A CFD Layout Sensitivity Study to Identify Optimum Safe Design of a FPSO
Authors: Berg J.T. , Bakke J.R. , Fearnley P. , Brewerton R.B. 
Published: October 2008

GexCon, the gas explosion consultancy, at Christian Michelsen Research has undertaken a layout sensitivity study on an FPSO. The objective of the study was to quantify the potential overpressures for a typical FPSO and quantitatively assess a number of explosion risk reduction measures. This is to aid the development of guidance to designers and legal assessors in order to produce inherently safe designs of FPSOs.

In the study FLACS simulations were performed using a full geometry model of an FPSO. In this detailed model a number of risk reduction measures were assessed:Effect of grating between the lower and upper process deck, when considering explosion at the lower deck,Effect of three different barrier walls on the explosion development, by considering either a weak barrier wall or a strong blast wall,Effect of two different separation gap configurations on an end ignition explosion case,Effect of high-module configurations with or without separation gap. This was excluded in this paper.

The study will act as quantitative guidance to designers in a critical phase of the design of the FPSO.

At the present time the optimum solution appears to be sectionalisation of the process area to limit the size of the gas cloud or the effective length of the flame path in congested areas. However, the effectiveness of the barrier walls hinges upon their ability to contain the gas cloud prior to ignition. If this cloud size reduction is not successful then the vent gap is the optimum solution.

The work provides the designer with information which has not been available before on significant matters relating to the inherent safe design and layout of FPSOs.

Introduction

Objective

Floating production systems are increasingly becoming the favoured solution for both marginal fields and developments in harsh environments. From an explosion safety viewpoint FPSOs have a number of advantages over enclosed modules. FPSOs are normally open with good venting of potential flammable gasses and ample explosion relief areas. This will help to limit the amount of flammable gas from releases and reduce the consequences of any explosions. However, FPSOs also have large and very congested process areas that have the potential for large gas clouds and long flame travel distances. This will tend to increase the potential overpressures. Due to the fact that there are limited data available for explosion simulations on FPSOs it is difficult to qualitatively recommend risk reduction measures in view of the above dilemma.

A recently completed Joint Industry Project has shown that severe explosions up to 9.4 barg and above can be generated in a test rig (28×12×8m3) which is significantly smaller than a typical process area on an FPSO. It was also observed in these experiments that the addition of small obstacles such as pipework and scaffolding more than doubled the overpressures1.

The objective of the present study is to quantify the potential overpressures for a typical FPSO and quantitatively assess a number of explosion risk reduction measures.