Study On Fatigue Damage Evolution Of Offshore Platform Structures Under Ocean Environmental Loads

Compared with the terrestrial environment，the loads acting on the underwaterstructures are more complex and harsh. The reson of the huge catastrophic accidentsoccurred on marine structures over the past decade is summarized in this paper.Investigation showed that the fatigue damage is the main factor of structure failure.The coupling effects from wave, current, wind, etc would impact on componentswhich are bearing random load for long time and lead to multi-axis characteristics tothe underwater structures. The damage would happen in varied directions and indifferent flats. The physical mechanism of the fatigue damage is much morecomplicated without any uniform theory and there are still many basic problems to besolved.K-type pipe joint is a key part of underwater structure and is also the weakestlink. This article introduced the study of evaluation mechanism of fatigue and damagefirstly, correlated the macroscopic marine environmental factors and the changes ofmicrostructure inner materials, and finally proposed the micro mechanism and thediscipline of the fatigue evaluation; there is a close relation between material’s fatigueand damage thermal dynamics. Under the effects of cyclic load, the atoms inmaterial will escape due to the phenomenon of thermal disturbance. The hole causedby escape of atom would not disappear completely due to the change of its balancedposition. Afterwards, the point defect would expand to a large area defect and finallyforms into a real defect whose further development is likely to cause the fatigue andfracture of material.According to the interaction between damage variables and numbers of thecycles under single load, the relation between approximation equations and theircoefficients could also be obtained. By drawing the approximation curve usingsimulation program such as Matlab, the correctness of this damage evolvement theorycould be verified.The calculation of marine environmental load based on the wave and currentcalculation equation. It quoted the form of wave load action from one certain seaarea in Nanhai Sea and applied the calculation results for the next experiment, whichmade the experiment conditions and environment more closed to real underwaterenvironment. By using the FEM software to do the fatigue simulation for components,the multi axis experiment method was proposed. By combining the application ofthermocouple instrument, strain measuring instrument, the bending stress strain fieldof the joints, thermal diffusion fatigue temperature field at the fatigue crack werestudied.By analyzing the bending strain in this experiment, it can be seen that in a longtime, the change of the bending strain was stable. When the load was increasedgradually, the fatigue would happen on the pipe with a trend that decreased at first, increased and then decreased. From analysis of the change of the thermal diffusiontemperature in the experiment, three obvious processes of the surface temperature ofthe sample were detected: initial temperature rise period, temperature stable periodand temperature dramatically rise period. At the beginning, surface temperature of thematerial rose modestly. In the middle time, temperature was much higher thanprevious, but would keep stable for a long time. The fatigue crack period was veryshort with a sharply change of temperature and the rise speed percent could reach19.2%.The study results enriched further the theory of fatigue damage, explained thephenomenon of fatigue damage under the condition which is far away from thefatigue limit, validated the effectiveness and correctness of the theoretical model andprovided the theoretical basis for its engineering application.