Table Of ContentFIRE AND BLASTI NFORMATION GROUP
FABIG Technical Note 3
TECmIC& NOTE AND WORKED EXAMPLES
To COMPLEMENTT HE INTERIGMU IDANCEN OTES
FOR THE DESIGNA ND PROTECTION OF TOPSIDE
STRUCTUREASG AINSTE XPLOSIOANN D FIRE
Use of Ultimate Strength
Techniques for
Fire Resistant Design
of Offshore Structures
This document is a deliverable of the Fire And Blast Information Group (FABIG) for the
year April 1994 - March 1995.
.
We wish to acknowledge and thank those FABIG members who contributed, reviewed and
commented on the draft version of this Technical Note.
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 Mr Sava Medonos, FABIG Project Manager at The Steel Construction Institute.
The information in this document is published with the intent of making it available
to members of the Fire And Blast Information Group (FABIG). The information is
available for use subject to copyright. The information presented here is expected
to contribute to the further improvement in safety. However, The Steel Construction
Institute will not accept any liability for loss or damage originating from the use of
the information herein.
The Steel Construction Institute, Silwood Park, Ascot, Berkshire, SL5 7QN, United Kingdom.
Tel: f44 (0) 1344 23345, Fax: +44 (0) 1344 22944
CONTENTS
Page
1. INTRODUCTION
2. BACKGROUND
3. PERFORMANCE STANDARDS
3.1 In General
3.2 Definition of Performance Standards
3.3 Terminology
4. SYSTEM RESPONSE
5. THERMAL RESPONSE
Heat Flux Loading
Thermal Models of the Fire
Thermal Response of Structures and Plant
Calibration of Heat Transfer Properties
Space Frame Thermal Model
Solid Thermal Model
Shell Thermal Model
Comparison of the Spaceframe, Solid and Shell Thermal Models
Thermal Analysis
Thermal Response of Individual Structural and Plant Components
6. STRUCTURAL RESPONSE
6.1 Modelling of the Progressive Collapse
6.2 Modelling of Thermal Effects
6.3 Structural Model
6.4 Structural Response of Components to Mechanical Loading from Fire
7. PREDICTION OF STRUCTURAL FAILURE
7.1 In General
7.2 A Simplified Approach to Structural Response
7.3 Failure Modes
7.4 Some Applications of Failure Criteria
8. COMBINED EFFECTS
REFERENCES
APPENDICES
A. EXAMPLE 1 - Firewall
B. EXAMPLE 2 - Design of Fire Resistant Platform Top Bde
C. EXAMPLE 3 - Design of Offshore Structures Subjected to Fire
Use of Ultimate Strength Technique
1. INTRODUCTION
Offshore structures are designed elastically for normal Various numerical systems, as referenced in [2], [3], [4]
operating and deadweight loads. They may be also [5] and [9],h ave been applied to carry out progressive
designed by a load factor method for the serviceability collapse analysis. In order to provide guidance for
limit condition which takes account of the deflections selection and use of a suitable progressive collapse
found, although these are always expected to be in the analysis system and progressive collapse modelling, this
elastic range. Other load conditions may also be document introduces the basic theory behind progressive
considered as well as the in-place case, e.g. collapse and presents various methods for performing a
transportation, to check that no permanent deformations progressive collapse analysis. It also gives practical
will occur. examples of application of progressive collapse
techniques.
For accidental loads, however, permanent deformations
may be allowed providing that the behaviour of The document is set out in a logical manner, Figure 1
structures does not contravene the acceptance criteria. and considers in turn the setting of performance
Performance standards are set for structures in terms of standards to meet the present goal setting approach to
their functionality, reliability, availability, survivability, safety on offshore installations, the thermal response of
interactions between systems and maintenance and structures and components, the strength response or
inspection as they influence the systems reliability and structures.a nd components, combined effects and finally
availability. The ultimate strength may be exceeded, for examples describing various approaches.
example, after a structire has fulfilled its survivability
function for the necessary period of time in an accident.
