Fatigue Life Prediction Methods For Very Large Floating Structure

Very large floating structures are artificial floating land parcels on the sea,which are more gigantic than ordinary platforms.They are of important research value for future ocean exploitation.Very large floating structures are exposed to long term wave loads.Wave induced stresses are the main source of fatigue damage.This paper investigates on a primarily designed very large floating structure for purpose of fatigue strength check.Based on hydrodynamic response calculation and spectral analysis,two fatigue analysis methods are intouduced for the local structures of the very large floating structure: the first one is S-N curve approach combined with cumulative fatigue damage theory;the second is fracture mechanics approach focused on fatigue crack propagation.Thus the procedure in this paper mainly consists of three stages: global structural response,S-N curve faigue analysis approach and fracture mechanics approach for for hot spots.Step by step,a research from hydrodynamic response calculation to local fatigue strength analysis on the very large floating structure is completed.In principle,this paper mainly covers:1)Background and significance of very large floating structure fatigue strength research is described.And research status of semisubmersible platform,including the very large floating structure,is reviewed.Fatigue strength research methods on semisubmersible platform are briefly introduced here.The most widely used method is based on S-N curve and cumulative fatigue damage theory.Besides,another prediction method based on fatigue crack propagation theory has the advantage that it can be carried out on the structures found with flaw.Improvement of fatigue crack propagation theory helps it to apply more on complex structures under complex loads2)Hydrodynamic response and wave induced loads calculation is assisted by ANSYS/AQWA to obtain structure response data in frequency domain.Then combining design wave parameters,hot spots for fatigue assessment will be located after global structural stress analysis.3)Then taking advantage of sub-model technique in ANSYS,finite element sub-models of the chosen key spot areas were built to obtain the stresses transfer function.Spectral-based method for fatigue analysis was used for the fatigue analysis of the key spots,including calculation of their cumulative fatigue damage and fatigue life.The analysis results show that the transverse wave,in general,may result in the most significant damage on the platform.And it is found that two spots,respectively located in connection parts between cross brace and floating body as well as connection parts between columns and upper box,may not satisfy the requirement of design fatigue life.4)Following above procedure,this paper provides a fatigue assessment for a very large floating structure using a spectral-based fracture mechanics approach that combines spectral analysis and fatigue crack propagation model.To predict fatigue crack growth in the critical welded joints,besides stress intensity factor calculation,this paper summarizes formulas associating equivalent stress intensity factor range with equivalent stress range of a given hot spot.Both are performed using spectral analysis in stochastic sea conditions.Then a unique curve model is introduced for crack propagation prediction.Thus,this paper provides a reasonable way to convert complex wave loads into crack growth,and can be widely applied to other marine structures.Compared to S-N curve approach,the result is in accordance.