The Behaviour of Offshore Structures in Fires
Authors: Donegan E.M. 
Published: October 2008

This paper gives a general overview of the analysis of offshore structures under fire loads. Attention is drawn to the different requirements of component fire testing, which may give misleading emphasis to certain types of analysis.

It is shown that the basis of structural behaviour under fire loading is the change in material properties. However, it is the effects of thermal strains which are most likely to promote structural instability, leading to collapse.

Non-linear structural analysis has ban USCXI to illustrate fire loading effects, but it is likely that the offshore design industry will employ linear elastic structural analysis in many designs.

The design of seeondary structures to reskt fire loadings is discussed.

INTRODUCTION

The design of steel offshore structures to resist fire loadings is a new technology. Analytical techniques are not particularly well developed [1,2] or understood by either practicing engineers who develop the designs or safety specialists, who overview the design process. What is well understood is the technology of fire component testing [3,4], and it is this knowledge which is perhaps misleading the development of the analysis of steel structures under fire loadings.

Recent research into structural fire performance has concentrated on the numerical simulation of furnace fire tests. Whereas such work [5] can lead to economies in component testing programs it does little to increase the fundamental understanding of full-scale structures in real fires.

Offshore fires involve a hydrocarbon inventory, and embrace jet, pool and running liquid fire types. The description and calculation of fire loadings is not addressed in this paper.

Material Behaviour

The behaviour of steel structures under fire loading conditions is determined largely by temperature related material properties. Foundation data is available from a number of sources [6,7,8,9]. It is conventional to split the material properties into two groups. These are then termed the thermal and the mechanical material properties.

Thermal properties are:specific heat,thermal conductivitycoefficient of thermal expansion.The mechanical properties are:yield stress (Fy),Youngs modulus (E),Shear modulus (G).

Almost without exception, the material properties of steel vary to a greater or lesser extent with temperature. The stress-strain behaviour of a typical structural steel is shown in Figure 1. Note the loss of a sharp yield point and the drop in performance between 400 deg C and 750 deg C.

When a steel type has been given enhanced properties by heat treatment, then the properties are likely to fall-back to those associated with the chemical composition when a threshold temperature of about 600 deg C is attained in a fire.

By contrast the material properties associated with microstructure and hence chemical composition are both enduring and pertinent to fire conditions. In Figure 2 the behaviour of Austenitic stainless steel, low carbon structural steel and 'fire-resisting' steel are compared. In the range 600 deg C to 800 deg C the stainless steel retains more of its basic strength.