Study On Mechanical Properties Of Steel Jacket Offshore Platform In Blast And Fire

Offshore platform is the basic infrastructure in exploration and development of themarine resource. It is the life and production base. It plays an important role at sea. Withthe feature of adaptable, safe, reliable, simple and low cost, jacket offshore platform hasbeen the most widely used structural form in offshore oil exploration. In recent years, fireand explosion accidents in offshore platform occurred frequently. It not only causedcasualties and significant economic losses, but also led serious pollution and ecologicaldestruction to the surrounding coastal and marine environment. Therefore, it is necessaryand valuable to carry out structural response of offshore platform in blast and fire onaccount of typical form.Based on reviewing of current status of the research in the related field at home andabroad, the experimental research, finite element simulation and theoretical analyticalmethod were used to carry out the research of the offshore platform under blast and fireconditions，including single members，tubular joints and integral structure. The main workand the corresponding conclusions are as follows:(1) Carrying out the dynamic mechanical test of Q345steel at different temperaturesand different strain rates. First the quasi-static compression and impact experiments studiesunder different stain rates of the Q345steel at ambient temperature were carried out. Theexperimental results show that: the dynamic strength of the Q345steel is markedly higherthan that under the quasi-static state. Separately in quasi-static and dynamic region, thestress levels of the Q345steel under different strain rates are so close to each other that itcan be taken for a non-sensitive material. Second the dynamic mechanical test which aimsto study the Q345steel’s mechanical properties at elevated temperatures under differentstain rates were carried out. Finally based on the experimental data, a constitutive model used for structure analysis under impact load was determined.(2) In order to study the mechanical behavior of the structural members in blast andfire, an experimental investigation has been carried out for a steel tubular T-joint withoutfire-proof coating. The specimen was impacted first and then heated up under constantloading. The dynamic response of the T-joint in impact load was obtained. The failuremode, the critical temperature, and the temperature distribution of the impacted T-joint atelevated temperatures were gained. The test results were then used to validate thenumerical analysis in Explicit-implicit sequence solution method, which laid a foundationfor further study of the parametric study of the T-joint.(3) On the basis of the experimental study and the numerical simulation method of theimpacted steel tubular T-joint in fire in previous chapter, the effects of the material strainrate and the different geometric parameters on the dynamic mechanical properties of theT-joint under blast load were studied. The dynamic response and the variation of dynamicamplification factor of T-joints were obtained. The effects of the prophase impact on themechanical properties of T-joints at elevated temperatures were also studied, and thechanging principle of critical temperature was gained.(4) An integral structure model of steel jacket offshore platform was established byusing the finite element software ANSYS/LS-DYNA. The dynamic response of the overalloffshore platform subjected to different blast load was simulated using dynamic nonlinearfinite element technique. The influence of strain rate strengthening effect was taken intoconsideration. Then the fire-resistant performance of the offshore platform after explosionwas analyzed systematically. The difference in mechanical properties of platformstructures at elevated temperatures in whether explosion or not was obtained. The effect ofthe previous explosion with different load intensity on the fire-resistance property of theoffshore platform was also studied, and its changing principle was gained.(5) The theoretical study on dynamic response of offshore platform structures was conducted in finite element method. Using the rate-dependent high-temperatureconstitutive model obtained in this paper, the equation of motion taken temperature andstrain rate into account was established, the stiffness equation and dynamicthermal-nonelastic stiffness matrix were derived. According to the characteristic offiber-beam element, the relevant explicit form of element including stiffness matrix,average strain increment, temperature load increment and the load increment due to strainrate changes was presented by further operation. A dynamic nonlinear element programwhich could be used to calculate the dynamic response of offshore platform structures inblast and fire was developed. The material nonlinearity and geometric nonlinearity wereconsidered, so as to the uneven distribution along the height and thickness of the steelmember cross-section. Finally, the applicability and reliability of the program has beenproved by the calculation example results.