A method of pile extraction for a multi-leg jack-up offshore platform

Abstract

The application discloses a pile pulling method for a multi-leg jack-up offshore platform, and comprises the following steps: S1, a ballast adjusting step; S2, an outer ring pile pulling step: controlling a lifting system to perform a pile pulling action on outer ring pile legs located at the outer ring of a platform hull, keeping at least one middle pile leg located at the geometric center of the platform static and supported on the seabed, and using the middle pile leg as a fulcrum to provide a support force in the same direction as the buoyancy to the platform hull; S3, an inner ring pile pulling step: after the outer ring pile legs are initially loosened, controlling the middle pile leg to perform a pile pulling action, locking the loosened outer ring pile legs through a locking device, keeping the loosened outer ring pile legs static and supported on the seabed, using the outer ring pile legs as a fulcrum to provide a support force in the same direction as the buoyancy to the platform hull, and using the reserve buoyancy of the platform hull to assist in loosening the middle pile leg; and S4, a full pile lifting step. The application has the advantages of simple and convenient construction, stability and reliability, shortened pile pulling time and reduced safety risk.

Description

Technical Field

[0001] This invention belongs to the field of offshore operation engineering technology for self-elevating offshore platforms, and specifically relates to a method for removing piles from a multi-leg self-elevating offshore platform. Background Technology

[0002] As an important piece of equipment for offshore oil and gas development and engineering operations, the typical operation process of a self-elevating offshore platform includes pile driving in the target sea area, standing operation, pile extraction and relocation, and placement at the new site. In conventional pile extraction operations, the industry generally adopts a combined approach of floating buoyancy and lifting unit lifting force, supplemented by a high-pressure pile driving system to jet fluidize the soil at the bottom of the pile shoe and the surrounding soil. This aims to reduce the adhesion force and lateral friction resistance of the seabed soil to the pile shoe, thereby achieving successful pile extraction.

[0003] However, in actual operations, due to severe sea conditions (such as storm surges), prolonged operation, or the consolidation effect of soft clay layers on the seabed, pile shoes often face extreme conditions such as excessive penetration depth or significantly increased soil shear strength. In these situations, the adsorption and gripping forces of seabed silt and soil on the pile shoes increase exponentially. If the traditional pile extraction technique is still used, the following significant drawbacks exist: 1. Low operational efficiency: The pile extraction process is extremely slow, with routine operations taking 24 to 48 hours, and in extreme cases even more than a week, which severely compresses the effective operation window and increases the platform rental and maintenance costs.

[0004] 2. High safety risks: Traditional methods mainly rely on the lifting unit to provide vertical tension, which can easily lead to uneven stress on the platform. Under huge abnormal loads, this can easily induce major safety accidents such as stress concentration in the pile legs, hull tilting, or even platform capsizing, posing a serious threat to the safety of people and property.

[0005] Therefore, optimizing the pile extraction sequence, scientifically utilizing the hull structure and fulcrum reaction forces, and disrupting the soil adsorption balance to significantly shorten the pile extraction cycle while ensuring platform safety has become a pressing technical challenge in this field. Based on this need, this invention proposes a novel "return"-shaped pile extraction method. Summary of the Invention

[0006] This invention aims to address the technical shortcomings of existing technologies, such as uneven stress distribution, long extraction cycles, and easy platform overturning, in cases of excessive mud penetration or soil consolidation. It provides a method for extracting piles from multi-legged self-elevating offshore platforms that utilizes the hull structure's fulcrum effect and releases soil adsorption force through a specific extraction sequence.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for removing piles from a multi-legged jack-up offshore platform, wherein the multi-legged jack-up offshore platform includes a platform hull, multiple legs, pile shoes corresponding to each leg, a lifting system, a ballast system, and a pile driving system. Each leg is distributed circumferentially along the platform hull and is equipped with an independent lifting unit and locking device. The method for removing piles includes the following steps: S1: Counterweight adjustment steps: Control the lifting system to lower the platform hull into the water, start the ballast system to discharge the ballast water in the ballast tank to reduce the total weight of the platform, and at the same time start the pile driving system to spray high-pressure water around the pile shoes. S2: Outer ring pile extraction procedure: Control the lifting system to first perform the pile extraction action on the outer ring pile legs located on the outer ring of the platform hull, while keeping at least one intermediate pile leg located at the geometric center of the platform stationary and supported on the seabed, using the intermediate pile leg as a fulcrum to provide a supporting force to the platform hull in the same direction as the buoyancy. S3: Inner ring pile extraction step: After the outer ring pile legs are initially loosened, control the middle pile leg to perform the pile extraction action, and at the same time lock the loosened outer ring pile legs through the locking device to keep the loosened outer ring pile legs stationary and supported on the seabed. Use the outer ring pile legs as fulcrums to provide support force to the platform hull in the same direction as the buoyancy, and use the platform hull's reserve buoyancy to assist in loosening the middle pile legs. After the intermediate pile legs are initially loosened, steps S2 and S3 are performed alternately until all pile legs are fully loosened, and it is ensured that none of the pile legs are completely detached from the hard soil layer of the seabed. S4: Full pile lifting procedure: Simultaneously start all lifting units, and with the reserve buoyancy of the platform hull, make all pile legs detach from the hard soil layer of the seabed, and drive the pile shoes to the zero position.

