thermal response of vessels and pipework exposed to fire

0T192 610

THERMAL RESPONSE OF VESSELS AND PIPEWORK

EXPOSED TO FIRE

This document was written by Dr J N Davenport

Shell Research Limited Dr S M Richardson and Dr G Saville

Imperial College for

The Steel Construction Institute Silwood Park

Ascot Berkshire SL5 7QN

This document London: HMSO Lc0t5 pages J

Health and Safety Executive - Offshore Technology In formation

Crown copyright 1992 Applications for reproduction should be made to HMSO

First published 1992 iSBN 0 11 8820990

This report is published by the Health and Safety Executive as part of a series of reports of work which has been supported by funds formerly provided by the Department of Energy and lately by the Executive. Neither the Executive, the Department nor the contractors concerned assume any liability for the reports nor do they necessarily reflect the views or policy of the Executive or the Department.

Publications in the Offshore Technology Information (OTT) series are intended to provide background information and data arising from offshore research projects funded by the Department, or the Executive, and major companies. Results, including detailed evaluation and, where relevant, recommendations stemming from their research projects are published in the 0TH series of reports.

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FOREWORD

This report is one of twenty six work package reports written as part of the Joint Industry Project on Blast and Fire Engineering for Topside Structures. The Project Phase 1 started in May 1990 to collate, appraise and disseminate information on blast and fire loads, and on the resistance of structures and facilities to these loads. The titles and numbers of the reports generated by this project are as follows.

List of Reports General 01192597 Behaviour of oil and gas fires'in the

(FL2) presence of confinement and obstacles. 01192585 Generic foundation data to be used in (01(a)) the assessment of blast and fire sce- 01192598 Current fire research, experimental,

narios. (FL3) theoretical and predictive modelling resources.

(02 included Typical structural details for primary, in the above secondary, and supporting structures/ Blast Resistance report) components.

OTT 92599 The effects of simplification of the 01192586 Representative range ofblast and fire (BR1) explosion pressure-time history. (01(b)) scenarios.

01192 600 Explicit analytical methods for deter 01192587 The prediction of single and two phase (BR2) mining structural response. (Gl(c)) release rates.

01192601 Computerised analysis tools for 01192588 Legislation, codes of practice and (BR3) assessing the response of structures (G3) certification requirements. subjected to blast loading. 01192589 Experimental facilities suitable for use 01192602 The effects of high strain rates on (04) in studies of fire and explosion hazards (BR4) material properties.

in offshore structures. 01192603 Analysis of projectiles.

01192590 The use of alternative materials in the (BRS) (05) design and construction of blast and

fire resistant structures. Fire Resistance Blast Loading 01192604 Experimental data relating to the

(FR1) performance of steel components at 01192591 Gas/Vapour build-up on offshore elevated temperatures. (BL1) structures.

01192605 Methodologies and available tools for 01192592 Confmed vented explosions. (FR2) the design/analysis of steel components (BL2) at elevated temperatures. 01192593 Explosions in highly congested 01192606 Passive fire protection performance (BL3) volumes. (FR3) requirements and test methods.

01192594 The prediction of the pressure loading 01192607 Availability and properties of passive (BL4) on structures resulting from an explo- (FR4) and active fire protection systems.

sion. 01192608 Existing fire design criteria for

01192595 Possible ways of mitigating explosions (FR5) secondary, support and system steel- (BL5) on offshore structures. work.

Fire Loading 01192609 Fire performance of explosion- (FR6) damaged structural and containment

01192596 Oil and gas fires - characteristics and steelwork. (FL1) impact. 01192610 Thermal response of vessels and

(FR7) pipework exposed to fire.

1

The purpose of these reports isto collate, review and critically appraise the current state of knowledge in each of the listed subject areas. They discuss the current state of technology as applicable to each of these areas. They also indicatetheareasofsignificantresidualuncertainty. The objectivesand scope ofeachreport are outlinedinthe 'Work Package Descriptions' included in each report

The reports are written for use by engineers with specialist responsibilities for specific tasks associated with safety against potential fires and explosions in the design ofTopsides on Of hore Structures (eg as part ofa Formal safety Assessment) or for the use by their expert advisors. They are intended to be both expert and authoritative.

Thesereportshavealsobeenusedasthebasis forestablishinginterimguidanceandto identifyanynecessaryfurther research or development work These are also project deliverables and are issued as separate documents.

This Project has been sponsored and funded by the following twenty nine oianisations:

AEA Technology (Safety and Reliability Directorate) Agip (UK) Limited

Amerada Hess Limited Amoco (UK) Exploration Company

Aico British Limited British Gas Plc

BP International Limited Chevron (UK) Limited Conoco (UK) Limited

Den Noiske Stats Oljeselskap AS The UK Department of Eneiy

Elf (UK) Plc Enterprise Oil Pie

ESSO Exploration & Production (UK) Limited Hamilton Brothers Oil & (las Limited

Kerr-McGee Oil (UK) Plc Marathon Oil (UK) Limited Mobil North Sea Limited

NorskHydro AS Occidental Petroleum (Caledonia) Limited

Petro-Canada Resources Phillips Petroleum Co (UK) Limited

Ranger Oil (UK) Limited Saga Petroleum AS

Shell UK Exploration and Production Texaco Britain Limited Total Oil Marine Plc

Ultramar Exploration Limited Unocal (UK) Limited

2

CONTENTS

Page

EXECUTIVE SUMMARY

1. DEFINITIONS 7

2: INTRODUCTION 9

2.1 The fire response work packages 9 2 . 2 Obj ectives of FR7 9 2.3 Background 9 2.4 Scope 10 2.5 Outline of the study 10

3. FIRES ON OFFSHORE PLATFORMS 12

3.1 Platform layout 12 3.2 Pipes and vessels 12 3.3 The hydrocarbon inventory 13 3.4 Fire scenarios 13 3.5 Fire loading of vessels and pipes 14 3.6 Mitigatory factors 14

4. PHYSICAL PROCESSES 17

4.1 External heat fluxes 18 4.1.1 Heat flux from the fire 18 4.1.2 Heat transfer to the pipe or vessel 18 4.1.3 Heat transfer to a water deluge 19 4.1.4 Heat losses to the surroundings 20

4.2 Heat transfer through the vessel/pipe wall 20 4.3 Heat transfer to the vessel/pipe contents 22

4.3.1 Heat transfer from wall to vapour 23 4.3.2 Heat transfer from wall to liquid 24 4.3.3 Heat, mass and momentum transfer 26

between liquid and vapour 4.4 Venting 28

4.4.1 Venting through the blowdown system 28 and a PSV

4.4.2 Venting of a flashing liquid 30 4.5 Failure mechanisms 31

4.5.1 Superposed pressure and thermal stress 32 4.5.2 Stress at failure 33

4.6 Summary 34 4.7 Sources of equations 35

5. CURRENT MODELLING CAPABILITIES 37

5.1 The LPG fire response models 37 5.1.1 Features of the LPG response models 37 5.1.2 Validation of the LPG response models 38

5.2 Blowdown models 40

3

5.2.1 Features of the blowdown models 40 5.2.2 Validation of the blowdown models 41

5.3 Summary of current capabilities 41

6. OUTSTANDING REQUIREMENTS 43

6.1 The requirements of predictive models 43 6.2 Physical processes 43 6.3 Predictive modelling 44

6.3.1 Non uniform heat fluxes 45 6.3.2 Models for pipework 46 6.3.3 Fire protection 46 6.3.4 Multicomponent fluids 46 6.3.5 Venting 47 6.3.6 Thermodynamic treatment 47 6.3.7 Vessel failure 48 6.3.8 Validation 48 6.3.9 Interconnected vessels 48 6.3.10 Partial damage 49 6,3.11 Heat transfer to the sea 49

7. CURRENT POSITION 50

8. AREAS OF UNCERTAINTY 54

REFERENCES 57

TABLES

FIGURES

APPENDICES

A FR7 Work Package

B Primary information on predictive models

C REVIEW PAPER BY K MOODIE

4

EXECUTIVE SUMMARY

EXECUTIVE SUMMARY

Fire attack of vessels and pipework containing hydrocarbon on offshor platforms can lead to escalation if loss of containment occurs. The appropriate emergency response depends on a firm understanding of the vulnerability of vessels and pipes to different fire types, a ranking of the equipment most at risk, predictions of the time available for countermeasures, and the hazard posed by rupture.

