A suction cylinder installation method suitable for clay-sand mixed strata in deep sea.

Abstract

This invention belongs to the field of marine engineering foundation construction technology, specifically relating to a suction cylinder installation method suitable for deep-sea clay-sand mixed strata. The method comprises four main stages of penetration: self-weight and suction penetration in the clay layer, interface separation and water pressure recovery, fracturing and seepage recovery, and suction penetration in the sand layer. By utilizing controllable interface separation (i.e., active cracking) and differential pressure-induced fracturing to enhance permeability, the passive failure risk at the clay-sand interface during suction cylinder penetration is smoothly addressed. This avoids problems that may arise with traditional construction techniques, such as soil plug heave, reverse end-bearing effect of the pile end under pull-out load, and seepage grid blockage. This installation method is economical, feasible, and easy to install, significantly reducing the total construction cost and strongly promoting the commercial development of offshore wind power.

Description

Technical Field

[0001] This invention belongs to the field of marine geotechnical and foundation engineering technology in the offshore wind power, photovoltaic, oil and gas and other energy development industries, and specifically relates to a suction cylinder installation method suitable for deep-sea clay-sand mixed strata. Background Technology

[0002] To accelerate the construction of a clean and low-carbon energy system, wind power is playing an increasingly important role in my country's energy structure adjustment. In recent years, with the gradual reduction of available onshore wind farms, my country's wind power development is shifting from onshore to offshore, and from near-shore and shallow waters to deep-sea areas. However, compared with onshore wind power, the high construction cost remains one of the major factors restricting the large-scale commercial development of offshore wind power, with more than 30% of the construction cost coming from the wind turbine foundation and its installation. Therefore, developing economically feasible and easy-to-install foundation types is of great significance for reducing the total construction cost.

[0003] Furthermore, the marine sedimentary environment results in heterogeneous geological formations at offshore wind farm sites, typically characterized by alternating layers of clay and sand. Although high-quality wind farms with single geological formations are initially prioritized for offshore wind development, the development and utilization of wind farms with mixed geological formations has become an inevitable trend as these high-quality resources are gradually depleted.

[0004] Suction pumps are a common type of marine foundation used in various marine engineering projects, offering advantages such as low cost, low installation noise, short offshore operation time, and recyclability. However, in seabed mixed strata consisting of clay overlying sand, when the suction pump penetrates to the interface between the two soil types, the sand layer struggles to seep through due to the poor permeability of the overlying clay layer. This seepage resistance reduction fails, preventing penetration into the underlying sand layer and hindering subsequent penetration, ultimately leading to suction pump failure. Increasing the suction force within the pump to overcome the penetration resistance could potentially lift the overlying clay layer, causing damage to the mixed strata.

[0005] Chinese invention patent (publication number CN114756948B) discloses a method for calculating the penetration resistance of a suction cylinder, which is used to calculate the penetration resistance of the suction cylinder when it penetrates in a clay-sand mixed soil layer. This provides data and theoretical support for the promotion and application of suction cylinder foundations in offshore wind power. However, this method requires a lengthy control system with extensive computing power and the installation of multiple sensors, which greatly increases the construction cost of the suction cylinder. Therefore, there is still a long way to go for the commercial development of offshore wind power.

[0006] Therefore, there is an urgent need for a suction cylinder installation method suitable for deep-sea clay-sand mixed strata that is economical, feasible, and easy to install, so as to reduce the total construction cost and promote the commercial development of offshore wind power. Summary of the Invention

[0007] To address the technical bottlenecks in the installation of suction cylinders in existing mixed seabed strata, this invention proposes a suction cylinder installation method suitable for mixed clay and sand strata in deep sea. This method can generate seepage in the sand layer overlying the clay layer and restore the seepage drag reduction effect, making it economical, feasible, and easy to install.

