Riser-free mud circulation system and control and operation methods thereof

Abstract

The present invention discloses a riser-free mud circulation system and its control and operation methods, relating to the field of oil and gas drilling technology. The system comprises a floating drilling platform, a wellhead suction device, and a drill string. The upper end of the vertically arranged drill string is disposed on the floating drilling platform, and the lower end of the drill string is passed through the wellhead suction device. A low-pressure mud treatment system is disposed on the floating drilling platform. The return line system comprises a lift pump, a rigid pipe, a first flexible pipe, and a second flexible pipe. The upper end of the vertically arranged rigid pipe is disposed on a drilling vessel. The lower inlet of the rigid pipe is connected to the outlet of the lift pump. The two ends of the first flexible pipe are respectively connected to the outlet of the wellhead suction device and the inlet of the lift pump. The two ends of the second flexible pipe are respectively connected to the upper outlet of the rigid pipe and the inlet of the low-pressure mud treatment system. The system can solve the problem that existing drilling platforms cannot store too many rigid pipes, and avoid the existing rigid pipes being easily twisted and broken, interfering with or colliding with drill string operations, and causing recovery accidents.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas drilling, and in particular to a riser-free mud circulation system and a control method and an operation method thereof. Background Art

[0002] Compared to conventional drilling methods, riser-less mud circulation technology can effectively match fracture pressure with formation pore pressure, effectively controlling annular pressure within the wellbore. It also simplifies the wellbore structure of deepwater wells and avoids the safety hazards associated with multiple layers of casing. Furthermore, the subsea drilling fluid lift system saves drilling time and costs, reduces pressure hazards, prevents lost circulation, and makes it easier to control shallow flow zones. Therefore, riser-less mud circulation technology is commonly used in oil and gas drilling operations.

[0003] Riser-less mud circulation technology typically uses flexible hoses as return lines in shallow waters. However, in deepwater, rigid pipes are connected to form a pipe string for return lines. Currently, its maximum application depth is around 1,500 meters. However, numerous challenges exist in applying this technology to deeper waters, making it difficult to expand its application to deeper drilling environments.

[0004] Among them, these problems are mainly manifested in the following aspects: (1) Since a large number of rigid pipes need to be connected into a return pipeline, and the deck operation area of ​​the drilling platform is limited, it is impossible to store more rigid pipes. (2) The rigid pipes and drill strings used as the return pipeline are operated together in the moon pool area. In order to separate the seabed suction module and the seabed pump by a certain distance, the return rigid pipe needs to be inserted into the seabed at a certain angle, and the seabed anchor module needs to be used to insert it into the seabed at the same time. This will lead to problems such as the seabed anchor module being difficult to stably insert into the seabed, and interference between the rigid pipe and the drill string operation. (3) When the bow direction of the ship needs to be rotated and adjusted due to the influence of wind and waves, there are problems such as the return rigid pipe being easily twisted and broken, and colliding with the drill string. (4) In emergency situations, it is difficult to safely recover the rigid pipe and the drill string at the same time, so there is a risk of accidents. Summary of the Invention

[0005] The present invention aims to address at least one of the technical problems existing in the prior art. To this end, the present invention proposes a riser-free mud circulation system and its control and operation methods. This system addresses the problem of existing drilling platforms' limited deck operating area, which prevents excessive storage of rigid pipes. It also prevents existing rigid pipes from twisting and breaking during drilling, interfering with or colliding with drill strings, and causing recovery accidents, thereby improving operational efficiency.

[0006] A first embodiment of the present invention provides a riser-free mud circulation system, comprising:

[0007] A drilling device comprising a floating drilling platform, a wellhead suction device and a drill string, wherein the drill string is arranged vertically, the upper end of the drill string is arranged on the floating drilling platform, and the lower end of the drill string is passed through the wellhead suction device;

[0008] drilling vessels;

[0009] a low-pressure mud processing system, which is provided on the floating drilling platform;

[0010] A return pipeline system includes a lift pump, a rigid pipe, a first flexible pipe and a second flexible pipe. The rigid pipe is arranged vertically, and the upper end of the rigid pipe is provided on the drilling ship. The lower inlet of the rigid pipe is connected to the outlet of the lift pump. One end of the first flexible pipe is connected to the outlet of the wellhead suction device, and the other end is connected to the inlet of the lift pump. One end of the second flexible pipe is connected to the upper outlet of the rigid pipe, and the other end is connected to the inlet of the low-pressure mud processing system.

[0011] The mud circulation system based on a watertight pipeless embodiment according to the first aspect of the present invention has at least the following beneficial effects: the drill string and the rigid pipe are respectively arranged corresponding to the floating drilling platform and the drilling ship, and the two ends of the first flexible pipe are respectively connected to the outlet of the wellhead suction device and the inlet of the lifting pump, and the two ends of the second flexible pipe are respectively connected to the upper outlet of the rigid pipe and the inlet of the low-pressure mud treatment system, and then the lower inlet of the rigid pipe is connected to the outlet of the lifting pump, so as to form a mud return pipeline; when the drilling device is running, the wellhead suction device can suck in and collect the mud returned from the wellbore, and then, under the operation of the lifting pump, the mud is prompted to flow out of the wellhead suction device, and flow through the first flexible pipe, the lifting pump, the rigid pipe and the second flexible pipe in turn, and finally flow to the low-pressure mud treatment system, so that the mud flows back from the seabed to the floating drilling platform, realizing the mud circulation function without a watertight pipe.

[0012] The present invention adopts a combination of a floating drilling platform and a drilling ship to separate and arrange the drill string and rigid pipe, both of which are in a vertical state, and effectively increase the horizontal spacing between the drill string and the rigid pipe. This can avoid the problems of interference and collision between the drill string and the rigid pipe during drilling operations, and the drill string and the rigid pipe being difficult to recover safely in emergency situations. It can also prevent the rigid pipe from twisting and breaking when the bow of the ship needs to be adjusted due to the influence of wind and waves. At the same time, it also solves the problem that the existing drilling platform cannot store too many rigid pipes due to the small deck operating area, thereby greatly reducing operational risks, improving operational efficiency, and promoting the widespread use of watertight pipe-free mud circulation technology in deeper ocean drilling environments.

[0013] In some embodiments of the present invention, the drilling ship has a high-pressure mud manifold, and the return pipeline system also includes a first control valve, a second control valve and a three-way manifold. The three-way manifold has three pipe interfaces, and the three pipe interfaces are respectively connected to the upper outlet of the hard pipe, the inlet of the second hose and the outlet of the high-pressure mud manifold. The first control valve is arranged at the upper outlet of the hard pipe, and the second control valve is arranged at the outlet of the high-pressure mud manifold.