Performance Standards
A structure subjected to a fire progressively loses its
strength until a global mechanism or a rupture develops
when the structure collapses and becomes unusable. Thermal Response of Structures and
Components
Present advanced numerical non-linear methods in I
structural engineering make an ultimate strength analysis
possible. In an ultimate strength analysis, the applied
Strength Response of Structures and
loads are increased from zero to some predetermined
Components
level, and the behaviour of the structure is monitored for
I
each loading increment. As such an analysis predicts
progressive failure of members, it is termed progressive k
Combined Effects
collapse analysis.
The Interim Guidance Notes (IGN) for the Design and
Protection of Topside Structures against Explosions and I I
Examples
Fire [I] makes only brief reference to progressive
collapse in Section 3.5. In the three years following the
publication of this document a significant amount of
Figure 1
work has been conducted, developing a greater
Document Layoul
understanding in the field of progressive collapse.
The objective of an analysis of progressive collapse
induced by fire is to determine the failure or the time to
failure of the structure as a result of the failure of its
members due to fire loading. This deterioration in the
load carrying capacity of members, and the structure as
a whole, can lead ultimately to structural collapse under
dead and operational loading.
This Technical Note has been prepared to focus on the
subject of progressive collapse and supplements Sections
3.5, 4.4 and 4.6 of the IGN, and also Sections 3, 7 and
8 of Technical Note 1 describing the “Fire Resistant
Design of Offshore Topside Structures”.
FABIG Technical Note - April 1995 Page 1
Use of Ultimate Strength Techniques
2. BACKGROUND
A platform topside resists a hydrocarbon fire as a whole
where the structural and plant components interact. A
partitioning (non-load bearing) wall, for example, can
provide a sufficient resistance to flame penetration, even
up to a temperature of 1000 deg centigrade, for a time
much longer than 60 minutes. Therewith, such a wall
provides considerable shielding for loadbearing
structures behind the wall, which increases the time to
collapse of the structures from re-radiated heat from the
wall and confirms longer survivability of the topside or
its parts.
The topside supporting structure itself has large thermal
and structural reserves when subjected to a hydrocarbon
fire. The structure can be analysed for the time-
dependent thermal response due to the fire and the fire
induced progressive collapse using numerical methods
providing that results of such analysis can be used with
confidence.
All the above thermal and structural effects may be
included in one overall numerical model of a platform
topside. Rating of all the existing walls may be
reviewed for their resistance against flame penetration.
Effects of shielding provided by the walls and other
significant objects may then be included in the model
whereby they provide thermal shielding until the time of
their collapse. Moreover, for improved efficiency, the
same model should be used to compute the time history
of temperatures and to simulate the resistance of the
topside by progressive collapse due to the rising
temperatures.
The theoretical background of this technique should be
carefully considered especially in respect of the accuracy
which may be feasible to achieve. The heat gain of the
structural and plant steel has been identifed as lacking
a sufficient accuracy. Specific thermal models need
therefore to be developed to provide realistic
temperature history for the thermdy induced collapse.
A topside normally requires only limited fire protection
if all the inherent reserves, which may be substantiated
by these techniques, are taken into consideration. Fire
protection can thus be optimised.
Page 2 FABIG Technical Note - April 1995
Use of Ultimate Strength Techniques
3. PERFORMANCE STANDARDS
3.1 in General
The present goal-setting approach to safety on offshore performance required of a system, item of equipment,
installations in the UK Continental Shelf means that new person or procedure and which is used as the basis for
oil and gas installations must be designed to sufficiently managing the hazard - e.g. planning, measuring, control
resist potential accidental loads. Furthermore, the or audit - through the lifecycle of the installation. The
operator’s management system must enable safe regulation does not specify what performance standards
operation of the installatibn and provide adequate should be - that is for the duty holder to decide, taking
response to accidents, should they occur. account of the circumstances on the particular
installation.