[0008] As a preferred embodiment of the present invention, the outer ring pile extraction step specifically includes: The piles were pulled out in alternating groups, with priority given to removing the pile legs that were deeper into the mud, until the depth of each pile leg into the mud was consistent. During the pile extraction process, the tilt angle of the platform hull is monitored in real time. When the tilt angle of the platform hull exceeds 2°, the pile extraction action of the current pile leg is suspended and the support status of other pile legs is adjusted until the tilt angle is restored to within 2°.

[0009] As a preferred embodiment of the present invention, in the outer ring pile extraction step, a diagonal alternating pile extraction method is adopted: When the load on one of the outer diagonal pile legs in a certain group drops to the first preset threshold, it is determined that the pile leg has become loose. The operation on that pile leg is immediately stopped, and the operation is switched to another group of diagonal pile legs for pile extraction.

[0010] As a preferred embodiment of the present invention, in the outer ring pile extraction step and / or the inner ring pile extraction step, when the pile extraction load is detected to drop below the second preset threshold, it is determined that the pile leg has been sufficiently loosened.

[0011] As a preferred embodiment of the present invention, in the outer ring pile extraction step and / or the inner ring pile extraction step, the pile extraction action is performed while the pile driving system is continuously activated to carry out jetting operation.

[0012] As a preferred embodiment of the present invention, the inner ring pile extraction step, which utilizes the reserve buoyancy of the platform hull to assist in pile extraction, specifically includes: When the intermediate pile leg is loosened, the ballast system is controlled to further discharge ballast water, and the upward buoyancy generated by the platform hull is used to offset part of the pile leg's self-weight and the resistance to pile extraction.

[0013] As a preferred embodiment of the present invention, an environmental assessment step is further included before performing the pile extraction action: Obtain tidal data and sea condition information for the operating area, select the high tide period as the window for pile extraction, and confirm that the freeboard height of the platform hull is always not lower than the maximum draft of the platform.

[0014] As a preferred embodiment of the present invention, the pile extraction method further includes a safety monitoring step: Real-time data on the mud penetration depth of each pile leg is acquired, and the lifting force of each lifting unit is dynamically adjusted to ensure that the platform hull is subjected to balanced forces during the pile extraction process.

[0015] As a preferred embodiment of the present invention, the multi-leg self-elevating marine platform has 6 or 8 legs arranged in a rectangular array.

[0016] The pile extraction method for a multi-legged self-elevating offshore platform provided by this invention has the following advantages compared with the prior art: by adopting a step-by-step pile extraction strategy, scientifically optimizing the pile extraction sequence, and making reasonable use of the middle and outer legs as fulcrum reaction mechanisms, combined with counterweight adjustment, pile driving system, and reserve buoyancy assistance, the adsorption force and lateral friction resistance of the seabed soil on the pile shoes are effectively weakened. Thus, efficient and safe pile extraction is achieved while ensuring the stability of the platform structure. It has the advantages of simple and convenient construction, stability and reliability, effective shortening of pile extraction time, and reduction of safety risks.

[0017] Furthermore, since the external force exerted during pile extraction is small, the impact on the pile legs and corresponding lifting units can be reduced, effectively extending the service life of the pile legs and corresponding lifting units. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings of the embodiments will be briefly described below.

[0019] Figure 1A top view of a multi-legged jack-up offshore platform provided in an embodiment of the present invention; Figure 2 A schematic diagram illustrating the descent of a multi-legged jack-up offshore platform into a floating state, as provided in an embodiment of the present invention. Figure 3 This is a schematic diagram of pile extraction operation (extracting the outer ring of pile legs first) for a multi-legged self-elevating offshore platform provided in an embodiment of the present invention. Figure 4 This is a schematic diagram of a multi-legged jack-up offshore platform with all legs pulled out, as provided in an embodiment of the present invention. Figure 5 This is a schematic diagram of a multi-legged jack-up offshore platform in the leg recovery operation state, as provided in an embodiment of the present invention. Figure 6 This is a schematic diagram of a multi-legged self-elevating offshore platform in a free-floating state, as provided in an embodiment of the present invention.

[0020] Marked in the image: Platform hull 10; lifting unit 20; outer ring legs 30; middle legs 40. Detailed Implementation

[0021] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0022] In the description of this invention, it should be understood that the orientation descriptions, such as up, down, front, back, left, right, etc., are based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0023] In the description of this invention, "several" means one or more, "more than" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. The use of "first" and "second" in the description is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0024] In the description of this invention, unless otherwise explicitly defined, terms such as "set up," "install," and "connect" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of the above terms in this invention in conjunction with the specific content of the technical solution.

