Floating type wind power asymmetric mooring system and optimization method thereof

Abstract

The invention discloses an optimization method of a floating type wind power asymmetric mooring system, and relates to a mooring system of an offshore wind power platform, the method comprises the following steps: obtaining the historical parameters of a mooring cable of an original mooring system, iteratively solving a top flare angle theta from the historical parameters of the mooring cable through a genetic iterative algorithm, obtaining the parameters of the mooring cable corresponding to the top flare angle theta, and obtaining the mooring cable parameters corresponding to the top flare angle theta; performing static analysis on a mooring cable of the asymmetric mooring system according to the mooring cable parameters to obtain environmental load parameters of the mooring cable; and arranging mooring cables of the asymmetric mooring system according to the environmental load parameters. The invention further discloses a floating type wind power asymmetric mooring system. According to the method, the efficiency of optimal design of the mooring system is improved, and computing resources are saved.

Description

Technical Field

[0001] The present invention relates to a mooring system for an offshore wind power platform, and more particularly to a floating wind power asymmetric mooring system and an optimization method thereof. Background Art

[0002] Floating wind turbine systems consist of multiple modules, including a floating foundation, wind turbines, towers, and mooring systems. The floating foundation is connected to the seabed via the mooring system, which resists motion responses caused by environmental loads, maintains the platform's range of motion, and achieves positioning. As the core equipment of floating wind turbine systems, the mooring system's construction cost accounts for one of the largest proportions of the total construction cost. Therefore, the optimal design of mooring systems is crucial to the development of floating wind turbines and holds significant scientific and practical value.

[0003] Factors that influence mooring system design typically include environmental conditions, operational requirements, and economic costs. In addition to environmental conditions, mooring performance depends largely on key mooring cable design parameters. For example, pretension significantly influences the dynamic tension response of the mooring cable. Generally speaking, smaller pretension leads to smaller mooring recovery stiffness and tension, as well as larger floating platform offsets. It also increases the variation amplitude and standard deviation of the mooring tension, which has an adverse effect on fatigue damage. Researchers compared and analyzed the advantages and disadvantages of catenary mooring systems and tensioned mooring systems from the aspects of mooring static and dynamic characteristics and mooring loads. The results show that compared with catenary mooring systems, tensioned mooring systems have better hydrodynamic performance and smaller mooring tension amplitudes, but require more complex technology and higher economic costs for the design, installation, and maintenance of the anchor foundation. Therefore, catenary mooring systems remain the primary choice for floating wind turbine mooring systems. The analysis and design of mooring systems involve multiple variables. Existing designs of mooring systems for floating wind turbines rely primarily on experience, often using a trial-and-error approach to optimize the design. This consumes significant computing resources and time during the design phase.

[0004] Furthermore, in most ocean areas, long-term statistics on wind, waves, and currents show that their energy is primarily concentrated in a dominant direction (such as prevailing winds, dominant wave direction, or ocean current path). For example, some ocean areas experience prevailing southwesterly winds and waves in winter, while certain coastal areas, influenced by the monsoon, experience southeasterly winds and waves in summer and northwesterly winds and waves in winter. Traditional symmetrically arranged mooring systems have excessive redundancy in directions less susceptible to environmental loads, resulting in material waste. Furthermore, symmetrical designs require more mooring lines and anchoring foundations, increasing material costs and installation complexity.

[0005] In order to improve the design and optimization efficiency of mooring systems, some researchers have used intelligent optimization algorithms to study the design optimization methods of mooring systems. Shafieefar and Rezvani [1] proposed a mooring system design method based on genetic algorithm. This method normalizes the objective function based on the weighted sum method and establishes a single-objective optimization problem for mooring system design with the goal of minimizing the longitudinal and transverse response.

