Abstract

Structural integrity assessment and fire response analysis of offshore topsides structures focus on protecting topsides primary structural steel, hydrocarbon equipment supports, secondary steel along the primary escape routes, pressurized hydrocarbon pipes and vessels from damage and fracture due to pool and jet fires, and providing safe escape routes for emergency evacuation of the personnel during a specific period of time. In order to achieve these purposes, it is very critical to apply sufficient amount of Passive Fire Protection (PFP) on the topsides structural steel members, pressurized vessels and piping to supplement active fire protection systems like deluge, foam systems, etc. On the other hand, excessive use of PFP on the structure results in considerable additional cost and extra added dead load to the structure.

Simplified and conservative approaches are available to estimate the extent and amount of the PFP on the offshore structures. However, the main concern with simplified approaches is that they can lead to over-application of the PFP resulting in substantial increase in the topsides weight. These methods may also result in under-estimation of the required amount of PFP, which can compromise the topsides structural integrity. With the use of advanced engineering analysis and knowledge of critical load path, an optimized PFP scheme can be developed that would maintain the structural integrity of the structure and also provide sufficient escape time for personnel during fire. The risk-based approach for PFP scheme development and optimization is introduced in this paper. In this method, ductility level fire response analysis is discussed, which is coupled with the probability of failure and probability of escalation to estimate risk to individual, asset, environment, etc. The ductility level analysis accounts for material and geometric nonlinear behavior of steel and load redistribution when heated members are over-utilized. The risk-based method is compared against the conventional method of PFP optimization using ductility level analysis recommended by API RP 2FB. It was concluded that using the risk-based approach can result in a significant reduction in the amount of required PFP.

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