Decks

Decks normally comprise a series of girders and a stiffened deck plate. Usually the PFP will be applied to limit the temperature rise of main girders and those secondary beams that support the higher categories of Safety Critical Equipment. Coat-back requirements are often relaxed to 50 mm or so hence the overall percentage of cover to deck steel work will be relatively small.

Of course it is not normal to coat the top surface of decks. In pool fires decks would not be expected to be heated if the fire is from above but this may not be the case with jet fire. The top flanges of girders can be weakened in such circumstances, particularly as the insulation to the girder below the deck plate will inhibit heat loss and allow higher top flange temperatures to occur.

These are potential considerations.

forces imposed during gross deformation of the secondary members they connect without fracture (as in earthquake-resistant design).

Potential deficiencies in fire resistance of decks which support SCE’s or which act as fire-boundaries can sometimes be overcome by the addition of deck to deck hangers. Where the critical fire is below the deck these hangers would not be weakened as they would be located in a different fire area. This is a solution that can be applied for retrofit situations.

It should be understood that there may be a problem of running pool fires on decks due to distortion of plates arising from the fire. All analyses are based on a circular or bunded fire and do not take into account potential running. The HSE are currently planning a test programme to investigate this effect.

4.9 Feedback from explosion testing at Spadeadam

Large scale fire testing has been carried out at the Spadeadam test establishment. In the context of testing of PFP, Spadeadam has developed a large scale explosion test of PFP material which subjects PFP covered samples to an explosion loading (including drag forces in the case of pipes or beams) and following the explosion test, the samples are subjected to a standard jet fire test.

There is an added level of acceptance for Duty Holders that the combined explosion/fire testing can be witnessed by a 3^rd Party such as the Verification Body, who have certified the results of the tests for a number of PFP manufacturers.

5. Derivation of fire loadings and heat transfer

5.1 Introduction

In the following sections generic fire types are identified for the types of fire that might occur on or near an offshore installation. The fire types are considered further in terms of their characteristic flame and how their behaviour might be affected by confinement and/or deluge. The parameters used to define the fire and the hazards presented by the fire in terms of thermal and smoke loading are defined.

Based on large scale experimental work (including unpublished studies by Advantica to which access has been granted for this guidance) and on the predictions of validated models developed and used by Shell and Advantica, typical fire loading data are summarised for the fire types.

Typical values can be used to assess the hazard to personnel and the likely effect on fire impacted obstacles using a simple calculation method.

Also considered are the effects of deluge on fire behaviour, the potential heat loads from fires and the effect on the temperature rise of an engulfed object, plus the manner in which PFP may limit the rate of temperature rise of an engulfed object and how blow-down may reduce the heat load and hence the likelihood of failure. Using these typical fire loadings and calculations of heat transfer to objects, the steps to prepare an initial scoping or indicative QRA of the fire hazards are also outlined.

5.2 Fire characteristics and combustion effects

5.2.1 General

In Section 3.2.35, potential fires on offshore installations were categorised into six fire types. In this section, each of these fire types is described in detail in terms of the likely nature of the flame and the thermal loading it may present to the surroundings. Where appropriate, the effect of active water deluge on the fire is discussed as is the effect of confinement.

5.2.2 Gas jet fire

5.2.2.1 Nature of the gas jet fire

An ignited pressurised release of a gaseous material (most typically natural gas) will give rise to a jet fire. A jet fire is a turbulent diffusion flame produced by the combustion of a continuous release of fuel. Except in the case of extreme confinement which might give rise to extinguishment, the combustion rate will be directly related to the mass release rate of the fuel.

In the offshore context, the high pressures mean that the flow of an accidental release into the atmosphere will be choked having a velocity on release equal to the local speed of sound in the fluid. Following an expansion region downstream of the exit the flame itself commences in a region of sub-sonic velocities as a blue relatively non-luminous flame. Further air entrainment and expansion of the jet then occurs producing the main body of the jet fire as turbulent and yellow. In the absence of impact onto an object, these fires are characteristically long and thin and highly directional. The high velocities within the released gas mean that they are relatively unaffected by the prevailing wind conditions except towards the tail of the fire. The fire size is predominantly

less luminous than jet fires involving higher hydrocarbons. CO concentrations in the region of 5 to 7 % v/v have been measured within a jet fire itself but this is expected to drop to less than 0.1 % v/v by the end of the flame.

