Research On Dynamic Catastrophe Mechanism And Survivability Of Jacket Platform

As the critical device for offshore oil exploration and development,the jacket platform isnot only subject to conventional environmental,but also to bear extreme environmentalconstantly during service time,such as extreme wind&wave or strong earthquake etc.Unlikethe mobile offshore platform,the jacket platform can only bear the extreme environment loadpassively.Once the resistance of jacket cannot meet the requirements of extreme loads,theplatform may be seriously damaged,which may cause inestimable devastating loss.Therefore,it is necessary to research the extreme dynamic catastrophe mechanism and survivability ofjacket platform under extreme environment,to provide technical support for the project ofjacket platform’s disaster-resistant design and safe operation.The dissertation is supported bythe National Natural Science Foundation Program of “Research on survivability and dynamiccatastrophe emergency measures for fixed platforms under extreme marine environment” and“Research on catastrophic collapse mechanism and robustness of aging offshore platform”.The extreme dynamic catastrophe mechanism and survivability of jacket platform has beenstudied systematically.The research has made some progress in the study of wind-inducedvibration of towering derrick,numerical of extreme wave in deck,survivability of platformunder extreme wind&wave,survivability of platform under strong earthquake,risk andcatastrophe emergency measures under extreme environment.The main work is summarizedas follows:1.Wind induced vibration of jacket platform topside moduleThe distribution of aerodynamic coefficient and base moment spectrum is obtainedthrough the scaled high frequency force balance tests under 0~360° wind directions.Themethod and formula for calculating the self/cross power spectrum of generalized modal forceis established and deduced,furthermore the frequency domain method for calculating thewind induced vibration is established by mode superposition method,also the impact of theparticipation mode and the vibration mode combination on the frequency results is discussed.The space distribution of along/across fluctuating wind load spectrum is deduced based on thebase moment power spectrum of scale model,then the time history of along/across fluctuatingwind load is obtained by improved harmony superposition method.The time domain analysisof wind induced vibration is studied by Newmark method,also the characteristics ofalong/across wind vibration of towering derrick and the change rules of along/across gustloading factor with the height and wind direction are emphatically discussed,which mayprovide valuable reference to the typhoon-resistant design of the topside towering structure ofplatform.Also this chapter can provide the basic wind load data for the survivabilityassessment of jacket platform under extreme wind&wave.2.Research on wave in deck of jacket platform under extreme waveBased on the wave potential theory,a numerical three-dimensional wave tank isdeveloped by the secondary development of FLUENT software.The accuracy andengineering applicability is validated through Airy and Stokes 5th wave theory.Themechanism of wave running up the legs is studied through the numerical wave tank,and theimpact of wave-steepness and the interference effects among legs on the running up height isdiscussed.The numerical result is compared with the empirical formula,and points that theempirical formula may produce big error in predicting the air gap height of platform.Furthermore,extreme wave in deck is studied through numerical wave tank,in order toreconstruct the change process of flow field around platform and obtain the characteristics ofhorizontal and vertical wave impact force during wave in deck process.The results arecompared with API code,and then the characteristic of horizontal and vertical wave impactforce of platform can be predicted accurately.Finally,this study can provide technical supportfor the survivability assessment of jacket platform under extreme wind&wave.3.Survivability assessment of jacket platform under extreme wind&waveBased on the nonlinear plastic-elasticity theory,the IPOA and IDPA method are providedin this paper,which are suitable for the performance status evaluation of platform underextreme wind&wave.Based on the performance design and evaluation principle,the damagestate of platform is divided into five grades(DS1~DS5 grades),also the assessment criteria ofultimate states(LS1~LS4)is constituted,and finally a new calculation method and process forevaluating the survivability of platform under extreme wind&wave is proposed.In connectionwith wind sensitivity for the topside towering derrick of jacket platform,the weakness inwind-resistant,main failure mode and viability envelope of towering derrick under extremewind&wave are obtained by IPOA and IDPA analysis.Based on the Stokes 5th wave theoryand wave-in-deck model,the weakness in wave-resistant,main failure mode and viabilityenvelope of the whole platform under extreme wind&wave are obtained by IPOA and IDPAanalysis.4.Seismic performance and survivability assessment of platform under strongearthquakeFollowing the structural feature of offshore jacket platform,the calculation methods forassessment of elasticity-plasticity seismic performance are provided,such as capacityspectrum method,MPA and IMPA.The main failure mode and seismic performance ofplatform are obtained through capacity spectrum method,MPA and IMPA analysis with theintensity of 8-degree(fortification and rare earthquake).The validity and accuracy of thesethree methods are summarized by comparing the results of the three methods.Considering theuncertainty and randomicity of earthquake,the seismic vulnerability of platform is studiedsystemically by IDA analysis based on large sample earthquake data.Furthermore,a newcalculation method and process for evaluating the survivability of platform under strongearthquake is proposed based on the seismic vulnerability design,and the survivability ofplatform under different exceedance probability of different damage state is emphaticallydiscussed.5.Research on risk and emergency framework of platform under extremeenvironmentBased on the failure modes and damage states of platform under extreme wind&wave,the accident scenarios of different failure modes and collapse chain scenario of toweringderrick and whole platform under extreme wind&wave are studied by event tree and safetybarrier method,and the risk levels of towering derrick and platform under extremewind&wave are determined.Considering the aleatory uncertainty of earthquake,the annualaverage risk level of different damage states and probabilistic risk level during service periodof platform are quantitatively evaluated based on the probabilistic seismic theory.Theemergency decision system framework of platform under extreme environment is establishedbased on the damage state,risk level,integrity management ideology,monitoring andearly-warning technology of platform under extreme environment,which can provideengineering guidance for the disaster-resistant risk evaluation and emergency decision ofplatform under extreme environment.