[0014] In some embodiments of the present invention, the mud circulation system based on a watertight riser also includes a control system, which is electrically connected to the first control valve, the second control valve and the lift pump respectively. The control system can control the first control valve and the lift pump to open and control the second control valve to close in a mud return condition, and can control the second control valve to open and control the first control valve and the lift pump to close in a replenishment condition.

[0015] In some embodiments of the present invention, the first hose is provided with a plurality of first buoyancy blocks along its extension direction; and / or,

[0016] The second hose is provided with a plurality of second buoyancy blocks along its extending direction.

[0017] In some embodiments of the present invention, the drilling ship has a drill pipe, a first derrick and a first top drive, the drill pipe is set as the hard pipe, the first top drive is drivingly connected to the hard pipe, and the first top drive is slidably connected to the first derrick in the up and down directions to drive the hard pipe to rise and fall; the drilling ship has a heave compensation system, and the heave compensation system is used to compensate for the heave movement of the hard pipe.

[0018] In some embodiments of the present invention, the rigid tube is provided with a plurality of tube fixing devices along its extending direction, and the plurality of tube fixing devices are detachably connected to the first flexible tube.

[0019] In some embodiments of the present invention, the tube fixing device includes a bracket, a first pin and a connecting rod, the bracket is U-shaped, one end of the connecting rod is hinged to one end of the bracket, the other end of the bracket and the other end of the connecting rod are both provided with a first socket, the first pin can be inserted into all of the first sockets, so that the connecting rod and the bracket together form a fixing cavity for fixing the first hose.

[0020] In some embodiments of the present invention, the pipe fixing device also includes a second pin, the connecting rod is arranged horizontally, the first pin and the second pin are arranged vertically, one end of the bracket and one end of the connecting rod are provided with a second socket, and the second pin can be inserted into all of the second sockets so that one end of the connecting rod is hinged to one end of the bracket.

[0021] A second embodiment of the present invention provides a control method for a riser-less mud circulation system, which is applied to the riser-less mud circulation system as described in the first embodiment, and includes the following steps:

[0022] After receiving the input command, determine the current working condition;

[0023] If the current operating condition is a mud return condition, the first control valve and the lift pump are controlled to be open, and the second control valve is controlled to be closed;

[0024] If the current operating condition is a supply condition, the second control valve is controlled to be open, and the first control valve and the lift pump are controlled to be closed.

[0025] According to the control method of the mud circulation system based on the watertight pipeless embodiment of the second aspect of the present invention, there are at least the following beneficial effects: according to the instructions input by the user, the current operating condition of the mud circulation system based on the watertight pipeless is judged; if it is judged that the mud circulation system based on the watertight pipeless is in the mud return condition, the lifting pump and the first control valve are ensured to be in the open state, and the second control valve is in the closed state, thereby enabling the mud to be returned from the seabed through the return pipeline system to the low-pressure mud treatment system on the floating drilling platform, thereby realizing the mud circulation operation without watertight pipe; if it is judged that the mud circulation system based on the watertight pipeless is in the supply condition, the second control valve is ensured to be in the open state, and the lifting pump and the first control valve are in the closed state, thereby enabling the supply fluids such as mud, cementing fluid and drilling water on the drilling ship to be transported from the high-pressure mud manifold through the second hose to the low-pressure mud treatment system on the floating drilling platform, thereby completing the supply work.

[0026] Such a design can adjust the working status of the first control valve, the second control valve and the lifting pump according to changes in the operating conditions, enabling the mud circulation system based on a watertight pipe to switch between mud return conditions and supply conditions.

[0027] A third embodiment of the present invention provides an operating method for a riser-less mud circulation system, which is applied to the riser-less mud circulation system as described in the first embodiment, and includes the following steps:

[0028] Driving the wellhead suction device to move downward and install it on the wellhead, and driving the drill string on the floating drilling platform to drill into the wellhead suction device;

[0029] driving the drilling vessel to sail a first distance toward the floating drilling platform, driving the hard pipe to move the lift pump and the first hose downward a second distance, and connecting the second hose to the low-pressure mud processing system;

[0030] After the first hose moves downward a second distance, driving the ROV to remove the first hose from the pipe fixture and connect the first hose to the wellhead suction device;

[0031] After the first hose and the second hose are installed, the drilling vessel is driven to sail a third distance away from the floating drilling platform, and the hard pipe is driven to drive the lift pump to move down a fourth distance;

[0032] Turning on the heave compensation system on the drilling vessel and driving the hard pipe to move the lift pump downward to the seabed;

[0033] The lift pump is started to perform mud return operation.

[0034] The operating method of the mud circulation system based on a watertight pipeless embodiment according to the third aspect of the present invention has at least the following beneficial effects: after the floating drilling platform is positioned relative to the wellhead on the sea surface, the wellhead suction device is moved downward from the floating drilling platform and set at the wellhead, and then the drill string is moved downward and screwed into the wellhead suction device, so that the drill string can perform drilling operations at the wellhead. During this process, the wellhead suction device can suck in and collect mud returned from the wellbore; the drilling ship is driven close to the floating drilling platform, and the horizontal distance between the drilling ship and the floating drilling platform is reduced so that the second hose can be installed in the low-pressure mud treatment system on the floating drilling platform. Moreover, after the hard pipe drives the lifting pump and the first hose to move down a second distance, the first hose is removed from the pipe fixing device on the hard pipe on the seabed with the help of ROV so that the first hose can be installed in the wellhead suction device. At this time, the work of connecting the return pipeline system to the wellhead suction device and the low-pressure mud treatment system respectively has been completed.

[0035] Then, the drilling ship is driven away from the floating drilling platform, and the horizontal distance between the drilling ship and the floating drilling platform is increased to avoid interference and collision between the drill string and the rigid pipe. Then, when the position of the drilling ship on the sea surface is confirmed, the rigid pipe is driven to drive the lifting pump down to an appropriate height from the seabed, and the heave compensation system on the drilling ship is activated. The rigid pipe is then driven to drive the lifting pump to continue to move down until the lifting pump is located on the seabed. At this time, the heave compensation system can realize the wave heave compensation function of the mud circulation system based on the watertight pipe, avoid the problem of the rigid pipe being pulled and damaged in strong winds and waves, and ensure that the mud circulation system based on the watertight pipe can smoothly carry out drilling operations. Once the return pipeline system is installed and the positions of the lifting pump and rigid pipe are set, the lifting pump can be operated to carry out the mud return process during the drilling process.