Knowledge of these factors is essential for the safe operation and shutdown of existing facilities, and should influence the design of new platforms.

This report examines the extent of knowledge of the physical processes which take place during fire attack, and compares the predictive capabilities of available models with the requirements. It gives a "snapshot" of the current position, concentrating on information which is within the public domain, and models which have been validated in large fires.

The relevance and applicability of the relationships used to describe physical processes is discussed, and reference is made to sources where the appropriate equations can be found.

This report includes summaries of the current position and areas of uncertainty, but the reader is encouraged to study it as a whole. Where possible the report is self contained, but it draws on findings of other Work Packages, to which reference should be made as appropriate.

The physical processes taking place when pressurised equipment containing hydrocarbons is exposed to a fire are relatively well understood, at least when the fluid is simple, and empirical relationships are available to describe the heat and mass flows. However, for the fluids of interest, which are mixtures such as condensate and crude oils, a physical description is far more complex, and there are many uncertainties.

Modelling the latter subject is very difficult, and has not yet been attempted as a whole. There are predictive computer codes for the simpler situations, and in that role they are satisfactory, but as might be expected, they have only limited application.

One of the most important questions to be answered is the likely time to failure of a vessel or pipe. The available models do not generally predict this, and the subject needs further attention.

It is possible that the existing computer codes can be used to rank vessels and pipework at risk from fire attack, to gain an

5

EXECUTIVE SUMMARY

idea of the relative importance of factors not yet included, and to aid the development of more appropriate models.

A great deal of research and development work may be needed, but it must be well focused to meet pre-identifled essential needs on time.

6

DEFINITIONS SECTION 1

1. DEFINITIONS

Organisations and Official Bodies

API American Petroleum Institute

ASTM American Society for Testing and Materials

BAM Federal Institute for Materials Research and Testing HMSO Her Majesty's Stationery Office

HSE Health and Safety Executive (UK)

Terms and Abbreviations

BLEVE A foiling iquid expanding apour xp1osion which results from the sudden failure of a vessel containing a pressurised saturated liquid/vapour at a temperature well above its normal (atmospheric) boiling point.

BLOWDOWN The controlled venting of a pressurised vessel or line on normal or emergency shutdown of a process.

CRITICAL HEAT FLUX (density) The heat flux (density) above which film boiling can occur.

CRITICAL PRESSURE The pressure below which a substance may exist as a gas in equilibrium with the liquid.

FIREBALL A fireball is the rapid turbulent combustion of fuel as an expanding, usually rising ball of flame.

HEAT FLUX (density) Heat flux density is an expression quantifying the rate of heat transfer per unit area normal to the direction of heat flow. A convenient unit in the present context is kW m2. (1 kW m2 = 317 Etu ft2 h').

7

DEFINITIONS SECTION 1

HYDROCARBON (fire) A hydrocarbon is a molecule comprised exclusively of carbon and hydrogen. There are an enormous number of such species; the petroleum industry commonly deals with hydrocarbons ranging from one carbon atom (methane, natural gas) through to waxy and bituminous mixtures with several tens of carbon atoms. From the fire point of view, there is also a wide spectrum of other organic chemicals, solvents, treatment agents etc., with similar combustion characteristics producing a rapid temperature rise and sustained high heat flux density. See Work Package FL1 Section 7.

JET FIRE A turbulent diffusion flame resulting from the combustion of a fuel continuously released with some significant momentum in a particular range of directions.

LPG (Liquefied Petroleum Gas) The term applied to those hydrocarbons which are in the gaseous state at normal temperature and pressure but which can be liquefied by compression and/or refrigeration. Propane (boiling point -42C) and butane (boiling point -0.5C) are the most common.

POOL FIRE For the purposes of this report, a pool fire is defined as a turbulent diffusion fire burning above an upward facing horizontal pool of vaporising fuel under conditions where the fuel vapour or gas has zero or very low initial momentum. See Work Package FL1 for details.

PSV (pressure safety valve) A relief valve which prevents the internal pressure of a vessel or pipe from exceeding an allowed limiting pressure.

TOPSIDE In this report, topside structures are taken to include the jacket, risers and conductors down to the waterline. The insides of legs below the waterline are also included.

8

INTRODUCTION SECTION 2

2. INTRODUCTION

2.1 The Fire Response york Packages

There are seven Fire Response work package reports:

FRi Experimental data relating to the performance of steel components at elevated temperatures (SCI).

FR2 Methodologies and available tools for the design/analysis of steel components at elevated temperatures (SCI).

FR3 Passive fire protection - performance requirements and test methods (Shell Research Ltd).

FR4 Availability and properties of passive and active fire protection systems (SCI).

FR5 Existing fire design criteria for secondary, support and system steelwork (Sd).

FR6 Fire performance of explosion-damaged structural and containment steelwork (SCI)

FR7 Thermal response of vessels and pipework exposed to fire (Shell Research Ltd)

2.2 Objectives of FR7

The formal work package objectives are:

To summarise knowledge on the thermal response of hydrocarbon containing vessels and pipework to fire effects.

To identify gaps in knowledge.

The FR7 work package is reproduced for reference in Appendix A.

2.3 Background

The rupture of pipework or process vessels through fire impact on an offshore platform can lead to escalation. To devise effective means of protection and control it is essential to understand the mechanisms involved.

There has to be a firm understanding of the vulnerability of vessels and pipework to particular fire events, and the

9


ability to predict which parts of the plant are most at risk, so that special protection or additional safety steps can be adopted, either retrospectively on an existing structure or at the design stage for a new platform. If a fire does develop, there needs to be an assessment of the time to possible failure, so as to implement the safest and most effective emergency procedures.

2.4 Scope

This study is based on the assumption that a platform fire has triggered the alarm system, and initiated an emergency shutdown sequence involving isolation and blowdown of the process trains, and activation of the water deluge system.

For vessels and pipework containing hydrocarbon, this is probably a more severe scenario than one in which the process plant is still in operation, as it precludes removal of heat by the flowing fluid.

It is recognised that the emergency shutdown may not occur in practice, for example the alarm system may fail to respond, and damage to equipment or communication lines might be so rapid and severe that shutdown does not happen as planned. However, such events are outside the scope of this study.

2.5 Outline of the study

This study has involved work in two main areas:

The state of knowledge of the physical processes which occur when vessels and pipework are exposed to fire, and the ability of models to predict the response.

The relevance and application of this knowledge to the safe operation of existing and future offshore platforms, and the identification of gaps or deficiencies where further work may be required.

The approach has been to appraise

The characteristics of vessels and pipework on the topsides of offshore platforms, the nature of potential fire loadings upon them, and the ways in which the effects of fire on vessels and pipework can be mitigated (Section 3).

The basic physical processes occurring when vessels and pipework containing hydrocarbons are exposed to fire loadings (Section 4).

10


The capabilities and extent of validation of models (probably, but not necessarily, involving the use of computer programs) for prediction of the response of vessels and pipework containing hydrocarbons to fire loadings (Section 5).

The differences between what is required of a comprehensive predictive model which has been fully validated and what is currently available, and ways in which those differences might be eliminated (Section 6).

The relevanceof fire response modelling work to the safety of offshore platforms, and in particular the ways in which future work might be targeted to improve understanding of the hazards involved. In this way platform design, operational practices and emergency response may be improved.

The primary information on the current models, which has been used for this section of the report, is given in summary form (Appendix B). A relatively recent paper, which includes a review of the modelling of the response of vessels engulfed in fires and of experimental validation of such modelling, is also given (Appendix C).

Parts 7 and 8 present the current position and areas of uncertainty.

11

FIRES ON OFFSHORE PLATFORMS SECTION 3

3. FIRES ON OFFSHORE PLATFORMS

All process plants dealing with flammable materials are vulnerable to fire attack of the pipework or process vessels. Loss of containment can quickly result in escalation of the fire and a loss of control. This is particularly so for offshore platforms, where restricted space means that vessels and pipes are very close together. In addition, there is often a large inventory of hydrocarbons, and escape to a safe distance is difficult.

Escalation of the fire through attack of a vessel or pipe can occur in several ways. Low momentum events such as leakage of hydrocarbon would increase the fuel available to a local pool fire or create an additional one, while high pressure gas or liquid releases might give rise to jet fires and fireballs. Explosion hazards such as blast waves and missiles can occur if rupture of the vessel or pipe is sufficiently fast.

