Research On Fatigue And Ultimate Strength Evolution Of Chains Considering Corrosion And Wear

The mooring chains work in the marine environment for a long time.The key chains in the marine environment are easily to fatigue damage because of alternating cyclic loading,which is caused by the combined action of wind,wave and periodic reciprocating motion of the offshore platform.At the same time,it is inevitable that chains will suffer from extreme conditions during the service period.The study of the ultimate strength of the mooring chains in the lifetime has also been a hot issue in the engineering field.However,at present,few studies take corrosion and wear damage into the fatigue analysis or ultimate strength analysis of chains.Mooring chains will inevitably suffer from severe corrosion when submerged in a complex marine environment for a long time.In addition,there will be wear damage between adjacent chain links.Therefore,it is very meaningful to evaluate the fatigue damage and ultimate strength evolution of the mooring chains considering the corrosion and wear during the lifetime.The purpose of this paper is to study the fatigue damage analysis and ultimate strength analysis of mooring chains considering corrosion and wear,and put forward a method for evaluating the safety of mooring chains that can be employed in practical engineering program.Through the comparative analysis of numerical examples,the effect of wear on the fatigue life and ultimate tension of the chains will be studied,which proves the necessity of the method proposed in this paper.After that,the practical value of this method is illustrated by taking the safety assessment of FPSO mooring chains under actual sea conditions.Based on the above goals,theoretic research was carried out firstly.The OPB fatigue analysis method of the key chains was summarized,and the mechanism of the OPB phenomenon of the chains was studied.A fatigue damage assessment method and a flow chart considering the corrosion and wear of the mooring chains during the lifetime was proposed.After that,the existing ultimate strength calculation methods of marine engineering field were summarized,and an ultimate strength evolution assessment method of the mooring chains during the lifetime was proposed.Finally,the phenomenon of corrosion and wear of the chains were summarized.The numerical simulation calculation method and the flow chart of the cumulative damage of the chain were given,which had considered the coupling effect of corrosion and wear to a certain extent.Secondly,a comparative example analysis was studied.A three-dimensional elastoplastic finite element model and a worn comparison model of the chains were employed in ANSYS.The stress comparison,OPB fatigue life comparison and ultimate tension attenuation comparison was studied respectively,which proved the necessity of considering wear damage in engineering.At last,the method proposed in this paper was employed to evaluate the safety of the FPSO mooring chains under actual sea conditions during the lifetime.Above all,the actual sea conditions were simplified and the time-history curves of tension and interlink angle were drawn.Then,the cumulative damage of corrosion and wear of the chains during the lifetime was studied by using numerical simulation,and the amount of damage was evaluated.Finally,the OPB fatigue damage evolution and ultimate strength evolution assessment of the chains were studied.The results show that the amount of damage of the chains increases gradually with the service time increasing.The amount of corrosion damage is more than wear damage,but the max depth of wear is deeper than corrosion.The fatigue damage of the mooring chains increases gradually during the lifetime,which proves that the influence of corrosion and wear needs to be considered when analyzing the fatigue damage of the chains.It is obvious that the ultimate tension attenuation of the chains during the lifetime,which proves the importance of the ultimate strength evolution analysis of the mooring chains.