[0008] Working principle: This invention proposes an innovative concept of "controllable interface separation - pressure difference-induced fracturing and permeability enhancement". By controlling the suction of the top surface, the interface of the overlying clay layer is controlled to separate. The pore pressure is restored by the seepage self-stabilization of the underlying sand layer. Then, a greater pressure difference is applied to induce vertical fracturing of the clay layer to increase permeability, forming a continuous and stable seepage channel. This restores the seepage drag reduction effect of the sand layer and enables the suction cylinder to be successfully penetrated in the mixed clay and sand strata.

[0009] The technical solution of the present invention is as follows: A suction cylinder installation method suitable for deep-sea clay-sand mixed strata, the method includes the following steps: S1. Self-weight and suction penetration stage in clay layer: The suction cylinder is hoisted to the seabed surface and relies on its own weight to embed the lower edge of the cylinder into the clay layer to form a seal. Then, the water inside the cylinder is pumped out to establish a pressure difference, so that the suction cylinder can continue to penetrate into the clay layer. S2, Interface Separation and Water Pressure Recovery Stage: When the suction cylinder penetrates to the thickness of the clay layer, the pumping rate is reduced and the suction is controlled to cause a controllable interface separation between the bottom surface of the clay layer and the top surface of the sand layer, forming a cavity. The water pressure in the cavity is then restored to the original pore water pressure of the sand layer by the seepage in the sand layer. S3, Fracturing and Seepage Recovery Stage: Increase the pumping volume to increase suction. By separating the cavity, the upper suction is transferred to the bottom of the clay layer, forming a pressure difference between the top and bottom surfaces in the clay layer. This induces vertical cracks in the clay layer to expand and connect, forming a crack network that penetrates the clay layer and constructs an effective seepage channel from the sand layer to the inside of the suction cylinder. S4. Suction Penetration Stage in Sandy Soil Layer: Maintain or adjust the pressure difference to form stable seepage in the sandy soil layer through the crack network. Utilize the seepage drag reduction effect to reduce penetration resistance, allowing the suction cylinder to continue sinking in the sandy soil layer until the top of the cylinder contacts the top surface of the clay layer or reaches the design depth, thus completing the overall installation of the suction cylinder.

[0010] Furthermore, during the controllable interface separation process, the overall instability of the mixed strata is avoided by controlling the upper limit of suction, suction is applied in stages to avoid one-time destruction, and the interface is separated in an orderly manner according to the dominant vertical connection direction.

[0011] Furthermore, the criteria for determining that the interface separation is complete in step S2 are: a significant increase in the amount of water seeping into the bottom of the cylinder or a significant change in the total resistance to penetration of the cylinder.

[0012] Furthermore, in step S2, the cavity is formed by the upward deformation of the overlying clay layer under the negative pressure inside the cylinder. The formation mechanism is as follows: the water pressure inside the cylinder is lower than the hydrostatic pressure outside the suction cylinder, and a pressure difference is formed at the bottom of the clay layer. The negative pressure inside the cylinder brings an upward load, causing the middle of the clay to deform upward. When the additional stress generated by the pressure difference between the inside and outside of the cylinder is sufficient to overcome the tensile strength of the soil layer contact surface, the two soil layers separate at the interface to form a cavity.

[0013] Furthermore, the basis for determining the establishment of the seepage channel in step S3 is that the rate of water level drop inside the cylinder is significantly accelerated.

