Study On Two Key Mechanical Problems Of Offshore Engineering Structures

With the development of offshore engineering,its superstructure is getting bigger and bigger,and the bearing capacity of pile foundation is also increasing.Concrete material is often used as the pile material of offshore engineering structure due to its good compression performance.The concrete material has different moduli in tension and compression,whose characteristic should be seriously considered when checking the strength of concrete legs.On the other hand,as the structure of wind turbine needs to withstand the increasing pressure from the top,the possibility of buckling instability will increase.Based on the above characteristics,the offshore structure is studied in two aspects of the static and dynamic analysis.And the two key mechanical problems are put forward.Among them,the first key mechanical problem mainly focuses on the influence of plain concrete structure with different modulus in tension and compression on the distribution and magnitude of structural stress.The main research content is as follows:For a member or structure of the isotropic materials with the property of different moduli in tension and compression,taking the positive and negative of the first invariant J1 of the stress as the criterion on different modulus properties of the material,the calculating and analysis program of three-dimensional complex multi-body structure with the material properties of different moduli is written by APDL language on the secondary development with ANSYS software as the platform.Taking the simple bending beam in uniaxial stress state and the simply supported beam by uniform load in biaxial stress state as examples,the finite element solutions of several different ratios of tension to compression modulus obtained by using the developed calculating and analysis program are compared with the analytical solutions in the previous literatures.The relative errors between the two are no more than 6%.It is verified that the calculation accuracy of developed programcan meet the engineering requirements.The calculating and analysis module of materials with different tensile and compressive modulus developed by ANSYS is used to analyze the Concrete-Filled Steel Tube legs of offshore wind turbines.Choosing the three cases of compressive elasticity modulus,average elastic modulus and elasticity moduli in different tension and compression for calculating,the results according to compressive and average elastic modulus are respectively compared with that according to different tension and compression modulus of elasticity.Along the path 3 in the thesis,the relative error of the maximum tensile stress in the z direction of the former is as high as 41.08%,and the relative error of the maximum tensile stress in the z direction of the latter reaches 20.98%.The calculated results based on a single elastic modulus(compressive elastic modulus or average elasticity modulus)can not meet the requirements for engineering accuracy.Therefore,considering the bimodulus properties of concrete materials will undoubtedly ensure that the simulation results have accuracy and effectiveness in the checking calculation of the concrete-filled steel tube legs of offshore wind turbines.The second key mechanical problem mainly discusses the non-destructive testing method for the buckling load of towering structures.The content of this research is as follows:In order to consider the influence of different constraint conditions on prediction of buckling load,the beam with anti-rotation spring constraint at both ends and the beam fixed at both ends are taken as examples.The relationship between axial pressure and natural frequency square of the two is very close to be linear.For the beam with anti-rotation spring constraint at both ends,buckling load obtained by analytical solutions between axial pressure and transverse natural frequency square will increase with the stiffness coefficient.When the stiffness coefficient at both ends approaches infinity,it is simplified as the beam fixed at both ends,and the instability load reaches the maximum.The tower of wind turbine is simplified to two models to analyze the relationship between the axial pressure and the natural frequency square,which are the first model is that the wind wheel mass of the wind turbine after assembled and the nacelle mass are reduced to the concentrated mass,and the tower is simplified as a beam structure whose bottom is simplified as a rigid fixed end;the second model is that the wind wheel mass of the wind turbine after assembled and the nacelle mass are reduced to the concentrated mass with eccentricity and moment of inertia,and the tower is simplified as a beam structure whose bottom is simplified as elastic constraint.There is an approximate linear relationship between axial pressure and square of transverse natural frequency of these two simplified forms.It can be seen that the latter simplified model is more reasonable and closer to the true value through the analysis.Based on the approximate linear relationship,selecting the experimental loads less than the critical value of instability,recording the natural frequencies of the structure under two kinds of load conditions,using the conclusion of the approximate linear relationship between the axial force and the square of natural frequency,the critical buckling load of the offshore wind turbine tower structure can be approximately determined.Taking steel pipe-concrete column as an example,the effects on modal shape and the natural frequency due to different moduli properties of the concrete are studied.The conclusion can be got that the modal shapes of steel pipe-concrete column are basically the same according to the same modulus and different moduli theory,and the property of different moduli in tension and compression of concrete has little effects on the first-order natural frequency.