3.1 Platform layout

The space available on the topsides of offshore oil or gas platforms is very restricted, typically some tens of metres long, wide and high. Almost without exception this means that the processing vessels and pipework are packed very closely. Thus the layout is very different from that in a comparable onshore process plant, and the type and degree of reliance on fire protective measures is not the same.

At a location such as a refinery, fire escalation can be minimised through well-defined hazardous and non hazardous areas and large safety distances, but for the offshore platform, far more reliance has to be placed on containment of the fire by barriers and water deluge systems.

3.2 Pipes and vessels

The vessels of interest in the present study range from ambient pressure storage tanks to high pressure separators, with varying capacities up to several tonnes. Usually the vessels are housed within specific modules, but in some cases they are large enough to span adjacent areas.

There are kilometres of interconnecting pipework, and in addition, there are the conductors and risers for oil/gas import and export.

The walls of the vessels and pipelines on an offshore platform are generally steel, but there may be additional, usually concentric, layers of different materials such as insulation on walls which can become very cold (for example downstream of Joule-Thomson (flash) and pressure relief valves) or as

12


thermal protection on walls which might become very hot in a fire.

Operating temperatures of the pipes and vessels can range from - 30 Celsius to +100 Celsius, with pressures up to several hundred bar, but generally below 200 bar.

3.3 The hydrocarbon inventory

The hydrocarbons of interest fall into two categories - those involved in the primary production process, and those which are used in support of the platform operation.

In the production process the hydrocarbons can range from natural gas (mainly Cl) through associated gases (mainly Cl to C4) and condensates to 'live' and 'dead' crude oils.

In addition there will be a variety of hydrocarbons used in support roles, for example diesel fuel for generators, methanol for gas processing, and aviation fuel for helicopter refuelling.

Thus the inventory of the pipes and vessels can be all vapour, all liquid or vapour plus liquid and can comprise hydrocarbons ranging from Cl to C30 and beyond. There may also be significant quantities of water, nitrogen, carbon dioxide and hydrogen sulphide.

On an oil platform in particular, the inventory may be quite large. Quite apart from the contents of the process vessels and pipework, which will amount to many tens of tonnes, there are large quantities held in the risers (this can usually be segregated by emergency shutdown devices). Much more detailed information on platform layout and inventories can he found in Work Packages Cl and G2.

3.4 Fire scenarios

A fire can be broadly characterised by the type, physical state and source of the fuel involved, the ventilation conditions, the intensity with which the fire burns, and its duration. The principal fire conditions to be considered on the basis of severity and in relation to the response of vessels and pipework relate to hydrocarbon pool fires and jet fires. Work Packages FL1 and FL2 provide a comprehensive review and appraisal of heat flux data relating to these fires.

13


3.5 Fire loading of vessels and pipes

Incident heat flux on a vessel or line will, of course, increase the wall temperature, and the temperature and pressure of the contents. Usually, the pressure rise will eventually trigger lifting of a pressure safety valve (PSV), and fire detection systems may have initiated external cooling from a water deluge system.

A PSV does not prevent weakening and failure of a vessel that becomes locally overheated and overstressed. It can only limit the internal pressure from rising beyond the allowable accumulation pressure.

A vessel or pipe y be protected against such failure by depressurising

and/or limiting the heat input The value of these mitigatory factors are considered in the next section.

3.6 Mitigatory factors

Controlled depressurising of a vessel (blowdown) reduces the internal pressure, and hence the stress in the vessel walls. It also reduces the potential addition of fuel to the fire should the vessel rupture.

Industry practice for the blowdown of process facilities has been led by blowdown criteria given in API Recommended Practice documents 520 and 521 [1,2]. This generally involves reducing the pressure from initial conditions to 50 % of the vessel's design pressure or 7 barg, whichever is the lower, within approximately 15 minutes.

If the blowdown operation is one which vents from the vapour space, then depending on how effective it is, the overall change of vessel pressure may be positive or negative. If blowdown is from the liquid then of course the liquid level drops. The relative amounts of liquid and vapour will change as a function of temperature and pressure, and indeed as the liquid temperature increases, the quantity of liquid may either rise or fall.

The strength of the vessel will decrease as the wall temperature rises. If the wall strength is no longer able to cope with the internal pressure then the vessel or line will fail, and the process may continue with other neighbouring vessels and pipework.

It is important to realise that failure may actually take place at a lower pressure than was originally present in the vessel, and that a blowdown sequence which gives a net

14


reduction of pressure with time is no guarantee of the vessel integrity.

If blowdown is activated as a result of a fire, it might increase the likelihood of vessel failure under some circumstances. For example, if the blowdown operation lowers the fluid level in a vessel with a fire loading, the increasing area of unwetted wall makes the risk of vessel failure greater.

Limiting the heat input from fires by external insulation reduces both the rise of the vessel wall temperature and the generation of vapour inside the vessel. The various types of fire protective materials which can be used to mitigate the effects of fire loading on vessels and pipework are discussed in Work Packages FR3 and FR4.

External deluges, generally of water, are often directed onto the upper parts of a vessel. Convective flux is reduced by heat transfer to the water, and radiative flux to the vessel wall is reduced because of high absorption by the water. Water deluge systems are discussed in Work Package FR4.

Layout of the platform can be improved, whereby the geometrical arrangement of vessels and lines is used to reduce the risk or consequences of fire spreading from one vessel or line to another.

All of these mitigatory factors have their uses on platform topsides, in various situations, but the most appropriate choice or combination requires knowledge of the most likely fire hazards and the response of the vessels and pipework both protected and non-protected.

There are other considerations as well. For example:

Fireproof layers on the outside of the wall can have low mechanical strength when exposed to severe fires. The high temperature properties of the materials have to be studied carefully, as radiative heat transfer within the insulation may become important.

Firewalls can be used as barriers for protecting particularly sensitive areas, but can hinder access or egress in an emergency. Design of firewalls must take into consideration the effect of compartmentalising any fire or explosion.

Water deluge systems are common, but have usually not been tested under jet fire conditions, where relatively high gas velocities in some fires might seriously affect performance. Scale and corrosion within the water system can affect operation. In addition, the deluge system can only work if

15


the seawater mains, and the pumps and associated power supply survive the fire.

Removal of contents tends to work only when the incident heat flux is relatively small, since the contents have to be removed sufficiently quickly and safely.

The use of different layouts, for example whether it is better to cluster high pressure vessels in one area and low pressure ones in another, rather than mix high and low pressure vessels together, has not yet been seriously investigated.

One novel form of heat absorption for vessels [3] involves a concentric inner shell, with apertures at the top and bottom. If fluid in the vessel boils, some is forced up the annular space, and back through the aperture into the body of the vessel. It is essentially an internal analogue of a water deluge, providing cooling to the upper part of a vessel, where it is generally most needed.

16

THE PHYSICAL PROCESSES SECTION 4

4. THE PHYSICAL PROCESSES

Exposure of a vessel or line to a fire involves up to six essentially independent units (the fire, the wall of the vessel or line, its vapour contents, its liquid contents, the vent and the surroundings). The units are interconnected by appropriate heat, mass and momentum fluxes, as shown below.

Fire Vessel or pipe vapour contents

t mass heat heat flux flux flux heat flux

[ Vessel or pipe wall >< mass flux Vent including insulation

momentum flux heat flux heat flux mass

4r flux

Vessel or pipe

I

Surroundings liquid contents

For the purposes of this review, the characteristics of the fire itself are regarded as known, this information being provided in Work Packages FL1 and FL2.

Thus the primary concern here is with:

The temperature response of the wall of the vessel or line, and the insulation if fitted (Section 4.2);

The temperature and pressure response of the contents of the vessel or line, with thermodynamic and phase consistency requirements if the contents are two-phase (Section 4.3);

The characteristics of the PSV and the blowdown sequence (Section 4.4);

The response of the pressure-containing wall of the vessel or line, in particular the manner in which it might fail (Section 4.5).

In what follows, no equations are given. To do so would vastly lengthen the report without a significant increase in useful information. The equations themselves are available in the literature, to which specific references are given (Section 4.6).

17


4.1 External heat fluxes

The external flux balance comprises the fire loading, the heat which is transferred to the pipework or vessel, heat losses to the surroundings, and heat transferred to the water deluge if present. These are illustrated in Figure 4.1.