A factor which is often overlooked is the noise produced by sonic gaseous releases. This is usually high pitched and so loud that it may prevent effective radio communication between personnel. As a result emergency actions could be hampered.

A further point to note is that some combinations of leak size and pressure will not give rise to an inherently stable flame. For hole sizes under 30 mm diameter, there is a lower bound pressure which high pressure releases must exceed to produce stable flames. Simplistically, this extends approximately linearly from 2 barg at 30 mm diameter to about 75 barg at 8 mm diameter. In practice this means that most small leaks (see Section 5.4.1) will be inherently unstable and will not support a flame without some form of flame stabilisation, such as the presence of another fire in the vicinity to provide a permanent pilot or stabilisation as a result of impact onto an object such as pipework, vessels or the surrounding structure. This implies that unstable flames may self-extinguish revert to leaks thus contributing to potentially explosive environments. This aspect of flame behaviour should be considered in determining major hazard scenarios and their escalation paths, however, in the highly congested environment offshore, impact within a short distance is very likely, and small leaks will most likely stabilise on the nearest point of impact.

Apart from providing flame stabilisation, impact onto an obstacle may also significantly modify the shape of a jet fire. Objects which are smaller than the flame half-width at the point of impact are unlikely to modify the shape or length of the flame significantly. However, impact onto a large vessel may significantly shorten the jet fire, and impact onto a wall or roof could transform the jet into a radial wall jet where the location and direction of the fire is determined by the surface onto which it impacts.

As noted above, the combustion process within a natural gas jet fire is relatively efficient and produces little soot (carbon). Consequently these flames are not as luminous as higher hydrocarbon flames. Radiation emissions from natural gas flames arise mostly from water vapour and carbon dioxide, except for very large releases where soot production starts to enhance the process. The long thin shape may also result in a flame path which is not optically thick. The net result is that the radiative heat transfer to the surroundings is lower than for higher hydrocarbon flames and this is reflected in the fraction of heat radiated, F, for such fires (see Section 5.3.1 “Fire loading to the surroundings”). Similarly, the radiative heat transfer to objects engulfed by the flame is generally lower than for higher hydrocarbon flames, but the high velocities within gas jet fires can result in high convective heat transfer to objects. Clearly the total heat flux which is imparted to an engulfed object will vary over the surface of the object. In addition, the relative proportions of convective and radiative heat flux will vary over the surface, with the highest convective component likely to be experienced close to the point of impact of a flame where the highest velocities occur, whereas the highest radiative heat load will be experienced where the more radiative part of the flame (usually nearer the end of the flame) is viewed by the object. As the more radiative part of the flame is closer to the tail, this can result in the highest overall heat fluxes being experienced on the rear surface of an engulfed object which may seem counter-intuitive. Neglecting such spatial variations, broadly speaking, for a given location of an object within a flame (as a proportion of flame length), the convective component is more or less constant with increasing size of release, whereas the radiative component increases with release rate as the flame becomes optically thick and more smoky. Hence the relative proportion of convective to radiative flux varies with fire size (see Section 5.4.2).

5.2.2.2 Effect of deluge on gas jet fires

The activation of general area deluge can adversely affect the stability of high pressure gas jet fires, particularly if the fire is not impacting onto an obstacle. However, in most practical cases, this undesirable effect is very unlikely to occur due to impact onto obstacles providing adequate flame stabilisation. Indeed, deluge has little effect on the size, shape and thermal characteristics of a high pressure gas jet fire. Therefore, the heat loading to engulfed obstacles is not diminished. The same

is true for dedicated vessel deluge systems; the water being unable to form a film over the vessel in the presence of the high velocity jet, and so dry patches form where the temperature rise is undiminished by the action of deluge.