[0036] Other features and advantages of the present invention will be described in the following description, and in part will become apparent from the description, or will be understood by practicing the present invention. The purposes and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 2 is a schematic structural diagram of a riser-free mud circulation system according to an embodiment of the present invention when the first hose and the second hose are in a non-use state;

[0038] Figure 2 1 is a schematic structural diagram of a riser-free mud circulation system according to an embodiment of the present invention when a first hose and a second hose are installed;

[0039] Figure 3 1 is a schematic structural diagram of a riser-free mud circulation system according to an embodiment of the present invention during mud return operation;

[0040] Figure 4 is a schematic structural diagram of a pipe fixing device provided in an embodiment of the present invention from a top view;

[0041] Figure 5 1 is a schematic diagram of a specific flow chart of a control method for a riser-free mud circulation system according to an embodiment of the present invention;

[0042] Figure 6 It is a specific flow chart of an operating method of a riser-free mud circulation system provided according to an embodiment of the present invention.

[0043] Figure numerals: 110, floating drilling platform; 120, drill string; 130, wellhead suction device; 140, second derrick; 150, second top drive; 200, wellbore; 310, drilling ship; 320, first derrick; 330, first top drive; 410, hard pipe; 420, lift pump; 430, first hose; 440, first buoyancy block; 450, pipe fixing device; 451, bracket; 452, connecting rod; 453, first pin; 454, second pin; 455, U-bolt; 456, connecting plate; 460, three-way manifold; 470, second hose; 480, second buoyancy block; 510, high-pressure mud manifold; 520, low-pressure mud processing system. DETAILED DESCRIPTION

[0044] The following describes embodiments of the present invention in detail. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended only to explain the present invention and are not to be construed as limiting the present invention.

[0045] In the description of the present invention, it should be understood that a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0046] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "connected" and "connection" should be understood in a broad sense. For example, they can refer to fixed connection, detachable connection, or integral connection; mechanical connection or electrical connection; direct connection or indirect connection through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0047] Conventional drilling techniques typically utilize single-gradient drilling methods. However, deepwater drilling presents the challenge of narrow formation pressure gaps, necessitating the use of multi-layer casing systems. The maximum water depth and well depth are constrained by various factors. Furthermore, large and long riser systems are subject to complex forces in harsh marine environments, making drilling accidents more likely.

[0048] Riserless Mud Recovery System (RMR), a dual-gradient drilling technology, does not use conventional marine risers during the drilling process. Instead, it uses a subsea suction module to divert the cuttings and drilling fluid returning from the wellbore annulus. Under the action of a subsea pump, the cuttings and drilling fluid are returned to the drilling platform from the seabed through a small-diameter return pipeline, thus realizing riser-free drilling fluid circulation.

[0049] Compared to conventional drilling methods, riser-less mud circulation technology can effectively match fracture pressure with formation pore pressure, effectively controlling annular pressure within the wellbore. It can also simplify the wellbore structure of deepwater wells and avoid the safety hazards caused by multiple layers of casing. Furthermore, the subsea drilling fluid lift system saves drilling time and costs, reduces pressure hazards, prevents lost circulation, and makes it easier to control shallow flow zones. Therefore, riser-less mud circulation technology is commonly used in offshore oil and gas drilling operations.

[0050] Riserless mud circulation technology typically uses flexible hoses as return lines in shallow waters. However, in deepwater, it uses rigid pipes connected into a string for return lines. Currently, its maximum application depth is approximately 1,500 meters. However, in deeper waters, riserless mud circulation technology faces numerous challenges, making it difficult to expand its application in deeper offshore drilling environments.

[0051] Specifically, these problems mainly include: (1) Since a large number of rigid pipes need to be connected into a return line, and the deck operating area of ​​the drilling platform is limited, more rigid pipes cannot be stored on the drilling platform. (2) The rigid pipes used as the return line and the rigid pipes used as the drill string are operated together in the moon pool area, that is, there are two rigid pipes from the sea surface to the seabed in the moon pool area. In order to separate the seabed suction module and the seabed pump by a certain distance, it is generally necessary to insert the return rigid pipe at a certain angle into the seabed, and at the same time use the seabed anchoring module to insert into the seabed. This will lead to problems such as the seabed anchoring module being difficult to stably insert into the seabed and interference between the two rigid pipe strings. (3) When the bow direction of the ship needs to be rotated and adjusted due to the influence of wind and waves, there are problems such as the return rigid pipe being easily twisted and broken and colliding with the drill string. (4) In the event of an emergency, it is difficult to safely recover the two rigid pipe strings at the same time, so there is a risk of accidents.

[0052] Based on the above technical problems, the present invention provides a mud circulation system based on a watertight pipe and its control method and operation method, which can solve the problem that the deck operating area of ​​the existing drilling platform is small and cannot store too many hard pipes, and can avoid the existing hard pipes from being easily twisted and broken during the drilling process, interfering with or colliding with the drill string operation, and recovery accidents, which is conducive to reducing operational risks and improving operational efficiency, and ultimately realizing that the watertight pipe-free mud circulation technology can be promoted and applied in deeper marine drilling environments.

[0053] Reference below Figures 1 to 6 The present invention describes a riser-less mud circulation system and a control method and an operation method thereof according to an embodiment of the present invention.

[0054] like Figures 1 to 4 As shown, the mud circulation system based on a watertight pipeless embodiment according to the first aspect of the present invention can be applied to ocean scientific drilling, deep-sea oil and gas exploration and development, and natural gas hydrate mining, and can perform watertight pipeless drilling fluid circulation during the drilling process.

[0055] like Figures 1 to 3 As shown, the riser-less mud circulation system of this embodiment includes a drilling device, a drilling vessel 310, a low-pressure mud processing system 520, and a return pipeline system.

[0056] The drilling rig includes a floating drilling platform 110, a wellhead suction unit 130, and a drill string 120. The drill string 120 is arranged vertically, extending vertically. The upper end of the drill string 120 is mounted on the floating drilling platform 110. The drill string 120 can follow the floating drilling platform 110 as it moves on the sea surface, allowing the floating drilling platform 110 to move the drill string 120 to an appropriate position for drilling operations. The lower end of the drill string 120 is inserted into the wellhead suction unit 130.

[0057] It will be appreciated that, in one embodiment, the floating drilling platform 110 is a semi-submersible drilling platform; in another embodiment, the floating drilling platform 110 is a drillship. Before drilling begins, the wellhead suction device 130 is positioned on the floating drilling platform 110 and is capable of moving with the floating drilling platform 110. The wellhead suction device 130 is lowered by a winch on the floating drilling platform 110 and positioned at the wellhead on the seabed. The wellhead suction device 130 is configured to suck in and collect mud returning from the wellbore 200.

