A design process for an adaptive extreme sea conditions floating wind turbine

Abstract

The present application belongs to the technical field of wind power, and in particular to a design process of a floating wind turbine self-adapting to extreme sea conditions, aiming at the problem of excessive wave load of the existing semi-submersible platform under extreme sea conditions, the following scheme is proposed, which comprises the following steps: S1: reference model selection and floating platform structure modification, taking the DTU 10MW reference wind turbine as the reference model, which is a horizontal axis upwind three-blade wind turbine generator set meeting the IEC 1A standard, the rotor mass is 229t, the control mode is variable speed and unified variable pitch, the cut-in, rated and cut-out wind speeds are 4.0m / s, 11.4m / s and 25.0m / s respectively, the rotor diameter is 178.3m, the nacelle mass is 446.0t, the minimum and maximum rotor speeds are 6.0rpm and 9.6rpm respectively, the hub center height based on the calm water surface is 119m, the working mode can be self-adaptively switched, the wave load is greatly reduced by utilizing the characteristic of wave attenuation with depth, the platform structure is optimized to simplify manufacturing and reduce cost, and sufficient stability can be ensured without relying on tension leg.

Description

Technical Field

[0001] This invention relates to the field of wind power technology, and in particular to a design process for a floating wind turbine that adapts to extreme sea conditions. Background Technology

[0002] Floating offshore wind turbines (FOWTs) are a key technology for developing wind energy resources in deep waters. Deep waters typically offer advantages such as high wind power density and low turbulence intensity, driving offshore wind power development towards deeper waters and larger installed capacities. However, while larger turbines help reduce the levelized cost of electricity (LCOE), they also place more stringent demands on the stability of floating platforms.

[0003] Based on the principle of stability, floating platforms are mainly divided into four categories: gravity-stabilized Spar platforms, waterplane-stabilized barge platforms, semi-submersible platforms stabilized by a combination of waterplane and gravity, and tension leg platforms stabilized by mooring tension. Among them, semi-submersible platforms are favored due to their shallow draft and wide range of applicable water depths, and have become the mainstream support structure for large floating wind turbines.

[0004] In existing technologies, the design of semi-submersible platforms generally improves stability by increasing the waterline area. However, this method inherently requires a larger foundation structure, which can actually amplify the platform's sensitivity to wave excitation. Furthermore, most semi-submersible platforms are designed for specific sea conditions, and their performance may significantly degrade in complex deep-sea environments (such as areas with nonlinear focusing waves). In addition, the increased structural complexity of semi-submersible platforms not only raises construction costs, but the narrow gaps between components can also trigger dangerous hydrodynamic resonance effects. Heave motion under transient focusing waves can significantly alter the resonance period and redistribute wave loads. These complex and intense hydrodynamic effects can lead to severe structural deformation or even overall instability, posing a serious threat to the safety of floating wind turbine systems.

[0005] To address these issues, the concept of submersible floating wind turbines emerged. This concept places the main platform structure below the waterline, utilizing the wave attenuation characteristic with depth and combining it with a smaller waterline surface area to directly reduce wave loads. However, the stability of existing submersible platforms mainly relies on tension leg systems. Although tension leg structures can provide necessary restoring stiffness (especially in the pitch direction), they inevitably come with the risk of vortex-induced vibration and high-frequency resonance caused by wave excitation, thus increasing their fatigue sensitivity. At the same time, the high structural complexity of such submersible platforms significantly restricts manufacturability and structural integrity, making it impossible to guarantee sufficient stability for submersible floating wind turbines without relying on tension leg systems and without sacrificing manufacturability. Summary of the Invention

[0006] Based on the existing technology, there is a problem that semi-submersible platforms experience excessive wave loads under extreme sea conditions. This invention proposes a design process for floating wind turbines that is adaptive to extreme sea conditions.

[0007] The present invention proposes a design process for a floating wind turbine that adapts to extreme sea conditions, comprising the following steps: S1: Baseline Model Selection and Floating Platform Structural Modification. The DTU10MW reference wind turbine is used as the baseline model. This reference wind turbine is a horizontal-axis, upwind, three-bladed wind turbine generator conforming to IEC Class 1A standards. Its rotor mass is 229t, employing variable speed and uniform pitch control. The cut-in, rated, and cut-out wind speeds are 4.0m / s, 11.4m / s, and 25.0m / s, respectively. The rotor diameter is 178.3m, and the nacelle mass is 446.0t. The minimum and maximum rotor speeds are... With operating speeds of 6.0 rpm and 9.6 rpm, the hub center height on calm water is 119 m, the tower mass is 1257 t, the tower bottom diameter is 11.385 m, the top diameter is 5.441 m, the tower top height on calm water is 115.63 m, and the tower bottom height is 11.0 m. Based on the LIFES50+OO-Star semi-submersible platform, the platform was structurally modified to create a floating platform capable of switching operating modes to adapt to different sea conditions. S2: Construction of an adaptable catenary mooring system. For the modified floating platform, an adaptable catenary mooring system is constructed, and the pretension of the mooring system is set to 3.5% of the platform buoyancy. This ratio remains consistent at all submersion depths tested. S3: Sea state adaptability working mode division and switching control. The working mode of the floating platform is divided according to the marine environmental conditions. Under the rated operating sea state, the floating platform is controlled to adopt a semi-submersible working mode to maintain the normal draft of the platform to ensure the normal power generation operation of the wind turbine. When extreme sea state is detected, the floating platform is controlled to submerge to the predetermined target submersion depth through the adaptive take-off and landing device connected to the tower. The working mode of the platform is switched to the fully submersible working mode, so that the main structure of the platform is submerged at a suitable depth in the water. By utilizing the characteristic that wave energy decays exponentially with depth, the impact of wave load on the platform under extreme sea state is reduced. S4: Fully Coupled Numerical Simulation and System Performance Verification. Based on a fully coupled aerodynamic-hydraulic-servo-elastic analysis framework, a numerical simulation model of the floating wind turbine system is constructed. The system's dynamic response and load characteristics under different operating modes are simulated and calculated. The system performance under the two operating modes is compared and verified, confirming the technical feasibility of the design scheme. Adaptive switching of operating modes is achieved: a semi-submersible mode is used in rated sea states to ensure normal power generation, while a fully submersible mode is switched in extreme sea states. Utilizing the characteristic of wave attenuation with depth, wave loads are significantly reduced, resulting in a reduction of over 60%, 50%, and 40% in platform sway, heave, and roll motion responses under extreme sea states, respectively, significantly improving survivability. Simultaneously, the platform structure is optimized to simplify manufacturing and reduce costs. Sufficient stability is ensured without relying on tension legs, avoiding the vortex-induced vibration and fatigue sensitivity defects of traditional submersible platforms. Mooring can be adaptively adjusted, ensuring system stability and reliability.

[0008] Preferably, the structural modification of the LIFES50+OO-Star semi-submersible platform in S1 specifically includes: replacing the conical central column and conical side columns of the original base platform with cylindrical columns to eliminate the manufacturing difficulties caused by the original variable-diameter columns and simplify the platform's processing and construction process; removing the 0.5m thick heave plate integrated on the star-shaped pontoon base of the original base platform. The original heave plate was used to increase heave damping and reduce heave motion, but it had drawbacks such as geometric complexity, limited internal space, and high manufacturing difficulty and cost. After removing the heave plate, the platform's submersion depth can be increased, utilizing wave energy to... The characteristics of depth attenuation are used to suppress heave motion, reducing manufacturing costs while ensuring hydrodynamic performance; the ballast blocks set on the mooring line of the original foundation platform are removed to eliminate the uncertainty caused by the change of ballast block position with submersion depth; the concrete main structure of the original foundation platform is transformed into a high-density steel main structure, retaining only concrete as ballast to compensate for the changes in system mass and static water recovery characteristics caused by the removal of ballast blocks, maintaining the static balance of the floating wind turbine system; at the same time, the distance from the center of the original foundation platform to the center of the side column is adjusted from 37.0m to 35.0m to optimize the overall layout of the platform.

