Fire loading and Structural Response

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1 TECHNICAL NOTE I I Fire loading and Structural Response March 20 10

2 This document is a deliverable of the Fire and Blast Information Group (FABIG). FABIG would like to encourage comment and feedback from its membership. If you have any comments on this Technical Note or any other FABIG activities please address them to the FABIG Project Manager at The Steel Construction Institute The Steel Construction Institute Neither this publication nor any part thereof may be reproduced, stored or transmitted, in any form or by any means - electronic, photocopy or otherwise, without the prior permission in writing of the Steel Construction Institute. Illustrations and tables may not be copied in part or in whole. This publication is provided for use by FABIG members and shall not be lent, re-sold, hired out or otherwise circulated without the prior written consent of the publishers. Although care has been taken to ensure, to the best of our knowledge, that all data and information contained herein are accurate to the extent that they relate to either matters of fact or accepted practice or matters of opinion at the time of publication, the Steel Construction Institute, the authors and the reviewers assume no responsibility for any errors in or misinterpretations of such data and/or information or any loss or damage arising from or related to their use. This publication is supplied to the members of the Fire and Blast Information Group. II FABIG Technical Note 11

3 FOREWORD This Technical Note has been prepared for FABIG members. It provides guidance on the design of steel structures to resist hydrocarbon fires, updating, where appropriate, relevant recommendations given in: Interim Guidance Notes for the Design and Protection of Topside Structures against Fires and Explosions (1992), FABIG Technical Note 1 : Fire Resistant Design of Offshore Topside Structures (1993), FABIG Technical Note 6: Design Guide for Steel at Elevated Temperatures and High Strain Rates (2001). The state of the art position on estimating hydrocarbon jet and pool fire loads is described, along with simplified guidance. Principles of passive fire protection and the new European Standards describing the testing and classification regime are summarised. New data for the mechanical properties at elevated temperature for structural stainless steels are presented. Guidance on the application of the Eurocode approach to structural steel fire resistant design is given, supplemented by two design examples. This Technical Note includes contributions from the following people: Geoff Chamberlain (visitingprofessor at Lo ugh borough University), Barbara Lowesmith (University of Loughborough), Richard Holliday (MMI Engineering Ltd), Asmund Huser (DNV), Fadi Hamdan (independent consultant) Nancy Baddoo (The Steel Construction Institute). This Technical Note was revised in March 2010 to include the following amendments: Modifications to the tabulated guidance provided in Tables 2.2 and 2.3 regarding the effect of confinement on jet fires. Modifications to some reduction factors for Grade duplex stainless steel in Table C.2 following a revision of the values in reference FABIG Technical Note

4 CONTENTS INTRODUCTION 1.1 Background 1.2 Scope 1.3 Fire hazard management SIMPLIFIED GUIDANCE ON ESTIMATING HYDROCARBON FIRE LOADS 2.1 Jet fires 2.2 Pool fires HEAT TRANSFER AND TEMPERATURE DEVELOPMENT 3.1 Introduction 3.2 Heat transfer to surrounding objects 3.3 Heat transfer to engulfed objects 3.4 Heat transfer by attachments to structural steelwork GENERAL PRINCIPLES OF PASSIVE FIRE PROTECTION 4.1 Objectives of passive fire protection (PFP) 4.2 Testing and classification of PFP systems 4.3 PFP performance standards 4.4 Coatback of secondary and tertiary attachments MATERIAL PROPERTIES AT ELEVATED TEMPERATURES 5.1 Mechanical properties for structural carbon steels 5.2 Thermal properties for structural carbon steels 5.3 Mechanical properties for structural stainless steels 5.4 Thermal properties for structural stainless steels 5.5 Mechanical properties for welds and bolts EUROCODE APPROACH TO FIRE RESISTANT DESIGN 6.1 Designing with the Eurocodes 6.2 Verification by partial factor method 6.3 Scope of Eurocode for structural fire design of steel structures 6.4 Fire design procedures in the Eurocodes 6.5 Verification of member resistances in fire EUROCODE SIMPLE DESIGN RULES FOR STRUCTURAL STEEL MEMBERS IN FIRE 7.1 Section classification 7.2 Critical temperature method 7.3 Design resistances of structural members 7.4 Design resistance of joints REFERENCES APPENDIX A A.l A.2 A.3 A.4 APPENDIX B B.l B.2 APPENDIX C c.1 c.2 PROBABILISTIC ASSESSMENT OF FIRE LOADS AND STRUCTURAL RESPONSE Introduction Section 1 : Determination of fire load Section 2: Structural response analysis Case study: a probabilistic assessment of fire load in a Cooler area PROPERTIES OF CARBON STEEL AT ELEVATED TEMPERATURE Mechanical properties Thermal properties PROPERTIES OF STAINLESS STEEL AT ELEVATED TEMPERATURE Mechanical properties Thermal properties Page FABIG Technical Note 11 V

