CFD analysis of a pool fire in an offshore platform

Abstract

One of the most important aspects in the design of the offshore platforms is related to natural hazards and catastrophic events such as wind storm, wave, earthquake, explosion and fire. Computational Fluid Dynamics (CFD) modelling, which enables to examine complex structures affected by selected fire scenarios, has become one of the most common tools in the fire engineering, also in case of offshore platforms. Fires on offshore platforms are large, in the open air, hydrocarbon usually with a very rapid heat release. Unfortunately, this type of field models results in problems with their validation since there is a huge gap in analytical methods that could be used for this purpose. For the fire risk assessment, numerous experiments were performed in order to introduce analytical methods. However, these analytical approaches are valid only for particular conditions. The limitations are associated with the experimental environment on which they are based. Regarding the analytical models, EN 1991-1-2 presents a simplified calculation model developed by Heskestad to assess localised fires in case of fire in open air. However, this method shows some limitations: only the temperature in the plume is calculated and, if the wind effect is considered, the method is no longer applicable. Moreover, it is questionable to use Heskestad approach for large pool fires, since the experiments on which the method is based were conducted for smaller scale fires. There are also some analytical models for determining the radiative heat flux: Shokri and Beyler model, Mudan and Croce model and Modified Solid Flame model. However, they also show some limitations, especially due to the shape of the fire area by assuming that the fire flame is cylindrical. Due to the complex conditions, validation and accuracy of offshore fires and their field models is still uncertain. In this thesis, a numerical simulation of a full-scale hydrocarbon pool fire is performed in FDS software. The study case corresponds to a “fictitious” fixed offshore platform which dimensions are based on the typical offshore platform. It is defined as an open structure composed by three main decks and two intermediate decks, steel frames, compartments and helideck. Fire scenarios assume an accidental crude oil leakage, which spreads and results in a localised pool fire. Modelled scenarios vary due to the localisation of the fire, its size, the total heat release rate and wind conditions. The results from the FDS model are compared with the available analytical methods. It is observed that smaller grid sizes in FDS models give better prediction of the temperature in the plume along its symmetrical vertical axis if compared to the temperatures assessed by the expression given in EN 1991-1-2 for localised fires. The outcomes for the radiative heat fluxes from FDS models are within the values predicted by the analytical models. However, the deviation in the results from all considered methods was observed which arises from different assumptions that the analytical correlation are based on as well as the input parameters used for the numerical modelling.