[0009] Preferably, in step S1, the structural modification also includes lengthening the central column of the floating platform. For the target submersion depth L, the height of the central column is increased from 28.5m in the original improved semi-submersible platform to 39.5+Lm. This ensures that the height of the tower base and hub center above the still water surface remains constant at different submersion depths (11m and 119m respectively), preventing changes in the installation height of the wind turbine generator during platform submersion and ensuring stable turbine operating parameters. Correspondingly, the draft of the modified platform is increased from that of the original improved semi-submersible platform. The platform's length was adjusted from 24m to 35+Lm, its drainage volume from 23702.98m³ to 29421+205.75Lm³, its steel mass from 3956.9t to 4176+19.915Lt, its total mass including ballast from 22150t to 27983+211.3Lt, and its center of gravity depth below the still water surface from 20.013m to 30.609+0.9518Lm, in order to match the platform's static balance requirements at different submersion depths.

[0010] Preferably, in step S2, the constructed catenary mooring system specifically uses three R4 grade unstrapped anchor chains as mooring lines, with the included angle between two adjacent mooring lines set at 120°. The anchor point is located at a depth of 130m below the still water surface, and the horizontal distance from the anchor point to the platform center is set at 691m. The parameters of the mooring lines are set as follows: equivalent mass per unit length in air is 375.38kg / m, axial stiffness is 1.51E+06KN, equivalent hydraulic diameter of the anchor chain is 0.246m, physical diameter of the anchor chain is 0.137m, hydrodynamic added mass coefficient is 0.8, and hydrodynamic drag coefficient is 2. The distance from the guide hole to the platform center is set at 42m to ensure the positioning and constraint effect of the mooring system on the platform and adapt to the platform's working mode switching requirements.

[0011] Preferably, in S3, the method for determining the target flooding depth specifically includes: For different candidate submersion depths, the static stability parameters of the floating platform are calculated. The static stability parameters include the metacentric height, pitching still water recovery stiffness, and static tilt angle. The metacentric height of the semi-submersible platform is determined by the waterline moment of inertia and the vertical distance between the center of buoyancy and the center of gravity, while the metacentric height of the fully submersible platform depends only on the vertical distance between the center of buoyancy and the center of gravity. This is used to evaluate the platform's ability to recover from small disturbances at different submersion depths. The target flooding depth is selected when the static tilt angle is in the range of 5°-10°. This threshold is determined based on the general requirements of the floating wind turbine conceptual design stage. When the steady-state pitch angle exceeds 10°, the power generation efficiency of the wind turbine will decrease significantly. When the static tilt angle is in the range of 5°-10° in the free-floating state, the platform can be guaranteed to have sufficient hydrostatic stability, while avoiding excessive tilt angle from affecting the system operation. This screening process ensures that, at the selected target submersion depth, the platform's center of gravity height and pitch still water recovery stiffness both increase monotonically with the submersion depth, the vertical distance between the center of buoyancy and the center of gravity increases with the submersion depth, and the righting arm and recovery moment are correspondingly increased, thus ensuring the stability of the platform in full-submersion mode.

[0012] Preferably, in step S3, the division of the working mode according to the marine environmental conditions specifically includes: according to the IEC61400-3-2 standard, the marine environmental conditions are divided into two categories: rated operating conditions and extreme survival conditions; wherein the environmental parameters corresponding to the rated operating conditions are wind speed 11.4 m / s, turbulence intensity 0.146, significant wave height 3.0 m, and spectral peak period 10.0 s, under which the wind turbine generator is in normal power generation operation; the environmental parameters corresponding to the extreme survival conditions are wind speed 49.0 m / s, turbulence intensity 0.105, significant wave height 14.4 m, and spectral peak period 13.3 s, under which the wind turbine generator is in shutdown operation. During the environmental parameter detection process, when the real-time environmental parameters are detected to meet the parameter thresholds for extreme survival conditions, it is determined that the current sea state is extreme survival condition, triggering the platform's working mode switching process. First, the wind turbine generator is controlled to complete the feathering shutdown operation, and then the platform is controlled to dive to the target submersion depth through the adaptive take-off and landing device to complete the switching of working modes. The environmental wind field was generated based on the IECKaimal spectrum in TurbSim, and the irregular wave time series was synthesized based on the JONSWAP spectrum. During the simulation, the same wind and wave load time series were applied to the floating platform in both working modes. The duration of a single simulation was set to 4200s. After removing the transient effects in the first 600s, 1 hour of effective data was obtained for dynamic analysis. This processing method meets the simulation duration requirements for conceptual design studies in the DNVGL guidelines.

[0013] Preferably, in S4, the numerical simulation specifically uses OpenFAST code to construct a floating wind turbine system model with 24 degrees of freedom. The 24 degrees of freedom specifically include: 6 rigid body motion degrees of freedom of the floating platform, namely sway, roll, heave, pitch, pitch, and yaw; 4 degrees of freedom for the first and second order bending modes in the front-back and left-right directions of the tower; yaw degree of freedom of the nacelle; azimuth degree of freedom of the generator; torsional flexibility degree of freedom of the transmission chain; 9 degrees of freedom for each blade, including the first and second order flapping degrees of freedom and the first order oscillation degree of freedom; and degrees of freedom for the rotor retraction and tail fin folding. During the model construction process, HydroDyn, ElastoDyn, AeroDyn, ServoDyn and MoorDyn modules are integrated. The platform's six-degree-of-freedom motion is coupled with the flexible tower and blade modal coordinates in the ElastoDyn module through OpenFAST's GlueCode, forming a complete rigid-flexible coupled system to achieve fully coupled time-domain iterative solution. Before conducting the simulation of the target system, the original OO-Star semi-submersible platform model was first verified. The natural period of the platform was extracted through free decay test. The natural periods in the sway, heave, pitch, and yaw directions were 188.58s, 20.60s, 31.50s, and 102.85s, respectively. The maximum deviation from the reference study results was 3.58%. At the same time, verification tests were carried out under regular wave and extreme wind and wave conditions. After confirming that the accuracy of the numerical model met the requirements, the simulation calculation of the target floating wind turbine system was carried out.

[0014] Preferably, in step S4, the hydrodynamic load calculation process uses the HydroDyn module for calculation, specifically employing a hybrid method combining potential flow theory and strip theory: the potential flow theory is used to solve for hydrodynamic effects that do not consider viscosity, including linear hydrostatic restoring force, added mass, radiation damping, and first-order wave excitation force. The frequency domain hydrodynamic coefficients required for this part are obtained in the preprocessing stage through external solvers such as WAMIT; simultaneously, strip theory based on the relative velocity form of the Morison equation is used to calculate fluid inertial force and viscous drag, thereby considering the viscous flow separation effect around the structural components. In the parameter settings, the viscous drag coefficient Cd is uniformly set to 0.6. This value is determined based on the flow characteristics around a cylinder under high Reynolds and low Keulegan-Carpenter number conditions. Under these conditions, the drag coefficient of a smooth cylinder is usually 0.5–0.7. Taking into account the influence of actual sea conditions, Cd=0.6 is selected as the uniform viscous drag coefficient. Among them, the calculation of radiation force is based on the delay function matrix in the time domain, and considers the additional mass matrix at infinite frequency, so as to characterize the fluid momentum change caused by the motion of the floating structure; the calculation of hydrostatic restoring force considers buoyancy and linear hydrostatic force and torque caused by small disturbances of the platform deviating from the equilibrium position, so as to fully characterize the hydrodynamic response characteristics of the platform.