[0058] The floating drilling platform 110 is equipped with a second derrick 140 and a second top drive 150. The second top drive 150, serving as a top-drive drilling device, is mounted on the top region of the second derrick 140 via a guide pulley assembly. The second top drive 150 is drive-connected to the drill string 120 and can rotate the drill string 120 during operation. The second derrick 140 provides structural support for the second top drive 150, bearing its own weight and the load of the drill string 120. The second top drive 150 can move up and down along guide rails provided within the second derrick 140, thereby controlling the raising and lowering of the drill string 120. The floating drilling platform 110 is conventional, and the second derrick 140, the second top drive 150, and the drill string 120 are ancillary equipment on the floating drilling platform 110. Those skilled in the art should understand their specific structures, connections, and operating principles, and will not be further described here.

[0059] During drilling operations on the drill string 120 of the floating drilling platform 110, a wellhead suction device 130 is primarily located at the wellhead, with the drill string 120 intersecting the wellhead suction device 130. The drill string 120 serves as a drilling fluid circulation channel, transporting mud to the bottom of the well. The mud, carrying cuttings, then passes through the wellhead suction device 130 (also known as a wellhead suction module or subsea suction module) for purification. The wellhead suction module utilizes cyclonic separation and other devices to remove impurities. Specifically, it diverts the cuttings and drilling fluid returning from the annulus of the wellbore 200, helping to maintain pressure balance within the closed drilling fluid circulation system.

[0060] In this embodiment, the drilling vessel 310 is a small drilling vessel located to the left and right of the floating drilling platform 110. When the floating drilling platform 110 is operating offshore, the drilling vessel 310 can serve as a supply vessel alongside the floating drilling platform 110, providing escort, crew rotation, and material replenishment.

[0061] The low-pressure mud treatment system 520 is installed on the floating drilling platform 110. It is understandable that the low-pressure mud treatment system 520 can play the role of waste liquid treatment and circulation purification, waste classification treatment, well pressure control, etc. For example, the low-pressure mud treatment system 520 can receive the waste mud, drill cuttings and waste liquid generated during the drilling process, and realize transfer and purification through a closed loop; perform graded solid-liquid separation through equipment such as vibrating screens, desanders, and desilters to reduce the solid phase content and maintain the rheological properties of the mud; and ensure that the treated mud can be re-injected into the drilling circulation by adjusting parameters such as density and viscosity, thereby reducing resource waste. For another example, the low-pressure mud treatment system 520 can adjust the density of the mud to balance the formation pressure, effectively preventing accidents such as blowouts or leakage.

[0062] The low-pressure mud treatment system 520 is an existing technology and an important component of the mud circulation system in drilling operations. It can realize the functions of mud purification, recovery and blending. Those skilled in the art should understand its specific structure and working principle, which will not be repeated here.

[0063] The return line system connects the wellhead suction device 130 and the low-pressure mud processing system 520, allowing the mud at the wellhead suction device 130 to return to the low-pressure mud processing system 520 through the return line system. The return line system includes a lift pump 420, a rigid pipe 410, a first hose 430, and a second hose 470.

[0064] The rigid pipe 410 is arranged vertically, that is, it extends in an up-and-down direction. The upper end of the rigid pipe 410 is mounted on the drill ship 310. The rigid pipe 410 can move with the drill ship 310 on the sea surface, allowing the drill ship 310 to drive the rigid pipe 410 to a suitable position, ensuring a safe distance between the rigid pipe 410 and the drill string 120. The lower end of the rigid pipe 410 is fixedly connected to the lift pump 420, which can move with the rigid pipe 410. The lower inlet of the rigid pipe 410 is connected to the outlet of the lift pump 420. The rigid pipe 410 has a lumen that communicates with the outlet of the lift pump 420, allowing the mud pumped by the lift pump 420 to flow upward through the lumen of the rigid pipe 410. The rigid pipe 410 can be made of, but is not limited to, titanium alloy or alloy steel.

[0065] One end of the first hose 430 is connected to the outlet of the wellhead suction device 130, and the other end of the first hose 430 is connected to the inlet of the lift pump 420, so that the lumen of the first hose 430 can be connected to the outlet of the wellhead suction device 130 and the inlet of the lift pump 420 respectively. Therefore, the mud at the wellhead suction device 130 can flow to the lift pump 420 through the first hose 430, and then through the pumping action of the lift pump 420, the mud flows upward along the extension direction of the rigid tube 410. One end of the second hose 470 is connected to the upper outlet of the rigid tube 410, and the other end of the second hose 470 is connected to the inlet of the low-pressure mud treatment system 520, so that the lumen of the second hose 470 can be connected to the rigid tube 410 and the low-pressure mud treatment system 520 respectively. Therefore, the mud at the rigid tube 410 can flow to the low-pressure mud treatment system 520 through the second hose 470, thereby enabling the mud to return from the wellhead suction device 130 to the low-pressure mud treatment system 520.

[0066] The hard pipe 410, the lifting pump 420 and the first hose 430 are lowered to an appropriate position in the seawater, and the upper outlets of the low-pressure mud processing system 520 and the hard pipe 410 are both located above the sea surface. Therefore, the first hose 430 is located underwater and the second hose 470 is located on the water surface.

[0067] The first hose 430 is detachably connected to the wellhead suction device 130, and the second hose 470 is detachably connected to the low-pressure mud processing system 520. Therefore, after the first hose 430 and the second hose 470 are detachably connected, the drilling vessel 310 can move freely on the sea surface with the return pipeline system.

[0068] In this embodiment, a dual-ship operation mode is adopted, the wellhead suction device 130 and the drill string 120 are arranged at the floating drilling platform 110, and the return pipeline system is arranged at the drilling ship 310, so as to decouple the drilling device from the return pipeline system. Therefore, it can solve the problem in the prior art that the deck operating area of ​​the drilling platform is small and cannot store enough hard pipes 410, which in turn makes it impossible to promote and apply the mud circulation technology without watertight pipes in deeper ocean drilling environments.

[0069] Moreover, since there is a sufficiently large horizontal spacing between the drill string 120 and the hard pipe 410, it is possible to solve the problems in the prior art of the seabed anchoring module being difficult to stably insert into the seabed and the operational interference between the drill string 120 and the hard pipe 410, and it is possible to solve the problem of the hard pipe 410 twisting and breaking or the hard pipe 410 colliding with the drill string 120 when the bow of the ship needs to be rotated and adjusted due to the influence of wind and waves. Moreover, in an emergency, the drill string 120 and the hard pipe 410 can be safely recovered at the same time to avoid collision accidents between the drill string 120 and the hard pipe 410 during the recovery process.

[0070] Generally, the daily operating cost of the drilling ship 310 is about one-tenth of the drilling equipment, which is relatively low. Therefore, when the drilling equipment uses the mud circulation technology without a watertight pipe for drilling operations, the drilling ship 310 can not only play the role of a supply ship, but also serve as a carrier ship for the return pipeline system.