[0025] Traditional methods for extracting piles from self-elevating offshore platforms are inefficient and time-consuming in extreme conditions such as excessive pile shoe penetration or significantly increased soil shear strength. Furthermore, these methods rely heavily on the lifting unit to provide vertical tension, which can lead to uneven stress distribution on the platform and pose safety risks such as stress concentration in the pile legs, hull tilting, or even platform capsizing.

[0026] For this, see Figures 1 to 6 This invention proposes a method for removing piles from a multi-legged jack-up offshore platform. The jack-up offshore platform is a type of offshore operational equipment, typically comprising a platform hull 10, multiple legs, corresponding pile shoes for each leg, a lifting system, a ballast system, and a pile-driving system. The platform hull 10 is the main structure of the platform, used to carry equipment and personnel; the legs support the platform hull 10 and anchor it to the seabed; the pile shoes are the structures at the bottom of the legs, in contact with the seabed; the lifting system controls the relative movement between the platform hull 10 and the legs, enabling the platform to rise and fall; the ballast system adjusts the draft and buoyancy of the platform hull 10; and the pile-driving system sprays high-pressure water around the pile shoes to reduce pile removal resistance. The legs are distributed circumferentially along the platform hull 10, and each leg is equipped with an independent lifting unit 20 (typically a rack and pinion lifting mechanism) and a locking device to achieve independent control and fixation of each leg.

[0027] The pile extraction method proposed in this embodiment of the invention includes the following steps: S1: Counterweight adjustment steps: Control the lifting system to lower the platform hull 10 into the water, start the ballast system to discharge the ballast water in the ballast tank to reduce the total weight of the platform, thereby reducing the total weight of the platform. At the same time, start the pile driving system to spray high-pressure water around the pile shoes to initially reduce the friction and adsorption force between the pile legs and the seabed.

[0028] Specifically, in the counterweight adjustment step S1, the platform hull 10 is first lowered into the water by controlling the lifting system. This lowering operation can be performed manually by controlling the lifting system to slowly descend the platform hull 10 to the predetermined draft depth. Subsequently, the ballast system is activated to discharge ballast water from the ballast tanks to reduce the platform's total weight. The discharge rate of ballast water can be set based on empirical values ​​or preset parameters, for example, by controlling the start and stop of the ballast water pump via a manual valve. Simultaneously, the pile driving system is activated to spray high-pressure water around the pile shoes. The pile driving system can use fixed nozzles or adjustable nozzles, and by activating the high-pressure water pump, the water flow is directly applied to the area where the pile shoes contact the seabed to reduce the soil's adhesion to the pile shoes.

[0029] S2: Outer ring pile removal procedure: Control the lifting system to perform the pile removal action on the outer ring pile legs 30 located on the outer ring of the platform hull 10. During this process, at least one intermediate pile leg 40 located at the geometric center of the platform needs to be kept stationary and supported on the seabed. The intermediate pile leg 40 is used as a fulcrum to provide a supporting force to the platform hull 10 in the same direction as the buoyancy, so as to assist in the loosening of the outer ring pile legs 30.

[0030] Specifically, in the outer ring pile extraction step S2, the lifting system first extracts the outer ring pile legs 30 located on the outer ring of the platform hull 10. This extraction can be performed by the operator selecting a single pile leg based on experience, or by sequentially extracting each outer ring pile leg 30 in a preset fixed order. During this process, at least one intermediate pile leg 40 located at the geometric center of the platform remains stationary and supported on the seabed. The stationary state of the intermediate pile leg 40 can be achieved by not activating its corresponding lifting unit 20, allowing it to continuously bear a portion of the load of the platform hull 10. Thus, the intermediate pile leg 40 acts as a fulcrum, providing a supporting force to the platform hull 10 in the same direction as buoyancy, thereby assisting in the loosening of the outer ring pile legs 30. For example, if the platform hull 10 tilts slightly when the outer ring pile legs 30 are extracted, the supporting force of the intermediate pile leg 40 can play a balancing role.

[0031] S3: Inner Ring Pile Extraction Steps: After the outer ring pile leg 30 is initially loosened, the middle pile leg 40 is controlled to perform the pile extraction action. Simultaneously, the loosened outer ring pile leg 30 is locked using a locking device, keeping it stationary and supported on the seabed. The outer ring pile leg 30 is then used as a fulcrum to provide support to the platform hull 10 in the same direction as buoyancy. The reserve buoyancy of the platform hull 10 is used to assist in loosening the middle pile leg 40. Reserve buoyancy refers to the additional upward buoyancy that the platform hull 10 can provide under a specific draft. During the pile extraction process, this reserve buoyancy can be used to help offset part of the pile leg's self-weight and the pile extraction resistance.