5 APPENDIX D FURTHER INFORMATION ON STRUCTURAL EUROCODES D.l List of structural Eurocodes D.2 Websites APPENDIX E EUROCODE DESIGN EXAMPLES vi FABIG Technical Note 11

1. INTRODUCTION 1.I Background The Interim Guidance Notes (IGNs) [ 11, published in 1992, provided guidelines for the protection of offshore structures against fires and explosions.

6 1. INTRODUCTION 1.I Background The Interim Guidance Notes (IGNs) [ 11, published in 1992, provided guidelines for the protection of offshore structures against fires and explosions. They summarised the state of knowledge following completion of the Joint Industry Project Blast and Fire Engineering for Topside Structures Phase I [2]. A year later, FABIG Technical Note 1 [3] was issued in order to give more information on the loading, response and protection of structures against fire, accompanied by worked examples. More recently, FABIG Technical Note 6 [4] was published in 2001 to provide material data on structural carbon steels and stainless steels used offshore. A very comprehensive update on recent developments in the fields of fire loading, fire response, explosion loading and explosion response was published by UKOOA in 2007 [5]. The last twenty years have seen intensive activity on the development of the structural Eurocodes. In 2005, the Eurocode dealing with structural fire design of steel structures, EN [6], was published as one of the many parts of the Eurocode for the design of steel structures, EN (Eurocode 3 Part 1)[7]. Eurocode 3 will replace the relevant parts of BS 5950 [S], the design standard for steel framed buildings in the UK, which is due to be withdrawn in March The Eurocodes are similarly being adopted in other countries of the European Union. 1.2 Scope This Technical Note updates and expands certain aspects of the guidance on fire engineering given in the Interim Guidance Notes, FABIG Technical Notes 1 and 6 and the UKOOA guidance. The contents are as follows: Section 2 covers hydrocarbon jet and pool fires, giving simplified guidance on estimating fire loads for design, Section 3 gives guidance on heat transfer and temperature development in steel members, Section 4 summarises general principles of passive fire protection (PFP), noting relevant standards, Section 5 gives strength and stiffness data for steel and stainless steel at high temperatures, Sections 6 and 7 describe the Eurocode basis of design and the process for determining the fire resistance of structural steel members in accordance with Eurocode 3, Appendix A describes a probabilistic approach for determining offshore fire loads, Appendices B and C give properties of carbon and stainless steel at high temperatures, Appendices D and E give further information on the structural Eurocodes and two design examples of fire resistant design to Eurocode 3. The scope of this document is limited to hydrocarbon fires and the response of steel members in the types of structures typically encountered in the oil and gas industry. 1.3 Fire hazard management As part of a fire hazard management strategy, it is necessary to identify and analyse all fire hazards and their associated effects and ensure that the risk corresponding to the fire hazards are as low as reasonably practicable (ALARP). The fire hazards should be prioritised and a combination of prevention, detection, control and mitigation systems should be implemented. These systems should be proportionate to the required risk reduction and supported throughout the life cycle of the structure. Fire protection on onshore structures is generally designed to ensure the structure survives the conflagration. If a fire occurs on an offshore structure, however, the priority is the safe evacuation of personnel, with long-term damage to the structure being of lesser importance, i.e. the escape routes and Temporary Rehge must be designed to survive a fire for the time required to evacuate the platform. The performance standards relating to fire hazards should be hlly defined at the commencement of design. For a structural member in an offshore platform, the performance standard is typically defined in terms of the length of time it is required to retain its load-bearing capacity. FABIG Technical Note 11 1

