Study On Spectral Analysis Method Of Multiaxial Fatigue Life Estimation For Marine Structure

Marine structures have been in service in harsh marine environments for a long period of time,subject to random wave loads,and the platform structure will cause severe fatigue damage.In addition,in the actual service process of the platform,due to the complicated geometry of the locally welded structure and the complexity of the marine environmental load,the platform structure is in a state of multi-axis fatigue damage.However,at present,uniaxial fatigue theory is used in engineering to evaluate the fatigue life of the platform structure,which leads to a large dispersion of the evaluation results.Therefore,it is of great significance to study the multiaxis fatigue damage analysis method of platform structure and establish the multi-axis fatigue life assessment method for marine structures under the load of complex marine environment,which is of great significance for improving the fatigue assessment accuracy of marine engineering structures.The difficulties of multiaxial fatigue life assessment of actual marine engineering structures mainly include the following three points: First,the hot spot stress method commonly used in engineering is difficult to obtain the multiaxial stress state at the fatigue assessment point,and the stress gradient effect in the plate thickness direction cannot be considered,especially It is because when the shape of the local welded structure is complex,the hot spot stress method is no longer convenient for the analysis of the multiaxial stress state at this place;the second is that the research on the multiaxial fatigue damage mechanism of the welded structure is still too few,although some more Shaft fatigue damage criterion,but the assessment accuracy is still insufficient,especially for multi-axis fatigue life assessment under nonproportional loading;third,the traditional spectral fatigue analysis method proposed for uniaxial fatigue in engineering is no longer applicable to multi-axis fatigue life Evaluation requires the establishment of a new multiaxial fatigue life assessment method for marine engineering structures.The main work of this paper is to use the zero-point structural stress method to analyze the multi-axial stress state of the fatigue area of the platform welded structure,and to calculate the multi-axial stress state of the zero position of the welded structure by establishing a multi-axis fatigue life assessment sub-model of the solid shell unit coupling The two-parameter critical surface MWCM method is used as the multiaxial fatigue damage criterion,the shear stress range on the critical surface is calculated as the multiaxial fatigue life control parameter,and the uniaxial fatigue design curve is corrected according to the critical surface stress ratio to obtain the multiaxial fatigue S-N curve;then Combined with the spectral analysis method widely used in engineering to calculate the probability density distribution of the shear stress range on the critical plane of the fatigue critical point of the welded structure under the wave load,and then use the modified multiaxial fatigue S-N curve to integrate the multiaxial fatigue damage to establish marine engineering Structural multi-axis fatigue life spectrum analysis method.Then,taking a deep-water semi-submersible offshore platform as an example,the multi-axial fatigue spectrum analysis method established in this paper is used to evaluate the multi-axial fatigue life of the platform structure,and the multi-axial fatigue life assessment for the marine engineering structure is realized.Finally,based on the method proposed in this paper,the corresponding automatic calculation module was developed using C # programming,and the multi-axis fatigue life spectrum analysis software for offshore engineering structures was developed with the help of the.Net platform,and the automatic evaluation of the multi-axis fatigue life of offshore structures was initially achieved.