Study of Progressive Collapse of a Flare Boom Structure

Abstract

Topside structures in offshore installations are subjected to deterioration mechanisms such as corrosion and fatigue. One of the topside structures more prone to deterioration by corrosion is the flare boom due to the combination of corrosive marine environment with cycles of high temperatures. This paper presents a structural reassessment of an existing flare boom structure under operation in a floating unit through a non-linear progressive collapse analysis. A traditional linear analysis was not considered since the original design criteria are not met in the presence of these damages and the results are inconclusive with respect to structural integrity. Therefore, it was decided to perform a progressive collapse analysis (nonlinear) where the loads are applied incrementally in the light of a LRFD methodology. In this way, the structural stiffness matrix is updated in each load step while loads are redistributed due to both large deflections and materials non-linearity. The variation (loss) of the mechanical properties with respect to the temperature field was also taken into account. This reassessment has considered, in addition to all design loads, several structural damages scenarios covering general and local corrosion. Since the actual corrosion pattern is not completely known due to the lack of inspection data, the definition of these assumed scenarios was necessary in order to investigate the global structural response in the presence of typical corrosion damages. Members buckling, yielding/plastification, joint failure and global collapse results have been evaluated for ULS conditions in which it could be observed that gravitational loads governs the structural collapse. SLS conditions have also been assessed in order to evaluate deflection results. Finally, the results allow identifying the most critical members of the flare boom with respect to structural integrity giving to the Operator a rational way to prioritize them for inspection purposes. Following this procedure, the most important members with respect to global structural integrity are then known. In other words, more conclusive results regarding the structural integrity in the presence of damages will be assessed.