[0071] It can be understood that, in the mud circulation system based on the watertight pipe provided by the embodiment of the first aspect of the present invention, the drill string 120 and the hard pipe 410 are respectively arranged corresponding to the floating drilling platform 110 and the drilling ship 310, and the two ends of the first hose 430 are respectively connected to the outlet of the wellhead suction device 130 and the inlet of the lifting pump 420, the two ends of the second hose 470 are respectively connected to the upper outlet of the hard pipe 410 and the inlet of the low-pressure mud processing system 520, and the lower inlet of the hard pipe 410 is connected to the lifting pump 4 The outlet of 20 can form a mud return pipeline; when the drilling device is running, the wellhead suction device 130 can suck in and collect the mud returned from the wellbore 200, and then, under the operation of the lifting pump 420, the mud is prompted to flow out of the wellhead suction device 130, and flows through the first hose 430, the lifting pump 420, the hard pipe 410 and the second hose 470 in sequence, and finally flows to the low-pressure mud processing system 520, so that the mud flows back from the seabed to the floating drilling platform 110, realizing the mud circulation function without a watertight pipe.

[0072] The present invention adopts a combination of a floating drilling platform 110 and a drilling ship 310 to separate and arrange the drill string 120 and the hard pipe 410, both of which are in a vertical state, and effectively increase the horizontal spacing between the drill string 120 and the hard pipe 410. This can avoid the problems of interference and collision between the drill string 120 and the hard pipe 410 during drilling operations, and the drill string 120 and the hard pipe 410 being difficult to safely recover in emergency situations. It can also prevent the hard pipe 410 from twisting and breaking when the bow of the ship needs to be adjusted due to the influence of wind and waves. At the same time, it also solves the problem that existing drilling platforms cannot store too many hard pipes 410 due to the small deck operating area, thereby greatly reducing operational risks, improving operational efficiency, and promoting the widespread use of watertight pipe-free mud circulation technology in deeper marine drilling environments.

[0073] In some embodiments, as Figures 1 to 3 As shown, the drilling vessel 310 has a high-pressure mud manifold 510. Furthermore, the return line system also includes a first control valve, a second control valve, and a three-way manifold 460. The three-way manifold 460 has three pipe interfaces, which are respectively connected to the upper outlet of the rigid pipe 410, the inlet of the second flexible pipe 470, and the outlet of the high-pressure mud manifold 510. The first control valve is located at the upper outlet of the rigid pipe 410, and the second control valve is located at the outlet of the high-pressure mud manifold 510.

[0074] It is understood that the three-way manifold 460 is fixedly mounted on top of the rigid pipe 410 and can move with the rigid pipe 410. Two of its pipe connections are directly connected to the upper outlet of the rigid pipe 410 and the inlet of the second hose 470, respectively. The other pipe connection of the three-way manifold 460 is connected to the outlet of the high-pressure mud manifold 510 via a hose. The high-pressure mud manifold 510 can transport replenishment fluids such as mud, cementing fluid, and drilling water from the drilling vessel 310 via the second hose 470 to the low-pressure mud processing system 520 on the floating drilling platform 110.

[0075] Of course, it is not ruled out that in other embodiments, two of the pipe interfaces of the three-way manifold 460 are directly connected to the inlet of the second hose 470 and the outlet of the high-pressure mud manifold 510 respectively, and the other pipe interface of the three-way manifold 460 is connected to the upper outlet of the hard pipe 410 through the hose.

[0076] The first control valve is used to control the opening and closing state of the upper outlet of the rigid tube 410, and the second control valve is used to control the opening and closing state of the outlet of the high-pressure mud manifold 510. The first control valve and the second control valve can be manual valves or electric valves. In this embodiment, the first control valve and the second control valve are electric valves, and the on-off control function of the first control valve and the second control valve can be realized by remotely controlling the on-off state of the first control valve and the second control valve. By switching the state of the first control valve and the second control valve, the inlet of the second hose 470 can be connected to the upper outlet of the rigid tube 410 or the outlet of the high-pressure mud manifold 510, thereby enabling the choice of transporting mud returning from the seabed to the drilling rig for mud return operation; or the choice of transporting mud flowing out of the high-pressure mud manifold 510 to the drilling rig for replenishment operation.

[0077] Furthermore, the riser-less mud circulation system also includes a control system. The control system is electrically connected to the first control valve, the second control valve, and the lift pump 420 , respectively. The control system can send control commands to the first control valve, the second control valve, and the lift pump 420 to control their opening and closing states.

[0078] The control system can control the first control valve and the lifting pump 420 to open and the second control valve to close during the mud return operation, so that the mud at the wellbore 200 flows through the wellhead suction device 130, the first hose 430, the lifting pump 420, the hard pipe 410, the three-way manifold 460 and the second hose 470 in sequence under the operation of the lifting pump 420 and the wellhead suction device 130, and finally flows to the low-pressure mud treatment system 520; in this process, since the second control valve is closed, it can prevent the supply fluid at the high-pressure mud manifold 510 from flowing to the low-pressure mud treatment system 520, thereby causing the problem of unstable mud return operation.

[0079] In addition, the control system can control the second control valve to open during the replenishment operation, and at the same time control the first control valve and the lift pump 420 to close, so that the replenishment fluid at the high-pressure mud manifold 510 can flow through the three-way manifold 460 and the second hose 470 in sequence, and finally flow to the low-pressure mud processing system 520; in this process, since the first control valve and the lift pump 420 are both in the closed state, the replenishment operation can be prevented from being affected by the mud backflow operation.

[0080] It is understood that the control system can be, but is not limited to, a host computer, a central control computer, or a handheld terminal device such as a controller of the drilling vessel 310. With the above-mentioned configuration, the dual operating modes of mud circulation and material replenishment can be switched in real time.

[0081] In some embodiments, as Figures 1 to 3 As shown, the first hose 430 is provided with a plurality of first buoyancy blocks 440 along its extending direction. The second hose 470 is provided with a plurality of second buoyancy blocks 480 along its extending direction.

[0082] It is understood that the number of first buoyancy blocks 440 and the number of second buoyancy blocks 480 can be selected based on actual needs and are not specifically limited herein. The first buoyancy blocks 440 can be fixed to the first hose 430 by means of a binding mechanism, and the second buoyancy blocks 480 can be fixed to the second hose 470 by means of a binding mechanism. The first buoyancy blocks 440 and the second buoyancy blocks 480 can be made of any material, including, but not limited to, existing composite foam buoyancy materials or hollow glass microsphere-reinforced epoxy resin-based materials.