[0014] Compared with the prior art, the advantages of the present invention are as follows: Existing technology cannot install suction cylinders in mixed clay and sand strata on the seabed. The key is that the overlying clay layer completely restricts seepage in the underlying sand layer. During installation, the seepage resistance reduction effect cannot be utilized, and the excessive penetration resistance in the sand layer ultimately leads to failure. The installation method provided by this invention overcomes the technical bottleneck by employing a "controllable interface separation-pressure differential fracturing and permeability enhancement" approach, solving the installation problem of suction cylinders in mixed clay-sand strata. This invention can achieve gradual separation of the clay and sand interfaces under pressure differential constraints, exhibiting characteristics of constraint (avoiding overall instability of the mixed strata through upper suction limit control), gradualness (applying suction in stages to avoid one-time destruction), and orderliness (separation along the dominant connectivity direction rather than random fracture). By adjusting the pressure differential, cracks can be created in the overlying clay layer, enabling crack propagation and connectivity, significantly improving the permeability of the clay layer and providing a key outlet for seepage in the underlying sand layer. It can restore the seepage drag reduction effect of the underlying sand layer, reducing the penetration resistance of the suction cylinder and enabling continued penetration after the suction cylinder penetrates the interface, ensuring good installability of the suction cylinder even in complex mixed strata. This installation method is economical, feasible, and easy to install, greatly reducing the total construction cost and strongly promoting the commercial development of offshore wind power. Attached Figure Description

[0015] Figure 1 This is a flowchart of the method of the present invention; Figure 2 This is a schematic cross-sectional view of the penetration process in the clay layer during the self-weight and suction settlement stages according to an embodiment of the present invention. Figure 3 This is a schematic cross-sectional view of the penetration process during the interface separation and water pressure recovery stages in an embodiment of the present invention; Figure 4 This is a schematic cross-sectional view of the penetration process during the fracturing and seepage recovery stages in an embodiment of the present invention; Figure 5 This is a schematic cross-sectional view of the penetration process in the suction penetration stage of the sand layer in an embodiment of the present invention. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "or / and" as used herein includes any and all combinations of one or more of the associated listed items.

[0018] The suction cylinder installation method proposed in this invention is applicable to suction cylinder construction and penetration in mixed clay and sand strata on the seabed. The flowchart of the method is as follows: Figure 1 As shown. The following is combined with... Figure 2 - Figure 5 This paper provides specific embodiments of the present invention, and describes the embodiments of the present invention in detail with reference to actual marine engineering experiments and numerical simulation data.

[0019] The first stage, the self-weight and suction penetration stage in the clay layer, includes two sub-stages: the self-weight penetration sub-stage and the suction penetration sub-stage.

[0020] The self-weight penetration stage is as follows: Select a construction site with a flat seabed and clear soil layer distribution. Confirm the thickness of the upper clay layer, overconsolidation ratio, burial depth of the lower sand layer, and related physical parameters through geological exploration. Hoist the suction cylinder to the designed location on the site and slowly lower it to the seabed surface. Adjust the level deviation of the bottom of the cylinder. Rely on the self-weight of the suction cylinder foundation (including the self-weight of the cylinder structure, the weight of the top structure, and additional counterweights) to carry out the initial self-weight penetration work. The initial self-weight penetration work is carried out in the upper clay layer, where the penetration resistance is small. Through the self-weight penetration work, the lower edge of the cylinder is cut and embedded into the clay layer to form a seal. The connection between the water inside the suction cylinder and the external seawater is cut off, and water exchange cannot be achieved, thus achieving a preliminary seal. During the self-weight penetration stage, the water level inside the suction cylinder is slightly higher than the seabed surface. After self-weight penetration, check the rate of water level drop inside the cylinder to confirm whether the water inside the cylinder has formed an effective seal.

[0021] The suction penetration sub-stage is as follows: The water pump device installed at the top of the suction cylinder is activated. Based on the relevant physical parameters obtained from the geological survey and in conjunction with the relevant specifications for suction cylinder installation, the corresponding pumping speed is determined. The drain outlet is set above the seabed surface to prevent backflow during the pumping process. The water inside the suction cylinder is discharged with the pumping. The pressure inside the cylinder is less than the external pressure. During this process, due to the extremely low permeability coefficient of the clay layer, the amount of water seeping into the cylinder from the bottom is much less than the amount of water pumped from the top. The pressure difference between the inside and outside of the suction cylinder is quickly established. A pressure difference is formed between the cavity inside the suction cylinder and the external sea surface, so that the applied suction is greater than the penetration resistance in the clay layer. The suction cylinder penetrates stably in the clay layer.