4.1.1 Heat flux from the fire

The principal external heat flux is from the fire to the wall of the vessel or pipe. The magnitude of the flux is dependent on the type of fire and relative positions of the flame and the object. If the vessel or pipe is partially or fully engulfed in the fire, heat transfer will be through radiation and convection; for a non engulfed object the convection term is insignificant.

Detailed information on this area is presented in Work Packages FL1 and FL2.

Fire attack of a vessel or pipe is likely to be accompanied by venting, through blowdown and/or the PSV to a flare, or through loss of containment. When venting is to flare, the additional heat flux can usually be neglected since the flare tip will almost certainly be far away, and there are likely to be intervening walls which shield the radiation. However, when venting is through a rupture it is possible and perhaps likely that the efflux will ignite, giving an additional heat flux. In principle this could be modelled in a similar way to venting through a valve or choke (see Section 4,4), but there are many uncertainties, such as the size of the rupture and the way it develops with time.

4.1.2 Heat transfer to the pipe or vessel

Transfer of heat to the outside of the pipe or vessel is by convection and radiation. Where engulfment does not take place, transfer is through radiation, the important factors being

* Magnitude of the incident radiative flux Variations of the flux in space and time Relative geometry of the fire and the vessel Absorptivity of the intervening air (perhaps

including combustion products) Absorption by a water deluge Absorptivity and emissivity of the wall Wall temperature

Radiative interchange factors have been calculated for several rather simple geometries (see, for example [4]), but the shape and position of the flame can be difficult to predict, and may

18

THE PHYSICAL PROCESSES SECTIOI'T 4

be more complex than the cylinder or cone which is often assumed, particularly if it is in an enclosure.

Areas o the pipe or vessel which are engulfed in the flame will additionally experience a convective heat flux, which can also show large variations spatially and perhaps also with time. Prediction of the heat fluxes within a flame is outside the scope of FR7 - see FL1 and FL2 for more details.

If we make the assumption that the radiative and convective fluxes present at the wall of the vessel or pipe are known, then relatively simple relationships (see, for example [6]) can be used to predict the magnitude of the heat transfer. However, it is important that due account is taken for the change in wall properties with time and temperature. There may be a layer of burnt paint, for example, or soot may be deposited on the wall surface, which can change the emissivity considerably. In addition, temperature dependant thermal properties are often unavailable for insulating materials - in particular, measurement of the radiation properties is a specialist area [7].

4.1.3 Heat transfer to a water deluge

Many pipes and vessels are equipped with a water deluge system, and additional water may be used as part of the emergency firefighting response. Heat transfer to the water may take place from the external heat flux or from the hot vessel wall.

For efficiency a water deluge is usually applied to the upper part of the containment, as drainage of water down the wall under gravity will then cool the lower part. More importantly, fire attack of the containment will give far higher shell temperatures where vapour is in contact with the inner surface than where there is liquid, because of the different efficiencies of heat transfer. Thus the upper part of the wall is generally more in need of cooling than the lower part.

Heat transfer to the water deluge is by radiation and forced convection, which leads to boiling. Heat transfer correlations are available (see, for example reference [6]), but the actual performance of the deluge in terms of mass flow rate, droplet size and distribution are often unknown, particularly in the presence of a high momentum release. The boiling regime, as discussed later in Section 4.3.2, is also important.

19


4.1.4 Heat losses to the surroundings

For most vessels and pipework on topsides the surroundings are module walls and the atmosphere, but there are obvious exceptions, notably risers, the substantial parts of which inevitably pass through the sea. The response of these parts of the risers may or may not be affected by fire attack on the tops ides

There are two basic modes of heat transfer from the wall of the vessel or pipe to the surroundings - radiation and natural convection - which combine additively to give the overall flux. The temperature dependance is such that when the wall is hot, most of the outward heat transfer is by radiation; when it is cool, most is by natural convection.

Of course, if a vessel is fully engulfed by fire, any heat transfer to the surroundings may be negligible compared with the fire itself. However, if there is partial engulfment, for example a pipeline, conduction of heat along the line may give significant levels of radiative flux from non-engulfed areas to the surroundings.

Modelling the radiative and convective heat losses to the surroundings is more straightforward than modelling the incident flux. A reasonable assumption is that the vessel or pipe is a black or grey body, for which well established relationships exist between wall temperature and the radiant flux emitted, eg [6]. However, the emissivity of the surface may not be known, particularly if is covered by soot or char.

Convection from the wall to surrounding air can be natural or forced, and correlations for the two cases exist (see, for example, reference [5]). However the initial air temperature and velocity can be important uncertainties, particularly if the fire is very intense or if the vessel or line is partially enclosed, as is often typical in the modules on topsides.

The completely dominant mode of heat transfer from a wall to the sea is natural convection. Radiation is negligible because of efficient cooling of the vessel or pipe wall. Forced convection can be important in certain sea states, but a conservative risk analysis would presume natural convection alone. The heat flux from such natural convection can be determined using well established correlations, for example from [6]. Unlike the case of heat transfer to the atmosphere, there is no difficulty in specifying the sea temperature, but the water velocity may be unknown.

4.2 Heat transfer through the vessel/pipe wall

The rigorous determination of heat transfer through the wall of the pipe or vessel to the fluid contents requires a

20


complete solution of the transient three-dimensional (radial, axial and azimuthal or circumferential) heat conduction equation. This is not trivial, and numerical methods involviftg discretisation in space and time must generally be applied. Currently, finite-difference methods are more common, but finite-element methods can also be used.

The data needed are the dimensions, specific heat and thermal conductivity of each part (steel, insulation and so on) of the wall. The thermal properties can vary significantly with temperature, and point values are probably not sufficiently accurate.

Because the numerical solution can involve significant quantities of computer time, approximations can sometimes be made which reduce the dimensionality of the problem. The basic geometry is essentially cylindrical, and it is tempting to suppose that a full three-dimensional analysis of heat transfer through the wall is not needed.

To examine the validity of such approximations, let us consider two scenarios:

1. A vertical vessel in a fully engulfing pooi fire.

2. A horizontal pipe in a non engulfing jet fire.

These two cases are illustrated in Figures 4.2 and 4.3.

In the first case, for simplification, we will assume that the heat flux to the entire vessel is uniform and constant. Thus the important modes of heat transfer are radial conduction to the liquid and vapour in the vessel, circumferential conduction, and axial or longitudinal conduction. Of course, these are closely coupled, but are considered separately for the time being:

As shown below (Section 4.3), the wetted and unwetted surfaces of the inner wall show the largest temperature difference, but each area is close to isothermal. Thus, the driving force for both axial and circumferential conduction along the wall is small, particularly if the assumption of uniform and constant heat flux is true. A useful simplification is to treat the two cases as isothermal zones, one between the incoming flux and the vapour within the tank - the other between the flux and the liquid. This assumption will give errors if the vessel is short and fat, rather than tall and thin, as there is a larger area of heat conduction near the ends than in the middle of the vessel.

Radial variations can be small within the two zones - one source is convection of the liquid inside the vessel. Neglect of radial conduction through the wall is possible, but doing so means that the heat flux through the wall can not be

21


evaluated, and in many cases it is variation of response with this same heat flux which is sought.

The second case, that of a pipe exposed to a non-engulfing jet fire, is more complex. The heat flux is almost entirely radiative, and there can be no assumption of spatial uniformity, as one side of the pipe may be receiving up to 100 kW m2, while the other is receiving nothing. Dividing the pipe into zones may still be possible - clearly there are radial areas such as wetted and non-wetted; high flux and low flux. However, there will also be large variations along the axial direction. The flux received is strongly dependent on distance from the heat source, and any objects between the source and the pipe effectively cast 'shadows'

Another, perhaps less important, problem in trying to reduce the dimensionality arises from the less than perfect shape of the pipes and vessels. Most vessels have geometrical irregularities such as inlets, outlets and supports, and lines have bends, tees and supports. There can be additional geometrical irregularities resulting from fire damage (for example partial loss of insulation), the effects of which need to be determined and accounted for.

4.3 Heat transfer to the vessel/pipe contents

In general, the contents of the vessel or pipework will comprise a vapour or gas region above a liquid region. (The case when there are two liquid regions, for example in primary separators, where there is a liquid hydrocarbon region above a water layer, will be ignored here). Heat transfer from the fire through the wall to the liquid causes it to heat up and then boil; heat transfer to the vapour causes it to heat up and increase in pressure. As the wall heats up, its strength decreases, perhaps to the point of failure.

