Research On Seismic Damage States Of Jacket Offshore Platform Structures

Steel jacket offshore platforms are widely employed in offshore petroleum activities throughout the oceans of the world at the present time. For a typical offshore structure located in a seismically active region, the environmental conditions affecting offshore platform structures are very complicated and harsh. And the earthquake ground motion becomes an equally important design load of structural excitations as well as the sea waves. This is due to abruptly changing stress amplitudes during the earthquake, which can cause serious damages although the earthquake duration is very short and does not occur so frequently as the wave. Moreover, it is a pity that there is no appropriate means appears to forecast the earthquake well and truly due to limitations of state-of-the-art technology. Whereas the platform is a very important structure, once collapse induced by severe earthquake exceeding the "strength-level", the loss as well as the social influence caused by the catastrophic results will be very grave.The purpose of this dissertation is to investigate collapse behaviors and damage states of jacket offshore platforms under the severe earthquake. The collapse behavior is divided into three stages and a brief summarize of available analysis method in respect to every response stage. And special emphasis is placed on the analysis of the dynamic response characteristics after structure goes into the damage stage. A characteristics time-varying analysis method along with the concept of structural seismic safety margin coefficient is proposed. Practical design example is given on collapse analysis of a jacket offshore platform structure under earthquake with different characteristics. And some conclusions are achieved. The analysis results give useful information to seismic design and assessment of the seismic safety of steel jacket offshore platform.The main results of this study are as follows:(1) Dividing the collapse response process into three stages: stage of intact condition, stage of plastic deformation, and stage of damage. A brief outline of features of system behavior in every response stage and corresponding available analysis method are discussed.(2) The dynamic response analysis after structure goes into the damage stage is performed introducing the proposed method, characteristics time-varying analysis method.The proposed method enables the effects of the structural damage to be considered during a collapse analysis. This way of taking damage into account is important because the structural configuration at the moment in which the damage occurs is different from the un-deformed configuration and considerable redistribution of forces takes place when the damaged member fails to accomplish its structural function. And in the modeling of damaged structural members, strength damage and buckling of steel tubular members are considered.(3) A theoretical algorithm for automatically analyzing kinematical instability of three-dimension space structures are introduced.(4) For the jacket offshore platform structure, the members of which are divided into three classes by means of the forced characteristics and significances. And the different damage state of a platform structure can be defined as the reparable damage state and the failure state.(5) The concept of structural seismic safety margin coefficient is proposed. And the corresponding expression of the coefficient according to different damage state is given.(6) Based on the above theory a computer program PHFX has been developed. The program is especially aimed at collapse analysis of jacket offshore platform structures under earthquake using the proposed characteristics time-varying analysis method.(7) Another program for computer dynamic simulation is worked out. The visualization of collapse process of three-dimension structures is carried out.