[0032] Specifically, the inner ring pile extraction step S3 is performed after the outer ring pile legs 30 have been initially loosened. The loosening state of the outer ring pile legs 30 can be determined by observing the load change of the lifting unit 20 or the slight displacement of the pile legs. Once the outer ring pile legs 30 are loosened, the intermediate pile legs 40 are controlled to perform the pile extraction action. At this time, the lifting unit 20 of the intermediate pile legs 40 can be manually operated to lift them. Simultaneously, the loosened outer ring pile legs 30 are locked by a locking device to keep them stationary and supported on the seabed. This locking device can be a mechanical locking pin or a hydraulic clamping mechanism to ensure that the outer ring pile legs 30 provide stable support when the intermediate pile legs 40 are being extracted. Thus, the outer ring pile legs 30 become a fulcrum, providing a supporting force to the platform hull 10 in the same direction as buoyancy. In addition, the reserve buoyancy of the platform hull 10 is used to assist in loosening the intermediate pile legs 40. For example, when the intermediate pile leg 40 is loosened, the ballast water is further discharged by controlling the ballast system, so that the platform hull 10 can obtain greater buoyancy, thereby using the upward buoyancy generated by the platform hull 10 to offset part of the self-weight of the pile leg and the resistance to pile extraction.

[0033] Similarly, the loosening state of the intermediate pile leg 40 can be determined by observing the load changes of the lifting unit 20 or the slight displacement of the pile leg. After the intermediate pile leg 40 is initially loosened, the outer ring pile extraction step S2 and the inner ring pile extraction step S3 need to be performed alternately until all pile legs are fully loosened. However, during this process, it is necessary to ensure that all pile legs are not completely detached from the hard soil layer of the seabed. This is the key to ensuring the balance of the platform hull 10: when all pile legs are in a critical loosening state of "almost detached but not detached", it indicates that the adhesion force of the hard soil layer has been effectively broken. At this time, the formal full lifting stage can be entered: the locking device is released from its fixed state, and all lifting units 20 are coordinated and controlled to lift synchronously and at the same speed, raising the entire platform to the predetermined height. Thus, the pile extraction operation smoothly transitions from the stage of overcoming static friction to the stage of dynamic friction control, which minimizes the risk of single-leg overload and ensures the overall stability of the platform under complex sea conditions.

[0034] S4: Full Pile Lifting Step: After confirming that all pile legs are in a sufficiently loosened state of "partial detachment," the full pile lifting stage begins. This stage employs a closed-loop coordinated control strategy, synchronously activating all lifting units 20, strictly controlling the lifting speed of each pile leg to be consistent, and utilizing the reserve buoyancy of the platform hull 10 to ensure all pile legs smoothly detach from the hard seabed soil layer, and to bring the pile shoes to the zero position, thus completing the entire pile extraction process.

[0035] Specifically, in step S4 of the full pile lifting process, the loosening status of all pile legs can be confirmed through simple sensor feedback. The operator releases all locking devices and issues a synchronous lifting command through the central control system. During lifting, the system monitors the horizontal attitude of the platform hull 10 and the load of each lifting unit 20 in real time. If an abnormal surge in load on a single leg is detected or the hull tilt exceeds a threshold, the system immediately pauses lifting and makes fine adjustments to prevent uneven stress on the pile legs due to residual negative pressure on the seabed or obstacles. The lifting operation continues, and the pile legs gradually detach from the seabed and retract upwards under the drive of the lifting units 20. As the pile shoe is fully raised above the water and further retracted into the locking position (zero position) of the pile fixing frame, the platform hull 10 is completely supported by its own buoyancy and floats, with the pile legs and pile shoe fixed as suspended loads within the lifting system at the bottom of the hull. At this point, the pile extraction operation is complete, and the platform is ready for free movement.

[0036] The pile extraction method in this embodiment employs a step-by-step extraction strategy: first extracting the outer ring pile legs 30, then the middle pile legs 40, and alternately using the remaining or loosened pile legs as fulcrums to provide stable support for the platform hull 10, effectively balancing the forces during the extraction process. Simultaneously, by combining the ballast system to adjust the platform's total weight and utilizing the platform hull 10's reserve buoyancy, the adhesion force and frictional resistance between the pile legs and the seabed are effectively reduced. This improves extraction efficiency, shortens the operation cycle, and reduces safety risks under extreme conditions while ensuring the platform's structural safety.

[0037] However, in actual operation, the depth of each pile leg into the mud may vary, or the platform hull 10 may tilt due to uneven stress during pile extraction. This may lead to low pile extraction efficiency, damage to platform stability, or even safety risks.