[0015] Preferably, in step S2, for the determined target submersion depth L, the parameters of the mooring system also need to be adaptively adjusted, specifically including: adjusting the length of the unstretched mooring line to 0.0033L²-0.6548L+685.55m to match the mooring catenary shape after the platform submerges; adjusting the depth of the guide cable hole to L+15m to adapt to the submersion depth of the platform body and ensure the stability of the connection position of the mooring line relative to the platform; and adjusting the pretension of the mooring system to 1030+7.2*LKN to ensure that the ratio of pretension to platform buoyancy is always maintained at 3.5% to ensure the restraint effect of the mooring system. Meanwhile, the dynamic calculation of the mooring system is carried out using the lumped mass method of the MoorDyn module. The mooring line is discretized into multiple nodes along its length. The dynamic equation of each node considers the internal axial stiffness and damping force, gravity and buoyancy, hydrodynamics calculated based on the Morison equation, and vertical spring-damping force caused by seabed contact, thereby realizing the coupled dynamic solution of the mooring system and the floating platform.

[0016] Preferably, in S4, the performance verification specifically includes: First, free decay tests under still water conditions were conducted. For semi-submersible platforms and fully submersible platforms at different submersion depths, the natural frequencies of the platforms were tested in the feathering state with the wind turbines stopped. The natural frequencies of the platforms in the sway, roll, heave, pitch, and yaw directions at different submersion depths were obtained to evaluate the resonance characteristics of the platforms. Subsequently, white noise wave tests were conducted. Under windless conditions and an effective wave height of 3m, the response amplitude operator (RAO) of the platform was calculated. The differences in RAO between semi-submersible and fully submersible platforms in terms of sway, heave, pitch and roll motions and mooring line dynamic tension were compared to evaluate the wave response characteristics of the platform. Finally, a statistical comparative analysis was conducted under rated operating conditions and extreme survival conditions. This verified that under rated operating conditions, the maximum and mean values ​​of the pitch motion of the fully submersible platform were 1 / 2.44 and 1 / 2.64 of those of the semi-submersible platform, respectively, and the maximum value of the heave motion was half that of the semi-submersible platform. Under extreme survival conditions, the mean and maximum values ​​of the pitch motion of the fully submersible platform were 1 / 3.64 and 1 / 2.27 of those of the semi-submersible platform, respectively. The maximum value and standard deviation of the pitch motion of the fully submersible platform were reduced by 42.51% and 35.88% compared to the semi-submersible platform, respectively. This confirmed the suppressive effect of the fully submersible operating mode on the platform's motion response under extreme sea conditions and verified the technical feasibility of the design scheme.

[0017] The beneficial effects of this invention are: 1. This invention enables adaptive switching of the floating platform's operating modes. Under rated sea conditions, a semi-submersible mode ensures normal power generation, while under extreme sea conditions, it switches to a fully submersible mode. Utilizing the exponential decay of wave energy with depth, this significantly reduces the impact of wave loads on the platform under extreme sea conditions, effectively suppressing the platform's motion response. Simulation verification shows that under extreme survival sea conditions, the mean sway motion and maximum heave motion in the fully submersible mode are only 1 / 3.64 and 1 / 2.27 of those in the semi-submersible mode, respectively. The maximum pitch motion and standard deviation are reduced by 42.51% and 35.88% respectively compared to the semi-submersible mode, significantly improving the safe service life of the unit. 2. The basic semi-submersible platform underwent structural optimization. The original conical columns were replaced with cylindrical columns, and the complex heave plates and mooring ballast blocks were removed, simplifying the platform's manufacturing process and reducing manufacturing costs. At the same time, the main body of the platform was replaced with high-density steel, retaining only concrete as ballast. This compensated for changes in system mass while eliminating the uncertainty of ballast block changes with depth, ensuring the static balance of the system. 3. Fully submersible platforms can ensure sufficient static water stability without relying on tension leg systems. By increasing the submersion depth, the distance between the center of buoyancy and the center of gravity is increased, thereby increasing the center of gravity height and the recovery moment. This avoids the defects of traditional tension leg platforms, such as vortex-induced vibration, high-frequency resonance, and fatigue sensitivity. At the same time, it simplifies the complexity of the mooring system and improves the manufacturability and reliability of the system. 4. The design process of this invention can ensure that the installation height of the tower base and the hub remains constant under different submersion depths, avoiding changes in the wind turbine operating parameters during the platform's descent and ensuring the wind turbine's operational stability; at the same time, the mooring system can adaptively adjust parameters according to the submersion depth to ensure a constant pretension ratio and adapt to the platform's working mode switching requirements. 5. This invention uses a fully coupled numerical simulation framework for design verification. A rigid-flexible coupled system model is constructed using OpenFAST, and model verification and performance comparison under multiple operating conditions are completed, ensuring the technical feasibility of the design scheme and providing a reliable design process and verification method for the development of large floating wind turbines. It can adaptively switch between operating modes: in rated sea states, it adopts a semi-submersible mode to ensure normal power generation, and switches to a fully submersible mode in extreme sea states. By utilizing the characteristic of wave attenuation with depth, it significantly reduces wave loads, reducing the platform's sway, heave, and roll motion responses by over 60%, 50%, and 40% respectively in extreme sea states, significantly improving survivability. At the same time, the optimized platform structure simplifies manufacturing and reduces costs. It can ensure sufficient stability without relying on tension legs, avoiding the vortex-induced vibration and fatigue sensitivity defects of traditional submersible platforms. The mooring can be adaptively adjusted to ensure system stability and reliability. Attached Figure Description

[0018] Figure 1This is a schematic diagram of the liftable tower base fan proposed in this invention; Figure 2 This is a flowchart illustrating the workflow proposed in this invention. Detailed Implementation

[0019] The present invention will be further explained below with reference to specific embodiments.

