Structure Strength And Damage Analysis Of Aged Jack-up Platform

Bohai8 jack-up platform was refitted from slot type to cantilever-beam type and is in its later service period now. The refitting changes the structure form and stress distribution and thus causes mismatching between carrying capacity and loads. According to the problems of BohaiS met in service,the platform that is in lift state is analyzed from aspects of static,fatigue,buckling and deformation. On this base,the structure strength check and damage evaluation are carried out and a proposal plan for improving strength of the platform structures in the next refittment is brought forward.First,integral special shell-beam element model of BohaiS is set up. After applying environment loads and work load and taking into account dynamic effect of drilling load,the stress distributions of the platform under various conditions are calculated by using finite element analysis software. Then the static strength of main hull and legs are checked.Second,a fatigue area of the main hull and cycling load that is different from previous (i.e. typical fatigue of tubular joints of legs) is put forward in this thesis. It is found through analysis that the fatigue happens at the vertical weld of yoke house and is caused by the fluctuation of drilling load. The fatigue accumulated damage is calculated and fatigue life estimated by adopting P - S - N curve method and linear accumulative damage theory. The affects of residual stress,stress concentration and mean stress are taken into account in calculation.On the base of experiment and comparative study,a dynamic crack propagation model of the yoke house fatigue area is established,which can be used to forecast the length of crack and estimate crack propagation life. By using residual strength interference theory and damage tolerance idea,the reliability of the structure is evaluated. Then the crack propagation rule and reliability index are modified through using field crack length measured data and Bayesian method. To deal with the deformation problem of machinery deck,global and localbuckling analysis are carried out by using theoretical and experiential formula in this thesis. Comparing with stress results from finite element analysis,the reason and type of buckling are confirmed and a plan to eliminate local plate buckling is given.At last,the maximum deflection and integral deformation of cantilever-beam structure are analyzed on the base of finite element analysis. Due to the requirements of the ship owner and structure characters,a structure refit scheme of letting up the deformation of the cantilever-beam is brought forward and quantitative calculation is conducted.