Analysis Of Fatigue Damage And System Reliability Of Mooring System Of Offshore Fish Cage

In recent years,the offshore aquaculture has gradually become an important approach to conduct the exploitation of marine fishery resources with the rapid development of the marine aquaculture industry.Thus the safety of offshore fish cage system,which is one of the main engineering structures of the offshore aquaculture facilities,is beneficial for the healthy development of the offshore aquaculture industry.And the mooring system plays an important role in the safety of offshore fish cage system.Under the long-term and complex environment conditions in deep sea,the extreme response of the mooring system may be likely to exceed the capacity and the fatigue failure of the mooring system may occur.And the failure of the mooring system will significantly threat to the safety of offshore fish cage system.Thus,it is of paramount importance to conduct the the investigation on the long-term fatigue damage and system reliability of mooring system for offshore fish cage.During the safety assessment of mooring system for ocean engineering,the assumptions introduced in the conventional methods may be not always reasonable,which depends heavily on the researcher's experience.And there is little attention on the uncertainty quantification of the load/response by the numerical models for aquaculture plant and its mooring system.And the research on the long-term fatigue damage and system reliability of mooring system for offshore fish cage system is very limited.This dissertation will devote to assess the long-term fatigue damage,the probability distribution of fatigue damage,extreme response distribution and model uncertainty of the grid mooring system for fish cage,and then conduct the reliability analysis of the grid mooring system for fish cage system under long-term sea conditions.The main contents of the dissertation are shown as follows:(1)An improved spectral moments equivalent method is proposed to calculate the long-term fatigue damage of mooring system.Based on the lumping block method,this method takes into account the wave period sensitivity of the fatigue damage of mooring line in the block and adopts the principle of slightly conservative estimation of fatigue damage in each block for reducing the number of sea states in the long-term wave conditions.And the accuracy and effiency of the new method is verified in comparison to the results for the original sea states.(2)The long-term fatigue damage of mooring system for fish cage under wave groups is studied.The hydrodynamic response of the fish cage under wave groups is modeled efficiently based on wave envelope spectrum.And the influence of wave groupiness factors on the stress range,fatigue damage curve,short-term and long-term fatigue damage of mooring line is investigated.(3)A new importance factor is proposed to represent the relative importance of the uncertainty of random variables to the fatigue reliability of mooring line for fish cage,and thus the assessment of system reliability of mooring system under fatigue limit state is conducted.The evolution of fatigue damage distribution with the number of load cycles is evaluated and the probability distribution of fatigue damage of mooring line can be calculated directly through the probability density evolution method.And then the system reliability of the mooring system under fatigue limit state is calculated by the equivalent extreme value event method.(4)The breaking strength of mooring chains is calculated through a nonlinear contact model of three chain links considering the effect of random corrosion model,and then the time dependent reliability analysis of mooring line for a single fish cage and a four cage group after corrosion is conducted.And the extreme value response distribution of the mooring system is obtained using the probability density evolution method,and the results are compared with those by the distribution fitting method.Based on the equivalent extreme value event method,the system failure probability of the grid mooring system is calculated under ultimate limit state.(5)The system reliability of mooring system under accidental limit state is analyzed based on probability density evolution method and virtual random process.And considering the progressive failure of the grid mooring system for fish cage,an improved reliability evaluation method for the mooring system is proposed.(6)The uncertainty of the response by numerical models for aquaculture plant and its mooring system is evaluated.And the probability distribution of the mean value and standard deviation of the model bias factor is calculated by bootstrap method.Considering the effect of the statistical uncertainty of model bias factor,the confidence interval of the reliability index of mooring line under ultimate limit state and fatigue limit state is quantitatively evaluated.