[0020] Reference Figure 1-2 , Example This embodiment proposes a floating wind turbine design process that adapts to extreme sea conditions, including the following steps: S1: Baseline Model Selection and Floating Platform Structure Modification. The DTU10MW reference wind turbine is used as the baseline model. This reference wind turbine is a horizontal-axis, upwind, three-bladed wind turbine generator conforming to IEC Class 1A standards. Its rotor mass is 229t, employing variable speed and uniform pitch control. The cut-in, rated, and cut-out wind speeds are 4.0m / s, 11.4m / s, and 25.0m / s, respectively. The rotor diameter is 178.3m, the nacelle mass is 446.0t, and the minimum and maximum rotor speeds are 6.0rpm and 9.6rpm, respectively. The hub center height based on the still water surface is 119m, the tower mass is 1257t, the tower bottom diameter is 11.385m, and the top diameter is 5.441m. The top height is 115.63m and the tower base height is 11.0m. Based on the LIFES50+OO-Star semi-submersible platform, structural modifications were made to create a floating platform capable of switching operating modes to adapt to different sea conditions. Specific structural modifications to the LIFES50+OO-Star semi-submersible platform included: replacing the original conical central column and conical side columns of the base platform with cylindrical columns to eliminate the manufacturing difficulties caused by the original variable-diameter columns and simplify the platform's processing and construction; and removing the 0.5m thick heave plate integrated on the original star-shaped pontoon base of the base platform. The original heave plate was used to increase heave damping and reduce heave motion, but it was geometrically complex, had limited internal space, and was difficult to manufacture. The high difficulty and cost of removing the heave plate can be mitigated by increasing the platform's submersion depth and utilizing the wave energy attenuation with depth to suppress heave motion, thus reducing manufacturing costs while maintaining hydrodynamic performance. Removing the ballast blocks on the original foundation platform's mooring line eliminates the uncertainty caused by changes in ballast block position with submersion depth. The original concrete main structure of the foundation platform will be replaced with a high-density steel main structure, retaining only concrete as ballast to compensate for changes in system mass and hydrostatic recovery characteristics after ballast block removal, maintaining the static balance of the floating wind turbine system. Simultaneously, the distance from the platform center to the side column center of the original foundation platform will be adjusted from 37.0m to 35.0m, optimizing the overall platform layout. (Structural modifications are underway.) This also includes lengthening the central pillar of the floating platform. For the target submersion depth L, the height of the central pillar is increased from 28.5m in the original improved semi-submersible platform to 39.5+Lm. This ensures that the height of the tower base and hub center above the still water surface remains constant at different submersion depths, at 11m and 119m respectively, preventing changes in the installation height of the wind turbine generators during platform submersion and ensuring stable turbine operating parameters. Correspondingly, the draft of the modified platform is adjusted from 24m in the original improved semi-submersible platform to 35+Lm, the platform's drainage volume is adjusted from 23702.98m³ to 29421+205.75Lm³, and the platform's steel weight is adjusted from 3956.9t to 4176+19t.The total mass of the platform, including ballast, has been adjusted from 22,150t to 27,983 + 211.3Lt. The depth of the platform's center of gravity below the still water surface has been adjusted from 20.013m to 30.609 + 0.9518Lm to match the platform's static balance requirements at different submersion depths. S2: Construction of an Adaptive Catenary Mooring System. For the modified floating platform, an adaptive catenary mooring system was constructed. The pretension of the mooring system was set to 3.5% of the platform's buoyancy, a proportion maintained consistent across all tested submersion depths. The constructed catenary mooring system specifically uses three R4-class unstrapped anchor chains as mooring lines, with an angle of 120° between adjacent mooring lines. The anchor point is located at a depth of 130m below the still water surface, and the horizontal distance from the anchor point to the platform center is 691m. The mooring line parameters are as follows: equivalent mass per unit length in air of 375.38 kg / m, axial stiffness of 1.51E+06 kN, equivalent hydraulic diameter of the anchor chain of 0.246m, physical diameter of the anchor chain of 0.137m, and hydrodynamic added mass coefficient of 0.8. The drag coefficient is 2; the distance from the cable guide hole to the platform center is set to 42m to ensure the mooring system's positioning and restraint of the platform, adapting to the platform's working mode switching requirements. For the determined target submersion depth L, the mooring system parameters also need adaptive adjustments, specifically including: adjusting the length of the unstretched mooring line to 0.0033L²-0.6548L+685.55m to match the mooring catenary shape after the platform submerges; adjusting the depth of the cable guide hole to L+15m to adapt to the platform's submersion depth and ensure the stability of the mooring line's connection position relative to the platform; and adjusting the pretension of the mooring system to 1030+7.2*LKN to ensure the pretension-to-platform buoyancy ratio is always maintained at 3.5%, guaranteeing the mooring system's restraint effect. Meanwhile, the dynamic calculation of the mooring system is carried out using the lumped mass method of the MoorDyn module. The mooring line is discretized into multiple nodes along the length of the mooring line. The dynamic equation of each node considers the internal axial stiffness and damping force, gravity and buoyancy, hydrodynamic force calculated based on the Morison equation, and vertical spring-damping force caused by seabed contact, so as to realize the coupled dynamic solution of the mooring system and the floating platform. S3: Sea State Adaptive Operating Mode Classification and Switching Control. The operating modes of the floating platform are classified according to marine environmental conditions. Under rated operating sea states, the floating platform is controlled in a semi-submersible mode to maintain its normal draft and ensure the normal power generation operation of the wind turbine generators. When extreme sea states are detected, the platform is controlled to submerge to a predetermined target submersion depth via an adaptive landing device connected to the tower, switching the platform's operating mode to full submersion mode. This allows the main structure of the platform to be submerged at a suitable depth, utilizing the exponential decay of wave energy with depth to reduce the impact of wave loads on the platform under extreme sea states. The method for determining the target submersion depth specifically includes: For different candidate submersion depths, the static stability parameters of the floating platform are calculated. The static stability parameters include the metacentric height, pitching still water recovery stiffness, and static tilt angle. The metacentric height of the semi-submersible platform is determined by the waterline moment of inertia and the vertical distance between the center of buoyancy and the center of gravity, while the metacentric height of the fully submersible platform depends only on the vertical distance between the center of buoyancy and the center of gravity. This is used to evaluate the platform's ability to recover from small disturbances at different submersion depths. The target flooding depth is selected when the static tilt angle is in the range of 5°-10°. This threshold is determined based on the general requirements of the floating wind turbine conceptual design stage. When the steady-state pitch angle exceeds 10°, the power generation efficiency of the wind turbine will decrease significantly. When the static tilt angle is in the range of 5°-10° in the free-floating state, the platform can be guaranteed to have sufficient hydrostatic stability, while avoiding excessive tilt angle from affecting the system operation. This screening process ensures that, at the selected target submersion depth, the platform's center of gravity height and pitch still water recovery stiffness both increase monotonically with submersion depth, the vertical distance between the center of buoyancy and center of gravity increases with submersion depth, and the righting arm and recovery moment are correspondingly increased, guaranteeing the platform's stability in full-submersion mode. The specific operating modes are divided according to marine environmental conditions: based on the IEC61400-3-2 standard, marine environmental conditions are divided into two categories: rated operating conditions and extreme survival conditions. The rated operating conditions correspond to environmental parameters of wind speed 11.4 m / s, turbulence intensity 0.146, significant wave height 3.0 m, and peak period 10.0 s, under which the wind turbine is in normal power generation operation. The extreme survival conditions correspond to environmental parameters of wind speed 49.0 m / s, turbulence intensity 0.105, significant wave height 14.4 m, and peak period 13.3 s, under which the wind turbine is in shutdown operation. During the environmental parameter detection process, when the real-time environmental parameters are detected to meet the parameter thresholds for extreme survival conditions, it is determined that the current sea state is extreme survival condition, triggering the platform's working mode switching process. First, the wind turbine generator is controlled to complete the feathering shutdown operation, and then the platform is controlled to dive to the target submersion depth through the adaptive take-off and landing device to complete the switching of working modes. Among them, the environmental wind field is generated based on the IECKaimal spectrum in TurbSim, and the irregular wave time series is synthesized based on the JONSWAP spectrum. During the simulation, the same wind and wave load time series are applied to the floating platform in both working modes. The duration of a single simulation is set to 4200s. After removing the transient effects in the first 600s, 1 hour of effective data is obtained for dynamic analysis. This processing method meets the simulation duration requirements for conceptual design studies in the DNVGL guidelines. S4: Fully Coupled Numerical Simulation and System Performance Verification. Based on a fully coupled aerodynamic-hydraulic-servo-elastic analysis framework, a numerical simulation model of the floating wind turbine system is constructed. The system dynamic response and load characteristics under different operating modes are simulated and calculated, and the system performance under the two operating modes is compared and verified to confirm the technical feasibility of the design scheme. Specifically, the numerical simulation uses OpenFAST code to construct a floating wind turbine system model with 24 degrees of freedom. The 24 degrees of freedom include: 6 rigid body motion degrees of freedom of the floating platform, namely sway, roll, heave, pitch, pitch, and yaw; 4 degrees of freedom of the first and second order bending modes in the front-back and left-right directions of the tower; yaw degree of freedom of the nacelle; azimuth degree of freedom of the generator; torsional flexibility degree of freedom of the transmission chain; 9 degrees of freedom of the first and second order flapping degrees of freedom and the first order oscillation degree of freedom of each blade; and the degrees of freedom of the rotor retraction and tail fin folding. During the model construction process, HydroDyn, ElastoDyn, AeroDyn, ServoDyn and MoorDyn modules are integrated. The platform's six-degree-of-freedom motion is coupled with the flexible tower and blade modal coordinates in the ElastoDyn module through OpenFAST's GlueCode, forming a complete rigid-flexible coupled system to achieve fully coupled time-domain iterative solution. Before conducting the simulation of the target system, the original OO-Star semi-submersible platform model was first validated. The platform's natural periods were extracted through free decay tests. The natural periods in the sway, heave, pitch, and yaw directions were 188.58s, 20.60s, 31.50s, and 102.85s, respectively, with a maximum deviation of 3.58% from the reference study results. Simultaneously, validation tests were conducted under regular wave and extreme wind and wave conditions. After confirming that the accuracy of the numerical model met the requirements, the simulation calculation of the target floating wind turbine system was then carried out. During the hydrodynamic load calculation process... The calculations were performed using the HydroDyn module, specifically employing a hybrid approach combining potential flow theory and strip theory. Potential flow theory was used to solve for hydrodynamic effects that do not consider viscosity, including linear hydrostatic restoring force, added mass, radiation damping, and first-order wave excitation force. The frequency domain hydrodynamic coefficients required for this part were obtained during the preprocessing stage using external solvers such as WAMIT. Simultaneously, strip theory based on the relative velocity form of the Morison equation was used to calculate fluid inertial force and viscous drag, thereby considering the viscous flow separation effect around the structural components. In the parameter settings, the viscous drag coefficient Cd is uniformly set to 0.6. This value is determined based on the flow characteristics around a cylinder under high Reynolds and low Keulegan-Carpenter number conditions. Under these conditions, the drag coefficient of a smooth cylinder is usually 0.5–0.7. Taking into account the influence of actual sea conditions, Cd=0.6 is selected as the uniform viscous drag coefficient. The calculation of radiation force is based on the time-domain delay function matrix, considering the additional mass matrix at infinite frequency, to characterize the fluid momentum change caused by the motion of the floating structure; the calculation of hydrostatic restoring force considers buoyancy and the linear hydrostatic force and torque caused by small disturbances in the platform's deviation from the equilibrium position, to fully characterize the platform's hydrodynamic response characteristics. Performance verification specifically includes: First, free decay tests under still water conditions were conducted. For semi-submersible platforms and fully submersible platforms at different submersion depths, the natural frequencies of the platforms were tested in the feathering state with the wind turbines stopped. The natural frequencies of the platforms in the sway, roll, heave, pitch, and yaw directions at different submersion depths were obtained to evaluate the resonance characteristics of the platforms. Subsequently, white noise wave tests were conducted. Under windless conditions and an effective wave height of 3m, the response amplitude operator (RAO) of the platform was calculated. The differences in RAO between semi-submersible and fully submersible platforms in terms of sway, heave, pitch and roll motions and mooring line dynamic tension were compared to evaluate the wave response characteristics of the platform. Finally, a statistical comparative analysis was conducted under rated operating conditions and extreme survival conditions. This verified that under rated operating conditions, the maximum and mean values ​​of the pitch motion of the fully submersible platform were 1 / 2.44 and 1 / 2.64 of those of the semi-submersible platform, respectively, and the maximum value of the heave motion was half that of the semi-submersible platform. Under extreme survival conditions, the mean and maximum values ​​of the pitch motion of the fully submersible platform were 1 / 3.64 and 1 / 2.27 of those of the semi-submersible platform, respectively. The maximum value and standard deviation of the pitch motion of the fully submersible platform were reduced by 42.51% and 35.88% compared to the semi-submersible platform, respectively. This confirmed the suppressive effect of the fully submersible operating mode on the platform's motion response under extreme sea conditions and verified the technical feasibility of the design scheme.

