Time domain fatigue stress algorithm for floating type fan foundation frame structure nodes

Abstract

The invention discloses a time domain fatigue stress algorithm for floating type wind turbine foundation frame structure nodes, and belongs to the technical field of ocean engineering structure fatigue analysis, and the algorithm comprises the following steps: floating type foundation multi-body time domain hydrodynamic analysis based on a hydroelastic model; obtaining a six-degree-of-freedom section force and an acceleration time domain result of a rod piece near a node of the floating foundation frame; loading the six-degree-of-freedom section force and the acceleration time domain result to the rod piece end part of the local model, and completing the finite element analysis of the fine mesh local model; obtaining a fatigue stress time domain result of a unit near a welding seam in the local model, and taking a proper amplification coefficient; performing rain flow counting statistics on the fatigue stress to obtain the relationship between the fatigue stress and the cycle index; and calculating the fatigue life. According to the method, only the rod piece section force is used as load input, complex outer wet surface water pressure calculation can be avoided, a load file is small, the loading and calculation speed is high, and more software can be used for extracting the rod piece section force in a hydrodynamic analysis result.

Description

Technical Field

[0001] The invention belongs to the technical field of fatigue analysis of marine engineering structures, and in particular relates to a time-domain fatigue stress algorithm for a floating wind turbine foundation frame structure node. Background Art

[0002] As offshore wind resources are increasingly used in deep seas, various types of floating wind turbine foundations have emerged. Under the action of alternating loads such as wave loads and wind loads, the floating wind turbine foundation structure suffers structural fatigue, and fatigue cracks are generated and expanded inside the steel, which in turn destroys the integrity of the floating foundation structure and can even cause the platform to capsize in severe cases.

[0003] The wind loads on traditional floating offshore oil and gas platforms usually do not exceed 5% of the wave and current loads, and the effects of wind loads on platform motion, mooring system and structural fatigue can be ignored. The wind loads on floating wind turbine platforms under power generation conditions are of the same order of magnitude as wave loads, and the effects of wind loads on the motion of the entire platform, the fatigue of the floating foundation structure and the fatigue of the mooring system can no longer be ignored. Compared with wave loads, wind loads are highly nonlinear. The wind turbine loads and the loads on the interface between the bottom of the wind turbine tower and the floating foundation are obtained using the time domain traversal method. Therefore, the spectral fatigue algorithm based on frequency domain hydrodynamic calculations and linear systems commonly used in traditional offshore floating platform structural fatigue cannot be applied to the fatigue analysis of floating wind turbine foundation structures. The specifications of various classification societies also require that the fatigue analysis of floating wind turbine foundation structures should be based on loads obtained by full coupling analysis in the time domain.

[0004] The current mainstream wind turbine design software has weak hydrodynamic analysis and mooring system analysis functions, while the floating foundation design software lacks aerodynamic analysis functions and wind turbine control logic. Therefore, the industry has little demand for a complete floating wind turbine foundation structure time domain fatigue analysis process. Figure 1 shown.

[0005] The second part of the process is to use the time-domain hydrodynamic analysis software to load the various loads borne by the floating foundation structure onto its finite element model based on the time-domain fully coupled analysis data output by the wind turbine software. The process also needs to calculate the time-domain hydrodynamics and finite element analysis of the coarse-grid overall model first, interpolate the node displacements of the fine-grid local model boundary from the coarse-grid finite element results, and calculate the time-domain hydrodynamic analysis of the local model again, assign hydrodynamic loads to the local model, and then calculate the fatigue stress of the local model. At present, there are very few software that can implement this step. Since the number of wet surface units in the overall structural model of the floating foundation exceeds 100,000, a load file of 3 hours and a time step of 0.1s may reach hundreds of GB. The subsequent overall structural fatigue stress analysis also has very high requirements on computer processor power, read and write speed, storage space and other resources, which greatly limits its promotion in engineering practice.

[0006] The main obstacles limiting the promotion of time-domain fatigue algorithms in engineering applications are the small number of available software, many load conditions, many model units, long calculation time, high storage space requirements, and low calculation efficiency. Therefore, it is urgent to design a time-domain fatigue stress algorithm for the nodes of the floating wind turbine foundation frame structure to solve the above-mentioned problems. Summary of the invention

[0007] The purpose of the present invention is to provide a time-domain fatigue stress algorithm for the nodes of the foundation frame structure of a floating wind turbine, which has the advantages of ensuring the fatigue stress calculation accuracy while greatly improving the calculation efficiency, and solves the problems mentioned in the background technology.

