| Fatigue analysis of the offshore wind turbine(OWT)substructure under complex loads is one of the key technical problems in offshore wind farm construction.The key joint of a tripod substructure is the three-planar tubular Y-joint,which bears various fatigue loads(such as wind,wave,current,etc.)during service life as a space joint.Therefore,the hot spot stress of this joint is affected by multiplanar interaction and reaction superposition effect.However,there is no systematic research and corresponding formula for this kind of joint to calculate the stress concentration factor(SCF)and the hot spot stress in codes and published literature.In order to solve the above problems,this paper studies the hot spot stress of the three-planar tubular Yjoint under complex loads by means of experiments,numerical and statistical methods.(1)Large scale physical model test of a three-planar tubular Y-jointAccording to the spatial tubular joint test requirements,a specially designed loading system,named the Multi-planar Complex Loading System for Space Structures,is designed and built.The system can be used for specimens with a complex spatial geometry structure.Moreover,complex loads,such as axial load,in-plane bending moment and out-of-plane bending moment,can be applied in multiple planes at the same time.Static tests and fatigue tests can both be implemented with this system.Based on this system,the large-scale physical model test of the three-planar tubular Y-joint is completed.Then,the characteristics of hot spot stress distribution under basic load cases and complex load cases are obtained.The test results revealed that it is necessary to study the interaction between loads and multiple planes quantitatively.These results are also the basis for deriving the hot spot stress formula.(2)Research on hot spot stress of three-planar tubular Y-jointsThe finite element(FE)models of three-planar tubular Y-joints is established based on ANSYS.The validity of the numerical analysis method is verified by the physical model test results.The range of commonly used geometric parameters is studied according to the design code of the OWT and actual engineering parameters.In this range,1920 sets of representative geometric parameters which are different from each other are selected.Then,the FE model database containing 1920 tubular joints is established accordingly.Three kinds of basic loads are applied to each model and calculated to obtain the finite element results database.The reaction superposition and multiplanar interaction mechanism are systematically studied by numerical method.The multiplanar interaction factor(MIF)is proposed to quantify this effect.The hot spot stress formula of spatial tubular Y-joints under multiplanar complex loads is derived.(3)Study on peak values of stress concentration factor and multiplanar interaction factorBased on the FE model database established in this paper,the influences of the length-toouter radius ratio of the chord ?,the brace-to-chord diameter ratio ?,the chord outer diameterto-chord thickness ratio ?,the brace-to-chord wall thickness ratio ?,and brace-to-chord inclination angle ? on SCF and MIF peak values under three basic loads are analyzed.A multivariate nonlinear regression analysis was performed to obtain the parametric formulas of SCF and MIF of three-planar tubular Y-joints under three basic loads.The accuracy of the formulas is evaluated according to the UK DoE criterion,which proves that it can be used in engineering design.Based on the finite element results,the error analysis is carried out.Sixteen groups of geometric parameters are selected in the range of applicable geometric parameters.The finite element model is established and calculated.The accuracy of the formula is proved by comparing the finite element results with the formula method.(4)Study on distribution of stress concentration factor and multiplanar interaction factorBased on the FE model database established in this paper,the influences of the length-toouter radius ratio of the chord ?,the brace-to-chord diameter ratio ?,the chord outer diameterto-chord thickness ratio ?,the brace-to-chord wall thickness ratio ?,and brace-to-chord inclination angle ? on SCF and MIF distribution along weld curves under three basic loads are analyzed.A multivariate nonlinear regression analysis was performed to obtain the parametric formulas of SCF and MIF of three-planar tubular Y-joints under three basic loads.The accuracy of the formulas is evaluated according to the UK DoE criterion,which proves that it can be used in engineering design.Based on the finite element results,the error analysis is carried out.Four groups of geometric parameters are selected in the range of applicable geometric parameters.The finite element model is established and calculated.The accuracy of the formula is proved by comparing the finite element results with the formula method.(5)The application process of proposed formulas with an engineering exampleTaking a completed offshore wind farm project in Jiangsu Province as an example.The load conditions of the three-planar tubular Y-joints are calculated according to the geological survey data and the load data provided by the wind turbine manufacturer.The hot spot stress is calculated according to the peak value formula method and the distribution formula method proposed in this paper.Then,the results of the two methods are compared and evaluated. |
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