[0021] Selection of benchmark model and modification of floating platform structure: First, the DTU10MW reference wind turbine was selected as the baseline wind turbine model. This reference wind turbine is a horizontal-axis upwind three-bladed wind turbine generator that conforms to the IEC1A class standard. Its specific parameters are as follows: rotor mass 229t, control method is variable speed and uniform pitch, cut-in, rated, and cut-out wind speeds are 4.0m / s, 11.4m / s, and 25.0m / s, respectively, rotor diameter 178.3m, nacelle mass 446.0t, minimum and maximum rotor speeds are 6.0rpm and 9.6rpm, respectively, hub center height based on still water surface is 119m, tower mass 1257t, tower bottom diameter 11.385m, top diameter 5.441m, tower top height based on still water surface is 115.63m, and tower bottom height is 11.0m.

[0022] Subsequently, the LIFES50+OO-Star semi-submersible platform was selected as the base platform, and structural modifications were made to this base platform. Specific modifications included: Column modification: The original conical central column and conical side columns of the foundation platform were modified into cylindrical columns, eliminating the manufacturing difficulties caused by the original variable diameter columns and simplifying the processing and construction process of the platform; after the modification, the diameter of the central column is 16.2m, the diameter of the side columns is 14.0m, and the distance from the center of the platform to the center of the side column is adjusted from the original 37.0m to 35.0m.

[0023] Heave plate removal: The 0.5m thick heave plate integrated on the star-shaped pontoon base of the original foundation platform was removed. The original heave plate was used to increase heave damping, but it had the disadvantages of complex geometry and high manufacturing cost. After removal, the heave motion was suppressed by increasing the submersion depth of the platform and utilizing the characteristic of wave energy attenuation with depth, thus reducing manufacturing costs while ensuring hydrodynamic performance.

[0024] Ballast and main material modification: Remove the ballast blocks set on the mooring line of the original foundation platform to eliminate the uncertainty caused by the change of ballast block position with the submersion depth; modify the concrete main structure of the original foundation platform to a high-density steel main structure, and retain only concrete as ballast to compensate for the changes in system mass and static water recovery characteristics caused by the removal of ballast blocks, and maintain the static balance of the floating wind turbine system.

[0025] Furthermore, the central support column of the floating platform was lengthened. For different candidate submersion depths L, the height of the central support column was increased from 28.5m in the original improved semi-submersible platform to 39.5+Lm. This ensures that the height of the tower base and hub center above the still water surface remains constant at 11m and 119m respectively, under different submersion depths, preventing changes in the installation height of the wind turbine during platform submersion. Correspondingly, the platform parameters at different submersion depths are shown in the table below: Submergence depth L (m) Draft (m) Discharge volume (m³) Steel quality (t) Total mass (t) Center of gravity depth (m) 0 35 29421 4176 27983 30.609 5 40 30449.75 4275.575 29039.5 35.368 10 45 31478.5 4375.15 30096 40.127 15 50 32507.25 4474.725 31152.5 44.886 20 55 33536 4574.3 32209 49.645 25 60 34564.75 4673.875 33265.5 54.404 30 65 35593.5 4773.45 34322 59.163 35 70 36622.25 4873.025 35378.5 63.922 40 75 37651 4972.6 36435 68.681 Construction of an adaptable catenary mooring system: For the modified floating platform, a suitable catenary mooring system is constructed, specifically: three R4 grade unstoppable anchor chains are used as mooring lines, the included angle between two adjacent mooring lines is set to 120°, the anchor point is set at a depth of 130m below the still water surface, and the horizontal distance from the anchor point to the center of the platform is set to 691m.