[0006] Felix-Gonzalez and Mercier [2] proposed a mooring design method based on static equivalence and genetic algorithm, and took the minimization of the six-degree-of-freedom motion effect as the optimization goal. The study showed that the selection of weight coefficients has an important influence on the design results of the mooring system. Brommundt et al. [3] established a mooring optimization problem based on four design variables and three constraints, and solved it using the Nelder–Mead simplex algorithm. However, the minimum length of the mooring cable is the only goal of the optimization design. Monteiro et al. [4] proposed a mooring system design method based on particle swarm optimization (PSO) method and differential evolution (DE) method, and normalized the objective function through an averaging method, and carried out optimization work with the goal of minimizing the platform motion range. Mirzaei et al. [5] considered the mooring cable angle as a variable, took the minimization of platform displacement as the optimization goal, and carried out optimization research based on genetic algorithm. Ryu et al. [6] considered the mooring cable length and diameter as variables, and carried out optimization design with the mooring system cost as the goal. In order to simplify the problem, the estimation of the mooring system cost only considered the cost of the mooring cable. Currently, there is limited research on the intelligent optimization design of mooring systems. Most studies focus on reducing motion amplitude or improving economic performance, with a lack of research on multi-objective intelligent optimization methods that comprehensively consider the motion, safety, and economic performance of mooring systems. Furthermore, current research on intelligent optimization design for mooring systems focuses on symmetrically designed mooring systems, without considering the intelligent optimization design of asymmetric mooring systems that match the main load direction of the marine environment. This approach, which significantly reduces the lifecycle cost while ensuring the safety of floating wind turbines, is not feasible.

[0007] References:

[0008] [1]Shafieefar M,Rezvani A.Mooring optimization of floating platforms using a genetic algorithm[J].Ocean Engineering,2007,34(10):1413-1421.

[0009] [2]Felix-Gonzalez I,Mercier R S.Optimized design of staticallyequivalent mooring systems[J].Ocean Engineering,2016,111:384-397.

[0010] [3]Brommundt M,Krause L,Merz K,et al.Mooring system optimization for floating wind turbines using frequency domain analysis[J].Energy Procedia,2012,24:289-296.

[0011] [4] Monteiro BF, de Pina AA, Baioco JS, et al. Toward a methodology for the optimal design of mooring systems for floating offshore platforms using evolutionary algorithms [J]. Marine Systems & Ocean Technology, 2016, 11(3-4): 55-67.

[0012] [5]Mirzaei M, Maimun A, Priyanto A, et al.Mooring pattern optimization using a genetic algorithm[J].Jurnal Teknologi,2014,66(2).

[0013] [6]Ryu S,Duggal AS,Heyl CN,et al.Mooring cost optimization viaharmony search[C] / / International Conference on Offshore Mechanics and ArcticEngineering.2007,42673:355-362. Summary of the Invention

[0014] The technical problem to be solved by the present invention is to provide a floating wind power asymmetric mooring system and an optimization method thereof in view of the deficiencies in the prior art.

[0015] The present invention provides an optimization method for an asymmetric mooring system for a floating wind turbine. The method obtains historical mooring cable parameters of an original mooring system, iteratively solves a top angle θ from the historical mooring cable parameters using a genetic iterative algorithm, obtains mooring cable parameters corresponding to the top angle θ, performs a static analysis on the mooring cables of the asymmetric mooring system based on the mooring cable parameters to obtain environmental load parameters of the mooring cables, and arranges the mooring cables of the asymmetric mooring system based on the environmental load parameters.

[0016] Preferably, the static analysis specifically includes:

[0017] The first step is to set N nodes on the mooring cable, wherein the N nodes divide the mooring cable into N-1 units; and set the angle between the first unit of the mooring cable and the horizontal direction as the top angle θ;

[0018] The second step is to simplify the gravity, buoyancy and drag forces on each unit to the center of the unit;

[0019] Step 3: Use the end node of the n-1th unit as the starting point of the nth mooring line unit; where N is 1-n;

[0020] Step 4: Calculate the unit tension t and the node coordinates of each unit on the mooring line according to the mooring line parameters;

[0021] Step 5: Check whether the vertical coordinate Z of the node coordinate in the Nth unit of the mooring cable is the set water depth h; if the vertical coordinate Z is equal to the water depth h, assign the top angle of the current unit to the included angle; otherwise, iteratively change the top angle θ in the mooring cable history parameters using the golden section method, and then return to the first step.

[0022] Preferably, the unit tension t is determined by the following formula:

[0023]

[0024] Where t is the unit tension; ds is the unit length of the mooring line; dt is the change in unit tension on ds; dθ is the change in inclination angle on ds; F and D are the tangential and normal drag forces per unit length, respectively; P is the buoyant density per unit length of the mooring line; and ε is the elongation per unit length of the mooring line.