[0008] To achieve the above objectives, the specific technical solution of a time domain fatigue stress algorithm for a floating wind turbine foundation frame structure node of the present invention is as follows:

[0009] A time domain fatigue stress algorithm for a floating wind turbine foundation frame structure node comprises the following steps:

[0010] S1. Time domain fully coupled analysis based on wind turbine design software;

[0011] S2, obtain time domain results such as overall motion, tower bottom load, mooring load, wave surface rise, etc.;

[0012] S3. Multi-body time-domain hydrodynamic analysis based on the hydroelastic model of floating foundation structure;

[0013] S4, obtaining the time domain results of the 6-DOF cross-sectional force and acceleration acting on the rod near the floating foundation node;

[0014] S5, loading the 6-DOF section force and acceleration time domain results to the ends of the rods in the local model, and completing the finite element analysis of the fine-grid local model;

[0015] S6. Obtain the time domain results of the stress component perpendicular to the weld direction in the unit near the weld in the local model, and take the amplification factor as 5%;

[0016] S7. Count the rain flow for the fatigue stress time domain results;

[0017] S8. Use deterministic fatigue algorithm to obtain fatigue life.

[0018] Furthermore, in S1, the generalized mass matrix, damping matrix, stiffness matrix, and motion response amplitude operator of the floating foundation structure are obtained through frequency domain hydrodynamic analysis, and then the time domain fully coupled analysis based on the wind turbine design software is completed.

[0019] Furthermore, in S1, it also includes time-domain full coupling analysis for wind turbine design software through wind turbine blade parameters, tower parameters, control logic, wind field parameters, etc.

[0020] Furthermore, the frequency domain hydrodynamic analysis includes the floating foundation surface model, Morrison model, and mass model.

[0021] Furthermore, in S2, time domain results such as overall motion, tower bottom load, mooring load, and wave surface rise are extracted from the time domain fully coupled analysis results of the wind turbine design software.

[0022] Further, in S3, the floating foundation structure hydroelastic model includes a plurality of rigid floating bodies connected by flexible rods.

[0023] Furthermore, the rigid floating body includes the floating foundation center column, buoy, and bypass structure, which can be replaced by its generalized mass matrix, damping matrix, first-order wave load transfer function, hydrostatic restoring force matrix and other parameters in the model.

[0024] Furthermore, the flexible rod includes a strut structure between rigid floating bodies, and is simulated in the model by a Morrison rod having mass information, stiffness information, inertia force coefficient, and drag force coefficient.

[0025] Furthermore, the flexible rod should be set with nodes at the loaded section of the local finite element model member for subsequent section force extraction.

[0026] Furthermore, in S3, the multi-body time-domain hydrodynamic analysis of the hydroelastic model of the floating foundation structure also needs to use the time-domain results obtained in S2 as load input.

[0027] Further, in S4, the multi-body time-domain hydrodynamic analysis of the hydroelastic model of the floating foundation structure gives the time-domain results of the 6-DOF loads of the flexible rod nodes corresponding to the cross-sectional positions of the local finite element model.

[0028] Furthermore, in S5, independent local finite element models are built for different frame nodes, the mesh is refined at the key weld positions, and a main node coupled with the section node is set at the center of the bar section to load the load extracted in S4.

[0029] Further, in S6, the time domain result of the stress component of the unit near the weld in the local model perpendicular to the weld direction is obtained, and the stress component is multiplied by the amplification factor of 5% to be used as fatigue stress.

[0030] Furthermore, in S7, after performing rain flow counting statistics on the fatigue stress time domain results, the relationship between the fatigue stress and the number of cycles is obtained.

[0031] Furthermore, in S8, when determining the fatigue algorithm, SN curve, stress concentration factor, and thickness correction are also included.

