Effect Of Joint Flexibility On Overall Behavior Of Jacket Offshore Platform In Pushover Analysis And A Simplified Calculation Model

The hollow circular tubular structure is widely used in the daily large-scale steel building and jacket offshore platform due to its superior performance,simple and quick construction and beautiful appearance.Generally,the pipe members of circular tubular structure are connected by welding,which are so-called intersecting joints.The radial stiffness of the chord is much smaller than the axial stiffness of the brace in tubular joint,so the wall of chord around intersecting line will be deformed when the load transfer from the brace to the chord and the joint appears semi-soft and semi-rigid features.In the actual engineering,the steel truss structure is often simplified to the ordinary beam element model which is simple in modeling and fast in analysis,but ignores the influence of local joint flexibility,so the accuracy of the analysis can not be ensured and the safety design of structure will be affected.Solid or shell element which is complicated in modeling for large tubular truss structure,occupying a large number of computer Memory and have low analytical efficiency will be necessary to accurately simulate the local joint flexibility in the calculation.In order to solve this problem,this paper proposes a method to introduce the fictitious beam element(FBE)in the ordinary beam element model.In the elastic deformation stage of the structure,the local axial and bending deformation of the joint are simulated by defining the axial stiffness and flexural rigidity of the fictitious beam element.The axial stiffness and flexural rigidity of the fictitious beam element are equivalent to the local joint flexibility value calculated by the Fessler parameter formula,the Fessler improved parameter formula,and the Ueda parameter formula.12 sets of tubular T-joint and Y-joint truss models with different geometric parameters are modeled by shell element in ABAQUS.Meanwhile,rigid frame program wrote by Fortran is used to construct the rigid frame model and simplified model.By comparing the results of three different modeling methods,the accuracy of the simplified model in the elastic deformation stage of the circular tubular truss structure is verified,and the influence of the local flexibility of the joint on the static analysis of the tubular truss structure is reflected.In the elasto-plastic deformation stage of the structure,the material properties of the fictitious beam element are redefined.Firstly,the finite element software is used to model the tubular joint and calculate the load-displacement curve.Then,the obtained curves are transformed into the stress-strain curves of the fictitious beam elements.The constitutive relation is input into the material property definition of the fictitious beam element when the simplified model is established so that the fictitious beam element can simulate the local deformation of the tubular joint in the plastic stage.Similar to the above,12 sets of tubular T-joint and Y-joint truss models with different geometric parameters are designed.The load-displacement curves calculated by shell element model,simplified model and the rigid frame model were used to verify that the simplified model could accurately analyzed in the elastic-plastic deformation stage of tubular truss structureIn order to study the effect of local joint flexibility on the pushover analysis of the jacket type offshore platform,this paper makes the static elasto-plastic analysis in a actual example of the jacket type offshore platform.The target displacement of the structure under the 8 degree rare earthquake is calculated by means of the capability spectrum in SAP2000 structure design software.Shell element model,simplified model and rigid frame model is established by ABAQUS finite element analysis software and pushover analysis is carried out to find the Pushover curve.It is found that the rigid frame model will overestimate the lateral stiffness of the structure and result in imprecise Pushover analysis which is not conducive to the safety and seismic evaluation of the structure.On the contrary,The Pushover curve calculated by simplified model has a high degree of coincidence with which in shell element model.The simplified model proposed by this paper can be used in the static elasto-plastic analysis of actual engineering.