[0025] Preferably, the node coordinates are determined by the following formula:

[0026]

[0027] Where X and Z represent the horizontal and vertical coordinates of the mooring line unit, respectively; dX and dZ are the changes of X and Z on ds, respectively; and ε is the elongation of the mooring line per unit length.

[0028] Preferably, the static restoring force of the mooring cable of the asymmetric mooring system is obtained, and the minimum unit tension t is extracted from the unit tension t of each unit on the mooring cable as the maximum tension of the mooring cable. It is judged based on the static restoring force whether the maximum tension of the mooring cable meets the design requirements; if the static restoring force is greater than the maximum tension, it is judged that the design requirements are met.

[0029] Preferably, the static restoring force is obtained in the following manner:

[0030] The first step is to obtain an initial transverse distance value and a maximum motion displacement of the mooring line from the original mooring system, and determine a maximum and a minimum transverse distance of the mooring line based on the initial transverse distance value and the maximum motion displacement; select multiple transverse distance intervals from the maximum and minimum transverse distance values of the mooring line, average the transverse distance points within the transverse distance intervals, and calculate the top tension and top angle values corresponding to each transverse distance point to obtain a mooring line transverse distance-top tension curve;

[0031] Step 2: Taking the initial position of the floating platform of the asymmetric mooring system as the origin, setting the horizontal movement distance of each step of the floating platform as dx, and calculating the new direction angle and new horizontal distance of each mooring cable at the new position of the asymmetric mooring system according to the law of cosines;

[0032] The third step is to calculate the static restoring force according to the mooring line transverse distance-top tension curve, the new direction angle and the new transverse distance.

[0033] Preferably, in the iterative solution of the top angle θ, if h-h'>10 -4 , then it is time to continue iterating. If h-h'≤10 -4 , the iteration ends; where h is the set target depth and h' is the depth corresponding to the top opening angle θ.

[0034] A floating wind power asymmetric mooring system includes a three-column structure consisting of three columns arranged below a floating wind power platform; the three columns of the three-column structure are respectively connected to a weight block located on the seabed through a mooring cable; the mooring cable is optimized using the optimization method of the floating wind power asymmetric mooring system.

[0035] Preferably, the weight is located on the lying section of the seabed.

[0036] Beneficial effects

[0037] The advantages of the present invention are:

[0038] 1) Based on the static analysis model of the mooring system and a genetic algorithm, the present invention realizes the intelligent optimization design of the mooring system. This solves the problem that the traditional trial-and-error method for mooring system design requires a large amount of computing resources and time costs, improves the efficiency of the mooring system optimization design, and saves computing resources.

[0039] 2) The present invention takes into account the optimized design of the floating wind power asymmetric mooring system that matches the main load direction of the marine environment, which significantly reduces the full life cycle cost while ensuring the safety of the floating wind power platform, and better serves actual engineering projects.

[0040] 3) The overall intelligent optimization method of the mooring system of the present invention comprehensively considers the safety performance, positioning performance and economic performance of the mooring system, and the optimization design goal is more complete and comprehensive. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is a schematic diagram of the force analysis of the mooring cable unit of the present invention;

[0042] Figure 2 A schematic top view of the mooring cable position of the present invention;

[0043] Figure 3 A top view of a simplified mechanical model of a three-column platform mooring system according to the present invention;

[0044] Figure 4 Schematic diagram of the catenary of the asymmetric mooring system of the present invention;

[0045] Figure 5 Schematic diagram of establishing a plane rectangular coordinate system at an anchor point according to the present invention.

[0046] Figure 6 This is a flow chart of the multi-objective optimization design of a mooring system based on a genetic algorithm according to the present invention; DETAILED DESCRIPTION

[0047] The present invention will be further described below in conjunction with the embodiments, but this does not constitute any limitation to the present invention. Any limited number of modifications made by anyone within the scope of the claims of the present invention are still within the scope of the claims of the present invention.