[0038] To address this, this invention further proposes a specific implementation method for the outer ring pile extraction step, including extracting piles in alternating groups, prioritizing the extraction of pile legs with deeper mud penetration, until the mud penetration depth of all pile legs tends to be consistent. Specifically, "extracting piles in alternating groups" means that when performing the extraction of the outer ring pile legs 30, not all outer ring pile legs 30 are extracted at once, but the outer ring pile legs 30 are divided into several groups, and these groups are extracted in turn according to a preset order or strategy. For example, based on the structural characteristics of the platform hull 10, the outer ring pile legs 30 can be divided into diagonal groups, adjacent groups, or symmetrical groups, and the pile extraction operation is performed on each group of pile legs in sequence. This alternating group strategy helps to disperse the local stress on the platform hull 10 during the pile extraction process, avoiding excessive stress on one side or local areas, thereby maintaining the overall balance of the platform. Meanwhile, "prioritizing the removal of pile legs with deeper mud penetration" means that in the strategy of alternating pile removal in groups, the system will prioritize the removal of pile legs with greater mud penetration based on real-time or pre-measured mud penetration data. This is because pile legs with deeper mud penetration usually have greater removal resistance, and prioritizing these pile legs helps balance the overall removal progress and avoids situations where shallow pile legs are loosened prematurely while deeply embedded pile legs remain firmly stuck. "Until the mud penetration depth of each pile leg tends to be consistent" is the goal of this pile removal strategy. This means that through the above-mentioned alternating grouping and depth-priority removal method, the difference in mud penetration depth among the outer ring pile legs 30 is gradually reduced, so that they reach a relatively uniform state before being loosened, thus laying the foundation for subsequent pile removal operations.

[0039] Furthermore, during the pile extraction process, this embodiment of the invention also proposes real-time monitoring of the tilt angle of the platform hull 10. When the tilt angle of the platform hull 10 exceeds 2°, the extraction of the current pile leg is paused, and the support status of other pile legs is adjusted until the tilt angle returns to within 2°. Specifically, "real-time monitoring of the tilt angle of the platform hull 10" refers to continuously acquiring the tilt angle data of the platform hull 10 on the horizontal plane by installing tilt sensors and other equipment on the platform hull 10. These sensors can provide high-precision real-time tilt angle information so that operators or automated systems can understand the platform's attitude changes in a timely manner. A safety threshold is set for "when the tilt angle of the platform hull 10 exceeds 2°". This threshold is an empirical safety tilt angle limit. When the tilt angle of the platform hull 10 exceeds this range, it indicates that the platform may have an unstable trend or uneven stress, requiring immediate intervention. This threshold can be adjusted according to platform design, sea conditions, and operational requirements. "Pause the current pile-pulling operation" means that once the tilt angle is detected to exceed a preset threshold, the lifting unit 20 of the pile leg currently undergoing pile-pulling operation is immediately stopped to prevent further tilting. "Adjust the support status of other pile legs" means that after pausing the current pile-pulling operation, the lifting units 20 of other pile legs that are not being pulled or have been loosened but are still supported on the seabed are controlled to fine-tune their lifting force or support height to correct the tilt of the platform hull 10. For example, the support force of the pile leg on the tilting side can be appropriately reduced, or the support force of the pile leg on the other side can be increased, thereby restoring the balance of the platform hull 10. "Until the tilt angle is restored to within 2°" is the adjustment target, ensuring that the platform hull 10 remains within an acceptable stability range during the pile-pulling process, thereby ensuring operational safety.

[0040] By employing the aforementioned technical solution, when extracting the outer ring of pile legs 30, a strategy of alternating groups and prioritizing the extraction of pile legs with deeper penetration into the mud can effectively balance the extraction progress of each pile leg. This avoids uneven stress on the platform hull 10 caused by premature or delayed extraction of a single pile leg, thereby improving the overall efficiency and controllability of the extraction process. Simultaneously, real-time monitoring of the platform hull 10's tilt angle, and timely suspension of extraction and adjustment of the support status of other pile legs when the tilt angle exceeds the safe range, effectively prevents excessive tilting of the platform hull 10, significantly improving the stability and safety of the extraction operation, reducing operational risks, and ensuring the stable operation of the platform hull 10 in complex sea conditions.