It is in the contents of the vessel or pipework that the most complex heat and mass transfer effects occur. Heat is transferred from the fire through the wall of the vessel or pipe to the liquid or the vapour or both, depending on the nature of the fire. Heat may also be transferred from either or both through the wall to the surroundings or to external cooling. Heat and mass will be transferred between the two regions as they move towards thermodynamic and phase equilibrium. (Mass - and inevitably heat - will also be transferred out of the vessel or line through any vent; see Section 6.4.). The various heat and mass fluxes are illustrated in Figures 4.4 and 4.5.

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4.3.1 Heat transfer from wall to vapour

Heat transfer from the wall to the vapour region is largely by radiation and natural convection, although blowdown may increase gas velocities sufficiently for there to be forced convection. Because there is generally poor heat transfer between the wall and the vapour (though there is good heat transfer from the wall to the liquid, as already noted), the region of the wall exposed to the fire and in contact with the vapour is generally very hot (and much more so than that in contact with the liquid). Hence radiation can often dominate natural convection.

The radiative flux can be modelled using the same theory as is used for transfer to the outside of the vessel. However, the geometry inside the vessel makes this more difficult, as shown in Figure 4.4. Heat is radiated from the inside wall and some of it is absorbed by the vapour; the rest is reflected, or absorbed and then re-emitted by other parts of the wall (some parts of which can be relatively cool if they are not exposed to the fire), and the vapour-liquid interface, and so on. It can generally be assumed that the inside of the wall is a grey body and that most of the radiation incident on the vapour-liquid interface is absorbed by the (relatively cool) liquid. The data needed for the model are:

the wall emissivity and absorptivity; the vapour absorptivity;

and the geometry of the vessel or pipework. The convective flux cannot be determined using an exact theory. Instead, empirical correlations have to be used (see, for example reference [5]), generally leading to a heat transfer coefficient and hence to the heat flux. The correlations always require knowledge of the physical properties of the vapour. Because the pressure in the vessel or pipework can be relatively high, it is important to use appropriate estimates of the physical properties. For the multicomponent fluids on a typical platform these data will vary with time.

Given information on both modes of heat transfer, the temperature of the vapour can be determined. Of course, this temperature is not in fact spatially uniform: it will be higher near parts of the wall engulfed in fire and less near parts of the wall being cooled, for example by a water spray. These processes lead to stratification, and it may not be sufficient to assume that natural convection within the vapour will be sufficient to cause good mixing and spatial isothermality.

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4.3.2 Heat transfer from wall to licuid

As discussed above, if the unwetted wall of the pipe or vessel is exposed to the fire, it will quickly become hot, producing radiative heat transfer to the liquid surface. However, the predominant source of heat to the liquid is through the wetted wall, the mode of heat transfer depending on the boiling regime.

As shown in Figure 4.6, there are two principal types of boiling - nucleate boiling and film boiling (between them lies an area known as transitional boiling). With low incident heat fluxes, conventional nucleate boiling occurs, in which the liquid is largely in contact with the heated surface, and bubbles carry the vapour to the liquid surface.

As the heat flux is increased, the rate of vapour production rises, together with the difference in temperature between the heated surface and the bulk liquid boiling point. The maximum boiling rate occurs at a point known as the critical heat flux, and at higher fluxes the bubbles nucleating at the hot surface coalesce to form a continuous layer of vapour. This is the film boiling regime.

It should be recognised that the curves shown in Figure 4.6 and 4.7 will not be followed in a real fire. If the critical heat flux is exceeded, the film of vapour drastically reduces the heat transfer coefficient between the wall and the liquid, and unless the incident flux is reduced, the wall temperature rises sharply, and may approach the value of the unwetted wall.

In practice this means that partial fire attack below the liquid level can pose as severe a hazard as engulfment of the unwetted wall, if film boiling does indeed occur.

It will generally be the case that the liquid in the vessel or pipework will be sub-cooled in the initial stages of fire attack, and under these conditions the critical heat flux is unlikely to be reached (see [8,9]). Thus, the predominant mode of heat transfer will be natural convection from the wetted wall (or possibly forced convection). Empirical correlations can be used (see, for example reference [5]), generally leading to a heat transfer coefficient and hence to the heat flux.

As the liquid temperature rises, the values of critical heat flux decrease, and film boiling becomes more probable. If film boiling takes place, an alternative correlation for convective transfer must be used, as the wall of the vessel or pipe is no longer wetted, and radiative transfer has to be included as for the vapour region, but with the simplifying assumption that all the radiation is absorbed by the

24


(relatively cool) liquid and hence that none passes through it.

Because of the severe difficulties in examining stable film boiling, there are few experimental data. Indeed, none have been found for materials such as condensate or crude oil. However, the evidence suggests that film boiling is possible in a platform fire. For example, Figure 4.7 gives an experimental boiling curve, produced from the data in references [10] and [11], and showing a critical heat flux of about 265 kW m2 for toluene at a pressure of 3 bara.

There have been many attempts at deriving or producing correlations for the critical heat flux, see for example references [12] to [14]. However, these have only been validated for a limited range of fluids.

The critical heat flux is strongly dependent on pressure, and it has been found [15,16] that the curve depicted in Figure 4.8, which is for butane, is common, with minimum flux at the critical pressure, a somewhat higher value at very low pressures, and a maximum when the pressure is about one third of the critical pressure. Indeed, for a rather restricted range of substances it was found [16]) that the flux-pressure curves could be reduced to a single one by plotting critical heat flux divided by critical pressure against reduced pressure.

Thus, taking Figure 4.8 as an example, film boiling is more likely to occur at low or high pressures, and less likely at intermediate values. This means that for light hydrocarbons, at least, the risk of rupture of the vessel or pipeline may actually increase if the net result of the fire and the blowdown sequence is a reduction in pressure.

No critical heat flux data has been found for heavy hydrocarbons. However, as the molecular weight increases, the critical pressure falls dramatically [17], and hence it is reasonable to suspect that the range of absolute pressures over which film boiling can occur will be greater for heavier materials. There is little information on critical heat flux for mixtures, and none found for a multicomponent system.

Given information on heat transfer through convection and boiling, the temperature of the liquid can be determined. Of course, the liquid temperature is not spatially uniform in practice, as it will be higher near the.vessel wall than in the centre. There is some limited evidence (see reference [18] and also Appendix B, ff Bl4) that natural convection within the liquid is insufficient to cause good mixing of the liquid everywhere, and that thermal stratification occurs.

This arises because boiling liquid rises up along the wall of the vessel or tank and forms a lighter, hotter layer above the

25


bulk liquid: temperature differences of up to around 20 K are possible. While it is possible for such zonal stratification to be modelled, presumably with inter-zone heat transfer by natural convection (though the way in which this would be done is not entirely obvious), it is not clear that such an additional complication is warranted. Thus for most purposes, it is probably sufficient to assume that the liquid is spatially isothermal.

4.3.3 Heat, mass and momentum transfer between licuid and vapour

During blowdown there is in general both evaporation of the relatively lighter liquid components as the pressure drops and condensation of heavier vapour components as the temperature drops. When there is a fire load on a vessel or line, irrespective of whether there is simultaneous blowdown, there is always evaporation of lighter liquid components; there can also be condensation of heavier vapour components on cooler areas of the wall, for example if there is a water spray and only partial engulfment.

When the more volatile components of the liquid evaporate, there is a mass flux from the liquid to the vapour. In addition there is a heat flux, caused by the energy change on evaporation. Similarly, if the less volatile components of the vapour condense, there will be a mass flux and an accompanying heat flux from the vapour to the liquid. There is also a heat flux between the liquid and the vapour induced by the (natural) convection in both. This can be modelled using an empirical correlation to give a heat transfer coefficient (see reference [5J) and hence the heat flux.

Determination of the mass and heat fluxes requires information on the thermodynamic and phase behaviour of the fluid in the vessel or line. If it is a single component fluid, then matters are straightforward and relatively simple methods can be used to determine the boiling point and vapour pressure as a function of temperature.

However, the fluids are generally multicomponent, which makes the modelling much more complex. The phase behaviour of a multicomponent fluid is described by a surface in the three dimensional pressure-temperature-composition phase space. For a fluid of a particular overall composition, the relative amounts of liquid and vapour, and their compositions are dictated by the temperature and pressure of the system, A section through such a surface is shown on Figure 4.9

The combined effects of the incoming heat flux and the blowdown change the pressure and temperature simultaneously, and the problem is in calculating how the phase behaviour will be affected. If the conditions are far from critical and do not involve trajectories in phase space which traverse the

26


liquid-vapour boundary obliquely (see Figure 1), then straightforward methods, based for example on cubic equations of state (such as Soave-Redlich'-Kwong), can be used - see references [19] and [20].