[0048] Example 1

[0049] The present invention provides an optimization method for a floating wind turbine asymmetric mooring system. The method comprises: obtaining historical mooring cable parameters of an original mooring system; iteratively solving a top angle θ from the mooring cable historical parameters using a genetic iterative algorithm; obtaining mooring cable parameters corresponding to the top angle θ; performing a static analysis on the mooring cables of the asymmetric mooring system based on the mooring cable parameters to obtain environmental load parameters of the mooring cables; and arranging the mooring cables of the asymmetric mooring system based on the environmental load parameters.

[0050] The optimization method of the asymmetric mooring system of the present invention mainly considers three indicators: the positioning performance, safety performance and economic performance of the mooring system. The safety performance verification of the mooring system mainly includes the ultimate limit state verification (ULS), the fault state verification (ALS) and the fatigue failure state verification (FLS). Among them, the ultimate limit state verification is to ensure that the mooring line has sufficient strength to withstand the load under extreme environmental conditions. The extreme environmental load condition is generally selected with a 50-year recurrence period. The fault state verification means that when the mooring system loses any mooring line, it can still withstand the load under extreme environmental conditions and will not cause the anchor dragging phenomenon.

[0051] In the mooring system optimization method of the present invention, the positioning performance of the mooring system can be characterized by the horizontal displacement of the mooring system. Before the optimization begins, a time-domain analysis must be performed using the existing mooring system. The maximum horizontal displacement and direction of the floating structure are statistically determined. Based on this maximum horizontal displacement and direction, the angle between the top element of the mooring cable and the horizontal direction, i.e., the top tension angle θ, can be obtained. Based on this top tension angle θ, the corresponding mooring cable parameters can be retrieved from the mooring cable's historical parameters. Static analysis of the mooring cable of the asymmetric mooring system based on these mooring cable parameters can be performed to obtain the environmental load parameters of the asymmetric mooring system, including the top tension angle θ, the top tension, and the tension and coordinates of each element on the mooring cable. In the mooring system, these parameters are primarily used to measure the positioning performance of the mooring system. The safety performance of the mooring system is represented by a safety factor. Based on the maximum horizontal displacement of the mooring system, the top tension of each mooring cable at that displacement can be determined. The maximum value of these top tensions is the maximum pretension of the mooring system, and the ratio of the minimum breaking force (MBS) of the mooring cable to this value is the safety factor.

[0052] In actual situations, since the platform displacement has an acceleration in the opposite direction to the environmental load when it reaches its maximum value, the maximum pretension obtained by static calculation in the optimization program is smaller than the maximum pretension value in actual situations. Therefore, the safety factor must be corrected and compensated in the static calculation, that is, the safety factor used in the optimization should be larger than the safety factor required in the time domain calculation.

[0053] Specifically, the asymmetric mooring system optimization method proposed in the present invention combines the static analysis of the mooring system with a genetic algorithm (GA).

[0054] The present invention establishes a static analysis model of the mooring system based on the segmented extrapolation method. The segmented extrapolation method discretizes the mooring cable into n mooring cable units (Segment), and each unit is regarded as a massless spring, connected by nodes (Node) with mass. Node1 represents the node where the mooring cable is connected to the floating platform fairlead hole, and Node n represents the node where the mooring cable is connected to the anchor foundation. The buoyancy, gravity and fluid drag force of the mooring cable unit are all equivalently concentrated at the center of the unit. During the calculation, only the axial stiffness of the mooring cable is considered, and the bending stiffness and torsional stiffness of the mooring cable are ignored. The schematic diagram of the force analysis of any unit on the mooring cable is as follows: Figure 1 shown.

[0055] In the static analysis of the mooring system, it is assumed that the mooring cable is subjected only to tension and no pressure, bending moment, or torque. When performing numerical analysis on the mooring cable, appropriate boundary conditions are set to verify whether the final shape of the mooring cable meets the requirements. The static analysis model of the mooring system uses water depth as the boundary condition. The solution steps are as follows:

[0056] The first step is to set multiple nodes on the mooring cable and divide the mooring cable into several units for each node. It is assumed that the angle between the first unit (i.e., the top unit) of the mooring cable and the horizontal direction is the top angle θ.

[0057] The second step is to simplify the gravity, buoyancy and drag forces on each unit to the center of the unit.