[0083] The first buoyancy block 440 enhances the underwater buoyancy of the first hose 430, making it easier to transport the first hose 430 using an existing ROV (Remotely Operated Vehicle) and install it at the outlet of the wellhead suction device 130. The second buoyancy block 480 enhances the buoyancy of the second hose 470 on the surface of the water. This allows the second hose 470 to float on the sea surface, reducing the pulling force exerted by the two ends of the second hose 470 on the low-pressure mud processing system 520 and the three-way manifold 460, respectively.

[0084] In some embodiments, as Figures 1 to 3 As shown, the drilling vessel 310 has a drill pipe, which is configured as a hard pipe 410. It is understood that in this embodiment, the existing drill pipe on the drilling vessel 310 is directly used as the hard pipe 410 to provide a mud return channel, without the need for separate research, development, manufacturing, or purchase, thus saving costs.

[0085] Furthermore, the drilling vessel 310 includes a first derrick 320 and a first top drive 330. The first top drive 330 is drivingly connected to the rigid pipe 410 and is slidably connected to the first derrick 320 in an up-and-down direction to drive the rigid pipe 410 up and down. Specifically, the first top drive 330 can be mounted on the top area of ​​the first derrick 320 via a guide pulley assembly. The first derrick 320 can provide support for the first top drive 330. The first top drive 330 can move up and down along a guide rail provided within the first derrick 320 to achieve the lifting and lowering control of the rigid pipe 410.

[0086] It is understood that the first derrick 320, drill pipe, and first top drive 330 are ancillary equipment for the drillship 310. Those skilled in the art will appreciate their specific structures, connections, and operating principles, and are not detailed here. By directly utilizing the automated handling equipment already on the drillship 310, such as the power catwalk, iron roughneck, first derrick 320, and first top drive 330, the rigid pipe 410 can be quickly disassembled and lowered for recovery, significantly saving costs and improving operational efficiency and safety.

[0087] Furthermore, the drilling vessel 310 has a heave compensation system, which is used to compensate for the heave movement of the rigid pipe 410 .

[0088] It is understandable that in the prior art, to overcome the heave effect of waves, a telescopic sleeve structure is typically used in the seabed anchoring module to achieve simple heave compensation. However, in conditions of strong winds and waves and prolonged heave, the telescopic sleeve can easily become damaged or the heave compensation speed cannot keep up, resulting in the return pipe 410 being pulled and damaged. Therefore, in this embodiment, the existing heave compensation system on the drilling vessel 310 is directly utilized to enable underwater equipment such as the lift pump 420 and the pipe 410 to overcome the heave effect of waves, allowing them to sit stably on the seabed and operate safely, thereby achieving the wave heave compensation function of the riser-less mud circulation system.

[0089] Heave compensation systems are currently available and can be hydraulic, active, passive, or semi-active. As a key component of drillship 310, the heave compensation system is used to isolate the rigid pipe 410 from the effects of wave motion on the hull and compensate for the heave of the rigid pipe 410. This embodiment does not make any structural improvements to the heave compensation system. Those skilled in the art will appreciate its specific structure and operating principles, and will not be elaborated upon here.

[0090] Similarly, the floating drilling platform 110 is equipped with a heave compensation device. The main function of the heave compensation device is to block the impact of the movement of the hull under the action of waves on the drill string 120, compensate for the heave of the drill string 120, reduce the wear between the drill string 120 and the blowout preventer, and ensure safe and reliable drilling operations.

[0091] In some embodiments, as Figures 1 to 3 As shown, the hard tube 410 is provided with a plurality of tube fixing devices 450 along its extension direction. The tube fixing devices 450 are fixedly connected to the hard tube 410 and can move with the hard tube 410 . Moreover, the plurality of tube fixing devices 450 are detachably connected to the first hose 430 .

[0092] It is understood that the function of the pipe fixing device 450 is to securely fix the first hose 430 to the rigid pipe 410. The first hose 430 can extend along the length of the rigid pipe 410. This prevents the first hose 430 from being easily affected by waves and swinging during the lowering of the rigid pipe 410, the lift pump 420, and the first hose 430, which could damage the lift pump 420 or cause instability in the rigid pipe 410, the lift pump 420, and even the drilling vessel 310. This ensures the safe lowering of the rigid pipe 410, the lift pump 420, and the first hose 430. The number of pipe fixing devices 450 can be selected according to actual needs and is not specifically limited herein.

[0093] During lowering, the first hose 430 is fixed to the rigid pipe 410 by the pipe fixing device 450. When the rigid pipe 410 is about to be lowered to the seabed, the ROV is operated to unlock all the pipe fixing devices 450 on the rigid pipe 410 to release the fixing effect of the pipe fixing devices 450 on the first hose 430. Then, the ROV installs one end of the first hose 430 to the outlet of the wellhead suction device 130.

[0094] Specifically, such as Figure 4 As shown, the tube fixing device 450 includes a bracket 451, a first latch 453, and a connecting rod 452. The bracket 451 is U-shaped, and one end of the connecting rod 452 is hingedly connected to one end of the bracket 451 via a hinge axis, allowing the connecting rod 452 to swing relative to the bracket 451. The other ends of the bracket 451 and the connecting rod 452 are both provided with first insertion holes, and the first latch 453 can be inserted into all of the first insertion holes, so that the connecting rod 452 and the bracket 451 together form a fixing cavity for fixing the first hose 430.

[0095] In this embodiment, the rigid tube 410 is provided with several connecting devices, which are arranged in a one-to-one correspondence with the tube fixing device 450. The connecting devices include U-bolts 455, connecting nuts, and connecting plates 456. The connecting plates 456 have mounting holes for the ends of the U-bolts 455 to pass through. After the ends of the U-bolts 455 pass through the mounting holes of the connecting plates 456 and are threadedly connected to the connecting nuts, the connecting devices can be locked to the rigid tube 410. Of course, it is not ruled out that the tube fixing device 450 can be fixed to the rigid tube 410 by welding, bolting, or snap-fitting.

[0096] The bracket 451 is fixedly connected with the U-bolt 455. On the drilling ship 310, the first pin 453 is inserted into all the first holes to enable the pipe fixing device 450 to lock the first hose 430. When the rigid pipe 410, the lifting pump 420 and the first hose 430 are lowered to the appropriate position on the seabed, the first pin 453 is pulled out by the ROV, and the connecting rod 452 is driven to rotate to open the fixing cavity. After all the first pins 453 are pulled out and all the connecting rods 452 are rotated into place, the ROV can remove the first hose 430 from the rigid pipe 410 and install it at the wellhead suction device 130.

[0097] It is understood that the first latch 453 can be arranged vertically, tilted, or horizontally. Of course, a torsion spring can be added at the hinge between the connecting rod 452 and the bracket 451. The torsion spring is mounted on the hinge shaft, and the two torsion arms of the torsion spring are respectively fixedly connected to the connecting rod 452 and the bracket 451. Under the action of the torsion spring, the connecting rod 452 can automatically reset so that the connecting rod 452 and the bracket 451 form a fixed cavity.

