Study On Multi-scale Simulation Of Structural Damage And Mechanical Properties Analysis Of Offshore Derrick In-service

Offshore derrick is an important equipment for the development of offshore oil resources,which has been working in the harsh marine environment for a long time,and the corrosion problem is serious.In addition,the offshore derrick will also bear strong winds,strong waves and other extreme environmental loads,so that its structure will be damaged inevitably.Therefore,it is great significance to study the influence of structural damage on the mechanical properties of derrick.In the previous research,the full beam element is mostly used to simulate all kinds of damage through equivalent simplification or model modification,which is difficult to reflect the local details.In this paper,multi-scale finite element method is used to simulate the structural damage of offshore derrick in-service and analyze its mechanical properties.The main research contents and results are as follows:Firstly,the multi-scale analysis method and multi-scale model construction method of offshore derrick are studied.The damage characteristics and multi-scale characteristics of offshore derrick are analyzed,and the cross-scale interface connection between full-scale and meso-scale is realized by using the theory of displacement coordination and stress continuity,and multi-scale model construction method considering damage is proposed.Taking the derrick of an offshore platform as the research object,the multi-scale model of derrick considering column and joint characteristics are established.The full beam model and two multi-scale models are analyzed and compared.the results show that the multi-scale model can well reflect the stress distribution in the local details of the derrick,and verify the feasibility of the multiscale analysis method.Secondly,the multi-scale simulation and mechanical performance analysis of offshore derrick under pitting damage are carried out.The corrosion characteristics of offshore derrick are analyzed.Taking the pitting damage as the research object,the regular spherical pitting pit model is adopted.A multi-scale model of single point corrosion damage of derrick is established.The pitting damage is simulated under different pitting parameters.The evolution of pitting damage is analyzed.The effects of pit’s width,depth,width-depth ratio and volume on the stress concentration factor,stiffness and bearing capacity of the structure are obtained.On this basis,the random pitting damage characteristics of offshore derrick are analyzed.The random pitting model of derrick under multi-point erosion is established.The effects of whole distribution pattern,local distribution pattern,overall position and distribution density of random pitting on the bearing capacity of derrick are analyzed.The results show that the overall position and distribution density of random pitting pits have a great influence on the bearing capacity of derrick.Finally,the multi-scale simulation and mechanical performance analysis of offshore derrick under beam-column joint damage are carried out.The characteristics of joint damage of offshore derrick are analyzed.Taking the welded joint as the research object,the weld defect model is simplified,just as using hollow ball instead of slag inclusion.The weld cracking at the beam-column joint of derrick is studied.A multi-scale model of joint damage of derrick is established.Based on the theory of fracture mechanics,the location and depth of cracks are simulated and analyzed.The influence of the location of the crack and the depth of the crack on the joint is obtained.The results show that the location and depth of the crack have a great influence on the mechanical properties of the joint.