A goal-setting approach also allows other “goals” such
as environmental issues or continuity of hydrocarbon The performance standards are installation-unique. They
production targets to be defined and then integrated and should be defined by the duty holder on each project and
optimised within the whole design and operation of an the following three levels:
oil or gas installation. One of the major advantages this
Risk based performance standards which are
approach offers is common defined standards or criteria
quantitative and spec@ levels of individual risk,
which both risk practitioners and engineers can
Fatal Accident Rate or similar which have to be
understand and relate to.
satisfied;
In the past, design criteria for fire and explosions have Scenario based performance standards which can be
been prescriptive and component based. In reality, either qualitative or quantitative, and which set an
however, an oil or gas installation fulfils its objectives as overall target or objective for management of a
a whole system, i.e. all its parts and components act particular hazard or set of hazards;
together, often in a time-dependent manner.
and
Performance standards or criteria can be established for System based performance standards which specify
an oil or gas installation on the basis of reservoir data a level of performance or competence to be
productlcharacteristicsa nd commercial constraints which achieved by the system required to manage or to
may exist for the development of a hydrocarbon field. In respond to the hazard.
addition, the duty holders’ risk criteria will give
information for performance standards for systems under The performance standards address:
accidental load conditions. Overall systems, e.g. the
All major hazards:
installation, the temporary refuge, etc., can then be
Fire;
designed to achieve the performance standards for
Explosions;
accidental conditions utilising the full interaction
between systems and components of the installation. Dropped objects;
Vessel impact;
The performance standards are focused on the key Toxic release;
contributions to safety on an installation whereby the
Corrosion, fatigue, extreme weather, extreme seas,
duty holders must demonstrate that risks to personnel
overloading, etc.;
have been reduced to ALARP, stated in “A guide to the
Aircraft impact;
Offshore Installations (Safety Case) Regulations 1992”.
Loss of station - keeping (Mobile/Floating units only);
Under the ALARP principle duty holders may discharge
their responsibilities when they can show that there Effects from nearby installations (e.g . undersea releases,
would be a gross disproportion between the cost of oil slicks etc);
additional preventive or protective measures, and Construction.
reduction in the risk they would achieve.
All disciplines:
Process;
Piping;
3.2 Definition of Performance Standards
HVAC;
A definition of performance standards is contained in
Mechanical;
guidance to the Prevention of Fire and Explosions, and
structural;
Emergency Response (PFEER), i.e.
Electrical;
A performance standard is a statement, which can be Control & Insnumentation.
expressed in qualitative or quantitative terms, of the
FABlG Technical Note - April 1995 Page 3
Use of Ultimate Strength Techniques
and they will include the following characteristics of approved codes of practice with an appropriate input also
systems: from guidance material. Performance standards are
developed for each installation individually on the basis
Functionality;
of the same documents, reservoir data and commercial
Reliability;
constraints.
Availability;
Survivability;
Interactions between systems; 3.3 Terminology
and
For the purpose of major hazards management and in
Maintenance and inspection as they influence the systems respect to the above definitions of performance standards
reliability and availability. for systems, the following terminology and defintions
may be used:
The application of the performance standards approach
Functionality:
is illustrated in Figure 2. The available documentation
on the major fire and explosion hazards related to The ability of a system to perform as specified for its
offshore design and operational activities may be divided intended purpose.
in the following two generic tiers:
ReliabiLity:
Tier 1 - Various legislative documents which include:
.
The probability that a system is able to perform a
Acts of Parliament required function under stated conditions for a stated
.
Regulations period of time or for a stated demand.
Statutory Instruments
Availabiity :
Also included in this category are the Approved Codes
The proportion of the total time that a component,
of Practice.
equipment or system is performing in the desired
manner.