[0098] Further, such as Figure 4 As shown, pipe securing device 450 also includes a second latch 454. Connecting rod 452 is arranged horizontally, while first latch 453 and second latch 454 are arranged vertically. Second insertion holes are provided at one end of bracket 451 and one end of connecting rod 452. Second latch 454 can be inserted into all of these second insertion holes, allowing one end of connecting rod 452 to be hinged to one end of bracket 451. Both first latch 453 and second latch 454 serve as hinge axes between bracket 451 and connecting rod 452. If one of first latch 453 and second latch 454 becomes difficult to remove, the ROV can remove the other.

[0099] like Figures 1 to 5 As shown, the control method of the riser-free mud circulation system according to the second embodiment of the present invention is applied to the riser-free mud circulation system according to the first embodiment. Specifically, the control method includes the following steps:

[0100] Step S11: After receiving the input instruction, determine the current working condition.

[0101] Step S12: If the current operating condition is a mud return condition, the first control valve and the lift pump 420 are controlled to be open, and the second control valve is controlled to be closed.

[0102] Step S13: If the current operating condition is the replenishing condition, the second control valve is controlled to be open, and the first control valve and the lift pump 420 are controlled to be closed.

[0103] The control system can execute steps S11, S12, and S13. Based on the user input, the control system can determine the current operating condition of the riser-less mud circulation system and determine whether the current operating condition is a mud return condition, a replenishment condition, or a shutdown condition, so that the control system can issue a corresponding response instruction.

[0104] If it is determined that the mud circulation system based on no watertight pipe is in the mud return condition, the instructions issued by the control system ensure that the lifting pump 420 and the first control valve are in the open state, and the second control valve is in the closed state, thereby enabling the mud to return from the seabed through the return pipeline system to the low-pressure mud processing system 520 on the floating drilling platform 110, thereby realizing the mud circulation operation without watertight pipe.

[0105] If it is determined that the mud circulation system based on the watertight pipe is in the supply condition, the instructions sent by the control system ensure that the second control valve is in the open state, and the lifting pump 420 and the first control valve are in the closed state, thereby enabling the supply fluids such as mud, cementing fluid and drilling water on the drilling ship 310 to be transported from the high-pressure mud manifold 510 through the second hose 470 to the low-pressure mud processing system 520 on the floating drilling platform 110, thereby completing the supply work.

[0106] If it is determined that the mud circulation system based on the riser-free system is in a shutdown condition, the control system can control the first control valve, the second control valve and the lift pump 420 to close, so that the mud return work and the supply fluid delivery work are both in a stopped state.

[0107] Such a design can adjust the working status of the first control valve, the second control valve and the lift pump 420 according to the changes in the operating conditions, enabling the mud circulation system based on a watertight pipe to switch between mud return conditions and supply conditions.

[0108] like Figures 1 to 4 、 Figure 6 As shown, the operating method of the riser-less mud circulation system according to the third embodiment of the present invention is applied to the riser-less mud circulation system according to the first embodiment. Specifically, the operating method includes the following steps:

[0109] Step S21 : driving the wellhead suction device 130 to move downward and install it on the wellhead, and driving the drill string 120 on the floating drilling platform 110 to drill into the wellhead suction device 130 .

[0110] Step S22 : driving the drilling vessel 310 to sail a first distance toward the floating drilling platform 110 , driving the hard pipe 410 to move the lift pump 420 and the first hose 430 downward a second distance, and connecting the second hose 470 to the low-pressure mud processing system 520 .

[0111] Step S23 : After the first hose 430 moves downward by the second distance, the ROV is driven to remove the first hose 430 from the pipe fixing device 450 , and connect the first hose 430 to the wellhead suction device 130 .

[0112] Step S24: After the first hose 430 and the second hose 470 are installed, the drilling vessel 310 is driven to sail a third distance away from the floating drilling platform 110, and the hard pipe 410 is driven to drive the lifting pump 420 to move downward a fourth distance.

[0113] Step S25: The heave compensation system on the drilling vessel 310 is turned on, and the hard pipe 410 is driven to drive the lift pump 420 downward to the seabed.

[0114] Step S26: Start the lift pump 420 to perform mud return operation.

[0115] It is understandable that the specific values ​​of the first distance, the second distance, the third distance and the fourth distance are not limited and can be set according to actual conditions.

[0116] Before step S21, when a riser-free mud circulation operation is required, the floating drilling platform 110 and the drilling ship 310 can operate in parallel, and both the floating drilling platform 110 and the drilling ship 310 can turn on their own dynamic positioning functions to separate the floating drilling platform 110 and the drilling ship 310 by a certain horizontal distance, such as 100 meters.

[0117] In step S21, after the floating drilling platform 110 completes its positioning relative to the wellhead on the sea surface, the wellhead suction device 130 is moved downward from the floating drilling platform 110 with the help of a winch on the floating drilling platform 110 and is set at the wellhead at the seabed mudline. Then, using the second derrick 140 and the second top drive 150, the drill string 120 is moved downward and screwed into the wellhead suction device 130, enabling the drill string 120 to perform drilling operations at the wellhead. During this process, the wellhead suction device 130 can suck in and collect mud returned from the wellbore 200.

[0118] In step S22, the existing automated processing equipment on the drilling ship 310, such as the power catwalk, iron roughneck, first derrick 320 and first top drive 330, can be used to connect the first hose 430 tied with the first buoyancy block 440, the lifting pump 420, the hard pipe 410 and the three-way manifold 460 and other components, and lower them. By moving downward a second distance, the height position of the lifting pump 420 is made to be 10 meters away from the seabed mud line.

[0119] Before or after lowering rigid pipe 410, drillship 310 is moved closer to floating drilling platform 110, reducing the horizontal distance between them so that second hose 470 can be installed in low-pressure mud processing system 520 on floating drilling platform 110. Specifically, drillship 310 is slowly moved a first distance to a certain distance, such as 10 meters, from floating drilling platform 110. Drillship 310 then activates its dynamic positioning function to maintain a stable position. At this point, second hose 470 can be installed in the inlet of low-pressure mud processing system 520.

[0120] In step S23, after the rigid pipe 410 drives the lifting pump 420 and the first hose 430 to move downward the second distance and the drilling vessel 310 moves the first distance, the ROV is activated and moved to a position near the rigid pipe 410. With the help of the ROV, the pipe fixing devices 450 are opened from top to bottom on the seabed to remove the first hose 430 from the pipe fixing devices 450 on the rigid pipe 410, and the first hose 430 is lowered to the seabed. Then, the ROV drives one end of the first hose 430 to move to connect the first hose 430 to the outlet of the wellhead suction device 130. At this time, the work of connecting the return pipeline system to the wellhead suction device 130 and the low-pressure mud treatment system 520 respectively has been completed.