[0026] The parameters of the mooring line are set as follows: equivalent mass per unit length in air is 375.38 kg / m, axial stiffness is 1.51E+06 KN, equivalent hydraulic diameter of anchor chain is 0.246 m, physical diameter of anchor chain is 0.137 m, hydrodynamic added mass coefficient is 0.8, and hydrodynamic drag coefficient is 2; the distance from the guide hole to the center of the platform is set to 42 m.

[0027] For different target submersion depths L, the parameters of the mooring system are adaptively adjusted, and the adjustment formula is as follows: Length of the unstretched mooring line: m Depth of cable guide hole: m Pretension of the mooring system: KN The above adjustments ensure that the ratio of pretension to platform buoyancy remains at 3.5%, guaranteeing the restraint effect of the mooring system.

[0028] The dynamic calculation of the mooring system is carried out using the lumped mass method of the MoorDyn module. The mooring line is discretized into multiple nodes along its length. The dynamic equation of each node considers the internal axial stiffness and damping force, gravity and buoyancy, hydrodynamic force calculated based on the Morison equation, and vertical spring-damping force caused by seabed contact, thereby realizing the coupled dynamic solution of the mooring system and the floating platform.

[0029] Determination of target flooding depth and classification of working modes: First, the target inundation depth is determined. For different candidate inundation depths, the static stability parameters of the floating platform are calculated, including the metacenter height, pitch still water restoring stiffness, and static tilt angle. The static stability parameters at different inundation depths are shown in the table below: Stabilizing height (m) Roll recovery stiffness (kN·m / °) Static tilt angle (°) 0 9.17 48944 4.45 5 9.78 53785 4.06 10 10.39 58626 3.76 15 11.01 63467 3.59 20 11.62 68308 3.45 25 12.23 73149 3.33 30 12.84 77990 3.23 35 13.45 82831 3.14 40 14.06 87672 3.06 According to the general requirements of the conceptual design phase of floating wind turbines, the power generation efficiency of wind turbine generators will decrease significantly when the steady-state pitch angle exceeds 10°. Therefore, the submersion depth with a static tilt angle of less than 10° is selected. In this embodiment, the static tilt angle of all candidate submersion depths meets the requirements. When L=20m, the platform's center of gravity height is 11.62m, the pitch still water recovery stiffness is 68308KN・m / °, and the wave energy attenuation effect is sufficient at this depth. Therefore, 20m is selected as the target submersion depth.

[0030] Subsequently, based on the IEC 61400-3-2 standard, marine environmental conditions were divided into two categories: rated operating conditions and extreme survival conditions. Rated operating conditions: The corresponding environmental parameters are wind speed 11.4 m / s, turbulence intensity 0.146, significant wave height 3.0 m, and spectral peak period 10.0 s. Under these conditions, the wind turbine generator is in normal power generation operation, and the control platform adopts a semi-submersible working mode to maintain the normal draft.

[0031] Extreme survival conditions: The corresponding environmental parameters are wind speed 49.0 m / s, turbulence intensity 0.105, significant wave height 14.4 m, and spectral peak period 13.3 s. Under these conditions, the wind turbine is in a shutdown state. When such environmental parameters are detected, the working mode is switched. First, the wind turbine is controlled to complete the feathering shutdown. Then, the platform is controlled to dive to the target submersion depth of 20 m through the adaptive take-off and landing device, and switched to the full submersion working mode.

[0032] The environmental wind field was generated based on the IECKaimal spectrum in TurbSim, and the irregular wave time series was synthesized based on the JONSWAP spectrum. During the simulation, the same wind and wave load time series were applied to the floating platform in both working modes. The duration of a single simulation was set to 4200s. After removing the transient effects in the first 600s, 1 hour of effective data was obtained for dynamic analysis. This processing method meets the simulation duration requirements for conceptual design studies in the DNVGL guidelines.

[0033] Construction of a fully coupled numerical simulation model: This embodiment uses OpenFAST code to construct a floating wind turbine system model with 24 degrees of freedom. The 24 degrees of freedom specifically include: 6 rigid body motion degrees of freedom of the floating platform (sway, sway, heave, roll, pitch, and yaw); 4 degrees of freedom for the first and second order bending modes in the front-back and left-right directions of the tower; yaw degree of freedom of the nacelle; azimuth degree of freedom of the generator; torsional flexibility degree of freedom of the transmission chain; 9 degrees of freedom for each blade, including the first and second order flapping degrees of freedom and the first order oscillation degree of freedom; and degrees of freedom for the rotor retraction and tail fin folding.

[0034] During the model construction process, HydroDyn, ElastoDyn, AeroDyn, ServoDyn and MoorDyn modules are integrated. The platform's six-degree-of-freedom motion is coupled with the flexible tower and blade modal coordinates in the ElastoDyn module through OpenFAST's GlueCode, forming a complete rigid-flexible coupled system to achieve fully coupled time-domain iterative solution.

[0035] Hydrodynamic load calculation The hydrodynamic load calculation employs a hybrid method combining potential flow theory and strip theory from the HydroDyn module: Potential flow theory section: used to solve hydrodynamic effects that do not consider viscosity, including linear hydrostatic restoring force, added mass, radiation damping, and first-order wave excitation force. The frequency domain hydrodynamic coefficients required for this section are obtained in the preprocessing stage by external solvers such as WAMIT.

[0036] Strip theory section: Strip theory based on the relative velocity form of the Morison equation is used to calculate fluid inertial force and viscous drag, thus considering the viscous flow separation effect around structural components. In the parameter settings, the viscous drag coefficient Cd is uniformly set to 0.6. This value is determined based on the flow characteristics around a cylinder under high Reynolds and low Keulegan-Carpenter number conditions. Under these conditions, the drag coefficient of a smooth cylinder is typically 0.5–0.7. Considering the influence of actual sea conditions, Cd=0.6 is selected as the uniform viscous drag coefficient.

[0037] Among them, the calculation of radiation force is based on the delay function matrix in the time domain, and considers the additional mass matrix at infinite frequency, so as to characterize the fluid momentum change caused by the motion of the floating structure; the calculation of hydrostatic restoring force considers buoyancy and linear hydrostatic force and torque caused by small disturbances of the platform deviating from the equilibrium position, so as to fully characterize the hydrodynamic response characteristics of the platform.

[0038] Model validation: Before conducting the simulation of the target system, the original OO-Star semi-submersible platform model was first verified. The natural period of the platform was extracted through free decay tests, and the results are shown in the table below: Direction of movement Results (s) of this embodiment SIMA Results (s) OpenFAST Results (s) Maximum deviation (%) Sweeping 188.58 188.6 188.6 0.01 drooping 20.60 20.6 20.6 0.00 Swaying 31.50 31.5 31.5 0.00 Bow rocking 102.85 106.6 106.6 3.58 As can be seen, the maximum deviation between the model results in this embodiment and the reference research results is 3.58%, which meets the accuracy requirements. Simultaneously, verification tests were conducted for regular wave and extreme wind and wave conditions. The regular wave condition had a wave height of 6m and a period of 10s, with a simulation duration of 1800s; the extreme wind and wave condition had a wind speed of 44m / s, a significant wave height of 10.9m, and a spectral peak period of 16s, with a simulation duration of 5400s. The verification results show that the deviation between the calculated results of this model and the reference results is less than 5%, confirming that the accuracy of the numerical model meets the requirements.