[0058] The third step is to use the end node of the previous mooring line unit as the starting point of the next mooring line unit.

[0059] Step 4: Calculate the tension t of each unit on the mooring cable and the coordinates of the unit nodes.

[0060] According to the basic assumption of static solution, at any time, any unit on the mooring cable is in static equilibrium. Figure 1 The static equilibrium state of the unit can be described by the control equations. After neglecting the second-order infinitesimal quantities, the mooring cable control equations are as follows:

[0061]

[0062]

[0063] Where t is the unit tension; ds is the unit length of the mooring line; dt is the change in unit tension on ds; dθ is the change in inclination angle on ds; F and D are the tangential and normal drag forces per unit length, respectively; P is the buoyant density per unit length of the mooring line; and ε is the elongation per unit length of the mooring line.

[0064] in,

[0065]

[0066] Where A is the cross-sectional area of the mooring line element; ρ is the seawater density; V c is the ocean current speed; C N is the normal drag coefficient; C T is the tangential drag coefficient.

[0067] According to the geometric relationship of the mooring line unit, we can get:

[0068]

[0069] Where X and Z represent the horizontal and vertical coordinates of the mooring line unit, respectively. dX and dZ are the changes of X and Z on ds, respectively. The coordinates of any point on the anchor line can be obtained from the above formula.

[0070] Step 5: Verify the water depth boundary condition. This means checking whether the vertical coordinate Z of the last element of the mooring line is equal to the water depth h. If Z is equal to the water depth h, the water depth boundary condition is satisfied, and the current top angle of the mooring line is assumed to be θ, and the calculation ends. Otherwise, the top angle θ in the mooring line's historical parameters is iteratively modified using the golden section method, then the calculation is repeated in step 1.

[0071] Based on the above calculation method and algorithm flow, after giving the mooring line properties in the static analysis program and assigning a value to the initial span of the mooring line, the corresponding top tension angle θ and top tension, as well as the tension and coordinates of each unit on the mooring line, can be obtained.

[0072] In actual engineering, the planar motion range of a floating platform is mainly affected by the restoring stiffness of the mooring system, while the restoring performance in the vertical and rotational directions mainly comes from its own structure. Since the vertical length of the mooring cable is basically unchanged, the influence of the vertical displacement of the floating platform is generally not considered when calculating the static restoring force provided by the mooring system. In an asymmetric mooring system, the static restoring force calculated based on the above-mentioned static analysis model of the mooring system should be greater than the maximum pretension to ensure the reliability of the mooring cable. Based on the above-mentioned static analysis model of the mooring system, the calculation process of the static restoring force of the mooring system is as follows:

[0073] Step 1: Obtain the horizontal distance-top tension relationship curve of a single mooring line:

[0074] The material properties, initial span, and water depth parameters of the mooring cable are known. The corresponding top tension of the mooring cable can be calculated based on the given span. When the platform is in its initial equilibrium position, the top tension of the mooring cable corresponding to the initial span value is the pretension. Based on the initial span value of the mooring cable and the maximum possible motion displacement during the platform's movement, the maximum and minimum span values can be determined. To calculate the span-top tension curve of the mooring cable, it is first necessary to select a suitable span interval. The span interval selection criterion is to ensure that the maximum and minimum values of the platform's motion range are contained within the interval, while not being too large. An excessively large span interval not only causes the calculated motion range to exceed the design requirements, wasting time and cost; it also increases the amount of calculation and reduces the interpolation accuracy. After determining the span interval, average points within the interval and calculate the top tension and top angle values corresponding to each span to obtain the mooring cable span-top tension curve.

[0075] Step 2: Calculate the horizontal distance of each mooring line corresponding to the horizontal displacement of the floating platform:

[0076] Taking the initial position of the platform as the origin, the initial direction angle of the mooring cable is known, and the horizontal movement distance of each step of the floating platform is defined as dx. The offset distance can be controlled by the number of movement steps. As the mooring system moves dx with the floating platform, the new direction angle and new horizontal distance of each mooring cable at the new position can be calculated according to the cosine theorem. The top view diagram of the mooring cable position is shown as follows: Figure 2 shown.

