Multi-scale Numerical Analysis Of Jacket Platform Structures

Offshore platform is an important infrastructure in offshore petroleum development engineering.Because of its harsh service environment,complex and diverse loads,the local components such as tubular joints,which are the key parts of the platform,are vulnerable to damage due to stress concentration,and may cause the damage of the whole platform.If the platform is damaged,it will lead to catastrophic consequences.Therefore,numerical simulation will be one of the important means to evaluate the safety of the platform structure.At present,for large-scale offshore platforms,the whole structure is usually established as a macro-model of tube element and beam element,but it will be difficult to reflect the details of structural damage.If the fine model of solid element is adopted,the difficulty of modeling and calculation will be increased.Therefore,multi-scale modeling method is an effective modeling method to solve this kind of problem,which can not only obtain the response of structural details,but also grasp the overall macro-information.Based on the above analysis,the multi-scale method is introduced into the numerical analysis of offshore platform structure.Aiming at the three scenarios of K-joint reinforcement,tubular joint corrosion and ship-platform structure collision,the multi-scale numerical models of platform are established by using finite element software ANSY and LS-DYNA.The method of interface connection between different scale models is studied,and its correctness and computational efficiency are verified.Based on this,the multi-scale numerical analysis of the mechanical behavior of offshore platform structures in different scenarios is carried out,the response of local joints is simulated,and the influence of local damage on the overall mechanical properties of the platform is studied.The main contents of this paper are as follows:(1)Taking the circular tube model as an example,the multi-scale model composed of solid and pipe element is established by using the multi-scale method(MPC).At the same time,the fine model of the solid element and the macro model of the pipe element are established.The simulation results of the three models under different loads are analyzed and compared.It is proved that the results of the multi-scale model are accurate and less time-consuming,which provides a theoretical basis for the application of the multi-scale modeling method below.(2)A K-tubular joint area in jacket platform model is choosing to establish the solid element model.MPC method is used to couple tubular joints into the whole jacket.The accuracy of the multi-scale model is verified by comparing with the tubular jacket model.The reinforcement model of tubular joints is established by using the method of rib plate reinforcement.The selection of rib plate parameters is discussed,and the local reinforcement effect and the overall reinforcement effect are analyzed respectively.(3)The multi-scale modeling method for pitting damage of offshore platform structures is studied and analyzed.A circular tube model with pitting pits is established by using shell element model and solid element model,which verifies the correctness of pitting damage model established by node offset method.Taking K-type tubular joints at pile legs in offshore platform structures as the research object,a model of offshore platform with locally pitted corrosion is established by using MPC method.The influence of pitting distribution at different locations on the ultimate bearing capacity and modality of the platform is analyzed.The situation of pitting location in the main pipe is selected.The influence of pitting depth and pitting radius on the ultimate bearing capacity of the platform is discussed.(4)Multi-scale validation model is established in LS-DYNA explicit analysis software by nodal rigidity method.Compared with the fine model with the same size,the validity of multiscale model is verified by the consistency of damage deformation,collision force and energy change.A multi-scale model of collision between ship and offshore platform is established.The initial velocity and collision ship mass are taken as parameter variables.The impact law of the change of parameters on collision process is studied through the change of collision force and platform damage deformation during collision.