Explosion safety evaluation for congested offshore platforms based on CFD simulations

This thesis is to evaluate the safety of congested offshore platforms subjected to vapour cloud explosion. A new confinement specific correlation was developed to predict explosion consequence based on computational fluid dynamics (CFD). A data-dump technique was proposed to improve accuracy of CFD modelling in investigating safety gap effect on blast wave propagation. Overpressure mitigating measures such as safety gap and blast wall in a cylindrical floating liquefied natural gas (FLNG) vessel were studied. Through comprehensive CFD modelling, a quantitative explosion risk assessment method was developed to provide up-to-date techniques on safety design of large scale offshore structures.
One of the major goals of this study is to develop an efficient vapour cloud explosion overpressure prediction approach in a confined and congested environment. A confinement specific correlation (CSC) was developed based on large-scale numerical simulations carried out for offshore structures with complex components. By using the commercial software of FLACS, the CSC produced a significant improvement of the Guidance for the Application of the Multi-Energy method (GAME), especially for highly congested and confined environment. The parameters used to define the confinement, the volume blockage ratio, and the flame path distance etc. were redefined and validated by using the CSC model. The proposed CSC correlated better with the CFD simulation results along with the same calculation efficiency of the GAME correlation. The irregularity effect of the congestions was further investigated by simulating artificial and realistic platform modules.
The investigation of the effectiveness of overpressure mitigating measures is another major goal in this research. A data-dump technique, which modifies the calculation of the turbulence length scale in different safety gap simulations, was firstly proposed to improve the explosion overpressure calculation accuracy during the investigation of the cutting-edge explosion mitigating measure – safety gap. Furthermore, the safety gap effect was evaluated in the vapour cloud gas dispersion and explosion simulations on a cylindrical FLNG platform. The worst scenario studies indicated that the safety gap is active in reducing gas cloud size and mitigating gas explosion overpressures by using proper safety gap designs. Finally yet importantly, a probabilistic approach, which offers more detailed risk analysis data, was performed to assess the blast wall effect on overpressure mitigation on the cylindrical FLNG platform. By considering various uncertainties, exceedance curves were derived and the most efficient blast wall design configuration was achieved.