Development of a probabilistic fire demand model and a fire protection for performance-based fire design of petrochemical plants

Structures and fire have always been a deep concern for humans. That is even truer for structures dedicated to host a large number of people as high-rise buildings or to deal with hazardous materials as industrial components. Past events have shown how fire can cause severe damages in such structures and trigger devastating consequences in terms of fatalities and losses. To control and prevent fire events, scientists have been providing practitioners with mathematical tools facilitating the study of fire behaviour. These tools have been used to develop fire models, design guidelines and more practical applications such as fire protections for structural members. Therefore, fire engineering community currently aims to cover and enlarge these three fields. Among these fields two unexplored issues relative to fire are identified and appear meaningful to be addressed. They concern the development of additional design guidelines for industrial plants and the design of an innovative fire protection system for steel columns. This thesis is therefore composed of two parts addressing these fire engineering issues by adopting different design approaches. On one hand, a probabilistic fire demand model (PFDM) is developed to study steel pipe-rack structure behaviour exposed to localised fires with a performance-based approach, whereas on the other hand, a fire protection is developed using a prescriptive-based approach. The first part of the thesis studies the behaviour of a structural steel pipe-rack exposed to localised fires. Petrochemical plants are locations highly exposed to severe fire incidents due to the nature of materials that are processed and contained within it. It was observed in the past that critical components as tanks and pipes can fail and lose their containment in case of extreme events like earthquakes or operational accidents. Important localised fire scenarios can result from the ignition of a leaking fuel likely to occur in industrial environment. Since pipe-racks are transporting pipes on long distance within plants, these structures are usually unprotected and more exposed to potential localised fires. For these reasons, a methodology is developed to build a PFDM to investigate the structural behaviour of a steel pipe-rack exposed to a localised fire. To that end, a pipe-rack from an industrial plant in Italy is considered as case study. This structure is then analysed when exposed to 539 different localised fire scenarios which introduced uncertainties for the PFDM. Localised fires are defined with different severity levels by varying three parameters: fire diameter, fire-structure distance and fuel. Parametric analyses are performed to quantify the liquid outflow from orifices in tanks and pipes which facilitates the definition of plausible fire diameters. Thermo-mechanical behaviour of the pipe-rack is analysed with the LOCAFI localised fire model by adopting finite element methods. The purpose of the PFDM is to derive fire fragility functions that can be used by practitioners. For the proper PFDM development, structural analysis outcomes are analysed through cloud analysis (CA) considering different engineering demand parameter (EDP) and intensity measures (IM), characterising the structural response and the fire severity, respectively. CA reveal that the most efficient EDP-IM pair is the interstorey drift ratio (ISDR) - average heat flux impinging the structure (HFavg) pair. The resulting PFDM is then used to derive fire fragility curves considering two predefined structural damages states. The second part of the thesis presents the development of an innovative and cost-effective fire protection system for steel columns. The fire protection is designed to be a plug-and-play system easy and quick to install and dismantle. Therefore, it is composed by two identical components made of high-density rock wool and steel sheets. Steel sheets are bend with a U-shape and present connection claws at their extremities that allow the connection between both components. Rock wool boards are installed inside the steel plates to ensure the insulating efficiency of the protection. The system is designed to protect steel columns and maintain their temperature below 550°C when exposed on four sides for 120 min to standard fire. 550°C corresponds to the steel temperature where strength is reduced to 60%. The fire protection development relies on two experimental campaigns. In order to evaluate the behaviour and the insulating efficiency of the fire protection in early stage, 7 small-scale experimental tests are first performed in a furnace having reduced dimensions. 2D thermal models are developed with finite element methods and calibrated based on experimental results to understand and predict the fire protection efficiency. Predictions are compared against results obtained with the prescriptive analytical model provided in EN1993-1-2. Both numerical and analytical models facilitate the definition of the second experimental test. A large-scale experimental test is performed at an advanced stage of the protection development. That test is conducted according to the norm EN13381-4 and aims to assess the final version of the protection by testing simultaneously five specimens, including four thermal tests and one thermo-mechanical test. Experimental tests results are assessed against the norm and certify the efficiency of the system developed. The fire protection can therefore address steel profiles presenting I, H or hollow cross sections with box section factors going from 42 to 103 m-1. The development is concluded with a cost analysis attesting the competitivity of the plug-and-play fire protection by comparing direct- and indirect costs with existing solutions. In summary, the results of this thesis consist in the development of a PFDM for steel pipe-racks exposed to localised fires and in the development of an innovative and cost-effective plug-and-play fire protection system for steel columns. PFDM can be used to derive fire fragility curves that account for the uncertainty of localised fires severity. These curves constitute tools for practitioners to be applied in a probabilistic fire engineering framework or in a fire risk assessment. Furthermore, a valuable result from the development of the PFDM, remains the methodology adopted and the use of CA which reveal to be a suitable and versatile tool to build a PFDM. The fire protection is designed and developed against European norms certifying its ability to protect steel columns presenting box section factors going up to 103 m-1. The plug-and-play system ensures to keep steel temperature below 550°C for columns being exposed on four sides to standard fire for 120 min. It worth to mention that the good visual aspect of the fire protection brought by the use of steel sheet, make this system particularly appropriate for public and office buildings. Eventually, even though both research topics addressed in this thesis are not directly related, they present a potential synergy considering the fact that plug-and-play fire protections could be used to protect columns of steel pipe-racks and therefore mitigate the impacts of localised fires scenarios identified as critical by fire fragility curves.