Fire Loading and Structural Response

103 Pages·2009·9.482 MB·English

TECHNICAL NOTE I I
Fire loading and Structural Response
March 20 10
Fire Loading and Structural Response
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.
0 2009 The Steel Construction Institute
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II FABIG Technical Note 11
Fire Loading and Structural Response
FOREWORD
This Technical Note has been prepared for This Technical Note includes contributions from
FABIG members. the following people:
It provides guidance on the design of steel Geoff Chamberlain (visitingprofessor at
structures to resist hydrocarbon fires, updating, Lou gh borough University),
where appropriate, relevant recommendations
Barbara Lowesmith (University of
given in:
Loughborough),
Interim Guidance Notes for the Design and
Richard Holliday (MMI Engineering Ltd),
Protection of Topside Structures against Fires
Asmund Huser (DNV),
and Explosions (1992),
Fadi Hamdan (independent consultant)
FABIG Technical Note 1: Fire Resistant
Design of Offshore Topside Structures (1993), Nancy Baddoo (The Steel Construction
Institute).
FABIG Technical Note 6: Design Guidef or
Steel at Elevated Temperatures and High
Strain Rates (200 1).
This Technical Note was revised in March 2010
to include the following amendments:
The state of the art position on estimating Modifications to the tabulated guidance
hydrocarbon jet and pool fire loads is described, provided in Tables 2.2 and 2.3 regarding the
along with simplified guidance. Principles of effect of confinement on jet fires.
passive fire protection and the new European
Modifications to some reduction factors for
Standards describing the testing and classification
Grade 1.4462 duplex stainless steel in
regime are summarised. New data for the
Table C.2 following a revision of the values in
mechanical properties at elevated temperature for
reference 67.
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.
...
FABIG Technical Note 11 111
Fire Loading and Structural Response
CONTENTS
Page
1. INTRODUCTION 1
1.1 Background 1
1.2 Scope 1
1.3 Fire hazard management 1
2. SIMPLIFIED GUIDANCE ON ESTIMATING HYDROCARBON FIRE LOADS 3
2.1 Jet fires 3
2.2 Pool fires 20
3. HEAT TRANSFER AND TEMPERATURE DEVELOPMENT 33
3.1 Introduction 33
3.2 Heat transfer to surrounding objects 33
3.3 Heat transfer to engulfed objects 35
3.4 Heat transfer by attachments to structural steelwork 37
4. GENERAL PRINCIPLES OF PASSIVE FIRE PROTECTION 39
4.1 Objectives of passive fire protection (PFP) 39
4.2 Testing and classification of PFP systems 39
4.3 PFP performance standards 41
4.4 Coatback of secondary and tertiary attachments 41
5. MATERIAL PROPERTIES AT ELEVATED TEMPERATURES 44
5.1 Mechanical properties for structural carbon steels 44
5.2 Thermal properties for structural carbon steels 44
5.3 Mechanical properties for structural stainless steels 45
5.4 Thermal properties for structural stainless steels 46
5.5 Mechanical properties for welds and bolts 46
6. EUROCODE APPROACH TO FIRE RESISTANT DESIGN 48
6.1 Designing with the Eurocodes 48
6.2 Verification by partial factor method 50
6.3 Scope of Eurocode for structural fire design of steel structures 53
6.4 Fire design procedures in the Eurocodes 53
6.5 Verification of member resistances in fire 54
7. EUROCODE SIMPLE DESIGN RULES FOR STRUCTURAL STEEL MEMBERS IN FIRE 56
7.1 Section classification 56
7.2 Critical temperature method 56
7.3 Design resistances of structural members 58
7.4 Design resistance of joints 63
8. REFERENCES 64
APPENDIX A PROBABILISTIC ASSESSMENT OF FIRE LOADS AND STRUCTURAL RESPONSE 68
A.l Introduction 68
A.2 Section 1: Determination of fire load 70
A.3 Section 2: Structural response analysis 76
A.4 Case study: a probabilistic assessment of fire load in a Cooler area 77
APPENDIX B PROPERTIES OF CARBON STEEL AT ELEVATED TEMPERATURE 85
B.l Mechanical properties 85
B.2 Thermal properties 90
APPENDIX C PROPERTIES OF STAINLESS STEEL AT ELEVATED TEMPERATURE 91
c.1 Mechanical properties 91
c.2 Thermal properties 93
FABIG Technical Note 11 V
