Research On Explosion Risk Analysis And Mitigation Design Of Offshore Platforms

This dissertation is supported by“Safety Risk design and control technology”of the independent innovation engineering on the seventh generation deepwater semi-submersible platform from the Ministry of Industry and Information Technology of China,and“Research on Evolution of Chain Risk,Disaster Mechanism,and Response Mechanism of Major Accident for Oil and Gas production in Offshore Industry”from National Key R&D Plan of China.According to the site actual operational requirement of deepwater drilling in South China Sea,taking the 7th generation deepwater semi-submersible platform that is an original design as the main research object,a systematic research on Explosion Risk Analysis(ERA)methodology and explosion mitigation modelling of offshore platform is carried out.This research significantly contributes to the offshore platform design to mitigate the explosion risk including the flammable cloud volume esimation,methodology development of ERA,efficient design model for corrugations,robust vent area design model of hydrocarbon-air mixture inside the cubic enclosure with obstacles,optimal water spray design methodology.The main achievements of the paper are summarized as follows:1.ANN-based model to estimate the flammable cloud volume in offshore facilityResponse Surface Methodology(RSM)and Frozen Cloud Approach(FCA)have been widely used for Explosion Risk Analysis(ERA)to achieve a balance between simulation workloads and accurate results.However,the worse performance of both methods are obvious in terms of the flammable cloud volume estimation.The Artificial Neural Network(ANN)with higher accuracy is thereby introduced.Two case study of varied offshore platforms are conducted.The performance of two commonly used ANNs for function approximation,namely BRANN and LMANN are investigated.In addition,comparisons among two ANNs,RSM and FCA in terms of flammable cloud volume prediction are compared.At last,the BRANN model,which can be a robust and accurate alternative for the following ERA,is proposed.2.BRANN-based Explosion Risk Analysis methodology of offshore platformERA during the early design phase of offshore platform aims to quickly determine the optimal layout to mitigate the explosion consequence.Accordingly,the efficiency of the corresponding ERA procedure becomes a significant issue.This study firstly develops the ERA procedure based on BRANN model.Comparsions between BRANN-based ERA and RSM-based ERA are conducted and the more robustness of BRANN-based ERA for the early design phase is confirmed by a case study of ultra-deep-water submersible offshore platform.Additionally,considering the more accuracy of the ERA results during the detailed design phase and the overfitting problem that BRANN model may occur,the ABC-BRANN algorithm is firstly developed and the ABC-BRANN-based ERA procedure is proposed.A case study of fixed offshore is conducted to show the more accuracy of ABC-BRANN-based ERA over the FCA-based ERA.3.P-I model for efficient design regarding corrugations on offshore platform to improve the blast loading resistance capacityPressure-impulse(P-I)diagrams are commonly used for assessment on blast resistance capacity of corrugations on offshore platform for given blast-loaing scenarios or in the early design.Current methods such as experimental models,analytical models and Nonlinear Finite Element Analysis(NLFEA)models could be used to generate the P-I diagram.However,each single model has certain shortcomings in despite of its advantages.Based on the convenience of analytical model and the accuracy of numerical model,this study presents a numerical procedure to derive analytical formulae to easily generate P-I diagram for corrugations with NLFEA method.Numerical model of corrugations which considers strain rate effect,buckling and membrane effects and adopts an equivalent plastic strain based failure criterion is built.Parametric studies are performed to investigate the paramtric effects on the P-I diagram.Based on the numerical results,analytical formulae to predict the P-I diagram are derived.An experimental case study shows that the proposed analytical formulae can be easily used to generate P-I diagram for corrugations.The results are also compared with those obtained from the SDOF approach.It is shown the the proposed procedure gives better prediction of P-I diagram than the SDOF approach.4.An integrated model for vent area design of hydrocarbon-air mixture explosion inside cubic enclosures with obstaclesCurrently,three methods are commonly applied for vent area design of hydrocarbon-air mixture explosion inside cubic enclosures with obstacles.However,the performance of those methods is still poor under specific conditions.Accordingly,an integrated model,namely NFPA-68-BRANN model,is presented by combining the developed data-driven model by the BRANN with the revised NFPA-68 model.The publicly available tests of cubic enclosure with homogenous obstacle arrangements inside are summarized.Two current standards,namely the NFPA-68 2018(including the original and adjusted ones)and BS EN 14994:2007are examined with respect to the experimental data.The existing issue with NFPA-68 2018model is discussed and its specifically phenomenological equations are revised.Subsequently,the BRANN model presenting the relationship between the ratio X/X₀ and its affecting factors is determined.Accordingly,the NFPA-68-BRANN model is generated and Newton-Raphson optimization algorithm is applied to determine the vent size.The New Baker Test is used to validate the efficiency of the integrated model.This integrated model is validated to be a robust alternative,which can be used for vent design in combination with the above ERA procedure.5.Optimal design methodology for water spray regarding explosion mitigation of offshore platformConsidering the accuracy of FLACS to describe the 3D explosion scenarios with water spray of offshore platform,the optimal design methodology for water spray design regarding explosion mitigation is procedure by appling the ABC-BRANN-based ERA procedure Firstly,the feasibility of ABC-BRANN-based ERA procedure with water spray is demonstrated by performing a case study of ultra-deep-water submersible offshore platform.Subsequently,parametric sensitivity analysis of water spray on the DALs determine from the ABC-BRANN-based ERA procedure is conducted.Furthermore,the surrogated model is generated to present tne non-linear relationship among the DALs and the corresponding water spray paramters by employing the BRANN model.Evetually,the optimal parameters are determined by adopting the ABC algorithm.