The Research Of Jack-up Platforms In The Dynamic Response And Fatigue Analysis

In the complicated and changeable load under the action of the Marine environment，truss-type jack-up platform, which is greater flexibility of its own，is complex structuraldynamic response significantly. Especially long-term effects of this major load control loadsin waves, the platform leg is easy to crack, or even lead to fatigue failure. Therefore, the studyon dynamic responses of jack-up platform legs and fatigue analysis, which are necessary forthe safe usage of jack-up platform.In the software SACS of ocean engineering, this paper refers to the jack-up platform legsfor dynamic response analysis and fatigue research. First of all, comparing to its inherentfrequency and wave frequency, modal analysis was carried out on the operating mode, then tojudge the possibility of resonance; Secondly, the waves of the dynamic response of platformstructure is analyzed; Finally, considering the power amplification effect and the influence ofstress concentration, the fatigue life prediction of the platform legs is done by directlydeterministic method and dynamic spectrum analysis method.The main work is as follows:1) This paper states ocean environment load, the basic theory and method of fatigueanalysis, and introduces the process of fatigue analysis based on SACS, etc.;2) Considering the influence of pile-soil nonlinear boundary condition, oceanengineering software SACS was adopted to construct the three-dimensional finite elementmodel of jack-up platform. The connection method of the hull and leg structure and the chordmodel adopted the equivalent method to solve the problem of pile legs hydrodynamic areresearched, and then the structure strength and the bearing capacity of pile of the platformlegs are checked. Comparing with the wave frequency, the modal analysis method is appliedto computing the natural frequency of platform, to determine the possibility of resonance. Inaddition, under wave loading by comparing with the results and quasi-static analysis, thewave dynamic response analysis was carried out on the platform structure.3) Considering the impact of dynamic amplification effect and stress concentration factor,the direct deterministic method and spectral analysis methods were conducted forself-elevating platforms of fatigue analysis, and predict the fatigue life of the platform leg. When using direct deterministic method for fatigue analysis, One year higher wave frequencyof24groups selected from the occurrence of waves scatter plot, to predict the fatigue life ofthe platform leg. In the end, the dangerous region focuses on the port side of the splash zonelocation and leg junction with the main hull. When using spectrum analysis method to predictthe fatigue life of the platform, the sea state condition is the north sea-Bery&Frig site ofone-year wave scatter. Through calculating the stress transfer function, the response spectraldensity function of the structure of the tube node is obtained, according to S-N curve andMiner linear cumulative damage theory, then the fatigue life of the platform leg are obtained.By analyzing the calculation results of the above two methods, the calculation precision of themethod of spectral analysis is higher than the certainty method directly. but this certaintymethod is simple. Firstly the initial fatigue analysis of jack-up platform can use directlydeterministic method to determine more dangerous pipe node, then using the spectrumanalysis method of fatigue analysis for further, the results of this method meet the accuracyrequirement of the fatigue analysis of offshore platform.5) As the selection of S-N curve, the common wave spectral density (P-M spectrum, etc.),the calculation method of SCF and wave scatter diagram, the results of spectrum fatigueanalysis were discussed, and then the influence of the reasonable range of options andmethods are presented.