Study On Formal Safety Assessment Of LNG Carrier Structures After Accidents

In order to improve the maritime safety level,the International Marine Organization(IMO)began to promote the Goal Based Standard(GBS)in 2002,and proposed the Safety Level Approach(SLA)to gradually replace the empirice based prescriptive standards.With the promotion of SLA,the risk-based ship design method for single ship will play a more important role in the design and construction of inovative ships.Formal Safety Assessment(FSA)is an important universal tool to realize the SLA method.However,there are still many gaps to be bridged between the theoretical method of FSA and the application of FSA in risk-based ship design.For example,there aren’t risk analysis model and risk acceptable criterion for hull structural risks;it is lack of effective decision-making tools to support risk-based optimization design in FSA method system,etc.These issues mentioned have become main obstacle limiting the application of FSA and SLA method.As an important part of the high-tech ship research projects of “The FSA/SLA approach study” and “The FSA/SLA application study”granted by the Ministry of Industry and Information Technology of China,this dissertation is aimed to study how to improve the current FSA method system to make it more consistent with the needs of the risk-based design of ships and explore a feasible way to realize application of SLA in the design and construction process of ships.Liquefied Natural Gas(LNG)carriers are a key equipment to ensure the strategic security of energy in China.The design and construction of this shiptype is a high-tech process with many difficulties.And there is a wide space for the innovative design or the alternative options of this shiptype.Considering dangerous cryogenic LNG and complex cargo containment system,as well as,many non-traditional uncertainty factors in accidental safety assessment of this shiptype,LNG carrier is selected as the objective shiptype to study the structural risk analysis model and the corresponding risk acceptable criterion of the hull structure after accidents;improve cost-effectiveness anslysis method and risk-absed decision-making method in FSA,and some positive results are obtained.The main study works of this dissertation are as follows:(1)The accessible matrix method is used to bulid a hierarchical structure of 15 kinds of structural safety factors provided by IMO maritime safety committee(MSC),and to analyze the relation of the factors in structural safety system.Refering to the experience of risk-based design of offshore engineering,the main targets and the methodology of safety assessment of hull structures in accidental state are studied.On this basis,the framework of risk-based design of hull structures and the specific requirements of this framework for FSA method are proposed.(2)The method of simulating the temperature field distribution of LNG carrier by using the finite element analysis is studied.Moreover a nonlinear finite element analysis is used to calculating the residual utilmate strength of damaged LNG hull after grounding.In the calculation,based on the temperature field distribution,the cryogenic stress and brittle damage of LNG leakage on the hull are analyzed in some grounding conditions.(3)A structural reliability analysis(SRA)model based on a Bayesian belief network(BBN)is proposed for the hull girder collapse risk after accidents.The hull collapse risks of a membrane LNG carrier and a SPB LNG carrier after grounding are respectively analysed using the BBN-SRA model.(4)The structural risk indicators,average Potential Structural Failure of the hull girder in a period of time(PSF)and corresponding F-S(Frequency of occurrence-Structural failure probability)curve,are presented especially for the hull failure mode after accidents.Furthermore how to determine the risk acceptable criterion based on ALARP(As Low As Reasonable and Practicable)principle is studied.In a case study,the structural risk assessment of a SPB LNG carrier is done by using the PSF indicator and the F-S curve to verify the feasibilities of them..(5)The application of Utility Theory in risk-based ship design is studied to improve Cost Effective Analysis(CEA)of risk control options(RCOs).In a case study,a risk control option of LNG carriers for accident risks is assessed by the risk utility based CEA model to verify the feasibility of it.(6)A multi-attributes assessement model is proposed to select the more optimized RCOs.In this model,a dynamic multi-attributes system is designed based on ALARP principle.Through comparing the Utility Function Method and Fuzzy Comprehensive Evaluation,the former method is selected to synthesize the multi-attributes and a Grey Rational Analysis(GRA)model is used to improve the mothed.Two cases about LNG carriers and crude tankers are studied using the proposed model to verify the feasibility of it.A series of innovative studies are carried out in this dissertation to improve the traditional FSA method and make it more suitable for the risk-based design of ship.Firstly,in order to enable FSA to assess the structure failure risks of hull in accidental conditions,some innovative studies are done:(1)the BBN-SRA model is built to analyze the risk of hull structure failure after an accident.In this model,BBN is used to represent the random states of variable risk events after accidents,as well as the dependencies between events;and SRA is used to evaluate the failure probability of hull girders for each possible accident condition.(2)Two other functions of BBN-SRA model,the diagnosis of main risk factors and the rapid estimation of accident consequences with new evidence,are developed using BBN reverse reasoning algorithm.(3)When calculating the residual ultimate strength of the damaged hull after accidents,the method of simulating the cryogenic compact of LNG leakage on the hull is studied.(4)The PSF value and F-S curve of the hull failure risk after the accident are proposed,and the methods of determining the risk acceptable criterion are studied.Secondly,in order to make FSA more scientific and systematic for supporting the decision-making in risk-based design of ships,some other innovative studies are also done:(5)An algorithm of equivalent accident loss,including a risk utilization process and a risk anti-utilization process,is proposed to enable the traditional CEA model to concider the risk aversion.(6)In order to select the more optimized RCOs,a dynamic multi-attributes system and a comprehensive assessment model based on GRA theory are proposed,and that an integrated weighting method based on AHP-Game theory is studied to balance the subjective and objective weights of the muti-attributes.