Fire Loading and Structural Response
APPENDIX D FURTHER INFORMATION ON STRUCTURAL EUROCODES 95
D.l List of structural Eurocodes 95
D.2 Websites 96
APPENDIX E EUROCODE DESIGN EXAMPLES 97
vi FABIG Technical Note 11
Fire Loading and Structural Response
1. INTRODUCTION
1. I Background Sections 6 and 7 describe the Eurocode basis
of design and the process for determining the
The Interim Guidance Notes (IGNs) [ 11,
fire resistance of structural steel members in
published in 1992, provided guidelines for the
accordance with Eurocode 3,
protection of offshore structures against fires and
Appendix A describes a probabilistic
explosions. They summarised the state of
approach for determining offshore fire loads,
knowledge following completion of the Joint
Industry Project Blast and Fire Engineering for Appendices B and C give properties of
Topside Structures Phase I [2]. A year later, carbon and stainless steel at high
FABIG Technical Note 1 [3] was issued in order temperatures,
to give more information on the loading,
Appendices D and E give further information
response and protection of structures against fire,
on the structural Eurocodes and two design
accompanied by worked examples. More
examples of fire resistant design to
recently, FABIG Technical Note 6 [4] was
Eurocode 3.
published in 2001 to provide material data on
structural carbon steels and stainless steels used
The scope of this document is limited to
offshore. A very comprehensive update on recent
hydrocarbon fires and the response of steel
developments in the fields of fire loading, fire
members in the types of structures typically
response, explosion loading and explosion
encountered in the oil and gas industry.
response was published by UKOOA in 2007 [5].
1.3 Fire hazard management
The last twenty years have seen intensive activity
on the development of the structural Eurocodes. As part of a fire hazard management strategy, it
In 2005, the Eurocode dealing with structural fire is necessary to identify and analyse all fire
design of steel structures, EN 1993-1-2 [6], was hazards and their associated effects and ensure
published as one of the many parts of the that the risk corresponding to the fire hazards are
Eurocode for the design of steel structures, EN as low as reasonably practicable (ALARP). The
1993-1 (Eurocode 3 Part 1)[7]. Eurocode 3 will fire hazards should be prioritised and a
replace the relevant parts of BS 5950 [S], the combination of prevention, detection, control and
design standard for steel framed buildings in the mitigation systems should be implemented.
UK, which is due to be withdrawn in March These systems should be proportionate to the
2010. The Eurocodes are similarly being adopted required risk reduction and supported throughout
in other countries of the European Union. the life cycle of the structure.
1.2 Scope Fire protection on onshore structures is generally
designed to ensure the structure survives the
This Technical Note updates and expands certain
conflagration. If a fire occurs on an offshore
aspects of the guidance on fire engineering given
structure, however, the priority is the safe
in the Interim Guidance Notes, FABIG Technical
evacuation of personnel, with long-term damage
Notes 1 and 6 and the UKOOA guidance. The
to the structure being of lesser importance, i.e.
contents are as follows:
the escape routes and Temporary Rehge must be
Section 2 covers hydrocarbon jet and pool designed to survive a fire for the time required to
fires, giving simplified guidance on estimating evacuate the platform.
fire loads for design,
The performance standards relating to fire
Section 3 gives guidance on heat transfer and
hazards should be hlly defined at the
temperature development in steel members,
commencement of design. For a structural
Section 4 summarises general principles of member in an offshore platform, the performance
passive fire protection (PFP), noting relevant standard is typically defined in terms of the
standards, length of time it is required to retain its
load-bearing capacity.