[0041] Furthermore, in the aforementioned outer ring pile extraction steps, a diagonal alternating pile extraction method is preferred. Specifically, when a significant decrease in load is detected in one of the outer ring pile legs 30 in a certain diagonal group, it is determined that the pile leg has loosened, the operation on that pile leg is immediately stopped, and the extraction operation is switched to another diagonal group of pile legs. "Using a diagonal alternating pile extraction method" means that when performing the outer ring pile leg 30 extraction operation, the outer ring pile legs 30 are grouped according to the diagonal direction, and the pile extraction operation is performed sequentially on these diagonal groups of pile legs. This grouping method helps maintain the stress balance of the platform hull 10 during the pile extraction process, avoiding platform tilting or structural stress concentration caused by uneven stress on one side or in a localized area of ​​pile legs. For example, for a platform with rectangularly arranged pile legs, the diagonal group consisting of the left front and right rear pile legs can be extracted first, and after it loosens, the operation can be switched to the diagonal group consisting of the right front and left rear pile legs. "Detecting a significant decrease in load on one of the outermost pile legs 30 in a diagonal group" refers to real-time monitoring of the upward pulling or jacking force experienced by the pile leg during the extraction process using load or pressure sensors installed on each pile leg lifting unit 20. When this force value rapidly decreases from a relatively stable high level, and the decrease reaches a first preset threshold (e.g., a decrease of more than 5%), it is identified by the system as a significant decrease in load. This rapid change in load is a direct manifestation of a sudden reduction in frictional resistance or suction between the pile leg and the hard soil layer of the seabed. "Determining that the pile leg has loosened" is a judgment automatically made by the platform control system based on the detection result of the aforementioned significant decrease in load. Once the condition of load decrease is met, the system considers that the pile leg has overcome the constraint of the seabed and is in a loosened state, and no longer needs to apply excessive extraction force. "Immediately stopping the action on the pile leg" means that after determining that a pile leg has loosened, the platform control system will quickly send a stop command to the corresponding pile leg lifting unit 20, causing it to stop continuing to pull the pile leg upward. This measure aims to avoid unnecessary continuous work on loosened legs, thereby saving energy, reducing equipment wear, and preventing uneven stress on the platform that might occur after excessive lifting and separation from the hard seabed soil layer, which could affect subsequent construction. "Switching to another set of diagonal legs for pile extraction" means that after stopping the extraction of loosened legs, the platform control system automatically selects another set of diagonal legs that have not yet been loosened, according to a preset extraction strategy, and activates its lifting unit 20 to begin the extraction operation on these legs. This switching mechanism ensures the continuity of the extraction operation and maintains the overall balance and stability of the platform hull 10 throughout the entire outer ring pile extraction process.

[0042] Through the aforementioned technical solution, during the extraction of the outer ring pile legs 30, the loosening status of the pile legs can be accurately determined by real-time monitoring of changes in the pile leg load. Once a significant drop in the pile leg load is detected, work on that pile leg is immediately stopped, avoiding unnecessary power consumption and potential structural stress on the loosened pile leg. Simultaneously, work is immediately switched to another set of diagonally opposite pile legs, ensuring the continuity of the extraction process. More importantly, by alternating diagonally, the stress on the platform hull 10 is effectively distributed, reducing the risk of platform tilting during the extraction process, thereby improving the efficiency and safety of the extraction operation. This proactive loosening judgment and operation switching mechanism, compared to passive adjustments relying solely on tilt angle monitoring, allows for more refined management of the extraction process, optimizes the stress state of the platform hull 10, and effectively utilizes the capabilities of the lifting system.

[0043] For example, in the above-described pile extraction method, the outer ring pile extraction step S2 and the inner ring pile extraction step S3 are key steps in the pile extraction operation. Their core lies in accurately identifying whether the pile leg has reached a "sufficiently loosened" state, thereby determining when to stop the single-leg movement and switch to the next stage (e.g., the full pile lifting step S4). In this regard, this embodiment of the invention further proposes that, in the outer ring pile extraction step and / or the inner ring pile extraction step, when the pile extraction load is detected to drop below a second preset threshold, it is determined that the pile leg has been sufficiently loosened.

[0044] Specifically, the pile extraction load refers to the upward pulling force (defined as a negative value) exerted by the lifting unit 20 on the pile leg during the pile extraction process. This load is monitored in real time by a force sensor installed on the lifting unit 20 and transmitted to the control center. A second preset threshold is set (e.g., -500 tons), where the negative sign represents the direction of the upward pulling force. The setting of this threshold takes into account factors such as the self-weight of the pile leg, the size of the pile shoe, and the seabed soil. In the initial stage of pile extraction, in order to overcome the adhesion between the pile leg and the seabed, the lifting unit 20 needs to output a large upward pulling force, at which time the pile extraction load value is relatively low (e.g., -800 tons). As the pile leg is gradually loosened, the soil resistance decreases significantly, and the required upward pulling force decreases accordingly. When the monitored pile extraction load value increases to above the preset threshold (i.e., -500 tons) and the fluctuation tends to stabilize, it indicates that the adhesion between the pile leg and the hard soil layer of the seabed has been basically overcome, and the pile leg is in a fully loosened state. Based on this determination, the system confirms that the pile leg is no longer firmly bound by the soil and can be moved relatively easily. This not only avoids unnecessary energy consumption and equipment wear caused by excessive lifting of the lifting unit 20, but also provides precise execution basis for subsequent steps: for example, after confirming that the outer pile legs 30 have been loosened, it is safe to switch to inner pile extraction; after confirming that the inner pile legs have been loosened, the full pile lifting step can be initiated. Through this data-driven quantitative judgment, the pile extraction operation has been transformed from "experience-driven" to "precise control," improving the safety and efficiency of the operation.

[0045] For example, in actual operation, even after initial counterweight adjustment and preliminary jetting around the pile shoe by the pile driving system during the counterweight adjustment step, the pile leg may still encounter significant pile extraction resistance due to the adhesion and friction of the seabed soil in the subsequent outer and / or inner pile extraction steps. This persistent resistance not only increases the load on the lifting system, potentially leading to low pile extraction efficiency, but also poses a risk of equipment damage or operation interruption. To address this, this embodiment of the invention further proposes that, during the outer and / or inner pile extraction steps, the jetting system is continuously activated while performing the pile extraction action.