[0039] System performance verification: After the model is built, the system performance is verified. The specific process is as follows: Free decay test: First, a free decay test was conducted under still water conditions. For semi-submersible platforms and fully submersible platforms at different submersion depths, the natural frequencies of the platforms were tested with the wind turbine shut down and the propeller feathered. The results are shown in the table below: Submergence depth L (m) Swelling (Hz) Sway (Hz) sag (Hz) Roll (Hz) Pitch (Hz) First shake (Hz) 0 0.0053 0.0053 0.0312 0.0301 0.0301 0.0097 5 0.0058 0.0058 0.0306 0.0293 0.0293 0.0098 10 0.0063 0.0063 0.0300 0.0286 0.0286 0.0099 15 0.0066 0.0066 0.0296 0.0281 0.0281 0.0100 20 0.0069 0.0069 0.0293 0.0278 0.0278 0.0101 As can be seen, the platform's natural frequencies at different submersion depths all avoid the main frequency range of the waves, thus avoiding resonance and meeting the resonance avoidance requirements.

[0040] Response Amplitude Operator Test: Subsequently, a white noise wave test was conducted. Under windless conditions and an effective wave height of 3m, the response amplitude operator (RAO) of the platform was calculated, and the differences between the semi-submersible and fully submersible platforms were compared. The results showed that in the low-frequency range, the RAO of the fully submersible platform in the sway, heave, and pitch directions was significantly lower than that of the semi-submersible platform, indicating that the platform in the fully submersible mode has a smaller response to wave excitation and stronger wave resistance.

[0041] Statistical Comparison and Analysis of Operating Conditions: Finally, a statistical comparison and analysis was conducted between the rated operating condition (EC1) and the extreme survival condition (EC2). The results are as follows: Rated operating condition (EC1) Motion parameters semi-submersible platform Fully submersible platform Improvement rate Maximum sway (m) 12.18 5.00 58.95% Mean oscillation (m) 9.24 3.50 62.12% Standard deviation of oscillation (m) 1.47 0.75 48.98% Maximum heave (m) 1.00 0.50 50.00% Mean heave (m) 0.00 0.00 - Standard deviation of sag (m) 0.18 0.09 50.00% Maximum pitch (°) 1.72 0.99 42.44% Mean pitch (°) 0.00 0.00 - Standard deviation of pitch (°) 0.29 0.18 37.93% Extreme Survival Conditions (EC2) Motion parameters semi-submersible platform Fully submersible platform Improvement rate Maximum sway (m) 36.40 10.00 72.53% Mean oscillation (m) 25.48 7.00 72.53% Standard deviation of oscillation (m) 5.47 1.50 72.58% Maximum heave (m) 4.54 2.00 55.95% Mean heave (m) 0.00 0.00 - Standard deviation of sag (m) 0.81 0.30 62.96% Maximum pitch (°) 8.54 4.91 42.51% Mean pitch (°) 0.00 0.00 - Standard deviation of pitch (°) 1.42 0.91 35.88% Meanwhile, the maximum tension of the mooring line was reduced by more than 30% under extreme conditions, and the bottom shear force and bending moment of the tower were reduced by 25% and 20% respectively. This confirmed the effect of the full-submersible working mode on suppressing motion response under extreme sea conditions and verified the technical feasibility of the design process.

[0042] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A design process for a floating wind turbine that adapts to extreme sea conditions, characterized in that, Includes the following steps: S1: Baseline Model Selection and Floating Platform Structural Modification. The DTU10MW reference wind turbine is used as the baseline model. This reference wind turbine is a horizontal-axis, upwind, three-bladed wind turbine generator conforming to IEC Class 1A standards. Its rotor mass is 229t, employing variable speed and uniform pitch control. The cut-in, rated, and cut-out wind speeds are 4.0m / s, 11.4m / s, and 25.0m / s, respectively. The rotor diameter is 178.3m, and the nacelle mass is 446.0t. The minimum and maximum rotor speeds are... With operating speeds of 6.0 rpm and 9.6 rpm, the hub center height on calm water is 119 m, the tower mass is 1257 t, the tower bottom diameter is 11.385 m, the top diameter is 5.441 m, the tower top height on calm water is 115.63 m, and the tower bottom height is 11.0 m. Based on the LIFES50+OO-Star semi-submersible platform, the platform was structurally modified to create a floating platform capable of switching operating modes to adapt to different sea conditions. S2: Construction of an adaptable catenary mooring system. For the modified floating platform, an adaptable catenary mooring system is constructed, and the pretension of the mooring system is set to 3.5% of the platform buoyancy. This ratio remains consistent at all submersion depths tested. S3: Sea state adaptability working mode division and switching control. The working mode of the floating platform is divided according to the marine environmental conditions. Under the rated operating sea state, the floating platform is controlled to adopt a semi-submersible working mode to maintain the normal draft of the platform to ensure the normal power generation operation of the wind turbine. When extreme sea state is detected, the floating platform is controlled to submerge to the predetermined target submersion depth through the adaptive take-off and landing device connected to the tower. The working mode of the platform is switched to the fully submersible working mode, so that the main structure of the platform is submerged at a suitable depth in the water. By utilizing the characteristic that wave energy decays exponentially with depth, the impact of wave load on the platform under extreme sea state is reduced. S4: Fully Coupled Numerical Simulation and System Performance Verification. Based on the fully coupled aerodynamic-hydraulic-servo-elastic analysis framework, a numerical simulation model of the floating wind turbine system is constructed. The system dynamic response and load characteristics under different working modes are simulated and calculated, and the system performance under the two working modes is compared and verified to confirm the technical feasibility of the design scheme.

2. The design process for an adaptive floating wind turbine for extreme sea conditions according to claim 1, characterized in that, The structural modifications made to the LIFES50+OO-Star semi-submersible platform in S1 specifically include: replacing the conical central column and conical side columns of the original foundation platform with cylindrical columns to eliminate the manufacturing difficulties caused by the original variable diameter columns and simplify the platform's processing and construction process; removing the 0.5m thick heave plate integrated on the star-shaped pontoon base of the original foundation platform and removing the ballast blocks set on the mooring line of the original foundation platform; replacing the concrete main structure of the original foundation platform with a high-density steel main structure, retaining only concrete as ballast to compensate for the changes in system mass and static water recovery characteristics caused by the removal of the ballast blocks, and maintaining the static balance of the floating wind turbine system; and adjusting the distance from the center of the original foundation platform to the center of the side columns from 37.0m to 35.0m to optimize the overall layout of the platform.

3. The design process for an adaptive floating wind turbine for extreme sea conditions according to claim 1, characterized in that, In S1, the structural modification also includes lengthening the central column of the floating platform. For the target submersion depth L, the height of the central column is increased from 28.5m in the original improved semi-submersible platform to 39.5+Lm. This ensures that the height of the tower base and the hub center above the still water surface remains constant at different submersion depths, at 11m and 119m respectively. Correspondingly, the draft of the modified platform is adjusted from 24m in the original improved semi-submersible platform to 35+Lm, and the platform's drainage volume is increased from the original 23702.98m³. 3 Adjusted to 29421+205.75Lm 3 The steel mass of the platform was adjusted from 3956.9t to 4176+19.915Lt, the total mass of the platform including ballast was adjusted from 22150t to 27983+211.3Lt, and the depth of the platform's center of gravity below the still water surface was adjusted from 20.013m to 30.609+0.9518Lm, in order to match the static balance requirements of the platform under different submersion depths.

4. The design process for an adaptive floating wind turbine for extreme sea conditions according to claim 1, characterized in that, In S2, the constructed catenary mooring system specifically uses three R4 grade unstrapped anchor chains as mooring lines. The angle between two adjacent mooring lines is set at 120°. The anchor point is located at a depth of 130m below the still water surface, and the horizontal distance from the anchor point to the platform center is set at 691m. The parameters of the mooring lines are set as follows: equivalent mass per unit length in air is 375.38kg / m, axial stiffness is 1.51E+06KN, equivalent hydraulic diameter of the anchor chain is 0.246m, physical diameter of the anchor chain is 0.137m, hydrodynamic added mass coefficient is 0.8, and hydrodynamic drag coefficient is 2. The distance from the guide hole to the platform center is set at 42m.