7 Fire Loadina and Structural Response For a complete discussion of fire hazard management, reference should be made to the UKOOA Fire and Explosion Guidance [5]. Offshore facilities have limited space and therefore carehl layout design is essential to the overall safety of the installation. It is important that fire hazards are considered at the earliest stages of layout design. Where it is not possible to separate personnel from hazardous areas, protection by segregation behind fire walls and attention to escape routes is necessary. Key aspects are to keep living quarters and evacuation facilities away from the process and to provide a number of escape routes from modules and access platforms back to the Temporary Rehge or provide a suitable protected muster point. Section 3.2 of the UKOOA Guidance [5] gives detailed guidance on layout design to minimise the fire hazard. serious maintenance burden in the offshore environment and it is possible their performance will be impaired by a prior explosion. The choice between active and passive systems (or their combination) is influenced by the protection philosophy, the fire type and duration, the equipment or structure requiring protection, water availability and the time required for evacuation. In all cases, the specification must be matched to the fire type and exposure. PFP is generally preferred over deluge systems for protecting primary structural members since it is immediately available and has no moving parts to fail and prevent operation. Section 4 of this Technical Note gives guidance on the use of PFP; hrther information on mitigation of the effects of fire by deluge water systems can be found in Section 3.2 of the UKOOA Guidance [5]. Passive and active fire protection methods are used to mitigate effects of fire loads but should only be specified when essential as they carry 2 FABIG Technical Note 11

2. SIMPLIFIED GUIDANCE ON ESTIMATING HYDROCARBON FIRE LOADS This guidance summarises how to assess jet and pool fire hazards, including two-phase jet fires, the effect of confinement and behaviour of

8 2. SIMPLIFIED GUIDANCE ON ESTIMATING HYDROCARBON FIRE LOADS This guidance summarises how to assess jet and pool fire hazards, including two-phase jet fires, the effect of confinement and behaviour of jet and pool fires with water deluge. It updates and extends the UKOOA Guidance [5] and the jet fire overview by Lowesmith et a1 [9]. Offshore fire loads may also be determined by a probabilistic approach; Appendix A describes a procedure which is used in Norway by DNV. 2.1 Jet fires Jet fires can be produced following the pressurized release of a variety of fuel types. The simplest case is a pressurised gas giving rise to a gas jet fire. A pressurised liquidgas mixture (such as live crude or gas dissolved in a liquid) will give rise to a two-phase jet fire. The gas content and the mechanical energy in the stream atomize the liquid into droplets which are then evaporated by radiation from the flame. However, a pressurised release of a liquid can also give rise to a jet fire in which two-phase behaviour is observed if the liquid is able to vaporise quickly. This is most likely to occur when a liquid has a degree of superheat, i.e. it is released from containment at a temperature above its boiling point at ambient conditions whereupon flash evaporation occurs, and a flashing liquid jet fire results. Examples are releases of propane or butane. Non-volatile liquids (for example, kerosene, diesel, or stabilised crude) are unlikely to be able to sustain a two-phase jet fire, unless permanently piloted by an adjacent fire; even so, some liquid drop-out is likely and hence the formation of a pool Gas jet fires Nature and characteristics Containment pressures of greater than about 2 bara mean that the flow of an accidental pressurised gas release into the atmosphere will be choked, having a velocity on release equal to the local speed of sound in the fluid. Following an expansion region downstream of the release point, the flame itself commences in a region of subsonic velocities as a blue, relatively non-luminous flame. Further air entrainment and expansion of the jet then occurs producing the main body of the gas jet $re as a turbulent and yellow flame. The distance from the release point to the blue part of the flame is sometimes referred to as the lift-off. The blue part is not greatly radiative compared to the brighter, yellow, downstream part of the flame and so, particularly in jet fire modelling, the blue part is often ignored and the term lift-off is then applied to the distance from release to the start of the yellow flame. In the absence of impact onto an object, these fires are characteristically long and thin and highly directional. The high velocities within the released gas mean that they are relatively unaffected by the prevailing wind conditions, except towards the tail of the fire. By contrast, the lower exit velocities from flares or from containment pressures less than about 2 bara produce jet fires with shorter flame lift-offs and proportionately shorter and more buoyant flames overall. These lower velocities also result in fires that are more wind affected, and generally more luminous owing to less efficient burn-out of soot. Whether or not a stable jet fire will arise following the release of a pressurised hydrocarbon gas will depend principally upon the nature of the fuel, the size of the hole from which the release occurs and the geometry of the surroundings. In the case of natural gas, it has been found that, for free jets (not impacting), some combinations of hole size and pressure cannot produce stable flames [ 10,11,12]. Figure 2.1 shows that for hole sizes under 30 mm diameter, there is a pressure regime which natural gas releases must avoid to produce stable jet fires. In practice this means that most small leaks will be inherently unstable and will not support a flame without some form of flame stabilisation, such as the presence of another fire in the vicinity to provide a permanent pilot or stabilisation as a result of impact onto an object such as pipework, vessels, the surrounding structure, or by the wake of a wind-blown release [13]. FABIG Technical Note 11 3