In practice, however, trajectories are sometimes rather near critical (they are more often near critical when the vessel is being blown down in the absence of a fire loading) and often traverse the liquid-vapour boundary obliquely, the accuracy of such methods is often unacceptable. In that case, more complex - and computationally more expensive and time consuming - methods must be used, for example those based on the extended method of corresponding states (the extension being used to model non-spherical molecules, which is particularly important for the higher hydrocarbons) - see for example references [21] to [23].

It should be noted that physical property data (such as density, thermal conductivity and viscosity) are required to model heat and mass fluxes, both inside the vessel or line and outside it. It is generally important that such property data are consistent with the thermodynamic data. This can, of course, often - but by no means always - be guaranteed by using the same computer program for both thermodynamic and physical property data. Examples of such programs include PPDS (Institute of Chemical Engineers), PROCESS (SimSci: see Appendix B, ff B27) and PREPROP (Imperial College: see Appendix B, ff B6)

It should also be noted that there can be problems with multicomponent systems when using empirical correlations, which have usually been developed for single component systems. A particular example of this is in heat transfer during boiling, which generally involves the latent heat of the fluid. For multicomponent systems, the concept of latent heat is undefined, so it is not immediately obvious what to do. This problem may, however, be more apparent than real. Provided the energy change on evaporation/condensation does not vary too much with changes in composition then the latent heat can be identified with the energy change. When this is not the case, the penalty incurred by incorrect specification of a latent heat sometimes manifests itself only in a small error in temperature difference.

In addition to the thermodynamic and phase information, mass and energy balances are required on the liquid and vapour to complete the model. These are of the form:

mass balance on liquid: rate of change of mass of liquid I

= Inet rate of gain of mass by evaporation and condensation - Jrate of loss of mass through vent

27


mass balance on vapour: rate of change of mass of vapour I

net rate of gain of mass by evaporation and condensationl - irate of loss of mass through vent

energy balance on liquid: rate of change of energy of liquid

= boiling flux from walli + convection flux from wall + net rate of energy gain by evaporation and condensation - rate of loss of energy through vent

energy balance on vapour: rate of change of energy of vapour

= radiation flux from wall + convection flux from wall + net rate of energy gain evaporation and condensation I - rate of loss of energy through vent - rate of loss of energy by expansion

Furthermore, momentum balances on the liquid and vapour are required when venting lines. An important distinction between a line and a vessel is that there is a significant pressure drop (between the closed end and the vent) in the former but not in the latter. To determine the pressure drop, a relatively simple two-phase flow method can be used (based on the analyses of Friedl, Premoli et al and Lockhart & Martinelli; see, for example reference [13].

4.4 Venting

4.4.1 Venting through the blowdown system and a PSV

It is assumed in this study that the blowdown system has been activated by the fire. However, as heat is supplied to a vessel or line from a fire, the temperature of the contents rises and, because the volume of the vessel or line cannot increase significantly, there may still be a net increase in pressure. Venting through a PSV will occur when the pressure in the vessel or line exceeds the lift pressure of the valve. Sufficient venting may occur that the net pressure falls below the seat pressure of the valve (the lift pressure being perhaps 5% greater and the seat pressure perhaps 5% less than the set pressure of the valve) and the valve closes. The pressure will then increase again, because heat is still being

28


transferred to the contents of the vessel or line, and valve cycling may occur. However, seat wear and spring weakening often occur, and operation of the valve commonly deteriorates.

Clearly, if there is sufficient venting that the strength of the wall (which decreases with increasing temperature) is never exceeded, then failure of the vessel or line will not occur. However, it may be impossible to vent through a sufficiently large flow area to prevent failure in anything other than a very small fire. Despite this, the operation of the vent is important because it can have a significant effect on the time to failure.

To model venting, the following data are needed:

the characteristics of the blowdown choke; the characteristics of the PSV, in particular the

variation of the flow area with pressure, which includes specification of the lift and seat pressures (note that these may be modified, probably unpredictably, if there is fire damage to the valve);

the nature of the flow path downstream of the PSV and choke, where the effects of a significant back pressure would be important;

the nature of the flow through the valve and blowdown choke.

The last of these, the nature of the flow, is by no means trivial. If venting is only of vapour (not containing suspended droplets) or only of liquid (which does not flash to form vapour as the pressure drops through the valve) then matters are relatively simple.

This is true whether or not there is choking in the case of vapour venting, though the common assumption of perfect gas behaviour is generally inconsistent with the treatment of the thermodynamics and phase equilibria in the vessel or line and can give order of magnitude errors in predictions of flow rate.

Otherwise, however, matters are not simple and there is still neither a full theory nor a full set of empirical relations to cover all circumstances which can - and do - arise in practice, namely:

venting of vapour containing suspended liquid droplets; venting of liquid containing vapour bubbles; venting of liquid which flashes as it is vented.

The first of these can be treated crudely as a modification to the pure vapour case if the volume fraction of droplets is small, as is likely in practice. The second of these is likely only in a vessel or line in which the volume fraction of liquid is so large that de-entrainment of vapour bubbles

29


from the boiling liquid is impossible. It can be treated - albeit crudely - by modification of standard two-phase flow correlations. The third of these, though perhaps unlikely unless venting is from the bottom of the vessel or line, is perhaps the least predictable and merits special discussion.

4.4.2 Venting of a flashing liquid

The mechanism of flow of a flashing liquid through an orifice or short nozzle seems relatively well established; what is less certain is the rate of flow. The compressed liquid upstream of the orifice expands adiabatically and essentially reversibly until it reaches saturation. Flashing into the equilibrium two-phase state does not, however, take place at this point, since nucleation to form bubbles of vapour is a relatively slow operation. The liquid continues, therefore, to expand as before, passing into the metastable liquid region. This metastability can be maintained only for a short period of time before bubble growth has progressed sufficiently for significant amounts of vapour to form and the fluid reverts to the equilibrium state.

The time taken for an element of a fluid stream to pass through an orifice is comparable with the time taken for nucleation. Thus there is the possibility of the liquid passing completely through the orifice before any vapour is formed. However, the experimental evidence points to an even stronger conclusion, that no vapour is formed as long as the stream of liquid is converging. Vapour formation always takes place close to the point of minimum flow area of the stream of liquid and the flow rate through the orifice adjusts itself so that this is achieved.

The limited lifetime of a liquid in its metastable liquid state produces a situation very similar to the choking of a gas as it passes through an orifice or nozzle. The limiting factor in this latter case is the inability of the gas to exceed the local speed of sound as it passes through the nozzle. Thus, for small differences between the upstream and downstream pressures, the liquid will pass the narrowest part of the orifice without reaching the limit of metastability.

At some larger pressure difference this will not be true and the attainment of the limit of metastability will take place actually at this narrowest part. Further increases in pressure difference, such as by decreasing the downstream pressure, will not increase the flow through the nozzle any further, nor will it change the pressure actually in the throat of the nozzle. There will be free flashing expansion of liquid from the nozzle at its exit pressure to the downstream pressure. This behaviour is, therefore, exactly analogous to choked flow of a gas through a nozzle, but it is

30


important to remember that, in the case of a flashing liquid, consideration of the speed of sound is irrelevant.

A quantitative theory for the amount of metastability that a compressed liquid can undergo before phase separation necessarily takes place has been developed (see reference [24]) and applied to the release of high temperature water through nozzles and pipes. However, although the driving force for nucleation can be expressed unambiguously in terms of measurable physical properties of the fluid in question, this is not true of the extent to which nucleation must take place before phase separation takes over.

This means that this part of the theory can only be implemented by making a comparison of predictions with actual experiments. Nevertheless, the final theory is remarkably successful at representing the discharge of water through nozzles.

For more information on venting of a flashing liquid see references [24] to [30].

4.5 Failure mechanisms

The discussions of the previous sections are devoted to the determination of the temperature distribution over the surface of the pressurised equipment as a function of time. It might also lead to a knowledge of the temperature distribution through the wall of the vessel or pipe, but this is usually of secondary importance.