Tier 2 - Standards and various other forms of guidance
which try to document and reflect best practice. All
Survivability:
these documents are used by the duty holders to assist in
meeting the requirements established by legislation and The ability of a system to function through, and continue
the duty holders themselves. to perform adequately beyond, an event which it was not
intended for.
Standards and guidance are generally provided at two
levels:
Interaction:
Management Guidance The influence two or more systems have upon each
. Establishes the principles. other, and the response of the systems involved.
Technical Guidance
Maintenance:
Suggests an approach for design to fulfil the
established principles. The preventive or corrective actions performed to either
sustain a system at a specified condition or, return a
Management and technical guidance should complement system to a specified condition.
each other so as to satis@ the following safety life cycle
activities :
. Hazard identification
Risk assessment
Establishment of performance standards
Development and implementation of measures to
control risks
and
Verification that the performance standards are
met.
The duty holder’s risk criteria are established on the
basis of the legislation, regulations, standards and
Page 4 FABlG Technical Note - April 1995
Use of Ultimate Strength Techniques
I I
LEGISLATION (examples)
Health & Safety at Work Act, 1974.
Offshore Installations (Mineral Workings) Act, 1971.
HSC: Draft Offshore Installations (Prevention of Fire and Explosion, and
Emergency Response).
Offshore Installations (Safety Case) Regulations, 1992.
1
STANDARDS AND GUIDANCE (examples)
~~~~ ~
UKOOA: Fire and Explosion Hazards Management Guidance. PRINCIPLES
SCI: Interim Guidance Notes for the Design and Protection of Topside DESIGN
Structures Against Explosion and Fire. APPROACH
I
RESERVOIR DATA COMMERCIAL CONSTRAINTS DUTY HOLDER'S RISX
(HYDROCARBON CRITERIA
CHARACTERISTICS) Product demand
Sales price of products Safety
Oil/Gas TaxationlTariffs Environment
Reservoir Tie in to other prod. systems Assets
PressurelTemp. Others
Contaminants Connection to refining plant
Etc. I
I
PERFORMANCE STANDARDS
(rsvels ~ . .
Major Hazar d D l S Svstem Charactenms
Risk based Fire Process Functionality
Scenarios based Explosions Piping Reliability
Systems based Dropped Objects HVAC Availability
Ship impact, etc. Mechanical Survivability
Structural Interactions between Systems
Electrical Maintenance
Control & Instrumentation
F l p2
Illustration of Application of Pe@onnance Standards Approach
FABIG Technical Note - April 1995 Page 5
Use of Ultimate Strength 'Techniques
4. SYSTEM RESPONSE
In a fire accident, a platform topside behaves in a time- the effects of heat gain, conduction and heat loss, and
dependent manner. A gas leak ignites, a high intensity compute the temperature history in the structure.
flame is created which attacks plant and structure in its Finally, the strucmral strength response should take the
path and surroundings. The platform temperature history and combine it with other loads on
compartmentalisation, and fire and blast walls provide the structure or a plant component in order to simulate
the first protection of the escaping platform personnel. the timedependent strength behaviour.
Although existing walls, plant components and structures
provide shielding to each other, their temperature is At present, there is no single system available where all
rising with a progressive loss of integrity a$ a result. these phenomena would be included and the above
However, the platform safety shutdown functions limit requirement of an integrated analysis system is achieved
the size of the flame and as the hydrocarbon inventories by interlinking computer systems currently available.
are emptied, the flame recedes.
In an accidental event such as fire, permanent
deformations and/or other damage to the system or its
parts are acceptable providing that they do not
contravene, directly or indirectly, the performance
standards. A non-linear, time-history analysis is
therefore called for whereby the performance of the
entire system and its parts may be assessed throughout
the whole history of the event. This is providing that
the sufficiency and suitability of the analysis method
used can be demonstrated.
Also, both the scenario based and the system based
performance standards may vary with time.
Other reasons why the analysis should focus on the
whole system are:
The state of a system affected by fire at any instant
in time after the start of a fire depends upon the
history of the system's response.