Figure 1: Stability of Natural Gas Jet Fires // 10 ii 100 Vertical Horizontal Horizontal with at 12 I/m2/min Horizontal with at 24 I/m2/min deluge deluge 1 0 10 20 30 40 50 Diameter (mm) Figure 2.

9 Figure 1: Stability of Natural Gas Jet Fires // 10 ii 100 Vertical Horizontal Horizontal with at 12 I/m2/min Horizontal with at 24 I/m2/min deluge deluge Diameter (mm) Figure 2.1 Stability of natural gas jet fires (The points on the graph indicate the pressure and diameter where the flames blow themselves out.) Figure 2.1 also includes data from horizontal free jet fires without deluge and with general area deluge at two different deluge rates [14] from which it can be seen that deluge increases flame instability. However, in a highly congested environment, impact within a short distance is very likely, and hence small leaks are likely to stabilise on the nearest point of impact. The blow-out velocity ujb for vertical natural gas flames can be described by the empirical relationship, -= 'jb XU Rk1[-] Pair -1.5 where S, laminar burning velocity 4 pair RH is the expanded jet gas density, is the air density at ambient conditions, is the Reynolds number, H, the distance to the stoichiometric concentration, is given by: H= [(- 48:)! 7'1 - PJ d W Pair where 6' is the he1 mass fraction at the hole (equal to unity for pure hels) W is the he1 mass fraction in a stoichiometric mixture (equal to for methane and 0.06 for propane) d is the hole diameter or the expanded jet diameter for choked releases. Thus, accidental damage to small bore high pressure fittings might reasonably be expected not to result in a stable flame, except that the likelihood of flame stabilisation by impact on adjacent surfaces in a process unit is high. The flame stability curve shown in Figure 2.1 refers only to natural gas. The increased burning velocity S, associated with higher hydrocarbon gases results in greater stability and smaller critical diameters. For example, the critical diameter for propane vapour jet flames is about 12 mm, whereas for hydrogen it is 2 mm. Apart from providing flame stabilisation, impact onto an obstacle may modify the shape of a jet fire. Objects that are smaller than the flame half-width at the point of impact are unlikely to modify the shape or length of the flame to any great extent. However, impact onto a large vessel may significantly shorten the jet fire, and impact onto a wall or roof could transform the jet into a radial wall jet, where the location and direction of 4 FABIG Technical Note 11

10 the fire is determined by the surface onto which it impacts and its distance from the release point. In the case of high pressure releases of natural gas, the mixing and combustion is relatively efficient, resulting in little soot (carbon) formation, except for extremely large release rates. Hence, little or no smoke is produced by natural gas jet fires (typically <o.o~ gm-3). co concentrations in the region of 5 to 17% v/v have been measured within a jet fire flame but this drops to less than 0.1% v/v by the end of the flame, as it is converted to COz. Jet fire size is primarily related to the mass release rate. For gaseous releases this, in turn, is related to the size of the leak (hole diameter) and the pressure, which may vary with time as a result, for example, of emergency blow-down. Figure2.2 shows jet fire lengths for a range of hels plotted against the net power of combustion in megawatts, Q (= mass release rate x net calorijk value). The Figure includes a correlation based on the majority of the natural gas data, which is: L = Q where Q L is the net power of combustion (MW) is the jet fire length (m) Natural Gas. 1, PowerQ(MW) Propane A Butane Crude 0 Butane/NG mix KerosendNG mix x Crude/NG mix -Correlation Figure 2.2 Jet fire flame length FABIG Technical Note 11 5

REQUIREMENTS FOR VALIDATION OF MATHEMATICAL MODELS IN SAFETY CASES

REQUIREMENTS FOR VALIDATION OF MATHEMATICAL MODELS IN SAFETY CASES R F Evans Principal Specialist Inspector, Health & Safety Executive Offshore Safety Division, Bootle Merseyside. The Offshore Installations