There is some evidence that the deluge increases combustion efficiency resulting in lower CO and increased CO₂ levels within the flame.

The major benefit of area deluge with jet fires arises from the suppression of incident thermal radiation to the surroundings, which protect adjacent plant and in particular, aid escape by personnel. For a medium velocity type (e.g. MV57) nozzle operating at 12 l min^-1 m^-2, incident radiation levels can be reduced by about 20 % for a single row of nozzles, 30-40 % for 2 rows and 40-60 % for more than 2 rows (general area deluge). Increased deluge rates can further reduce incident radiation levels: 60-70 % at 18 l min^-1 m^-2; 80-90 % at 24 l min^-1 m^-2 for general area deluge. Nozzles producing smaller droplet sizes can have an enhanced mitigation effect, but there is an increased risk that the droplets will be blown away by the wind.

5.2.2.3 Effect of confinement on gas jet fires

The behaviour of a jet fire within a confined or partially confined area will depend upon the degree of confinement and the direction of the jet relative to the ventilation opening. If ventilation is plentiful or the jet is directed through a vent then there may be little difference in jet fire characteristics compared to an unconfined fire. However, if the release rate of gas is large relative to the size of the confinement or the ventilation openings are small then the fire may not be able to entrain enough air for complete combustion inside the compartment. This is likely to result in increased levels of incomplete combustion products such as CO, increased levels of smoke (soot) and increased flame temperatures, particularly in regions close to the ceiling of a compartment where hot combustion products may be trapped and recirculate. This leads to increased heat flux to objects and surfaces compared to an unconfined fire.

The location where combustion occurs and the hottest parts of the flame may also shift due to the confinement. In tests involving horizontal jet fires in a compartment incorporating a single wall vent, where the jet was directed away from the vent, increased temperatures were seen at the interface between the smoke layer leaving the compartment and the air layer entering the compartment, most particularly in the area furthest from the vent.

Unlike unconfined fires, the behaviour of under-ventilated confined fires changes with time as the air initially available within the compartment is consumed, and this may lead to ‘external flaming’

after a period of time when the body of flame moves through the vent in order to find the oxygen required for combustion. CO levels of up to 5 % v/v at the vent may occur but after the onset of external combustion the CO levels drop to typically less than 0.5 % v/v by the end of the flame.

Soot production is related to the equivalence ratio and hence the degree of ventilation and may range from about 0.1 g m^-3 at = 1.3 to up to 2.5 g m^-3 at = 2.

Certain ventilation patterns could lead to flame instability and extinguishment. The worst case condition is likely to occur if the jet fire is slightly under-ventilated as this leads to high heat release rates and enhanced soot production.

5.2.2.4 Confinement and deluge of gas jet fires

Deluge of a confined jet fire may lead to flame extinguishment and hence a serious explosion hazard from the continuing release. The likelihood of flame extinguishment is significantly

5.2.3 Two-phase jet fire

5.2.3.1 Nature of the two-phase jet fire

An ignited release of a pressurised liquid/gas mixture (such as ‘live crude’ or gas dissolved in a liquid) will give rise to a two-phase jet fire. The gas stream atomises the liquid into droplets which are then evaporated by radiation from the flame. However, a pressurised release of a liquid can also give rise to a jet fire in which two-phase behaviour is observed if the liquid is able to vaporise quickly. This is most likely to occur when a liquid is released from containment at a temperature above its boiling point at ambient conditions whereupon flash evaporation occurs, (for example propane, butane).

Pressurised releases of non-volatile liquids (for example, kerosene, diesel, or stabilised crude) are unlikely to be able to sustain a two-phase jet fire, unless permanently piloted by an adjacent fire;

even so, some liquid drop-out is likely and hence the formation of a pool. At high pressures, a spray of liquid droplets may be formed which can drift in ambient winds and become dispersed over a wide area.