[0032] The present invention has the following advantages: the structural analysis model in the algorithm of the present invention may only include several local models, and there is no need to complete the overall finite element analysis. The local model grid can well represent the local structural details and the number of units is relatively small. Only the cross-sectional force of the rod (which may also include the inertial force) is used as the load input, which can avoid the complex external wet surface water pressure calculation. The load file is small, and the loading and calculation speeds are fast. In addition, more software can be used to extract the cross-sectional force of the rod from the hydrodynamic analysis results. By only outputting the unit stress near the weld, the amount of calculation is further reduced and the analysis efficiency is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of the time domain fatigue analysis process of the floating wind turbine foundation structure in the prior art;

[0034] Figure 2 This is a schematic diagram of the second part of the time domain fatigue analysis process of the floating wind turbine foundation structure of the present invention;

[0035] Figure 3 This is a schematic diagram of a typical framework node;

[0036] Figure 4 It is a schematic diagram of the principal stress of key nodes under all loads in the overall model;

[0037] Figure 5 It is a schematic diagram of the internal forces at the ends of the frame members in the overall model;

[0038] Figure 6 Schematic diagram of principal stresses at key nodes where frame member section forces are applied to the local model. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0040] Those skilled in the art will appreciate that, although some embodiments herein include certain features included in other embodiments but not other features, the combination of features of different embodiments is meant to be within the scope of the present invention and form different embodiments. For example, in the claims, any one of the claimed embodiments may be used in any combination.

[0041] Please refer to the attached Figure 1 To Attachment Figure 6The invention describes a time domain fatigue stress algorithm for a floating wind turbine foundation frame structure node.

[0042] A time domain fatigue stress algorithm for a floating wind turbine foundation frame structure node comprises the following steps:

[0043] S1. Time domain fully coupled analysis based on wind turbine design software;

[0044] Specifically, the floating wind turbine wet surface model, Morrison model, and mass model are subjected to frequency domain hydrodynamic calculations to obtain the generalized mass matrix, damping matrix, stiffness matrix, and motion response amplitude operator, which are then used for time domain fully coupled analysis in the wind turbine design software;

[0045] S1 also includes time-domain fully coupled analysis of wind turbine design software through wind turbine blade parameters, tower parameters, control logic, wind field parameters, etc.

[0046] S2, obtain time domain results such as overall motion, tower bottom load, mooring load, wave surface rise, etc.;

[0047] S3. Multi-body time-domain hydrodynamic analysis based on the hydroelastic model of floating foundation structure;

[0048] Specifically, the multi-body time-domain hydrodynamic analysis based on the hydroelastic model of the floating foundation structure includes a plurality of rigid floating body hydroelastic models connected by flexible rods; the rigid floating bodies include the floating foundation neutral column, pontoon, and bypass structure, which can be replaced by their generalized mass matrix, damping matrix, first-order wave load transfer function, hydrostatic restoring force matrix and other parameters in the model; the flexible rods include the strut structure between the rigid floating bodies, which are simulated by Morrison rods with mass information, stiffness information, inertia force coefficient, and drag force coefficient; the flexible rods should set nodes at the loading section of the local finite element model rod for subsequent section force extraction; the multi-body time-domain hydrodynamic analysis of the hydroelastic model also needs to use the time domain results obtained in S2 as load input.

[0049] S4, obtaining the time domain results of the 6-DOF cross-sectional force (and acceleration) acting on the rod near the node of the floating foundation;

[0050] S5, loading the 6-DOF section force (and acceleration) time domain results to the ends of the rods in the local model, and completing the finite element analysis of the fine-grid local model;

[0051] S6. Obtain the time domain results of fatigue stress of the unit near the weld in the local model;

[0052] Specifically, the stress component of the unit near the weld perpendicular to the weld direction is taken as the fatigue stress.

[0053] Furthermore, an appropriate magnification factor (such as 1.05) is used when calculating fatigue stress.

[0054] S7. Count the rain flow for the fatigue stress time domain results;

[0055] S8. Use deterministic fatigue algorithm to obtain fatigue life.

[0056] Specifically, the fatigue algorithm is determined through the relationship between fatigue stress and the number of cycles, and then the fatigue life is obtained.

[0057] Furthermore, when determining the fatigue algorithm, SN curve, stress concentration factor, and thickness correction are also included.

[0058] Compared with the prior art, the present invention Figure 1 and Figure 2 As shown in the figure, the second part is adjusted and improved so that the structural analysis model in the algorithm of the present invention can only contain several local models, without the need to complete the overall finite element analysis. The local model grid can well represent the local structural details and the number of units is relatively small. Only the cross-sectional force of the rod (which may also include the inertial force) is used as the load input, which can avoid the complex calculation of the external wet surface water pressure, the load file is small, the loading and calculation speed is fast, and more software is available for extracting the cross-sectional force of the rod from the hydrodynamic analysis results. The amount of calculation is further reduced and the analysis efficiency is improved by only outputting the stress of the unit near the weld.