The pressurised equipment will fail, ie rupture, when it is subjected to a stress in excess of the strength of the material from which it is fabricated (for a thin-walled vessel or pipeline under normal conditions, this is usually taken as the ultimate tensile strength). This simple statement is, however, difficult to put into practice with a vessel subject to a non-uniform temperature distribution, because this creates thermal stresses, in addition to the stresses caused by the internal pressure, and also results in material of variable strength over the surface of the vessel.

As a first approximation, one might expect the vessel to fail at the point at which the total superposed stress - pressure and thermal - exceeds the material strength, but in practice the plastic deformation and hence stress relaxation which will have occurred before failure is reached will make this calculation difficult.

The effect of plastic deformation will be particularly noticeable if the vessel or pipe has locally high stresses, or equivalently, local regions of low strength. Thus, the effect of a fully engulfing pool fire will be very different from

31


that of localised fire engulfment. Little quantitative information is available as to the effect of this, and as a consequence, stress is usually calculated on the basis of elastic behaviour. Further work is needed to identify the true failure mechanism.

4.5.1 Superposed pressure and thermal stress

The principle of superposition in stress analysis maintains that in a vessel which is subject to two, or more, sources of elastic stress, the total stress is simply the sum of them. Thus we can consider pressure and thermal stress separately.

Provided the stresses remain elastic, the determination of pressure stress is a well established procedure and is the basis of the pressure vessel design codes, such as BS 5500. Some vessel designs may be too complex for these codes, in which case finite element techniques will be required, but this is an increase in computational complexity rather than a change in procedure.

The determination of thermal stresses for a vessel which remains elastic in situations in which there is a substantial degree of symmetry, e.g. a pressure vessel in which the inside temperature differs from the outside, but with no temperature variation over the surface, is also well established and analytic solutions are often possible.

Such solutions, however, are rarely available when there is significant temperature variation across the surface. This latter case is expected to be the norm where the temperature gradient is caused by flame impingement or by a pool fire under a large vessel. Solution of these cases is possible by finite element techniques, often using the same computer program as is used for the pressure stress calculations.

The relative magnitude of thermal stresses and pressure stresses varies enormously. For thin-wall vessels, the thermal stress caused by a temperature difference through the wall is relatively small, but temperature differences over the surface of the wall can lead to significant stresses.

As an example, a simple finite element model of a horizontal tank containing a volatile liquid subjected to a pooi fire and held at constant pressure through the action of a PSV, gave a maximum thermal stress (which was in the axial direction - see Figure 4.10) some 3 to 4 times the hoop stress due to pressure [31]

The large thermal stresses arose because the lower part of the vessel was held at essentially constant temperature (through a high heat transfer coefficient to the boiling liquid), while the temperature of the upper part of the vessel increased

32


rapidly. This gave rise to a large difference in thermal expansion between the lower and upper zones, and hence the high thermal stress. If, on the other hand, the vessel had contained only gas, the axial thermal stress would have been quite small compared with the hoop stress arising from the internal vessel pressure.

The wide variation in the relative magnitude of thermal and pressure induced stresses makes it difficult to make generalisations because if the direction of maximum stress changes, so also would one expect the direction of initial fracture to change, hence leading to a different mode of failure. The examples given above are the two extremes; there will be many intermediate cases which will be very difficult to quantify without a full finite element analysis.

4.5.2 Stress at failure

Pressurised equipment exposed to fire will experience a steady increase in temperature. This may result in a time-varying stress (see Section 4.5.1), and we expect that once this time-dependent maximum stress equals the material strength, the equipment fails. However, the material strength is itself decreasing in time as the material weakens with increasing temperature.

For carbon steel vessels or pipes built according to the usual codes, with a burst pressure at normal temperatures some 2.5 to 4.0 times the maximum working pressure, and operating at the maximum working pressure, the reduction in strength is such that failure would be expected at around 500 to 550 C if thermal stresses are negligible. If the vessel is operating below its maximum working pressure, the failure temperature is correspondingly higher, for example, if the pressure is 50% of the maximum working pressure, failure might be expected at 550 to 600 C.

The time for failure to occur depends on the severity of the fire, the extent and type of fire protection, and the pressure response of the vessel or pipework (including the blowdown system). This can vary between a few minutes and a few hours, and consequently the relevant strength criterion is more appropriately taken as the creep rupture strength, rather than the short term tensile strength (as the time to rupture goes to zero the creep rupture strength becomes equal to the short term tensile strength; the latter is just a particular case of the former).

It is important to realise that the use of creep rupture stress does not imply the presence of significant creep strain at failure. For the time periods of interest, the creep strain is likely to be small, but the creep rupture stress may well be significantly lower than the short term tensile

33


stress. Ignoring this may therefore result in an overestimate of the time to failure.

For a given vessel or pipe made of a given material at a fixed internal pressure (or, more accurately, stress) and temperature, the time to failure due to creep rupture can be determined by the Larson-Miller method (see, for example reference [32] and [33]).

This method is based on the assumption that creep is a rate process governed by an Arrhenius-type equation and the experimental observation that time to rupture is inversely proportional to creep strain rate. The method is generally expressed in the form of nomograms.

For a vessel or pipe exposed to fire, the temperature will not, however, be constant in time. While there is no properly validated way to account for this, a common and acceptably accurate way [33] is to use the Robinson Method (34] (or Life Fraction Rule). This is based on the assumption that the time to failure resulting from the overall pressure-temperature history of the vessel can be related to experimental failure times under particular values of pressure and temperature for a vessel of similar construction.

In view of the complexity of creep rupture calculations, the short term tensile failure criterion is often used, in which failure is presumed once the stress level reaches the short term ultimate tensile strength.

The overestimate of time to failure is probably not too important if the rate of temperature rise of the vessel or pipe is high. In this case, the final stages of the failure process are likely to be extremely fast, since strength decreases very rapidly with temperature when the material is hot. However, for slower temperature rises, for example where failure takes an hour or more, the creep rupture stress criterion should be used unless one is sure that this is not necessary.

4.6 Summary

In the scenario chosen for this study, fire attack occurs directly or indirectly on a vessel or pipework which may or may not be undergoing blowdown. The physical processes which take place are heat transfer to the exterior of the vessel, through any insulation to the interior wall and from there to the vessel contents. The temperature of any unwetted wall rises very rapidly, the wetted wall less so, the fluid temperatures increase, and fluid boiling occurs, resulting in an increase in the internal vessel pressure.

34


The individual steps in this overall process are relatively well understood, and for a single phase one component fluid it is not too difficult to derive expressions relating vapour pressure to the incoming heat flux. Not all of these are deterministic. However, there appear to be well established empirical relationships which describe the behaviour adequately.

If the fluid in the vessel is two-phase, or it develops two-phase behaviour in its temperature cycle, then it becomes more difficult to predict heat transfer. In particular, interphase heat and mass transfer is complex, the boiling regime is different, and vessel wall temperatures will not be the same.

Virtually all fluids to be found on an offshore platform will be in this latter category, and in addition will be multicomponent, rather than simple materials. This causes more serious problems, not least ofwhich is the fact that the empirical relationships such as heat transfer through convection are derived for single components, and their applicability for this case is uncertain.

In addition, if removal of vapour through blowdown is occurring, it is likely that fractional distillation of the liquid will occur, removing the lighter components and concentrating the heavier ones, and therefore reducing the rate of pressure rise. The lack of temperature-dependent physical and thermodynamic properties for these materials, which causes modelling problems, also makes a rigorous theoretical analysis significantly more difficult.

Nevertheless, if the required properties were determined, the thermodynamic equations of state of the fluid could be calculated, and with sufficient computing power it should be possible to carry out the task with some precision.

Thus, it appears that the physical processes involved in the fire attack and response of vessels and lines are relatively well understood, and that satisfactory relationships between attack and response can be derived. Models to describe vessel behaviour are, at least, feasible.

4.7 Sources of equations

There are four main references in which most of the equations needed for modelling the physical processes described here can be found. The most complete is reference [35]; two others which are less complete are reference [l8J and [36]. The fourth, which applies to blowdown as opposed to fire loading, is reference [37].

35


Reference [18] is good for:

mass and energy balances in the liquid and vapour contents of the vessel or line;

transient heat conduction through the wall (though horizontal variations are neglected).

References [35] and [36] are useful for:

radiation heat fluxes; natural convection heat transfer coefficients.

Reference [37 is useful for:

the mass flow rate for venting of a vapour with or without small quantities of suspended liquid drops.