As for the gas jet fire described above, the two-phase jet fire is a turbulent diffusion flame produced by the continuous combustion of a fuel at a rate directly related to the mass release rate, producing a fire which is long and thin (although generally wider than a gas only jet fire) and highly directional. The exception is when liquid drop-out occurs, leading to a potentially increasing accumulation of fuel as a pool. Two-phase jet fires (particularly those generated by flashing liquid releases) are significantly less noisy than gas jet fires. As for gas jet fires, impact onto an obstacle larger than the flame half- width at the point of impact may shorten or modify the shape of a fire significantly.

The liquid content results in relatively higher release rates for a given aperture and pressure compared to gaseous releases and, when the release is two-phase (such as may arise from a relatively long pipe connected to a liquid storage vessel), estimating the release rate is difficult. The output from Codes used to characterise 2-phase releases should be viewed with caution as these types of mixed fluid flows are very difficult to model accurately. The generally lower exit velocities from flashing liquid releases lead to shorter flame lift-offs and proportionately shorter and more buoyant flames overall. These lower velocities also make the fires more wind affected whilst the higher hydrocarbon content of these fuels increases the flame luminosity. However, two-phase releases involving gas dissolved in, or mixed with, a liquid can result in a jet fire which combines the worst aspects of both the gas jet fire and the flashing liquid jet fire, that is, high velocities and high flame luminosity.

The higher hydrocarbon content also results in more soot being formed than in a natural gas jet fire, although there is no available experimental data quantifying the difference. Measurements in the smoke downstream of a ‘live crude’ jet fire determined an optical obscuration factor of typically 10 % over a 200 mm path length. This corresponds to a visibility distance of about 5 m.

The soot produced then contributes significantly to the radiant emissions from the flame, resulting in a proportionately higher contribution of radiative flux to engulfed objects. However, the generally lower velocities arising from flashing liquid releases (such as propane or butane) results in a lower convective flux to engulfed objects. Impaction to an obstacle close to the leak can also result in a local cold spot and hence high temperature gradients to the surrounding hot areas, inducing thermal stress.

In the case of a pressurised gas-liquid mixture (such as ‘live’ crude), the high velocities may still occur and result in a high convective contribution, whilst the higher hydrocarbon content maintains a high radiative contribution; making these type of jet fires a ‘worst case’ in terms of total heat flux to engulfed obstacles. Experimental work suggests that the maximum combined fluxes occur for gas-liquid mixtures which are about 70 % by mass liquid.

A special case of interest at some installations is ‘live’ crude which includes a significant quantity of water. Experiments have shown that mixtures with a ‘water cut’ (defined as mass of water/mass of fuel x 100 %) of up to 125 % remain flammable, although not necessarily capable of supporting a stable flame in the absence of some other supporting mechanism. The inclusion of water also slightly increases flame length and flame buoyancy, and the amount of smoke produced reduces significantly. For water cuts under 50 % no significant reduction in heat fluxes to engulfed objects can be expected (<10 %). However, over 50 % the flames are significantly less radiative, and the overall heat flux to an obstacle can be reduced by 40 % or more.

5.2.3.2 Effect of deluge on two-phase jet fires

Compared to the situation with a gas jet fire, the use of dedicated vessel deluge to protect a vessel against a flashing liquid two-phase jet fire (e.g. propane, butane) can be more effective. The water interacts with the flame to some extent; reducing the flame luminosity and the amount of smoke produced. Nevertheless, at typical application rates (10 to 15 l min^-1 m^-2) it cannot be relied upon to maintain a water film over the vessel and hence to prevent vessel temperature rise in areas where

Compared to the situation with a gas jet fire, the use of dedicated vessel deluge to protect a vessel against a flashing liquid two-phase jet fire (e.g. propane, butane) can be more effective. The water interacts with the flame to some extent; reducing the flame luminosity and the amount of smoke produced. Nevertheless, at typical application rates (10 to 15 l min^-1 m^-2) it cannot be relied upon to maintain a water film over the vessel and hence to prevent vessel temperature rise in areas where