During the non-linear behaviour of the system,
loads are shed and the stiffness of structure or plant
re-distributes allowing for partial loss of strength in
some parts by carrying the load through parts less
affected; this leads to more realistic utilisation of
the available capacify and more realistic
survivability times.
The response of any part of the system at any
location is a result of the system behaving as a
whole, i.e. the response at any one location may be
the outcome of behaviour elsewhere in the system.
Ideally, all of the phenomena of fire, heating-up of the
structure and the progressive collapse of the structure
should be included in one computer system and one
computer model.
Within such an integrated system, the thennal model of
the fire should include the spatially varying heat
intensity, the three dimensional form of the progressively
receding flame as the inventory empties and the
shielding effects of existing walls and structural
members. The thermal structural response should include
-
Page 6 FABIG Technical Note April 1995
Use of Ultimate Strength Techniques
5. THERMAL RESPONSE
5. I Heat Flux Loading hydrocarbon inventories and therefore, the time
dependent nature of the resulting flame sizes.
The transfer of heat energy, per unit area is termed the
heat flux load. In order to determine the structural
Inventory sizing normally requires the accumulation of
response to this fue load, it is necessary to know the
inventory available in pipes and vessels which could
intensity, duration and variability with time and space,
contribute to a fire. Emergency Shutdown Valves
of the fire.
(ESDVs) are installed at strategic locations to reduce the
available ignitable inventory. This means in reality that
The heat balance equation is used to obtain the
only a fraction of the total inventory may be available to
temperature rise in a steel member or any other
fuel the fire. A smaller isolated inventory can also
component made of any material:
affect the size of fires. Rupture of a pipe or vessel can
quickly reduce the pressure of any hydrocarbons
contained within the plant. The decay in pressure may
be described as:
where:
9ir = the incident radiant heat flux, generally
given by the fire loading models;
where:
E = surface emissivity at surface reference
temperature (nondimensional) of the pt is the release pressure at time t;
body receiving the heat flux load;
9ic = the incident convective heat flux; po is the release pressure at time t = 0; this may be
taken as equal to the vessel pressure;
grad = the heat flux re-radiated from the surface;
CL is the decay constant, which can be fitted to the
9conv = the heat flux convected away from the time for the pressure to reduce to a fraction of its
surface;
original value.
gCod = the heat flux conducted away from the
surface (i.e. into the material); The release pressure and orifice size (hole diameter) can
be used to obtain the release rate of the hydrocarbon,
9s = the heat absorbed by the body receiving
and this can be related to a resulting jet flame length
it.
using expressions such as that developed by Wertenbach
PI
:
Both qir and qic depend upon the temperature difference
between the flame and the body receiving the heat. The
L = 18.5 m0.41 (3)
greater the difference in the respective temperatures, the
greater the energy transferred from the flame to the
where:
body. As time passes the temperature of the steel
increases, reducing the difference in the respective L is the jet flame length;
temperatures and consequently, the heat flux received by
m is the mass flow rate of the release c.~gs-').
the body decreases.
Note: this correlation has only been validated for vertical
The heat fluxes received by structural members or plant
jet releases. Care should be taken when using this
components are also affected by:
relationship beyond the validity range.
position of the member relative to the fire;
shielding; Thus the natural depressurisation of an inventory, as the
surface finish of the member; fuel is steadily released following rupture, can be related
type and quantity of any applied protection; to a flame length which will decrease with time, in a
hydrocarbon type; similar manner to the pressure.
and
The natural depressurisation of plant can also be assisted
size and nature of release. through 'blow-down' systems which purge the inventory
or enhance depressurisation. Gas inventories may be
[n.
See also [6] and directed to flare stacks or cold vent systems during
blow-down. It is unlikely that platforms will have
A second variable in the determination of structural similar systems available for the disposal of liquid
response is the time dependent nature of the available inventory in an emergency.