[0022] like Figure 2 As shown, the pressure inside the suction cylinder is P 1 refers to the suction force applied to the soil layer inside the cylinder.

[0023] The second stage, the interface separation and water pressure recovery stage: When the suction barrel's penetration depth approaches the thickness of the explored clay layer, the pumping rate is gradually reduced to decrease the pressure difference between the inside and outside of the barrel, thereby reducing the downward additional penetration load caused by the negative pressure and slowing down the barrel's settlement rate; the clay at the bottom of the barrel deforms under the previous negative pressure load, and the interface between the clay and the underlying sand layer gradually separates to form a bottom cavity (see...). Figure 3 (White area at the bottom of the middle cylinder). Excessive pressure difference will continuously expand the cavity. By reducing the pumping speed to decrease the upward additional stress, the degree of upward depression of the clay can be controlled. At the same time, the permeability coefficient of the underlying sand layer is much greater than that of the clay. Under the hydraulic gradient of the in-situ water pressure in the sand layer and the low pressure in the cavity, the pore water in the sand layer seeps into the cavity along the clay-sand layer void interface, inhibiting the unlimited expansion of the cavity. As seepage continues, the water pressure in the cavity gradually rises. When the cavity water pressure recovers to a level close to the in-situ hydrostatic pore water pressure in the sand layer and the hydraulic gradient across the clay layer approaches zero, the water pressure recovery stage is considered complete.

[0024] On-site layering identification: When the cylinder penetrates to the interface between the clay and sand layers, the seepage flow rate at the bottom of the cylinder will increase significantly; when the soil transitions from low-strength clay to high-resistance sand layer, the total resistance of the cylinder penetration will change significantly, which can be used as a basis for judging whether the interface has been reached.

[0025] Controllable interface de-voiding is achieved through graded control of negative pressure inside the cylinder: By dynamically limiting the peak suction and adjusting the pumping rate in stages, the deformation development of the clay and sandy soil interface is constrained, and gradual interface separation is achieved. This process has three main characteristics: ① Constraint: Set an upper limit for suction to control the downward additional stress generated by the pressure difference between the inside and outside of the cylinder, and prevent excessive deformation of the bottom clay from causing overall instability of the clay-sand mixed stratum; ② Gradual approach: When approaching the bottom of the clay layer, reduce the pumping rate in stages and gradually decrease the suction to avoid sudden large pressure difference causing one-time tension failure of the bottom clay; ③ Orderliness: Based on the distribution law of additional stress with large force in the middle of the bottom of the cylinder and small force at the edge of the cylinder, the void area expands in an orderly manner from the center of the cylinder to the circumference, avoiding irregular tension fracture of the soil.

[0026] It should be noted that the bottom cavity is actually formed by the upward deformation of the overlying clay layer under the negative pressure inside the cylinder. The specific mechanism is as follows: the water pressure inside the cylinder is lower than the hydrostatic pressure outside the suction cylinder, creating a pressure difference between the top and bottom of the clay layer; the negative pressure inside the cylinder brings an upward load, causing the middle of the clay to deform upward. When the additional stress generated by the pressure difference between the inside and outside of the cylinder is sufficient to overcome the tensile strength of the soil layer contact surface, the two soil layers separate at the interface to form a cavity; the pore water on the sand side will flow into the cavity through the gaps caused by the high and low water pressure difference.

[0027] However, it should be noted that if water is supplied solely through the interfacial voids and fissures to maintain sand seepage, the bottom cavity will continue to expand, making it difficult to control the settlement of the cylinder and preventing it from sinking to the designed installation depth, thus causing installation failure. In this case, generating a continuous network of fissures within the overlying clay layer becomes particularly crucial, which is also the core innovation of this technology.