[0059] In addition, according to the node construction characteristics of the floating wind turbine foundation frame structure, the present invention proves through calculation and analysis that in the local model near the key node, the stress distribution almost entirely depends on the cross-sectional force of the surrounding rods, and the stress generated by the local water pressure and inertia force in the key node position can be ignored. Conservative considerations can be made to use appropriate method coefficients to obtain the key node stress calculated based on the internal force of the rod to complete the subsequent fatigue analysis.

[0060] Typical node forms of floating wind turbine foundation frame structures are as follows: Figure 3 As shown. The common characteristics of these local structures are thick walls and dense internal reinforcement. The stress of the weld inside the fine mesh local model is generated by the following loads: inertial force, water pressure on the external surface, and cross-sectional force transmitted from both ends of the rod. The following example is used to illustrate:

[0061] The following table compares the principal stress values ​​of four typical locations under the combined action of all loads and the rod section force, where the first and third principal stresses with larger absolute values ​​are regarded as fatigue stresses (shown in bold in the table). By comparison, it can be seen that the principal stress results generated in the local model by applying only the rod section force have a maximum deviation of 1.75% from the fatigue stress generated under the combined action of the three loads.

[0062] The proportion of fatigue stress under the action of cross-sectional force

[0063]

[0064] Under the same working conditions, the external dynamic water pressure is 5380Pa, the node is a circular tube with a diameter of 3500mm and a wall thickness of 48mm, the stress generated by the water pressure in the circumferential direction of the tube wall is 1.96E5Pa, and the stress generated in the axial direction is 0.3*1.96E5Pa=5.88E4Pa, which is only 2.9% to 3.3% of the axial stress of the tube wall under the action of the cross-sectional force.

[0065] Combining the calculation results of other node forms, the following conclusions can be drawn: if the fatigue stress of the section force in the local model is amplified by 5%, it will be greater than the fatigue stress under the combined action of the three loads. Therefore, ignoring the effect of water pressure on local structural fatigue, only using the fatigue stress generated by the section force of the rod and amplifying it by 5% for local model fatigue analysis can meet the calculation accuracy requirements.

[0066] The specific implementation of the present invention is for Figure 1 In the second part of the process, the entire floating foundation model is discretized into multiple rigid floats and flexible rods in combination with the frame node positions (fine grid local model) that need to be analyzed. Each rigid float and flexible rod has a separate stiffness, mass, and hydrodynamic response. Time domain aerodynamic and hydrodynamic analysis software is used to complete the time domain coupling analysis including wind turbine aerodynamic information, floating foundation hydrodynamic information, mooring system, floating foundation stiffness information, and mass distribution information, and extract the time domain results of the cross-sectional force (and acceleration) of the rods around each frame node (fine grid local model). The time domain values ​​of the cross-sectional force (and acceleration) of the rods are loaded in each local model, and the finite element analysis of the local model is completed. The stress component perpendicular to the weld direction in the unit of each time step is amplified by 5% as the final fatigue stress, and then enters the rain flow counting statistics and fatigue analysis of the third part of the process.

[0067] The present invention has the following advantages:

[0068] (1) It is proved that the fatigue stress of the nodes of the floating wind turbine foundation frame structure is mainly generated by the cross-sectional force of the nearby rods. The influence of water pressure and inertia force on fatigue stress can be included by magnifying the stress generated by the cross-sectional force of the rods by 5%.

[0069] (2) When calculating fatigue stress, there is no need to calculate the time domain results of the water pressure load on the wet surface of the model. The structural finite element load file is small and the fatigue stress solution speed is fast.

[0070] (3) Use the time domain results of the cross-sectional force of the bars near the nodes extracted based on the time domain fully coupled analysis of the wind turbine, multiple buoys, and mooring system as the load input to calculate the time domain results of the fatigue stress at the key weld locations.

[0071] (4) It is not necessary to complete the overall finite element analysis of the floating foundation, and the loading and fatigue stress of multiple local models can be directly calculated in parallel.

[0072] (5) Certain continuous pontoon or column structures can also be used for fatigue stress calculation by reasonably discretizing the cross-sectional forces.

