| Under the combined action of wind,wave and current,mooring chains that in the harsh ocean environment will not only be subjected to the axial tensile load generated by the reciprocating motion of floating platform,but also be subjected to complex loads such as out-of-plane bending or twist.The stress of chain links in some key positions may exceed the design strength,resulting in the failure of mooring system,which in turn threatens the safety of entire floating platform.Heat treatment process and proof loading of mooring chains production will produce residual stress,which affects the stress characteristics and safety performance of chain links in service.Therefore,under the consideration of residual stress,it is of great significance to carry out stress analysis on chain links under different load modes.This paper aims to obtain the residual stress generated by heat treatment process and proof loading of mooring chains through numerical simulation.Based on this,the stress distribution and contact characteristics of chain links under different load modes are analyzed to study the influence of out-of-plane bending or twist with different angles,and the influence of residual stress on chain link stress.Based on the above research goals,firstly,with the help of finite element software Ansys Workbench 18.2,the finite element model of R4 grade 76 mm stud-less mooring chains is established.Indirect method is used to simulate the residual stress of heat treatment process,and it is found that heat treatment leaves considerable compressive residual stresses at the surface and tensile at the core.According to the DNV specification,the proof loading is applied,and it is found that there is large tensile residual stress at the edge of contact zone and the center line of section,and large compressive residual stress in bending area and crown.Then,three finite element models are established,which is the model do not consider residual stress,the model only consider residual stress of proof loading,and the model consider residual stress of heat treatment and proof loading,to study the influence of residual stress on stress distribution of chain links under axial tensile load.The results show that residual stress generated by heat treatment and proof loading has a significant impact on stress distribution of key position where the chain link is prone to damage.Finally,the differences of stress distribution,contact state and fatigue life of three models under out-of-plane bending and twist with different angles are analyzed.The results show that equivalent stresses of three models are approximately the same under out-of-plane bending with small angle,but are greater than those under axial tensile load.Equivalent stress of bending area on the side away from the bending direction increases with the increase of bending angle,and the trend of other side is opposite.Equivalent stress of three models increases as twist angle increases.The reason is that as twist angle increases,the contact state of links gradually changes from sticking to sliding.By comparing the stress characteristics of chain links under the same load with specific angle,it is found that equivalent stress of model b is the largest.With 2°out-of-bending angle,the equivalent stress of model A and model C is5.8% and 15.4% smaller than those of model B,respectively;with 10°twist angle,the equivalent stresses of model A and model C is 7.4% and 7.1% smaller than those of model B,respectively.This is because proof loading will generate tensile residual stress at the edge of contact zone,which will increases equivalent stress;while heat treatment process will form compressive residual stress on the surface of chain link,offset a part of tensile stress,and reduce overall equivalent stress.Calculation results of fatigue life show that the trend of overall fatigue life of chain link is opposite to the trend of equivalent stress.Therefore,proof loading and heat treatment process should be carefully considered in numerical simulation. |
PDF Full Text Request