[0028] During construction, the cavity size at the soil-layer interface is stably controlled within a reasonable range by adjusting the water pressure inside the cylinder: the cavity size will not be too large to prevent the cylinder from settling out of control and failing to reach the design depth; at the same time, the cavity size will not be too small to ensure that pore water in the sand can continuously flow into the cavity to form a stable seepage. On this basis, by controlling the pumping speed inside the cylinder (i.e., the rate of pressure difference change), the overlying clay is gradually tensioned to generate vertically dominant through-cracks, which ensure continuous and stable seepage of the underlying sand layer.

[0029] The actual negative pressure selected during construction and the setting of the negative pressure rise and fall curve depend on the site geological conditions and the structural parameters of the suction cylinder itself. These parameters can all be determined through multiple trial calculations using conventional and mature testing methods in this field, and will not be elaborated upon here. Figure 3 As shown, the bottom of the suction cylinder is located at the clay-sand interface, and the internal water pressure of the suction cylinder... P 1. (Generated by suction) Pore water pressure at the bottom P 2, its relationship with P 1. In the same direction, generally, suction is generated by applying force. P 1 is greater than the water pressure accumulated at the ultra-clean pore water pressure at the clay-sand interface. P 2, to ensure the subsequent suction penetration.

[0030] The third stage, the tension cracking and seepage reconstruction stage: After the water pressure recovery stage, the pumping flow rate is increased, and the negative pressure inside the cylinder is amplified. The low-pressure water inside the cylinder passes through the voids between the clay and sand, causing a synchronous decrease in the water pressure at the bottom of the clay layer. A significant water pressure difference is formed between the top and bottom surfaces of the clay layer, generating tensile stress within the overconsolidated clay. When the local tensile stress exceeds the tensile strength of the clay itself, micro-cracks first appear on the bottom surface of the stress-concentrated clay layer. These cracks extend upwards from the bottom of the soil layer, while tension cracks develop simultaneously on the top surface under the negative pressure. As the negative pressure inside the cylinder continues to increase, various cracks extend and intersect, forming a vertical crack network that runs through the entire clay layer. The overall equivalent permeability coefficient of the clay layer is significantly increased, and a stable seepage channel in the underlying sand layer is formally formed. Pore ​​water in the sand layer flows into the cylinder sequentially through interlayer voids and clay fissures; the rate of water level drop inside the cylinder accelerates significantly. Specifically, when the inflow rate into the cylinder continuously and steadily increases, and the rate of water level drop remains high for a long period without falling back, it can be determined that the stable seepage channels inside the soil layer are fully formed. Figure 4 As shown. The suction force generated P 1 ′ The water pressure accumulated at the ultra-clean pore water pressure at the clay-sand interface is greater than the water pressure accumulated at the clay-sand interface. P 2 ′ This is to ensure the formation and propagation of cracks.

[0031] The fourth stage, the negative pressure penetration stage in the sand layer: After the stable seepage channel in the sand layer is formed, the pumping speed is adjusted to maintain a stable negative pressure inside the cylinder. The low-pressure water inside the cylinder is conducted to the underlying sand layer through the cracks in the clay layer, creating a continuous and stable seepage within the sand layer. The seepage force generated by the seepage reduces the effective stress of the sand skeleton, thereby reducing the penetration resistance of the cylinder as it sinks within the sand layer. This stage continues with negative pressure penetration until the top of the cylinder sinks close to the original seabed surface, or the cylinder penetrates to the preset design depth. Pumping is then stopped and the pumping valve is closed. The pressure difference between the inside and outside of the cylinder resists the foundation's floating and sliding, ensuring the overall stability of the foundation after placement, thus completing all the penetration construction. Figure 5 As shown. At this point, there is no accumulation of ultra-clean pore water pressure at the clay-sand interface. Based on the geological exploration results, appropriate suction continues to be applied. P 3. In suction P Under the action of 3, the suction cylinder is stably penetrated until the predetermined depth is reached.