[0046] Specifically, during jet grouting, high-pressure water jets are sprayed through nozzles onto the soil surrounding the pile legs. The impact force of the water jets and the disturbance they create in the soil effectively disrupt the soil structure, reducing the adhesion and friction between the soil and the pile legs, thereby liquefying or loosening the soil around the pile legs. Here, "continuous operation" means that the jet grouting system does not only operate briefly before the pile extraction begins, but rather throughout the entire extraction process. This continuous jet grouting ensures that the soil around the pile legs remains disturbed and liquefied as they move upwards, minimizing extraction resistance, reducing the load on the platform hull structure and lifting equipment, and extending the equipment's service life. Furthermore, the extraction process becomes smoother and more efficient, reducing the risk of pile jamming or extraction interruption due to excessive resistance, and improving the safety and reliability of the operation.

[0047] For example, in actual operations, if the tidal changes and sea conditions of the operating area are not fully considered, the platform may face the risk of grounding, reduced operational efficiency, or even operational safety and stability due to adverse sea conditions during pile removal. To address this, the present invention further proposes an environmental assessment step before performing the pile removal operation: obtaining tidal data and sea condition information of the operating area, selecting the high tide period as the pile removal window, and confirming that the freeboard height of the platform hull 10 is always not lower than the platform's maximum draft.

[0048] Specifically, this environmental assessment step aims to comprehensively consider and predict the natural environmental conditions of the operating area before the formal commencement of pile extraction operations, ensuring that the operations can be carried out under the most favorable and safest environmental conditions. Its purpose is to mitigate potential environmental risks, optimize the operating window, and provide a basis for decision-making in subsequent pile extraction operations. Tidal data refers to the tidal rise and fall patterns in the operating area over a specific time period, including high tide, low tide, and their occurrence times. Sea state information encompasses marine environmental parameters such as wind speed, wind direction, wave height, swell cycle, and current speed and direction. This data and information can be obtained through various means, such as real-time collection by marine environmental monitoring equipment installed on the platform, obtaining authoritative forecast data from meteorological and oceanographic forecasting agencies, or combining historical data for analysis. Based on this, selecting a high tide period for pile extraction operations is primarily to utilize the higher water level to increase the buoyancy of the platform hull 10, thereby providing greater reserve buoyancy for the platform during pile extraction, helping to offset part of the self-weight of the pile legs and the pile extraction resistance, and reducing the load on the lifting system. Meanwhile, confirming that the freeboard height is not lower than the maximum draft before the pile extraction operation is to ensure that the platform maintains sufficient buoyancy reserve and wave resistance even if the draft increases for some reason during the pile extraction process, preventing the deck from being submerged by seawater, thereby ensuring the safety of the platform structure and the personnel. Therefore, this environmental assessment step provides a safe and efficient prerequisite for the subsequent pile extraction operation, improving the reliability and controllability of the entire pile extraction process.

[0049] For example, when a self-elevating offshore platform performs pile extraction operations, the complexity of factors such as the depth of each pile leg into the mud, geological conditions, and the order of pile extraction can easily lead to uneven stress on the platform hull 10 during the extraction process. This uneven stress may cause the platform hull 10 to tilt, or even cause potential damage to the platform structure, affecting operational safety and efficiency. To address this, the present invention further proposes a safety monitoring step before performing the pile extraction operation. This safety monitoring step aims to ensure balanced stress on the platform hull 10 during the pile extraction process through real-time monitoring and dynamic adjustment, thereby improving the safety and stability of the operation.

[0050] Specifically, the safety monitoring steps include real-time acquisition of the mud penetration depth data of each pile leg, and based on this, dynamically adjusting the jacking force of each lifting unit 20. This allows for real-time monitoring of the contact state and stress conditions between each pile leg and the seabed, providing precise feedback information for the pile extraction operation. Based on this real-time data, the control system can intelligently and dynamically adjust the jacking force of each lifting unit 20, actively compensating for uneven stress on the platform hull 10 caused by differences in geological conditions, varying pile leg mud penetration depths, or changes in the pile extraction sequence. This not only effectively avoids potential tilting and structural damage to the platform hull 10 during pile extraction, improving operational safety and stability, but also optimizes pile extraction efficiency, ensuring the smooth and controllable execution of the entire pile extraction process.

[0051] It should also be noted that the pile extraction method proposed in this embodiment is particularly suitable for jack-up offshore platforms with 6 or 8 legs arranged in a rectangular array (in a "U" shape). The rectangular array arrangement of the legs means that the legs form a rectangular support surface below the platform hull 10. This arrangement has good geometric symmetry, facilitating uniform load distribution during pile extraction. For example, when performing the outer ring pile extraction step described above, a diagonal alternating pile extraction method can be used based on the characteristics of the rectangular array, ensuring that the platform hull 10 maintains a relatively stable attitude throughout the extraction process. Furthermore, the rectangular array makes the definitions of "outer ring legs 30" and "middle legs 40" clearer, helping operators accurately identify and control legs in different positions, thereby simplifying the complexity of the pile extraction operation and improving its efficiency and safety.

