The prime objective of the fire resistance design of offshore topside structures is to retain stability and integrity of the Temporary Refuge (TR) and means of escape for a specified period of time. There is one additional prime objective of the fire resistant design of offshore topside structure, namely to prevent excessive deflections developing during the fire, which would cause loss of support of hydrocarbon equipment, new leaks forming resulting in additional explosion and fire. One of several measures taken to achieve this goal is to protect members required for safe operation of the TR with Passive Fire Protection (PFP). This selection of important members, which are either part of the framing which provide support to TR or carry heavy items, forms a stable minimal structure. The rest of topside can be allowed to degrade in a fire. The extent of PFP is kept as low as practicable to keep the deck weight down and to aid maintenance and structural monitoring. However, there unprotected members that are connected to this minimal structure and potentially could be transmit heat. To limit heat flow through these unprotected members into protected members of the minimal structure, PFP is applied to a part of the unprotected member where they are connected to the protected member. This partial coating is referred to as Coat-back. This paper addresses the problem of determining the optimal length for coat-back by deriving a closed form solution using the theory of heat transfer. Results of the analytical method are compared with experimental results, as well as those obtained using finite element methods. Correct calculation of the coat-back length has great economical and safety implications for a structure under fire attack. The application of the method is shown by several worked examples.

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