[0032] This invention proposes a suction cylinder installation method specifically for the settlement conditions of mixed strata with clay upper layer and sand lower layer. It employs a construction approach combining controlled interface separation with active crack creation, overcoming the drawbacks of traditional construction methods. By actively regulating the internal pressure of the cylinder, controllable cracking of the clay mass is achieved, constructing a continuous fracture network. This network of fractures restores the seepage system of the underlying sand layer, enabling coordinated and stable settlement of the soil layers. This effectively avoids the problems of interface stress imbalance, soil deformation instability, and settlement obstruction that easily occur in traditional construction methods, and avoids the risk of abnormal soil deformation and settlement failure caused by pressure differential imbalance. This method overcomes the technical limitations of traditional suction cylinder installation, which relies entirely on the natural permeability of the soil layer. It forms a novel construction mechanism of "controlled interface separation – pressure differential cracking and permeability enhancement," effectively breaking through the technical bottleneck of suction cylinder settlement in binary strata and properly solving the core problem of the difficulty in stable and complete settlement installation of suction cylinders in mixed strata with clay upper layer and sand lower layer.

[0033] The embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A suction cylinder installation method suitable for deep-sea clay-sand mixed strata, characterized in that, The method includes the following steps: S1. Self-weight and suction penetration stage in clay layer: The suction cylinder is hoisted to the seabed surface and relies on its own weight to embed the lower edge of the cylinder into the clay layer to form a seal. Then, the water inside the cylinder is pumped out to establish a pressure difference, so that the suction cylinder can continue to penetrate into the clay layer. S2, Interface Separation and Water Pressure Recovery Stage: When the suction cylinder penetrates to the thickness of the clay layer, the pumping rate is reduced and the suction is controlled to cause a controllable interface separation between the bottom surface of the clay layer and the top surface of the sand layer, forming a cavity. The water pressure in the cavity is then restored to the original pore water pressure of the sand layer by the seepage in the sand layer. S3, Fracturing and Seepage Recovery Stage: Increase the pumping volume to increase suction. By separating the cavity, the upper suction is transferred to the bottom of the clay layer, forming a pressure difference between the top and bottom surfaces in the clay layer. This induces vertical cracks in the clay layer to expand and connect, forming a crack network that penetrates the clay layer and constructs an effective seepage channel from the sand layer to the inside of the suction cylinder. S4. Suction Penetration Stage in Sandy Soil Layer: Maintain or adjust suction, form stable seepage in sandy soil layer through crack network, reduce penetration resistance by seepage drag reduction effect, and allow suction cylinder to continue sinking in sandy soil layer until the top of the cylinder contacts the top surface of clay layer or reaches the design depth.

2. The suction cylinder installation method according to claim 1, characterized in that, During the process of controllable interface separation, the overall instability of the mixed strata is avoided by controlling the upper limit of suction, suction is applied in stages to avoid one-time destruction, and the interface is separated in an orderly manner according to the dominant vertical connection direction.

3. The suction cylinder installation method according to claim 1, characterized in that, The criteria for determining that the interface separation is complete in step S2 are: a significant increase in the amount of water seeping into the bottom of the cylinder or a significant change in the total resistance to penetration of the cylinder.

4. The suction cylinder installation method according to claim 1, characterized in that, In step S2, the cavity is formed by the upward deformation of the overlying clay layer under the negative pressure inside the cylinder. The formation mechanism is as follows: the water pressure inside the cylinder is lower than the hydrostatic pressure outside the suction cylinder, and a pressure difference is formed at the bottom of the clay layer. The negative pressure inside the cylinder brings an upward load, causing the middle of the clay to deform upward. When the additional stress generated by the pressure difference between the inside and outside of the cylinder is sufficient to overcome the tensile strength of the soil layer contact surface, the two soil layers separate at the interface to form a cavity.

5. The suction cylinder installation method according to claim 1, characterized in that, The basis for determining the establishment of the seepage channel in step S3 is that the rate of water level drop inside the cylinder has significantly accelerated.

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