown in fig. 5, the third line 8 of the termination module 9 includes a kill line control valve 31 disposed in the first portion 23, a modular kill line 34, and a second flexible conduit 12. The kill line control valve 31 is connected between the inner pipe 3 and the modular kill line 34, which can be opened or closed to open or block the third line 8, and the modular kill line 34 is connected to the second flexible pipe 12. With the third pipeline 8 conductive, the control fluid can be fed from the drilling platform 17 to the riser-less subsea blowout preventer 15, or the drilling fluid can be returned from the borehole 40 to the drilling platform 17. For example, in the first case of a runaway drilling overpressure, higher pressure drilling fluid can be returned from the borehole 40 to the drilling platform 17 through the third line 8. In the second case of a runaway borehole overpressure, high pressure kill fluid can be fed from the rig 17 through this third line 8 to the riser-less subsea blowout preventer 15 and further into the borehole 40 to force excess fluid or gas into the formation, resuming pressure control in the borehole and continuing the operation.

The third line 8 of the termination module 9 further comprises a kill line pressure controller 27 disposed in the first section 23, the kill line pressure controller 27 being disposed on a module kill line 34. It is highly necessary to provide the kill line pressure controller 27 which is configured to control the pressure and volume of kill fluid pumped from the rig 17 through the riser-less subsea blowout preventer 15 into the borehole 40, and the pressure and volume of drilling fluid returned from the borehole 40 to the rig 17. The kill line pressure controller 27 is an important component equipped by the present disclosure, particularly for use in deep water, when the pressure in the borehole 40 is out of control, such as the first and second instances of borehole overpressure out of control, the non-riser subsea blowout preventer 15 locks the drill pipe 42 and even severs the drill pipe 42, thereby sealing the borehole 40 from communication with the outside seawater, and it is necessary to properly adjust the kill line pressure controller 27 so that high pressure kill fluid can smoothly enter the borehole 40 under high pressure, and in another instance, so that drilling fluid discharged from the borehole 40 can be returned to the drilling platform 17.

In an exemplary embodiment of the present disclosure, as shown in fig. 1, the drilling system further comprises a drilling template 41, the drilling template 41 being installed uphole of the borehole 40. The subsea riser-less blowout preventer 15 is also provided with a wellhead connector 49, which wellhead connector 49 is connectable to the drilling template 41. All of the components of the wellhead connector 49 are installed in the frame module to supplement the riser-less 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 configured to: before the riser-less subsea blowout preventer 15 is installed, it acts as a closed loop fluid return system for drilling operations and cementing operations of the surface interval of the borehole 40. After the riser-less subsea blowout preventer 15 is installed on the drilling template 41 and the drilling fluid accumulation controller 14 is installed above the riser-less subsea blowout preventer 15, riser-less drilling and drilling fluid return operations are performed for the remaining section of the borehole 40. By the above-described segmental operation, the marine riser-less blowout preventer 15 can be applied in a situation where the formation environment is more complicated, thereby making strain on various emergency situations.

In an exemplary embodiment according to the present disclosure, the first portion 23 further comprises a pressure and volume regulator 35 configured to control the volume and pressure of the high pressure gas within 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 is capable of pressure compensation when the double tube 2 and the end module 9 perform the first function.

In an exemplary embodiment according to the present disclosure, the bypass line 21 is also configured to circulate drilling fluid from the drilling platform 17 between the drilling platform 17 and the inner pipe 3. If the gas injection system 19 fails to inject air, it will cause drill cuttings of the air-drilling fluid mixture to settle 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, drilling fluid from the drilling platform 17 can be injected from the inner tubular 3 and flow into the annular space 5 to flush away drill cuttings deposits to achieve a circulating flow of the drilling fluid.

It will be appreciated by those skilled in the art that the embodiments described above are exemplary and can be modified by those skilled in the art, and that the structures described in the various embodiments can be freely combined without conflict in structure or principle.

While the present disclosure has been described in connection with the accompanying drawings, the embodiments disclosed in the drawings are intended to be illustrative of the preferred embodiments of the disclosure, and should not be construed as limiting the disclosure.

Although a few 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 in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.

It should be noted that the word "comprising" does not exclude other elements or steps, and the words "a" or "an" do not exclude a plurality. Additionally, any element numbers of the claims should not be construed as limiting the scope of the disclosure.

Claims

1. A drilling system for geological exploration drilling, comprising:

a drilling platform (17) that is capable of floating and moving on the water surface (20);

a mining section (4) comprising:

a borehole (40) formed in a subsea formation (13);

a drill pipe (42) extending downwardly from the drilling platform (17) into the borehole (40);

a riser-less subsea blowout preventer (15) disposed at a wellhead of the borehole (40) and through which the drill pipe (42) extends for drilling; and

a drilling fluid volume controller (14) disposed on top of the riser-less subsea blowout preventer (15),

wherein the riser-less 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 a pressure condition in the borehole (40); and

a drilling fluid return system (1) connected to the drilling platform (17) and spaced from the production section (4), comprising:

a double-layer tube (2) comprising an outer layer tube (65) and an inner layer tube (3), an annular space (5) being formed between the outer layer tube (65) and the inner layer tube (3); and

a termination module (9) connected to the lower end of the double pipe (2) and comprising a first line (6) arranged between the annular space (5) and the drilling fluid volume controller (14); a second line (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 riser-less 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 status in the borehole (40) to achieve different functions.

