Cost-Effective Explosion Risk Management of FPSO's
Authors: Huser Asmund , Rasmussen Olav 
Published: October 2008

Abstract
In the paper is presented applications of a new improved method for explosion risk management, applied primarily to FPSO's. First, the method will be described, then examples of field applications, further comparisons with older methods, and benefits of the new method.
The method can be used in all stages of an offshore process installation for explosion risk management, both FPSO's and other platforms. It is well suited for the early design phase, detail design phase, and during re-building of existing designs.

Introduction

HOLD
Applications of EXPRESS to fixed platforms is detailed in /10/.

Method
The explosion risk analysis involves collecting leak data environmental data, and ignition data, CFD modelling of gas dispersion and explosion, set-up of response surfaces and running scenario simulations in EXPRESS. A schematic flowchart of the procedure is given in Figure 1. For each new region, a few new CFD simulations are required in order to make the response surfaces. Dependent of the complexity of the area, from 10 to 20 dispersion scenarios and from 20 to 40 explosion scenarios are recommended. A database of general response surfaces of typical geometries has been developed. This is applied as a starting point when new analyses are performed. A detailed description of each main component of the analysis is given below.
The response surfaces are sets of mathematical equations which give the cloud sizes and the explosion pressures as functions of the dependent variables 8. The general shape ofthe mathematical equations have been determined from the present development work using regression analysis to curve fit simplified mathematical equations to the CFD results. In this work also physical evidence, symmetry, and engineering judgement have been applied to form the response surfaces. The response surface technique is applied because it represents a fast method to calculate consequences for the large number of scenarios, which are applied in the Monte Carlo simulations. Further, the present technique for developing response surfaces has been developed to minimise the number of CFD simulations 3.
Figure 1 Schematic data flow in explosion risk analyses.(available in full paper)

Leak
Leak calculations are performed as in a typical risk analysis giving the probabilistic distribution of leak rates. Typically in the risk analysis 3 leak categories are applied. In the explosion analysis the number of categories is flexible, 9 leak categories are recommended by the major Norwegian oil companies 2. A continues leak rate distribution can be defined from such discrete distributions. Leak durations are specifiedaccording to shutdown and blowdown times as well as segment inventories.

Ventilation and dispersion
The ventilation and dispersion analysis gives the transient development of gas clouds, both for the explosive mass of gas and the size of ignitable clouds. A set of pre-defined response surfaces have been developed and implemented in EXPRESS. In total 64 dispersion simulations have been applied together with physical evidence, dimensional analysis, and engineering judgement when developing generic response surfaces. Adetailed description of the response surfaces for dispersion is given in 3 and only a short description is given here.