The Approach To Passive Fire Protection Design For The North Everest And Lomond Platforms
Authors: Krisa T.E. , Nottingham O.F. , Medonos Sava , Prager Jacob , Yasseri S.F. 
Published: October 2008

Traditional methods of determining the extent and thickness of passive fire protection (PFP) on offshore platform structures are simplistic and provide little opportunity for design optimization. This paper presents a review of alternative methods of determining the use of PFP which have been tried to overcome these disadvantages. The method progressed and used for North Everest and Lomond was based upon the modeling of credible fire scenarios, the calculation of steel temperatures and the progressive collapse analysis of topsides structure under rising temperature. The selected method allowed the design engineer to optimize the use of PFP while instilling confidence that the, design met specified safety objectives.

INTRODUCTION

Autumn 1992saw the installation of North Everest and Lomond platforms in the UK sector of the North Sea. Both are operated by Amoco (UK) Exploration Company (AMOEX) and are similar production, utility and accommodation platforms. The topsides operating weight is approximately 16,000 tonnes. Both platforms produce natural gas. Drilling is by a jack-up in the "kidded-off" mode.

For the control and mitigation of the fire hazard, the design of the platform topsides includes hydrocarbon fire and blast resistant walls to separate main working areas and to provide protection to the accommodation module and other emergency facilities which are at the hydrocarbon free west end of the platform (refer Figure 1). An emergency shutdown (ESD) system segregates the process into a number of subsystems and isolates main process vessels. In the event of a confirmed fire, the ESD system will shut in the process and blow down the gaseous hydrocarbon inventory. Automatic deluge systems are also installed.

The above mentioned facilities and other fire hazard control and mitigation measures were found by assessment to provide substantial safety. However, preliminary analysis showed that additional fire protection of main structure within wellbay and process areas was necessary to ensure structural stability of the accommodation module and means of evacuation during a fire for a sufficient time commensurate with safe evacuation. Hence, it was decided to apply passive fire protection to the structure which maintains this stability. The passive fire protection is a sprayed on material which insulates structure from the heat of a fire, so preventing critical loss of structural strength on rising temperature.

However, the quantity of passive fire protection (PFP)determined by the use of the temperature limit method, which was usual industry approach at that time, appeared onerous. The temperature limit method is to apply a sufficient thickness of PFP so that the protected structure is maintained below a selected temperature limit (typically 400°C) for a defined duration during a hydrocarbon fire, Subsequent analyses confirmed that the quantity of PFP determined by this method was indeed more than required to meet safety objectives and substantial rationalisation resulted.

Several alternative approaches to PFP design analysis were used and this paper describes the advantages and disadvantages of each. The method which was finally selected for design is described in more detail (thermal modeling and progressive collapse analysis).