2. A drilling system according to claim 1, the terminal module (9) comprising a first portion (23) and a second portion (24) detachably connected together, the first portion (23) being connected to the lower end of the double pipe (2), and the second portion (24) being connected to the drilling fluid accumulation controller (14) by a first flexible pipe (10) and to the riser-less subsea blowout preventer (15) by 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. A drilling system according to claim 2, wherein the first line (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 pipe (2), which can be opened or closed to conduct or block the first line (6); and

wherein the drilling fluid transfer pump (28) is connected between the drilling fluid control valve (29) and the first flexible conduit (10) and is configured to draw drilling fluid from the drilling fluid volume controller (14) through the first flexible conduit (10) in a state in which the drilling fluid control valve (29) is opened to conduct the first line (6).

4. A drilling system according to claim 3, wherein the first line (6) of the termination module (9) further comprises a drilling fluid line pressure controller (33) arranged in the first section (23),

the drilling fluid line 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) above the seawater hydrostatic pressure, thereby avoiding the suction of seawater from the drilling fluid volume controller (14) into the drilling fluid return system (1) due to the U-tube effect.

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

6. A drilling system according to claim 2, wherein the third line (8) of the terminal module (9) comprises a kill line control valve (31) and an intra-module kill line (34) provided in the first portion (23), and the second flexible pipe (12);

the kill line control valve (31) is connected between the inner pipe (3) and the modular kill line (34) which can be opened or closed to open or close the third line (8), and the modular kill line (34) is connected to the second flexible pipe (12);

with the third line (8) conducting, either a control fluid can be supplied from the drilling platform (17) to the borehole (40) or drilling fluid can be returned from the borehole (40) to the drilling platform (17).

7. A drilling system according to claim 6, wherein the third line (8) of the termination module (9) further comprises a kill line pressure controller (27) provided in the first section (23),

the kill line pressure controller (27) is disposed on the modular kill line (34) and is configured to control the pressure and volume of kill fluid pumped from the drilling rig (17) through the riser-less subsea blowout preventer (15) into the borehole (40), and the pressure and volume of drilling fluid returned from the borehole (40) to the drilling rig (17).

8. A drilling system according to claim 2, wherein the first portion (23) and the second portion (24) are removably connected together by an emergency disconnect system (25);

the emergency disconnect system (25) is configured to: enabling separation of the first portion (23) and the second portion (24) in the event of inopportune inclement weather or emergency departure of the drilling platform (17) from the drilling site;

the second portion (24) is provided with a buoyancy block (46); and

the buoyancy block (46) is configured to: after performing an emergency disconnect, the emergency disconnect system (25) will be kept in a horizontal state so that the first part (23) can be reconnected to the second part (24).

9. The drilling platform as claimed in claim 1, the drilling platform (17) being provided with: a gas injection system (19) for supplying high-pressure gas into the inner pipe (3); a control fluid supply and choke system (58) for supplying control fluid into the inner tubular (3); and a drilling fluid separator (18) for separating gas from the high pressure gas-drilling fluid mixture.

10. A drilling system according to claim 9, further comprising a selector and pressure control manifold (59) connected to the top end of the double pipe (2) and connected to the gas injection system (19), the kill fluid supply and choke system (58) and the drilling fluid separator (18);

the selector and pressure control manifold (59) is configured such that the double pipe (2) is selectively connectable to the gas injection system (19), the kill fluid supply and choke system (58) or the drilling fluid separator (18).

11. The drilling system of claim 9, wherein,

in the event that pressure within the borehole is normal, the riser-less 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 a first instance of a loss of control of borehole overpressure, the riser-less subsea blowout preventer (15) is operated to seal the borehole (40) such that drilling fluid in the borehole (40) cannot enter the drilling fluid volume controller (14) and the drill pipe (42) is not severed; and

in a second situation of uncontrolled borehole overpressure, the riser-less subsea blowout preventer (15) is operated to: -upon sealing the bore hole (40), also cutting the drill rod (42);

wherein the pressure in the borehole in the second case of uncontrolled borehole overpressure is higher than the pressure in the borehole in the first case of uncontrolled borehole overpressure, and the pressure in the borehole in the case of normal pressure in the borehole is lower than the pressure in the borehole in the first case of uncontrolled borehole overpressure.

