Numerical Simulation of Accidental Explosions in Offshore Production Plant
AUTOR(ES)
Sávio Souza Venâncio Vianna
DATA DE PUBLICAÇÃO
2009
RESUMO
The production of oil and gas is an inherently hazardous task. Therefore it is crucial to provide reliable estimates of the risks involved. The major contributors to the risk level of an offshore installation, for example, arise from accidents involving explosion and fire. Computational Fluid Dynamics (CFD) can be a powerful tool to help with the calculation of accidental explosion scenarios. In this context, the present work suggests a novel implementation of a model based on a modified Porosity Distributed Resistance (MPDR) approach within an unstructured 3D Navier-Stokes solver. The model operates by representing parts of the filtered geometry from the original model through porosity values attributed to an unstructured tetrahedral mesh. Extra resistance terms are added in the momentum equation as well as extra sources of turbulence. Two extra sources of turbulence are modelled. The first of these is due to the shear layers of the non-resolved obstacles, whilst the second is due to the presence of wakes behind the non-resolved obstacles. Combustion is treated using a laminar flamelet approach based on the Bray, Moss and Libby (BML) formulation. In flame propagation, the burning velocity increases due to flame instabilities, gas dynamics, body forces and diffusive-thermal instabilities. During the early stages of an explosion, the flame propagates in a quasi-laminar regime. Proper modelling of this initial laminar phase is a key aspect in order to predict the peak pressure and the time to peak pressure. The present work also suggests a modelling approach for the initial laminar phase in explosion scenarios. Findings are compared with experimental data. A detailed analysis of the threshold for the transition from laminar to turbulent regime is also carried out. Results for 2D and 3D test cases are compared against both experimental data and simulations with fully resolved geometry and good agreement is observed. Due to the high cost of numerical simulation of explosion scenarios, the combination of CFD and mathematical response surface has emerged as a promising approach to help on the design of industrial facilities. Findings from the CFD modelling implemented in this work are used to investigate the behaviour of different mathematical expressions. Such formulations are used to predict peak pressure as a function of gas cloud size and ignition location. Polynomial functions of first and second order with and without cross product of the independent variables are used. Peak pressure predicted values by the polynomial expressions are compared with CFD results. The analysis has shown that the second order polynomial expressions with cross product are suitable to predict peak pressure from accidental scenarios. An example of how this polynomial expression can be used is discussed.
ASSUNTO(S)
computational fluid dynamics - cfd segurança análise de risco engenharia mecanica combustion explosion fluid mechanics offshore