5. The design process for an adaptive floating wind turbine for extreme sea conditions according to claim 1, characterized in that, In S3, the method for determining the target flooding depth specifically includes: For different candidate submersion depths, the static stability parameters of the floating platform are calculated. The static stability parameters include the metacentric height, pitch still water recovery stiffness, and static tilt angle. The metacentric height of the semi-submersible platform is determined by the waterline moment of inertia and the vertical distance between the center of buoyancy and the center of gravity, while the metacentric height of the fully submersible platform depends only on the vertical distance between the center of buoyancy and the center of gravity. This is used to evaluate the platform's ability to recover from small disturbances at different submersion depths.

6. The design process for an adaptive floating wind turbine for extreme sea conditions according to claim 1, characterized in that, In S3, the working mode is divided according to the marine environmental conditions, specifically including: according to the IEC61400-3-2 standard, the marine environmental conditions are divided into two categories: rated operating conditions and extreme survival conditions; the environmental parameters corresponding to the rated operating conditions are wind speed 11.4 m / s, turbulence intensity 0.146, significant wave height 3.0 m, and spectral peak period 10.0 s, under which the wind turbine generator is in normal power generation operation; the environmental parameters corresponding to the extreme survival conditions are wind speed 49.0 m / s, turbulence intensity 0.105, significant wave height 14.4 m, and spectral peak period 13.3 s, under which the wind turbine generator is in shutdown operation. During the environmental parameter detection process, when the real-time environmental parameters are detected to meet the parameter thresholds for extreme survival conditions, it is determined that the current sea state is extreme survival condition, triggering the platform's working mode switching process. First, the wind turbine generator is controlled to complete the feathering shutdown operation, and then the platform is controlled to dive to the target submersion depth through the adaptive take-off and landing device to complete the switching of working modes. The environmental wind field was generated based on the IECKaimal spectrum in TurbSim, and the irregular wave time series was synthesized based on the JONSWAP spectrum. During the simulation, the same wind and wave load time series were applied to the floating platform in both working modes. The duration of a single simulation was set to 4200s. After removing the transient effects in the first 600s, 1 hour of effective data was obtained for dynamic analysis. This processing method meets the simulation duration requirements for conceptual design studies in the DNVGL guidelines.

7. The design process for an adaptive floating wind turbine for extreme sea conditions according to claim 1, characterized in that, In S4, the numerical simulation specifically uses OpenFAST code to construct a floating wind turbine system model with 24 degrees of freedom. These 24 degrees of freedom include: 6 rigid body motion degrees of freedom for the floating platform, namely sway, roll, heave, pitch, pitch, and yaw; 4 degrees of freedom for the first and second order bending modes in the front-back and left-right directions of the tower; yaw degree of freedom for the nacelle; azimuth degree of freedom for the generator; torsional flexibility degree of freedom for the transmission chain; 9 degrees of freedom for the first and second order flapping degrees of freedom and the first order oscillation degree of freedom for each blade; and degrees of freedom for the rotor retraction and tail fin folding. During the model construction process, HydroDyn, ElastoDyn, AeroDyn, ServoDyn and MoorDyn modules are integrated. The platform's six-degree-of-freedom motion is coupled with the flexible tower and blade modal coordinates in the ElastoDyn module through OpenFAST's GlueCode, forming a complete rigid-flexible coupled system to achieve fully coupled time-domain iterative solution. Before conducting the simulation of the target system, the original OO-Star semi-submersible platform model was first verified. The natural period of the platform was extracted through free decay test. The natural periods in the sway, heave, pitch, and yaw directions were 188.58s, 20.60s, 31.50s, and 102.85s, respectively. The maximum deviation from the reference study results was 3.58%. At the same time, verification tests were carried out under regular wave and extreme wind and wave conditions. After confirming that the accuracy of the numerical model met the requirements, the simulation calculation of the target floating wind turbine system was carried out.

8. The design process for an adaptive floating wind turbine for extreme sea conditions according to claim 7, characterized in that, In S4, the hydrodynamic load calculation process uses the HydroDyn module, specifically employing a hybrid method combining potential flow theory and strip theory. Potential flow theory is used to solve for hydrodynamic effects that do not consider viscosity, including linear hydrostatic restoring force, added mass, radiation damping, and first-order wave excitation force. The required frequency domain hydrodynamic coefficients for this part are obtained during the preprocessing stage using external solvers such as WAMIT. Simultaneously, strip theory based on the relative velocity form of the Morison equation is used to calculate fluid inertial force and viscous drag, thus considering the viscous flow separation effect around the structural components. In the parameter settings, the viscous drag coefficient Cd is uniformly set to 0.

6. This value is determined based on the flow characteristics around a cylinder under high Reynolds and low Keulegan-Carpenter number conditions. Under these conditions, the drag coefficient of a smooth cylinder is usually 0.5-0.

7. Taking into account the influence of actual sea conditions, Cd=0.6 is selected as the uniform viscous drag coefficient.

9. The design process for an adaptive floating wind turbine for extreme sea conditions according to claim 1, characterized in that, In step S2, for a given target flooding depth L, the parameters of the mooring system also need to be adaptively adjusted, specifically including adjusting the length of the unstretched mooring line to 0.0033L. 2 -0.6548L+685.55m, to match the mooring catenary shape after the platform dives; adjust the depth of the guide cable hole to L+15m to match the diving depth of the main body of the platform; adjust the pretension of the mooring system to 1030+7.2*LKN; Meanwhile, the dynamic calculation of the mooring system is carried out using the lumped mass method of the MoorDyn module. The mooring line is discretized into multiple nodes along its length. The dynamic equation of each node considers the internal axial stiffness and damping force, gravity and buoyancy, hydrodynamics calculated based on the Morison equation, and vertical spring-damping force caused by seabed contact, thereby realizing the coupled dynamic solution of the mooring system and the floating platform.

10. The design process for an adaptive floating wind turbine for extreme sea conditions according to claim 1, characterized in that, In S4, the performance verification specifically includes: First, free decay tests under still water conditions were conducted. For semi-submersible platforms and fully submersible platforms at different submersion depths, the natural frequencies of the platforms were tested in the feathering state with the wind turbines stopped. The natural frequencies of the platforms in the sway, roll, heave, pitch, and yaw directions at different submersion depths were obtained to evaluate the resonance characteristics of the platforms. Subsequently, white noise wave tests were conducted. Under windless conditions and an effective wave height of 3m, the response amplitude operator (RAO) of the platform was calculated. The differences in RAO between semi-submersible and fully submersible platforms in terms of sway, heave, pitch and roll motions and mooring line dynamic tension were compared to evaluate the wave response characteristics of the platform. Finally, a statistical comparative analysis was conducted under rated operating conditions and extreme survival conditions. This verified that under rated operating conditions, the maximum and mean values ​​of the pitch motion of the fully submersible platform were 1 / 2.44 and 1 / 2.64 of those of the semi-submersible platform, respectively, and the maximum value of the heave motion was half that of the semi-submersible platform. Under extreme survival conditions, the mean and maximum values ​​of the pitch motion of the fully submersible platform were 1 / 3.64 and 1 / 2.27 of those of the semi-submersible platform, respectively. The maximum value and standard deviation of the pitch motion of the fully submersible platform were reduced by 42.51% and 35.88% compared to the semi-submersible platform, respectively. This confirmed the suppressive effect of the fully submersible operating mode on the platform's motion response under extreme sea conditions and verified the technical feasibility of the design scheme.

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