[0077] Step 3: Calculate the static restoring force provided by the mooring system:

[0078] According to step 2, the new horizontal distance of each mooring cable corresponding to each displacement dx of the floating platform can be calculated. Using step 1, the top tension angle, top tension and direction angle of each mooring cable can be obtained, thereby calculating the static restoring force provided by the mooring system at the current position.

[0079] Among them, in the process of solving the static restoring force, the calculation method of the top tension and top angle value corresponding to the horizontal distance in step 1 and the solution of the static restoring force in step 3 are both existing technologies, and the present invention does not improve them. For details, please refer to "Fan Tianhui. Static and low-frequency damping equivalent test method for anchoring truncation of deep-water semi-submersible platform [D]. Dalian University of Technology, 2016."

[0080] In the optimization method of the present invention, it is difficult to maintain static balance in an asymmetric mooring system arrangement, so it is necessary to find the corresponding parameters of static balance through iterative calculation.

[0081] The floating wind power platform of the asymmetric mooring system of the present invention is designed with three columns. The simplified mechanics model of the mooring system is shown in the top view. Figure 3As shown. F1, F2, and F3 are the resultant horizontal forces of the mooring tension acting on each column. The system is most likely to reach static equilibrium when the directions of F1, F2, and F3 are 0 degrees, 120 degrees, and 240 degrees. Therefore, the following calculations assume that the directions of F1, F2, and F3 are 0 degrees, 120 degrees, and 240 degrees. When the resultant force on the X-axis and Y-axis is 0, that is, ∑FX=0 and ∑FY=0, F1=F2=F3. Therefore, the platform is in force equilibrium if and only if the resultant forces on the three columns are equal.

[0082] Let the number of mooring cables on the three columns be x n (n=1,2,3), the angle between the mooring line and the direction of the resultant force is α n (n=1,2,3). Assume that the pre-tension of the mooring lines on all columns is equal, and the value is T n (n=1,2,3), the resultant force is T; the top angle is θ n (n=1,2,3). There are:

[0083] When x n =3, T is calculated as follows:

[0084] T n COSθ n +2T n COSθ n COSα n =T.

[0085] When x n =2, T is calculated as follows:

[0086] 2T n COSθ n COSα n =T.

[0087] The catenary of the asymmetric mooring system is as follows Figure 4 As shown in Figure 2. In each optimization calculation, changing the pre-tension of the target mooring cable will produce a change in pre-tension ΔT. Assuming static equilibrium in the initial state, it is known that T n (n=1,2,3),α n (n=1,2,3), ΔT2. Where, T n is the pre-tension of the mooring line on the nth column, and ΔT2 is the increment of the anchor chain tension on column 2. To ensure static equilibrium, T1 and T3 must also generate corresponding increments ΔT1 and ΔT3. To solve ΔT1 and ΔT3. According to Equations 1 and 2, we have:

[0088] 2ΔT2COSθ2COSα2=ΔT1COSθ1+2ΔT1COSθ1COSα1;

[0089]

[0090] The same applies to ΔT3.

[0091] After modifying T1 to (T1+ΔT1), the top angle will change from θ1 to θ'1, resulting in:

[0092] ΔT1COSθ1′+2ΔT1COSθ1′COSα1≠2ΔT2COSθ2COSα2.

[0093] That is, static imbalance. Therefore, it is necessary to iteratively solve to find the optimal ΔT1.

[0094] The calculation logic in the iterative calculation is to iteratively solve the top angle θ and find the depth h' corresponding to the top angle θ. Set the accuracy to 10 -4 , if h-h'>10 -4 , then it is time to continue iterating. If h-h'≤10 -4 , then the iteration ends, and the obtained top angle θ is used as the top angle of the anchor chain under the given mooring radius, diameter, and pretension conditions, and is substituted back to calculate the relevant parameters of the anchor chain.

[0095] When calculating the shape, tension and other results of the mooring cable, it is necessary to solve the catenary equation. The catenary equation without considering elasticity (i.e. the axial stiffness of the mooring cable) is as follows:

[0096]

[0097] Where a is the distance from the vertex of the curve to the horizontal axis.