Section 5 gives strength and stiffness data for
steel and stainless steel at high temperatures,
FABIG Technical Note 11 1
Fire Loadina and Structural Response
For a complete discussion of fire hazard serious maintenance burden in the offshore
management, reference should be made to the environment and it is possible their performance
UKOOA Fire and Explosion Guidance [5]. will be impaired by a prior explosion. The choice
between active and passive systems (or their
Offshore facilities have limited space and combination) is influenced by the protection
therefore carehl layout design is essential to the philosophy, the fire type and duration, the
overall safety of the installation. It is important equipment or structure requiring protection,
that fire hazards are considered at the earliest water availability and the time required for
stages of layout design. Where it is not possible evacuation. In all cases, the specification must be
to separate personnel from hazardous areas, matched to the fire type and exposure. PFP is
protection by segregation behind fire walls and generally preferred over deluge systems for
attention to escape routes is necessary. Key protecting primary structural members since it is
aspects are to keep living quarters and evacuation immediately available and has no moving parts to
facilities away from the process and to provide a fail and prevent operation. Section 4 of this
number of escape routes from modules and Technical Note gives guidance on the use of PFP;
access platforms back to the Temporary Rehge hrther information on mitigation of the effects of
or provide a suitable protected muster point. fire by deluge water systems can be found in
Section 3.2 of the UKOOA Guidance [5] gives Section 3.2 of the UKOOA Guidance [5].
detailed guidance on layout design to minimise
the fire hazard.
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
Fire Loading and Structural Response
2. SIMPLIFIED GUIDANCE ON ESTIMATING
HYDROCARBON FIRE LOADS
This guidance summarises how to assess jet and subsonic velocities as a blue, relatively
pool fire hazards, including two-phase jet fires, non-luminous flame. Further air entrainment and
the effect of confinement and behaviour of jet expansion of the jet then occurs producing the
and pool fires with water deluge. It updates and main body of the gas jet $re as a turbulent and
extends the UKOOA Guidance [5] and the jet fire yellow flame. The distance from the release point
overview by Lowesmith et a1 [9]. to the blue part of the flame is sometimes referred
to as the lift-off. The blue part is not greatly
Offshore fire loads may also be determined by a radiative compared to the brighter, yellow,
probabilistic approach; Appendix A describes a downstream part of the flame and so, particularly
procedure which is used in Norway by DNV. in jet fire modelling, the blue part is often
ignored and the term ‘lift-off is then applied to
2.1 Jet fires
the distance from release to the start of the yellow
flame.
Jet fires can be produced following the
pressurized release of a variety of fuel types. The
In the absence of impact onto an object, these
simplest case is a pressurised gas giving rise to a
fires are characteristically long and thin and
gas jet fire. A pressurised liquidgas mixture
highly directional. The high velocities within the
(such as ‘live crude’ or gas dissolved in a liquid)
released gas mean that they are relatively
will give rise to a two-phase jet fire. The gas
unaffected by the prevailing wind conditions,
content and the mechanical energy in the stream
except towards the tail of the fire. By contrast,
atomize the liquid into droplets which are then
the lower exit velocities from flares or from
evaporated by radiation from the flame.
containment pressures less than about 2 barA
However, a pressurised release of a liquid can
produce jet fires with shorter flame lift-offs and
also give rise to a jet fire in which two-phase
proportionately shorter and more buoyant flames
behaviour is observed if the liquid is able to
overall. These lower velocities also result in fires
vaporise quickly. This is most likely to occur
that are more wind affected, and generally more
when a liquid has a degree of superheat, i.e. it is
luminous owing to less efficient burn-out of soot.
released from containment at a temperature
above its boiling point at ambient conditions
Whether or not a stable jet fire will arise
whereupon flash evaporation occurs, and a
following the release of a pressurised
flashing liquid jet fire results. Examples are
hydrocarbon gas will depend principally upon the
releases of propane or butane. Non-volatile nature of the fuel, the size of the hole from which
liquids (for example, kerosene, diesel, or
the release occurs and the geometry of the
stabilised crude) are unlikely to be able to sustain
surroundings. In the case of natural gas, it has
a two-phase jet fire, unless permanently piloted
been found that, for free jets (not impacting),
by an adjacent fire; even so, some liquid drop-out
some combinations of hole size and pressure
is likely and hence the formation of a pool.
cannot produce stable flames [ 10,11,12].
