Load characteristics of steel and concrete tubular members under jet fire: An experimental and numerical study

Abstract

The aim of this study was to evaluate the load characteristics of steel and concrete tubular members under jet fire, with the motivation to investigate the jet fire load characteristics in FPSO topsides. This paper is part of Phase II of the joint industry project on explosion and fire engineering of FPSOs (EFEF JIP) (Paik and Czujko, 2009, Paik, 2010). To obtain reliable load values, jet fire tests were carried out in parallel with a numerical study. Computational fluid dynamics (CFD) simulation was used to set up an adiabatic wall boundary condition for the jet fire to model the heat transfer mechanism. A concrete tubular member was tested under the assumption that there is no conduction effect from jet fire. A steel tubular member was tested and considered to transfer heat through conduction, convection, and radiation. The temperature distribution, or heat load, was analyzed at specific locations on each type of member. ANSYS CFX, (2008) and KFX, 2007 codes were used to obtain similar fire action in the numerical and experimental methods. The results of this study will provide a useful database to determine design values related to jet fire.

Introduction

The Piper Alpha accident of 1988 was the world’s worst offshore oil disaster in terms of both lives lost and impact on the industry (Cullen, 1990). The total insured loss was about £1.7 billion (US$ 3.4 billion), and 167 people were killed. Other major fire accidents caused by gas leaks include the two incidents that occurred on the Enchova Central offshore oil platform in Brazil in 1984 and 1988, the first of which killed 42 people (Alvaro et al., 2001).

The possibility of fire hazard has grown as the number of offshore structures has increased. Accurate safety design values are thus required for guidelines on the design of passive fire protection (PFP) and firewalls. Fire action cannot be represented by an analytical expression that can be handled easily. To overcome this problem, an approach based on the numerical calculation for fire loads is needed along with the development of a suitable computing system.

Further, there is a lack of standardized methods and coordination for the calculation of fire loads acting on structures and equipment and their corresponding consequences. To provide robust guidance on the design of steel structures to resist jet fires, the characteristics of both fire action and its effects must be identified (Czujko, 2001, Czujko, 2005, Czujko, 2007, Paik and Thayamballi, 2003, Paik and Thayamballi, 2007, Paik, 2010).

This paper focuses on the load characteristics of steel and concrete tubular members under jet fire. Generally, fire involves the combination of a combustible vapor or gas with an oxidizer in a combustion process that is manifested by the evolution of light, heat, and flame (Nolan, 1996). Fig. 1 shows the shape of a jet fire with a specific leak direction.

Jet fires can arise following the pressurized release of various fuel types (FABIG 2009). The simplest case is a pressurized gas giving rise to a gas jet fire. A pressurized liquid/gas mixture (such as “live crude” or gas dissolved in a liquid) gives rise to a two-phase jet fire. The gas content and mechanical energy in the stream atomize the liquid into droplets, which are then evaporated by radiation from the flame. However, the pressurized release of a liquid causes rapid vaporization. This is most likely to occur when a liquid undergoes some degree of superheating, i.e., when it is released from containment at a temperature above its boiling point in ambient conditions, whereupon flash evaporation occurs and a flashing liquid jet fire results. This event may arise from the release of propane or butane. Non-volatile liquids (for example, kerosene, diesel, or stabilized crude) are unlikely to be able to sustain a two-phase jet fire, unless permanently piloted by an adjacent fire, but even so some liquid drop-out is likely, leading to the formation of a pool.

In the present study, steel tubular members are used in place of real tubular members of offshore installations to evaluate the jet fire load, including the effects of conduction, convection, and radiation. However, it is assumed that there is no conduction effect on concrete tubular members, as computational fluid dynamics (CFD) codes set up an adiabatic wall boundary condition.

The main objectives of the study were as follows:

• To obtain the ideal jet fire load through numerical and experimental methods, taking into consideration heat transfer such as conduction, convection, and radiation on steel and concrete tubular members.

• To study a CFD technique with ANSYS CFX, (2008) and KFX, 2007 for modeling FPSO and offshore structures under jet fire action.