Passive fire protection (PFP) is consistently used in the oil and gas industry as a method to avoid and or delay global collapse of offshore installations. Current practice is to leave the top flanges of passively fire protected beams on coated, commonly named 3-sided protected beams. The reason for this is to ease the installation of grating on top of those beams. Although discussed in guidance documents such as API 2218 there is, to the author's knowledge, no defined design guidance or standard that allows structural steel designers to calculate the response of such configurations and assure that partially fire protected beams survive the design fire scenarios. It is therefore of interest to gain more knowledge on the heat up process and failure mechanisms of these members. Partially 3-sided protected beams engulfed by hydrocarbon and jet fires are subject to large thermal gradients on their cross-section, high temperatures which in turn cause the beams to be susceptible to a series of possible failure mechanisms such as local buckling, loss of bending or shear capacity and more commonly lateral-torsional buckling.
This paper discusses the thermal and structural response observed on typical 3-sided PFP beams used in the offshore industry. Several heat transfer finite element (FE) analyses were performed on a range of rolled I sections and plate girders, the results provide an understanding of the influence of conduction, convection and radiation in the heat up mechanism of the beams.
This study provides an understanding of the structural response and failure mechanism of 3-sided protected beams. From these results a series of design solutions are proposed and their structural performance analyzed. The pros and cons of each solution are identified as well as their range of applicability. Finally recommendations and conclusions on analysis and design approaches for partially protected beams are made based on the results of this study.