Figure 2.1 shows that for hole sizes under 30 mm
2.1.1 Gas jet fires
diameter, there is a pressure regime which natural
Nature and characteristics gas releases must avoid to produce stable jet
fires. In practice this means that most small leaks
Containment pressures of greater than about
will be inherently unstable and will not support a
2 barA mean that the flow of an accidental
flame without some form of flame stabilisation,
pressurised gas release into the atmosphere will
such as the presence of another fire in the vicinity
be choked, having a velocity on release equal to
to provide a permanent pilot or stabilisation as a
the local speed of sound in the fluid. Following
result of impact onto an object such as pipework,
an expansion region downstream of the release
vessels, the surrounding structure, or by the wake
point, the flame itself commences in a region of
of a wind-blown release [13].
FABIG Technical Note 11 3
Fire Loading and Structural Response
Figure 1: Stability of Natural Gas Jet Fires
100
Vertical
Horizontal
//
Horizontal with deluge
at 12 I/m2/min
10
ii Horizontal with deluge
at 24 I/m2/min
1
0 10 20 30 40 50
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 where
jet fires without deluge and with general area
6' is the he1 mass fraction at the hole
deluge at two different deluge rates [14] from
(equal to unity for pure hels)
which it can be seen that deluge increases flame
instability. However, in a highly congested W is the he1 mass fraction in a
environment, impact within a short distance is stoichiometric mixture (equal to 0.055
very likely, and hence small leaks are likely to for methane and 0.06 for propane)
stabilise on the nearest point of impact.
d is the hole diameter or the expanded jet
diameter for choked releases.
The blow-out velocity for vertical natural gas
ujb
flames can be described by the empirical Thus, accidental damage to small bore high
relationship, pressure fittings might reasonably be expected
not to result in a stable flame, except that the
-1.5
likelihood of flame stabilisation by impact on
-'j=b 0.0028Rk1[-]
adjacent surfaces in a process unit is high. The
XU Pair
flame stability curve shown in Figure 2.1 refers
where only to natural gas. The increased burning
velocity S, associated with higher hydrocarbon
S, laminar burning velocity
gases results in greater stability and smaller
4 is the expanded jet gas density, critical diameters. For example, the critical
diameter for propane vapour jet flames is about
is the air density at ambient conditions,
pair 12 mm, whereas for hydrogen it is 2 mm.
RH is the Reynolds number,
Apart from providing flame stabilisation, impact
H, the distance to the stoichiometric onto an obstacle may modify the shape of a jet
concentration, is 7g'1iv en by: fire. Objects that are smaller than the flame
[( 48:)! half-width at the point of impact are unlikely to
+ modify the shape or length of the flame to any
H= - -PJ 5.81 d
great extent. However, impact onto a large vessel
W
Pair
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
Fire Loading and Structural Response
the fire is determined by the surface onto which it the pressure, which may vary with time as a
impacts and its distance from the release point. result, for example, of emergency blow-down.
In the case of high pressure releases of natural Figure2.2 shows jet fire lengths for a range of
gas, the mixing and combustion is relatively hels plotted against the net power of combustion
efficient, resulting in little soot (carbon) in megawatts, Q (= mass release rate x net
formation, except for extremely large release calorijk value). The Figure includes a correlation
rates. Hence, little or no smoke is produced by based on the majority of the natural gas data,
<o.o~ co
natural gas jet fires (typically gm-3). which is:
concentrations in the region of 5 to 17% v/v have
been measured within a jet fire flame but this L = 2.8893Q 0.3728
drops to less than 0.1% v/v by the end of the
where
flame, as it is converted to COz.
Q is the net power of combustion (MW)
Jet fire size is primarily related to the mass
release rate. For gaseous releases this, in turn, is L is the jet fire length (m)
related to the size of the leak (hole diameter) and
1000 -
100
10
1 ,
1 10 1 00 1000 10000 100000 1000000
.
PowerQ(MW)
+ Natural Gas 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