12. The drilling system of claim 11,

the marine riser-free subsea blowout preventer (15) comprising:

a fastening seal ram (51) disposed proximate to the drilling fluid volume controller (14);

a shear ram (50) disposed proximate to the borehole (40); and

a gate valve module (16) disposed between the clinch seal gate (51) and the shear gate (50);

wherein the riser-free subsea blowout preventer (15) is configured to:

-in the event of normal borehole pressure, deactivating said snap-on seal rams (51) and said shear rams (50) to allow low pressure drilling fluid in said borehole (40) to be collected in said drilling fluid volume controller (14);

in a first case of uncontrolled drilling overpressure, actuating the buckling sealing rams (51) to hug or buckle the drill pipe (42) to completely seal the borehole (40) such that drilling fluid in the borehole (40) cannot enter the drilling fluid accumulator (14), and

after completely sealing the borehole (40), in a second case of uncontrolled borehole overpressure with further increase in pressure in the borehole, the shear ram (50) is also actuated to shear the drill rod (42), wherein the lower end of the severed drill rod (42) is retained in the ram valve module (16).

13. The drilling system of claim 11,

-in the case of normal pressure in the borehole, the double pipe (2) is connected to the gas injection system (19), the first (6) and second (7) lines being in a conducting state and the third line (8) being in a blocking state;

the low pressure drilling fluid collected in the drilling fluid volume controller (14) is conveyed through the first line (6) into the annular space (5) of the double pipe (2); and

the high pressure gas injection system (19) injects high pressure gas into the terminal module (9) through the inner pipe (3) of the double pipe (2) and circulates there into the annular space (5) so that the high pressure gas mixes with low pressure drilling fluid, thereby recovering the low pressure drilling fluid to the drilling platform (17).

14. The drilling system of claim 11,

in a first case of uncontrolled borehole overpressure, the double pipe (2) is connected to the drilling fluid separator (18), the first (6) and second (7) lines are in a blocked state, and the third line (8) is in a conducting state;

high pressure kill fluid is injected into the borehole (40) through the drill pipe (42), and drilling fluid in the borehole (40) is conveyed to the drilling fluid separator (18) through the riser-less subsea blowout preventer (15), the third pipeline (8), and the inner pipe (3) of the double pipe (2).

15. The drilling system of claim 11,

in a second case of uncontrolled drilling overpressure, the double pipe (2) is connected to the control fluid supply and choke system (58), the first (6) and second (7) lines are in a blocked state, and the third line (8) is in a conducting state;

the control fluid supply and choke system (58) injects high pressure control fluid through the inner pipe (3) of the double pipe (2) which is injected through the third pipe (8) into the riser-less subsea blowout preventer (15) and further into the borehole (40) to restore hydrostatic control within the borehole (40) to continue drilling.

16. The drilling system of claim 3, wherein,

the drilling fluid volume controller (14) comprises a low-pressure drilling fluid receiving space (38) which is open and is in communication with seawater (39), wherein the low-pressure drilling fluid is delimited from the seawater (39) at a contact surface (37);

wherein the rotational speed of the drilling fluid transfer pump (28) is controlled on the basis of a level sensor (381) measuring the level of the contact surface (37) such that no seawater (39) is sucked into the first line (6).

17. The drilling system of claim 12,

the riser-less subsea blowout preventer (15) comprising a primary control system (56) and a secondary control system (55),

the primary control system (56) controls the clinch seal rams (51), the shear rams (50) and the ram valve modules (16) based on acoustic signals from the drilling platform (17), and

the secondary control system (55) controls the fastening seal ram (51), the shear ram (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,

the marine riser-less subsea blowout preventer (15) further comprises: a blowout preventer control fluid accumulator (57) for storing and supplying drive 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 makeup line (64) configured to: for supplementing the blowout preventer control fluid accumulator (57) with drive fluid when required;

wherein the accumulator makeup line (64) is connected to the terminal module (9) to receive drive fluid from the terminal module (9).

19. The drilling system of claim 1, further comprising: a drilling template (41), the drilling template (41) being mounted uphole of the borehole (40);

the drilling fluid volume controller (14) and the drilling fluid return system (1) are configured to: prior to installing the riser-less subsea blowout preventer (15), as a closed loop drilling fluid return system for use in drilling and cementing operations of a surface section of the borehole (40);

subsequent to installing the riser-less subsea blowout preventer (15) on the drilling template (41) and the drilling fluid volume controller (14) over the riser-less subsea blowout preventer (15), riser-less drilling and drilling fluid return operations are performed for the remaining section of the borehole (40).

20. The drilling system of claim 2,

the first section (23) further comprises a pressure and volume regulator (35) configured to control the volume and pressure of high pressure gas within low pressure drilling fluid injected into the annular space (5) so that low pressure drilling fluid can be lifted to the drilling platform (17).

21. A drilling system according to claim 5, wherein the bypass line (21) is further configured to circulate drilling fluid from the drilling platform (17) between the drilling platform and the inner pipe (3),

wherein if an air injection failure occurs in the gas injection system (19), drilling cuttings of an air-drilling fluid mixture are deposited in the annular space (5), and drilling cuttings deposits are flushed away by closing the kill line control valve (31) and the drilling fluid control valve (29) and opening the bypass valve (32) to enable drilling fluid to be injected from the inner pipe (3) and flow into the annular space (5) to effect a circulating flow of the drilling fluid.