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CURRENT MODELLING CAPABILITIES SECTION 5

5. CURRENT MODELLING CAPABILITIES

Because of the complexity of the physical processes (see Section4), all of which are coupled, most predictive models of the thermal response to Lire loadings are computer-based. The main source of information on current capabilities has been obtained by direct contact with some organisations (see Appendix B), and by conducting a computer-based literature survey.

Itis important to recognise that this list of models is not all-inclusive. For example, there are some models which touch on the area of interest in a peripheral way, and have been ignored for that reason. There may well be other, more important models, which have escaped our search.

In this context it is noteworthy that the fire response models identified are all based on work for the onshore LPG industry. Thus, the application is primarily for propane or butane at ambient temperature and at its saturated vapour pressure - the vessels for such storage generally have rather lower pressure ratings than those found offshore.

The LPG models were never intended for use in predicting the response of a vessel on an offshore platform, and as might be expected, their predictive ability falls short of the requirements for that application. However, the models are a good starting point, and we first need to assess how well they perform in their intended role.

5.1 The LPG fire response models

Six computer programs for simulating the thermal response of LPG vessels to fire loading have been identified:

ENGULF II from AEA Technology Safety & Reliability Directorate;

HEATUP from Shell Research Ltd; TCTCM from Queen's University Ontario; PIA from SINTEF Applied Thermodynamics Division; PLGS-I from the University of New Brunswick; VT*VESSEL from VERITEC.

5.1.1 Features of the LPC response models

The principal features of the computer programs are summarised in Table 5.1, with a key given in Tabie 5.3 (see also Table C2 of Appendix C). The table reveals the following:

There is an almost universal ability to simulate the response of vessels to a fire of reasonably uniform flux not exceeding 100 kW m2 (i.e. a totally engulfing pool fire), but

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no proven ability to model non uniform fluxes or fires of greater heat flux density;

There is a widespread ability to simulate radiative and convective heat transfer from the fire to the wall of the vessel;

There is some ability to simulate radiative and convective heat transfer to the atmosphere from the vessel;

There is a widespread ability to handle all orientations of vessels;

There is a widespread ability to simulate two-dimensional, but not three-dimensional heat conduction in the wall of the vessel, including insulating layers;

There is a universal ability to simulate two-phase single component fluids in the vessel;

There is a widespread ability to simulate convective heat transfer to the vapour in the vessel and convective and boiling heat transfer to the liquid in it;

There is a common tendency to use semi-rigorous thermodynamics and phase equilibria for the vessel contents with perfect gas behaviour for venting vapour, which is thermodynamically inconsistent (and can lead to large errors);

There is a very patchy ability to simulate all possible modes of pressure relief valve operation and venting from the vessel;

There is very little attempt to predict stresses within the wall of the vessel, or to predict vessel failure times or modes. There is also no agreement on the failure criterion to be adopted;

No models for the fire response of pipes have been identified.

5.1.2 Validation of the LPG response models

There has been considerable experimental validation of the programs, largely with vessels containing propane. The most complete tests have been carried out by:

Federal Institute for Materials Research & Testing (BAN); UK Health & Safety Executive, in conjunction with Shell

Research Ltd; Queen's University Ontario; University of New Brunswick.

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Detailed information on these research programmes is given in Appendix B. Their principal features are now discussed in turn.

BAM have conducted four series of tests on 4.85 cubic metre horizontal cylinders containing liquid propane, generally totally engulfed in pool fires. One series was for an unprotected vessel, one for an insulated vessel, one for a vessel protected by a conventional water spray and one for a vessel protected by an upgraded water spray. No complete program has yet been compared with the experimental data, however.

The UK Health & Safety Executive and Shell Research Ltd have used the HSE facilities at Buxton to validate both ENGULF II (AEA Safety & Reliability Directorate) and HEATUP (Shell Research Ltd). Experiments have been conducted on 0.25, 1.0 and 5.0 tonne tanks containing propane in totally engulfing pooi fires.

Queen's University Ontario have validated their program TCTCM using experimental data from full scale and one fifth scale tests on tanks apparently containing propane. The tests were conducted by the Transportation Development Centre of Transport Canada, for whom TCTCM was developed initially.

The University of New Brunswick have conducted a series of experimental tests on a 37.4 litre horizontal cylindrical vessel partially filled with refrigerant Rll to validate their program PLGS-I. Multiple heater elements were used to simulate the effect of an external heat load and it is claimed that totally and partially engulfing pool fires and jet fires can be simulated by appropriate use of the heater elements. It is not clear, however, whether the spatially non-uniform heat fluxes occurring in partially engulfing pool fires and jet fires can indeed be simulated in this way, either in terms of heat flux density or the flux distribution.

It emerges from these sets of experimental tests that:

There are extensive validatory data for cylindrical vessels in totally engulfing pool fires, but none for vessels in partially engulfing pool fires or in jet fires; With a totally engulfing pool fire loading, the computer codes are capable of good predictions for the pressure and temperature within the vessel, and the vessel wall temperature, until such time as a relief valve opens, venting the hydrocarbon inventory.

The flow through the valve and hence the response of the vessel and its contents after the valve has lifted cannot always be predicted accurately, and there is often no attempt to predict the time to failure.

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There is a limited amount of validatory data on vessels with wall insulation and water spray protection;

As expected, there have been no validatory tests on vessels containing multicomponent fluids;

There are no validatory data for pipework exposed to fire loading.

5.2 Blowdown models

In addition to the LPC fire response models, there are several codes which simulate the blowdown of a vessel. These share many features with the programs for simulating thermal response to fire loading. (N.B. It is claimed that SAFIRE can simulate thermal response to fire loading, but it does so in such a crude and unvalidated manner that it was not appropriate to include it in Section 5.1.)

Four blowdown models have been identified

VENTFLO from British Gas London Research Station; SAFIRE from Fauske & Associates; BLOWDOWN from Imperial College; PROCESS from SimSci.

5.2.1 Features of the blowdown models

The principal features of the four blowdown programs are shown in Table 5.2 (key given in Table 5.3), and can be summarised as follows

There is a widespread ability to simulate convective heat transfer to the atmosphere from the vessel. However, the less general case of heat transfer to the sea is not covered;

There is a universal ability to handle both horizontal and vertical vessels;

There is a universal ability to simulate one-dimensional, but not two-dimensional or three-dimensional heat conduction through the wall of the vessel;

There is a universal ability to simulate blowdown of two-phase multicomponent fluids but very little ability to simulate heat transfer to the contents properly or to handle multicomponent vapour-liquid equilibria in a rigorous manner;

There is a widespread ability to simulate venting of vapour but very little ability to simulate multi-phase venting;

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5.2.2 Validation of the blowdown models

The blowdown models have received far less validation than those for fire response. Indeed, only one has been tested in this way.

Experimental tests and validation of the BLOWDOWN model have been carried out by Imperial College and Shell Research Ltd.

Imperial College have conducted a series of blowdown tests using a 36.4 litre horizontal or vertical cylinder containing nitrogen or a mixture of nitrogen and carbon dioxide. An extensive additional series of tests has been conducted by Shell Research Ltd using a 2.85 cubic metre vertical cylinder containing methane, nitrogen, and mixtures of methane/propane and methane/propane/carbon dioxide.

The results of these tests showed that a rigorous thermodynamic treatment can give good agreement between prediction and experiment for the wall temperature, and the composition, pressure and temperature of a multicomponent fluid.

There have also been experimental tests conducted on the blowdown of pipelines but, with the exception of one brief paper (reference [38]), these are as yet unpublished.

5.3 Summary of current capabilities

The status of current predictive modelling capabilities in this area can be summarised as follows:

We have not been able to identify a validated model which can be applied to the fire response of a vessel or pipework under blowdown conditions. However, models do exist for the separate processes of fire attack on a vessel, and for the fluid response to blowdown.

The validated models for fire response which have been identified are all based on fire attack of a vessel containing LPC (more strictly, propane or butane) with a simple venting system. There has been extensive validation from large scale tests in fully engulfing pool fires.

The fire response models can be used with confidence to predict the pressure and temperature in and the wall temperature of a vessel containing a one-component but possibly two-phase hydrocarbon in a fire of reasonably uniform flux not exceeding 100 kW m2, until such time as a relief valve opens, venting the hydrocarbon inventory. The flow through the valve and hence the response of the vessel and its contents after the valve has lifted cannot always be predicted

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accurately, however, and the modelling of time

thermal response of vessels and pipework exposed to fire

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