Study On Decrease In Fatigue Strength Throughout The Life Cycle Of Platform Structures Based On Monitoring

Fatigue is one of the most significant failure modes for offshore platform structures,and fatigue strength is a crucial matter assuring safety and reliability in platform structure design.The disorderly changing ocean environment loads induce irregular fluctuations in structural stress response,hence statistical parameters are generally used to describe the aforementioned fatigue loading in structural design stage.Due to uncertain factors such as the form and parameters of fatigue loading,large scatter of fatigue life prediction results is often found according to traditional numerical methods.Structural monitoring systems have been extensively implemented among large-scale engineering structures and infrastructures in recent years.The real-time mechanical responses acquired continuously by the sensors can be beneficial in reducing the inherent uncertainties associated with loads in the process of structural safety assessment.However,with regard to structural monitoring-based fatigue life assessment among existing engineering structures,the mutually complementary sensor arrangement scheme and safety assessment method in any particular case are closely related to the characteristics of the structure itself,therefore cannot be copied to offshore platform structures indiscriminately.Consequently,taking the load-bearing and environmental characteristics of offshore platforms into consideration,it is necessary to propose a suitable fatigue damage monitoring method,as well as develop a legitimate fatigue life assessment process,in order to perceive the condition of decrease in fatigue strength.The article study the method of perceiving decrease in fatigue strength throughout the life cycle of platform structures based on structural monitoring.Firstly,a fatigue damage monitoring method is proposed,capturing the stress distribution on a structural cross section away from hot-spot location.A monitoring data processing method consistent with sensor arrangement is developed through finite element analysis,obtaining the complete stress state of structural stress at the fatigue critical position.A detailed fatigue damage monitoring scheme is designed for two typical platforms structures.The validity and precision of proposed method is verified by finite element analysis,and a concrete implementation strategy is given accordingly.Secondly,considering the influence of corrosion on the mechanical response and fatigue performance,the monitoring-based time-domain analysis of fatigue damage for platform structures is accomplished.Considering the influence of external environmental load variation and corrosion factors,a conservative estimating model for fatigue cumulative damage is established,predicting fatigue life of platform structures throughout the life cycle.Finally,in consideration of the variable amplitude multiaxial nature of platform structure fatigue,the multiaxial fatigue criterion Modified W?hler Curve Method(MWCM)is introduced to achieve the monitoring-based multiaxial fatigue damage analysis of platform structures.The fatigue damage monitoring method proposed in the article can effectively obtain the time history of the authentic stress state of structural stress at fatigue critical positions for platform structures.With further analysis of uniaxial or multiaxial fatigue damage,and prediction of fatigue life,the condition of decrease in fatigue strength throughout the life cycle of platform structures can be better perceived,additionally providing epistemic knowledge regarding the genuine safety state of platform structures,demonstrating promising applicability for engineering practice.