[0052] Compared with traditional methods, the pile extraction method proposed in this invention effectively disperses the pile extraction load by using a step-by-step, alternating approach, utilizing fulcrum reaction forces and reserve buoyancy. This reduces the resistance of single pile extraction, avoids the risk of uneven stress and tilting of the platform as a whole, and improves the efficiency and safety of pile extraction. Furthermore, this pile extraction method involves less external force during the extraction operation, reducing the impact on the pile legs and corresponding lifting units, and effectively extending the service life of the pile legs and corresponding lifting units.

[0053] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and substitutions can be made without departing from the technical principles of the present invention, and these improvements and substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A method for extracting piles on a multi-legged jack-up offshore platform, characterized in that, Includes the following steps: S1: Counterweight adjustment steps: Control the lifting system to lower the platform hull into the water, start the ballast system to discharge the ballast water in the ballast tank to reduce the total weight of the platform, and at the same time start the pile driving system to spray high-pressure water around the pile shoes. S2: Outer ring pile extraction procedure: Control the lifting system to first perform the pile extraction action on the outer ring pile legs located on the outer ring of the platform hull, while keeping at least one intermediate pile leg located at the geometric center of the platform stationary and supported on the seabed, using the intermediate pile leg as a fulcrum to provide a supporting force to the platform hull in the same direction as the buoyancy. S3: Inner ring pile extraction step: After the outer ring pile legs are initially loosened, control the middle pile leg to perform the pile extraction action, and at the same time lock the loosened outer ring pile legs through the locking device to keep the loosened outer ring pile legs stationary and supported on the seabed. Use the outer ring pile legs as fulcrums to provide support force to the platform hull in the same direction as the buoyancy, and use the platform hull's reserve buoyancy to assist in loosening the middle pile legs. After the intermediate pile legs are initially loosened, steps S2 and S3 are performed alternately until all pile legs are fully loosened, and it is ensured that none of the pile legs are completely detached from the hard soil layer of the seabed. S4: Full pile lifting procedure: Simultaneously start all lifting units, and with the reserve buoyancy of the platform hull, make all pile legs detach from the hard soil layer of the seabed, and drive the pile shoes to the zero position.

2. The pile extraction method according to claim 1, characterized in that, The outer ring pile extraction step specifically includes: The piles were pulled out in alternating groups, with priority given to removing the pile legs that were deeper into the mud, until the depth of each pile leg into the mud was consistent. During the pile extraction process, the tilt angle of the platform hull is monitored in real time. When the tilt angle of the platform hull exceeds 2°, the pile extraction action of the current pile leg is suspended and the support status of other pile legs is adjusted until the tilt angle is restored to within 2°.

3. The pile extraction method according to claim 2, characterized in that, In the outer ring pile extraction step, a diagonal alternating pile extraction method is adopted: When the load on one of the outermost diagonal pile legs in a certain group drops to a first preset threshold, it is determined that the pile leg has become loose. The operation on the pile leg is immediately stopped, and the operation is switched to another group of diagonal pile legs for pile extraction.

4. The pile extraction method according to claim 1, characterized in that, In the outer ring pile extraction step and / or the inner ring pile extraction step, when the pile extraction load is detected to drop below the second preset threshold, it is determined that the pile leg has been sufficiently loosened.

5. The pile extraction method according to claim 1, characterized in that, During the outer ring pile extraction step and / or the inner ring pile extraction step, the pile extraction action is performed while the jetting system is continuously activated for jetting operation.

6. The pile extraction method according to claim 1, characterized in that, The inner ring pile extraction step, utilizing the platform's reserve buoyancy to assist in pile extraction, specifically includes: When the intermediate pile leg is loosened, the ballast system is controlled to further discharge ballast water, and the upward buoyancy generated by the platform hull is used to offset part of the pile leg's self-weight and the resistance to pile extraction.

7. The pile extraction method according to claim 1, characterized in that, An environmental assessment step is also included before the pile extraction operation is carried out: Obtain tidal data and sea condition information for the operating area, select the high tide period as the window for pile extraction, and confirm that the freeboard height of the platform hull is always not lower than the maximum draft of the platform.

8. The pile extraction method according to claim 1, characterized in that, It also includes security monitoring steps: Real-time data on the mud penetration depth of each pile leg is acquired, and the lifting force of each lifting unit is dynamically adjusted to ensure that the platform hull is subjected to balanced forces during the pile extraction process.

9. The pile extraction method according to any one of claims 1 to 8, characterized in that, The multi-leg jack-up offshore platform has 6 or 8 legs arranged in a rectangular array.

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