Research On Time-varying Dynamic And Operation Safety Assessment For Deepwater Jack-up Unit

Accidents of Jack-up unit under operation condition frequently occur in recent years.The accidents for the elevating and ballasting conditions show the characteristics of uncertainty,abruptness and time-varying,especially,which pose a serious safety hazard to Jack-up facilities.With the increase of offshore oil exploration and production,the development of Jack-up unit trends to deeper water,higher efficiency.So it?s necessary and essential to investigate the time-varying dynamic behavior and operation safety assessment for Jack-up unit.The dissertation is supported by the National Natural Science Foundation Program of “Research on survivability and dynamic catastrophe emergency measures for fixed platforms under extreme marine environment” and the Fundamental Research Funds for the Central Universities of “Research on Time-varying Dynamic Behavior for Jack-up unit in the Elevating Condition”.The time-varying dynamic behavior and safety assurance technology for Jack-up unit are studied systematically in this work.The research covers some hot issues including the key mechanical properties,elevating dynamics analysis,dynamic catastrophe evolution analysis,time-dependent reliability evaluation and health status inversion for Jack-up unit.Operational mode design method and early warning of accident system for Jack-up unit under elevating and ballasting conditions are achieved from this research.The related findings can provide a theoretical basis for operation safety assessment and management of deepwater Jack-up unit.The main work is summarized as follows: 1.Research on mechanical behavior of jacking systems and spudcan-soil interactionKey mechanical properties of Jack-up unit are investigated for jacking system and spudcan-soil interaction.Numerical analytical model and simulation model for jacking system dynamics analysis is built.In terms of staggered tooth meshing optimization scheme,dynamic characteristics such as meshing strength,comprehensive meshing stiffness and dynamic response in pinion and rack drive process are studied systematically.The advantages of staggered tooth meshing optimization in the bearing capacity and transmission stationarity are discussed.We also verify the accuracy of the simulation model.Based on the simulation model,the influence of different engineering empirical formula on the spudcan parameters,stratum influence mechanism and soil backflow characteristic is investigated.Additionally,the applicability of bearing capacity prediction for double-layered soil foundation is investigated.The nonlinear analytical model of spudcan-soil coupling is established based on Coupled Euler-Lagrange method(CEL).Taking foundation centrifuge experiments as research object,the soil flow characteristics and penetration resistance change are analyzed in the process of spuncan penetration into soil process.Comparison of simulation and experimental results for double-layer soil foundation are carried out to show the validation of the proposed model.Finaaly,engineering cases study of spuncan penetration analysis is carried out by using CEL method,and the influence mechanism between each soil layers in multilayer soil foundation is investigated.In this way,the advantage of CEL method is illustrated in the prediction precision comparing with engineering empirical formula.2.Integrated dynamics analysis of deepwater Jack-up unit in elevating conditionBased on dynamic model of jacking system,a dynamic simulation model for Jack-up unit under elevating condition is established using rigid-flexible coupling dynamic analysis method.The modal reducing method is used to truncate the system modal,and in addition,the influence of modal reduction on the analysis accuracy and computational efficiency is discussed.The influence of flexible and rigid leg on the dynamic performance of Jack-up unit under elevating condition is compared,and the sensitivity of the structural response of the truss and shell leg platform to the leg flexibility is revealed.According to the emergency conditions of gear drive failure and unsynchronization,transmission stability and time-varying regularity of gear load distribution are analyzed and summarized.Elevating control and dynamic parameter monitoring device are integrated to build the global elevating dynamics test system,and dynamic performance test research in normal and punch-through elevating condition are carried out.By contrasting the test data and FEA results,the applicability of the simulation model is verified through the research on dynamic parameter change.Taking into account both influence degree and sensitivity of each safety evaluation index on different elevating conditions,a time-varying security decision model during ballasting&elevating operation is established based on Multi-Attribute Decision-Making Theory.Instability risk of each task sequences is brought forward as optimization control object function,and a decision-making method of leveling plan is proposed.3.Dynamic catastrophic evolution analysis of deepwater Jack-up unit in emergency conditionBased on spudcan-soil dynamic coupling analysis method,dynamic catastrophic evolution mechanism are investigated for survival and punch-through conditions.Accurate description of damage occurrence and evolution in component level are derived based on shear failure criterion.Additionally,platform overall failure criteria is proposed.Then a system dynamic disaster analysis model is established considering pile-soil coupling.The influence of the damage evolution and spudcan-soil restraint on the failure of the platform is discussed,and system resistance criterion is proposed based on the definition of structural resistance and foundation resistance.The evolution model of system resistance is revealed under emergency conditions of storm survival and punch-through,and further comparative analysis of dynamic disaster development process is performed.According to the damage platform after emergency operation,analysis system of residual bearing capacity is established,and the development of system dynamic parameters is revealed during punch-through.Depending on the criterion of performance requirements,safety critical state is defined in three aspects: normal operation,emergency operating and ultimate bearing.Considering the change of residual bearing capacity,formulation method of operation window is proposed in order to explore the distribution of operating window in different safety critical states.4.Time-dependent reliability analysis of deepwater Jack-up unit during punch-throughTime-dependent reliability is investigated based on the model of dynamic catastrophic evolution and spudcan-soil interaction.The reliability analysis model is established for Jack-up unit under punch-through condition.On the basis of the latin hypercube sampling,structural vulnerability is studied using the traditional Monte Carlo reliability analysis method,and the differences of vulnerability curves under a variety of sample numbers are discussed.In view of the limitation of the traditional reliability analysis method,two novel approach called HOMM-Pushover method and SAE-MC method are proposed considering reducing the number of samples and constructing the implicit limit performance function,which promote the improvement of the method for reliability solution.According to different failure states,the influence of model parameters and sample number on the prediction accuracy of structural vulnerability is discussed.Comparing the analysis results between traditional method and improved method,the advantages of improved reliability analysis method are verified.A system reliability analysis method is proposed by considering the random parameters coupling effect of spudcan and foundation.The system failure probability is evaluated quantitatively under the condition of complex foundation in different risk levels.System risk evolution process is revealed with ballasting load as a variable,and the sensitivity of randomness for structure and foundation to the system reliability is compared.5.Research on health status inversion of Jack-up unit based on structural monitoringIntelligent monitoring method is investigated based on theoretical analysis results of dynamic behavior and safety performance.The health state inversion system of Jack-up unit structure is established based on discrete inversion method.The method of deep belief networks is investigated to extract the feature of the non-stationary output signal to improve inversion method.Damage inversion of Jack-up unit structure is studied based on simulation data.Integer-coded genetic algorithms is used to optimize the sensor layout on the basis of the determination of optimization criteria.The applicability of the improved inversion algorithm for single damage and multi-damage combination is discussed,and the influence of modal order on the identification accuracy is revealed.Structural safety inversion experimental system is built,and random wave force excitation is simulated by the force-control output of the shaking table.Inversion test is designed considering punch-through and structural damage,and comparative analysis of identification accuracy is conducted from different inputs,which are acceleration signal,strain signal and synthetic signal.Principal component analysis is used to extract the abstract feature of damage,which is compared with the traditional damage identification feature to illustrate the advantages of improved inversion method.The core algorithm of positive and inverse problem analysis for structural mechanics is integrated to develop deepwater Jack-up unit elevating safety monitoring system.