[0121] In step S24, after the installation of first hose 430 and second hose 470 is completed, drilling vessel 310 is moved away from floating drilling platform 110 to increase the horizontal distance between drilling vessel 310 and floating drilling platform 110 to avoid interference or collision between drill string 120 and rigid pipe 410. Specifically, drilling vessel 310 is slowly driven a third distance to a position a certain distance, such as 50 meters, from floating drilling platform 110. Then, the dynamic positioning function of drilling vessel 310 is activated to maintain a stable state on the sea surface.

[0122] After the position of the drilling ship 310 on the sea surface is confirmed, the existing automated processing equipment on the drilling ship 310, such as the power catwalk, iron roughneck, first derrick 320 and first top drive 330, drives the hard pipe 410 to drive the lift pump 420 to slowly move down a fourth distance until the height position of the lift pump 420 is 2 meters from the seabed.

[0123] In step S25, the heave compensation system on the drilling ship 310 is first started, and then the hard pipe 410 is driven to drive the lifting pump 420 to continue to move slowly downward until the lifting pump 420 falls on the seabed. At this time, through the dynamic positioning function of the drilling ship 310 and the role played by the heave compensation system, the wave heave compensation function of the mud circulation system based on the watertight pipe can be realized, avoiding the problem of the hard pipe 410 being pulled and damaged in the case of strong winds and waves, and ensuring that the mud circulation system based on the watertight pipe can smoothly carry out drilling operations.

[0124] In step S26, after the return pipeline system is installed and the positions of the lift pump 420 and the hard pipe 410 are set, the drill string 120, the wellhead suction device 130 and the lift pump 420 can be coordinated to perform the mud return process during the drilling process.

[0125] Throughout this specification, reference to terms such as "one embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples" means that a specific feature, structure, material, or characteristic described in conjunction with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, illustrative uses of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0126] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to the embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the claims and their equivalents.

Claims

1. A riser-free mud circulation system, characterized in that: include: A drilling device comprising a floating drilling platform, a wellhead suction device and a drill string, wherein the drill string is arranged vertically, the upper end of the drill string is arranged on the floating drilling platform, and the lower end of the drill string is passed through the wellhead suction device; drilling vessels; a low-pressure mud processing system, which is provided on the floating drilling platform; A return pipeline system comprising a lift pump, a rigid pipe, a first flexible pipe, and a second flexible pipe, wherein the rigid pipe is arranged vertically, the upper end of the rigid pipe being disposed on the drilling vessel, the lower inlet of the rigid pipe being connected to the outlet of the lift pump, one end of the first flexible pipe being connected to the outlet of the wellhead suction device, and the other end being connected to the inlet of the lift pump, and one end of the second flexible pipe being connected to the upper outlet of the rigid pipe, and the other end being connected to the inlet of the low-pressure mud processing system; The drilling vessel has a high-pressure mud manifold, and the return pipeline system further includes a first control valve, a second control valve, and a three-way manifold. The three-way manifold has three pipe interfaces, which are respectively connected to the upper outlet of the rigid pipe, the inlet of the second flexible pipe, and the outlet of the high-pressure mud manifold. The first control valve is provided at the upper outlet of the rigid pipe, and the second control valve is provided at the outlet of the high-pressure mud manifold. The mud circulation system based on a watertight riser also includes a control system, which is electrically connected to the first control valve, the second control valve and the lift pump respectively. The control system can control the first control valve and the lift pump to open and the second control valve to close in the mud return condition, and can control the second control valve to open and the first control valve and the lift pump to close in the replenishment condition.

2. The riser-free mud circulation system according to claim 1, characterized in that: The first hose is provided with a plurality of first buoyancy blocks along its extension direction; and / or, The second hose is provided with a plurality of second buoyancy blocks along its extending direction.

3. The riser-free mud circulation system according to claim 1 or 2, characterized in that: The drilling ship has a drill pipe, a first derrick and a first top drive. The drill pipe is set as the hard pipe. The first top drive is drivingly connected to the hard pipe and is slidably connected to the first derrick along the up and down directions to drive the hard pipe to rise and fall. The drilling ship has a heave compensation system, which is used to compensate for the heave movement of the hard pipe.

4. The riser-free mud circulation system according to claim 3, characterized in that: The hard tube is provided with a plurality of tube fixing devices along its extending direction, and the plurality of tube fixing devices are detachably connected to the first flexible tube.

5. The riser-free mud circulation system according to claim 4, characterized in that: The tube fixing device includes a bracket, a first pin and a connecting rod. The bracket is U-shaped, one end of the connecting rod is hinged to one end of the bracket, and the other end of the bracket and the other end of the connecting rod are both provided with a first socket. The first pin can be inserted into all the first sockets, so that the connecting rod and the bracket jointly form a fixing cavity for fixing the first hose.

6. The riser-free mud circulation system according to claim 5, characterized in that: The pipe fixing device also includes a second pin, the connecting rod is arranged horizontally, the first pin and the second pin are arranged vertically, one end of the bracket and one end of the connecting rod are provided with a second socket, and the second pin can be inserted into all of the second sockets so that one end of the connecting rod is hinged to one end of the bracket.

7. A control method for a riser-less mud circulation system, applied to the riser-less mud circulation system according to claim 1, characterized in that: The steps include: After receiving the input command, determine the current working condition; If the current operating condition is a mud return condition, the first control valve and the lift pump are controlled to be open, and the second control valve is controlled to be closed; If the current operating condition is a supply condition, the second control valve is controlled to be open, and the first control valve and the lift pump are controlled to be closed.

8. An operating method for a riser-less mud circulation system, applied to the riser-less mud circulation system according to any one of claims 4 to 6, characterized in that: The steps include: Driving the wellhead suction device to move downward and install on the wellhead, and driving the drill string on the floating drilling platform to drill into the wellhead suction device; driving the drilling vessel to sail a first distance toward the floating drilling platform, driving the hard pipe to move the lift pump and the first hose downward a second distance, and connecting the second hose to the low-pressure mud processing system; After the first hose moves downward a second distance, driving the ROV to remove the first hose from the pipe fixture and connect the first hose to the wellhead suction device; After the first hose and the second hose are installed, the drilling vessel is driven to sail a third distance away from the floating drilling platform, and the hard pipe is driven to drive the lift pump to move down a fourth distance; Turning on the heave compensation system on the drilling vessel and driving the hard pipe to move the lift pump downward to the seabed; The lift pump is started to perform mud return operation.

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