Offshore Explosion Hazard Evaluations using a 3D CFD Code with Reduced Combustion Combined with a 1D CFD Code or Blast Curves
Authors: Fitzgerald G.A. 
Published: October 2008

A 3D CFD model with a reduced combustion capability can be used with a dispersion model and either a 1D CFD code or a blast curve method to provide an accurate explosion hazard evaluation early in the life of an offshore project. The analysis using this alternate approach is more accurate than using the 1D CFD code or blast curves alone because it accounts for blast wave interactions with structures, actual vent surfaces, venting into other areas of congestion and provides a more realistic blast wave decay for the specific scenario.

Introduction

The "North Sea" approach to predicting a vapor cloud explosion (VCE) in offshore applications has been the use of traditional three-dimensional computational fluid dynamics (3D CFD) models. Traditional 3D CFD models can be costly to obtain and are time consuming to operate. It would be reasonable to expect several man-weeks per scenario to build the traditional 3D CFD model and many days for the model to run. Although this would provide the most accurate prediction available, this evaluation is typically performed late in the life of a new project because only then are sufficient details known to develop the model. Unfortunately, this is also when modifications are difficult and expensive to make. The tools discussed in this alternate approach idealize the process layout, hence a general knowledge of the process layout will allows the using this alternate approach much sooner in project life than typical for a traditional CFD analysis. This has the potential for significant savings by discovering problems early in project life. The proposed alternate approach also results in a faster "what-if" analysis to evaluate potential corrective actions.

An example application of this alternate approach is presented for illustration. While this alternate approach is primarily targeted for a new offshore construction project, it also can also apply to existing offshore installations needing an explosion hazards evaluation. This alternate approach does not preclude the use of traditional CFD as a final validation of the explosion hazards.

Analysis Description

This alternate approach is broken into three tasks:a vapor cloud size evaluation;a peak pressure analysis; andan actual venting and blast wave interaction analysis.

The vapor cloud size evaluation uses engineering judgement, a dispersion model, ventilation calculations, or a combination thereof to evaluate the vapor cloud size. The peak pressure analysis is performed using either the 1D CFD model SCOPE [1] (Shell Code for Overpressure Prediction in gas Explosions), for confined explosions, or a simple blast curve method such as the latest Multi-Energy Method [2] (MEM2) or Congestion Assessment Method [3] (CAM2), for unconfined explosions. The actual venting and blast wave interaction analysis is performed using the 3D CFD model CEBAM [4] (Computational Explosion and Blast Assessment Model) with a simplified ("reduced") combustion computation. This alternate approach uses the strengths of each analysis method to overcome inherent potential errors each method has in order to maintain simplicity. These steps are described in more detail in the following sections.