FABIG Technical Note - April 1995 Page 7
Use of Ultimate Strength Techniques
The release pressure will have no significant effect on connect the various braces. All hollow members contain
the burning rate of a pool fire, though the release an internal member element so that the effect of internal
pressure may define the location and size of an area material (gas, water ....) can be simulated.
where fuel can drop out and develop into a pool fire.
Flame characteristics for a pool fire are largely Phenomenological Models
dependent upon the mass burning rate, ventilation and
pool diameter. A link program [lo] reads jet fire data from a database
created by a phenomenological model. From two-
dimensional flame information the flame axis is
5.2 Thermal Models of the Fire calculated. Using the flame axis as the centre-line the
two-dimensional data is manipulated to obtain a three-
The fire intensity is normally established using
dimensional flame. The three-dimensional flame is then
phenomenological models or numerical models. The
mapped onto a spaceframe heat conduction model of the
phenomenological models may be applied to problems
platform. Heat is transferred by means of radiation and
covered by the range of the tests used for development
convection links between the flame and the structure.
of the models. The numerical models are normally
Re-radiation fiom heated members is also considered
based on Computational Fluid Dynamics (CFD) which
where these members pass out of the flame as the flame
involve solution of the governing equations for
recedes. Conduction of heat through members is also
momentum, continuity and energy. CFD models are
included. The thermal model takes into account the
more generally applicable, but the confidence put on
shielding effect of deck plates or partitioning/fue walls,
their results need to be verified.
reducing the incident flux on otherwise impinged
members. This reduction is full or partial, to account for
As already mentioned in Section 4, there is no single
the fact that as the deck plate or a wall heats up due to
computer system available at present, where all the
flame impingement on the “hot” side, it may re-radiate
phenomena of heat transfer from the fie to the structure
this energy on the “cold” side. Modelling of this effect
and the structural progressive collapse would be
is accomplished by moddying the incident heat flux from
available. Nearest to such a system is the software
the flame to the shielded structural elements.
described in [9]. A limited number of suites of inter-
linked computer programmes exists where some of these
Finite Qement Models
capabilities are available, [ 101 and [l1 1, Figure 3. Some
of the link programmes connecting the suites are briefly
The thermal model of the fire [1 11 is constructed using
described below.
solid elements to model each member. This enables any
shape of fire to be modelled. Fire intensity can be more
0-Based Modelling
conveniently specified in terms of a flame temperature
related to the surface emissive power (SEP) by the
A CFD-based code for the computation of frre intensities
Stefan-Boltzmann law, than by SEP itself:
coupled with a heat conduction solver for temperature
histories with a further fire-induced progressive collapse
of spaceframes is available [9].
(4)
A heat transfer simulation connects the fire simulation
package with the structural response analysis package. where:
The gas temperature and radiation at each time step are
Q = surface emissive power of the fie;
given in a grid which envelops the entire structure,
Figure 4. From this the actual heat loads are determined u = Stef~-Boltunann~nstant
and the temperature in each structural member is (5.669 X lo4 W/m2 OK4);
calculated. The mean temperature and gradients over
E = surface emissivity (taken as 1).
the cross-section are stored and subsequently retrieved
by the structural response package.
Currently, the flame radiative properties and the
attenuation effects of the intervening atmosphere between
For the heat transfer and conduction analysis as
a flame and an external object are characterised by
implemented in [9], a new finite element model is
global empirical parameters:
created based on the space frame model generated by the
structural response package. It consists mainly of 4 The fraction of combustion energy released as
node quadrilateral elements. The transfer from beam radiation, the ‘F factor”;
element to surface elements is illustrated in Figure 5 for
The surface emissive power, SEP;
an ‘I*, ‘H’ and tubular profile, respectively. Special
elements are automatically introduced in tubular joints to The atmospheric transmissivity.
Page 8 FABIG Technical Note - April 1995