[0073] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A time domain fatigue stress algorithm for the nodes of a floating wind turbine foundation frame structure, characterized in that: The following steps are involved: S1. Time domain fully coupled analysis based on wind turbine design software; S2, obtain time domain results such as overall motion, tower bottom load, mooring load, wave surface rise, etc.; S3. Multi-body time-domain hydrodynamic analysis based on the hydroelastic model of floating foundation structure; S4, obtaining the time domain results of the 6-DOF cross-sectional force and acceleration acting on the rod near the floating foundation node; S5, loading the 6-DOF section force and acceleration time domain results to the ends of the rods in the local model, and completing the finite element analysis of the fine-grid local model; S6. Obtain the time domain results of the stress component perpendicular to the weld direction in the unit near the weld in the local model, and take the amplification factor as 5%; S7. Calculate the rain flow count statistics for the fatigue stress time domain results; S8. Use deterministic fatigue algorithm to obtain fatigue life.

2. The time domain fatigue stress algorithm of the floating wind turbine foundation frame structure node according to claim 1 is characterized in that: In S1, the generalized mass matrix, damping matrix, stiffness matrix, and motion response amplitude operator of the floating foundation structure are obtained through frequency domain hydrodynamic analysis, and then the time domain fully coupled analysis based on the wind turbine design software is completed.

3. The time domain fatigue stress algorithm of the floating wind turbine foundation frame structure node according to claim 2 is characterized in that: S1 also includes time-domain fully coupled analysis of wind turbine design software through wind turbine blade parameters, tower parameters, control logic, wind field parameters, etc.

4. The time domain fatigue stress algorithm of the floating wind turbine foundation frame structure node according to claim 2 is characterized in that: Frequency domain hydrodynamic analysis includes floating foundation surface model, Morrison model, and mass model.

5. The time domain fatigue stress algorithm of the floating wind turbine foundation frame structure node according to claim 1 is characterized in that: In S2, time domain results such as overall motion, tower bottom load, mooring load, and wave surface rise are extracted from the time domain fully coupled analysis results of the wind turbine design software.

6. The time domain fatigue stress algorithm of the floating wind turbine foundation frame structure node according to claim 1 is characterized in that: In S3, the floating foundation structure hydroelastic model includes a plurality of rigid buoys connected by flexible rods.

7. The time domain fatigue stress algorithm of the floating wind turbine foundation frame structure node according to claim 6 is characterized in that: The rigid floating body includes the floating foundation center column, buoy and bypass structure, which can be replaced by its generalized mass matrix, damping matrix, first-order wave load transfer function, hydrostatic restoring force matrix and other parameters in the model.

8. The time domain fatigue stress algorithm of the floating wind turbine foundation frame structure node according to claim 6 is characterized in that: The flexible rod includes a strut structure between rigid floating bodies, and is simulated in the model by a Morrison rod having mass information, stiffness information, inertia force coefficient, and drag force coefficient.

9. The time domain fatigue stress algorithm of the floating wind turbine foundation frame structure node according to claim 6 is characterized in that: The flexible rod should be set with nodes at the loading section of the local finite element model rod for subsequent section force extraction.

10. The time domain fatigue stress algorithm of the floating wind turbine foundation frame structure node according to claim 1 is characterized in that: In S3, the multi-body time-domain hydrodynamic analysis of the hydroelastic model of the floating foundation structure also requires the time-domain results obtained in S2 as load input.

11. The time domain fatigue stress algorithm of the floating wind turbine foundation frame structure node according to claim 1 is characterized in that: In S4, the multi-body time-domain hydrodynamic analysis of the hydroelastic model of the floating foundation structure gives the time-domain results of the 6-DOF loads at the flexible rod nodes corresponding to the cross-section positions of the local finite element model.

12. The time domain fatigue stress algorithm of the floating wind turbine foundation frame structure node according to claim 1 is characterized in that: In S5, independent local finite element models are built for different frame nodes, the mesh is refined at the key weld positions, and a main node coupled with the section node is set at the center of the bar section to load the load extracted in S4.

13. The time domain fatigue stress algorithm of the floating wind turbine foundation frame structure node according to claim 1 is characterized in that: In S6, the time domain result of the stress component perpendicular to the weld direction of the unit near the weld in the local model is obtained, and the stress component is multiplied by the amplification factor of 5% to be used as fatigue stress.

14. The time domain fatigue stress algorithm of the floating wind turbine foundation frame structure node according to claim 1 is characterized in that: In S7, after performing rain flow counting statistics on the fatigue stress time domain results, the relationship between fatigue stress and cycle number is obtained.

15. The time domain fatigue stress algorithm of the floating wind turbine foundation frame structure node according to claim 1 is characterized in that: In S8, when determining the fatigue algorithm, SN curve, stress concentration factor, and thickness correction are also included.

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