[0098] When considering the axial stiffness of the mooring cable, the linear density of the mooring cable will change with the change of tension. Let the axial stiffness of the mooring cable be EA, and the linear density of the mooring cable when it is not deformed be ρ0. Establish a plane rectangular coordinate system at the anchor point, such as Figure 5 As shown. At this time, when the mooring line unit is subjected to tension T, the linear density becomes:

[0099]

[0100] To generalize this formula, let k = EA, where E is the elastic modulus of the mooring line element material and A is the cross-sectional area of the mooring line element.

[0101] According to the geometric relationship of the tension force components, we have:

[0102]

[0103] Where y is the equation to be solved with respect to x. y' is the first derivative of y with respect to x. dx is the horizontal component of the mooring cable element, and dy is the vertical component of the mooring cable element. The coordinate axes are as follows: Figure 5 shown.

[0104] Consider the horizontal force relationship:

[0105]

[0106] To organize it into a general formula, record Integrating the above formulas we have:

[0107]

[0108] Where y' is the second-order derivative of y with respect to x.

[0109] By transforming the above formula, we can obtain the following differential relationship:

[0110]

[0111] Integrating both sides and combining the lowest point boundary conditions, we can get:

[0112] x=a(sinh -1 y′+by′);

[0113]

[0114] Let y′ = sinht, then the above formula can be written as:

[0115] x=a(t+b sinht)

[0116]

[0117] This is the catenary parametric equation that takes elasticity into account. In the present invention, the finite difference method is used to convert the differential equation above into an iteratively updated numerical equation for approximation and iterative solution. An unconstrained single-variable optimization numerical method is used in the iterative calculation to quickly approach the target value. By solving the relevant parameters of the catenary mooring cable, such as the span and top angle, the force conditions at each point on the catenary can be obtained, thereby analyzing the desired positioning performance and safety performance of the mooring system.

[0118] The present invention is based on a genetic algorithm. By modifying the mooring line parameters (chain diameter, stiffness, mass, length), pre-tension, number of weights arranged and the method of weighting within a certain range, and calculating the safety factor and steel consumption, the present invention finds the anchor chain arrangement method with the best economic benefits (i.e. the anchor chain parameters, pre-tension, number of weights arranged and the method of weighting mentioned above) within this range while ensuring safety performance, so as to achieve optimal calculation.

[0119] The genetic algorithm is based on natural selection and genetic theory, and is an efficient global optimization search algorithm that combines the survival of the fittest in the biological evolution process with the random information exchange mechanism of chromosomes within the group. The genetic algorithm abandons the traditional search method, draws on Darwin's theory of evolution and Mendel's genetics, simulates the biological evolution process in nature, and uses artificial evolution to repeatedly perform genetic-based operations (inheritance, crossover, and mutation) on the group. Each individual is evaluated according to the predetermined target fitness function, and according to the evolutionary rules of survival of the fittest and survival of the fittest, a better group is continuously obtained. At the same time, a global parallel search method is used to search for the best individual in the optimization group to obtain the optimal solution that meets the requirements. Its optimization design process is as follows: Figure 6 shown.

[0120] Example 2

[0121] A floating wind power asymmetric mooring system includes a three-column structure consisting of three columns arranged below a floating wind power platform; the three columns of the three-column structure are respectively connected to a weight block located on the seabed through a mooring cable; the mooring cable is optimized using the optimization method of the floating wind power asymmetric mooring system.

[0122] In previous studies, it was believed that placing the weights in the lying section would have a better effect on the performance of the mooring system. Therefore, in the optimized design of this invention, all the weights are placed in the lying section of the anchor chain, and the starting position is the anchor chain touchdown point. To facilitate the optimization design, the volume of a single weight is 1m 3 , the density is the standard density of steel 7850kg / m 3 The influence of additional mass is not considered for now.

[0123] The above is only a preferred embodiment of the present invention. It should be pointed out that for those skilled in the art, several modifications and improvements can be made without departing from the structure of the present invention. These modifications and improvements will not affect the effect of the implementation of the present invention and the practicality of the patent.

Claims

1. A method for optimizing a floating wind turbine asymmetric mooring system, characterized in that: Obtaining historical parameters of a mooring cable of an original mooring system, iteratively solving a top angle θ from the historical parameters of the mooring cable using a genetic iterative algorithm, obtaining mooring cable parameters corresponding to the top angle θ, and performing a static analysis on the mooring cable of the asymmetric mooring system based on the mooring cable parameters to obtain environmental load parameters of the mooring cable; The mooring lines of the asymmetric mooring system are arranged according to the environmental load parameters.

2. The optimization method of a floating wind turbine asymmetric mooring system according to claim 1, characterized in that: The static analysis specifically includes: The first step is to set N nodes on the mooring cable, wherein the N nodes divide the mooring cable into N-1 units; and set the angle between the first unit of the mooring cable and the horizontal direction as the top angle θ; The second step is to simplify the gravity, buoyancy and drag forces on each unit to the center of the unit; Step 3: Use the end node of the n-1th unit as the starting point of the nth mooring line unit; where N is 1-n; Step 4: Calculate the unit tension t and the node coordinates of each unit on the mooring line according to the mooring line parameters; Step 5: Check whether the vertical coordinate Z of the node coordinate in the Nth unit of the mooring cable is the set water depth h; if the vertical coordinate Z is equal to the water depth h, assign the top angle of the current unit to the included angle; otherwise, iteratively change the top angle θ in the mooring cable history parameters using the golden section method, and then return to the first step.

3. The optimization method of a floating wind turbine asymmetric mooring system according to claim 2, characterized in that: The unit tension t is determined by the following formula: Where t is the unit tension; ds is the unit length of the mooring line; dt is the change in unit tension on ds; dθ is the change in inclination angle on ds; F and D are the tangential and normal drag forces per unit length, respectively; P is the buoyant density per unit length of the mooring line; and ε is the elongation per unit length of the mooring line.

4. The optimization method of a floating wind turbine asymmetric mooring system according to claim 2, characterized in that: The node coordinates are determined by the following formula: Where X and Z represent the horizontal and vertical coordinates of the mooring line unit, respectively; dX and dZ are the changes of X and Z on ds, respectively; and ε is the elongation of the mooring line per unit length.

5. The optimization method of a floating wind turbine asymmetric mooring system according to claim 2, characterized in that: obtaining a static restoring force of a mooring cable of the asymmetric mooring system, extracting a minimum unit tension t from the unit tensions t of each unit on the mooring cable as the maximum tension of the mooring cable, and determining whether the maximum tension of the mooring cable meets design requirements based on the static restoring force; If the static restoring force is greater than the maximum tension, it is determined that the design requirements are met.

6. The method for optimizing a floating wind turbine asymmetric mooring system according to claim 5, characterized in that: The static restoring force is obtained as follows: The first step is to obtain an initial transverse distance value and a maximum motion displacement of the mooring line from the original mooring system, and determine a maximum and a minimum transverse distance of the mooring line based on the initial transverse distance value and the maximum motion displacement; select multiple transverse distance intervals from the maximum and minimum transverse distance values of the mooring line, average the transverse distance points within the transverse distance intervals, and calculate the top tension and top angle values corresponding to each transverse distance point to obtain a mooring line transverse distance-top tension curve; Step 2: Taking the initial position of the floating platform of the asymmetric mooring system as the origin, setting the horizontal movement distance of each step of the floating platform as dx, and calculating the new direction angle and new horizontal distance of each mooring cable at the new position of the asymmetric mooring system according to the law of cosines; The third step is to calculate the static restoring force according to the mooring line transverse distance-top tension curve, the new direction angle and the new transverse distance.

7. The optimization method for a floating wind turbine asymmetric mooring system according to claim 1, characterized in that: In the iterative solution of the top angle θ, if h-h'>10 -4 , then it is time to continue iterating. If h-h'≤10 -4 , the iteration ends; where h is the set target depth and h' is the depth corresponding to the top opening angle θ.

8. A floating wind power asymmetric mooring system, characterized in that: It comprises a three-column structure consisting of three columns arranged below a floating wind power platform; the three columns of the three-column structure are respectively connected to a heavy block located on the seabed through a mooring cable; the mooring cable is optimized and designed using the optimization method of the floating wind power asymmetric mooring system as described in any one of claims 1 to 7.

9. The floating wind power asymmetric mooring system according to claim 8, characterized in that: The weight is located at a lying section on the seabed.

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