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RAPID PHASE TRANSITION OF LNG

TRANSITION RAPIDE DE PHASE DE GNL

Dr. Philip CleaverDr. Carol Humphreys

BG Plc, Research and Technology

Michel GabillardDominique Nedelka

Gaz de France, Service Etudes Cryogeniques

Roy Scott HeierstedStatoil Research Centre

Jan DahlsveenNTNU, MTF-Fluiddynamics Division

ABSTRACT

Increasingly, as more reserves of natural gas are discovered in regions remote frompotential users, large quantities of LNG are being transported by sea from remote gasfields to distant markets. The transfer of LNG to and from an LNG carrier whilst loadingor unloading, provides the potential for spillage of LNG onto water to occur. It has beendemonstrated that under certain conditions, when LNG is spilled onto water, thevaporisation rate produced can be so high that physical explosions, termed rapid phasetransitions (RPTs), can occur. These RPTs can generate air and underwater blastpressures which could damage adjacent plant or structures.

BG and Gaz de France, both companies which were involved in the early developmentof LNG transportation technology, have been engaged in research into the factorsinfluencing the occurrence and the possible consequences of RPTs. The majority of thisresearch has been conducted on a collaborative basis with other organisations involved inthe operation of LNG importation and exportation facilities including Statoil and the GasResearch Institute in the USA.

As a result of this on-going research programme, it is possible to provide a descriptionof RPT phenomena and an insight into the possible hazards presented by RPT events,during ship to shore transfer of LNG and other cryogenic liquids

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RESUME

Du fait des découvertes grandissantes de réserves de gaz naturel éloignées desconsommateurs potentiels, d'importantes quantités de GNL sont transportées par voiemaritime, des champs de productions aux lieux de consommation. Lors des opérations dechargement ou de déchargement d'un navire méthanier, le déversement de GNL sur l'eauest potentiellement possible pendant le transfert du GNL. On a pu démontrer que, danscertaines conditions, lors de tels déversements de GNL sur l'eau, le taux de vaporisationpeut être si élevé que des explosions violentes, appelées communément TransitionsRapides de Phase (TRP), peuvent être générées. Ces TRP génèrent des ondes de pressionsaériennes et sous-marines, qui pourraient endommager les équipements ou structuresavoisinantes.

Les deux compagnies gazières, BG et Gaz de France, qui ont été impliquées dans lespremiers développements de la technologie du transport de GNL, se sont engagées dansun programme de recherche sur les facteurs influençant l'occurrence et les conséquencespossibles des TRP. La majorité de ce programme de recherche a été mené dans le cadred'une collaboration avec d'autres organismes impliqués dans l'exploitation des terminauxd'importations ou d'exportations de GNL, en particulier Statoil et le Gas ResearchInstitute aux U.S.A.

De ce programme de recherche en cours, résulte une description du phénomènedeTRP, ainsi qu'un aperçu des risques potentiels liés aux explosions durant le transfert deGNL ou d'autres liquides cryogéniques.

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RAPID PHASE TRANSITION OF LNG

1. INTRODUCTION

Exploitation of reserves of natural gas in regions which are remote from potentialmarkets, necessitates the transport of large quantities of LNG by sea bourne LNG carriers.Any future development of offshore liquefaction plant for the exploitation of smalleroffshore gas fields will also increase the quantities of LNG transported by sea. Thetransfer of LNG to and from an LNG carrier provides the potential for spillage of LNGonto the water. It has been demonstrated that under certain conditions, when LNG isspilled onto water the vaporisation rate produced can be so high that physical explosions,termed rapid phase transitions (RPT), can occur.

RPTs can, under some conditions, generate high air and underwater blast pressureswhich could damage adjacent plant or structures. Similar RPT phenomena have beenobserved in the nuclear, metal casting, and paper industries where liquids of widelydiffering temperatures and boiling points can come into contact.

BG, Gaz de France and Statoil have been involved in research into the factorsinfluencing the occurrence and the possible consequences of RPTs, as part of their interestin safety and development of LNG transportation technologies. The majority of thisresearch has been conducted on a collaborative basis with other organisations involved inthe operation of LNG importation and exportation facilities including the Gas ResearchInstitute of America.

As a result of this research programme it is possible to provide a description of RPTphenomenon and give an insight into the potential hazards associated with an RPT event.This relevant to any operation in which LNG (or other cryogenic liquid) could come intocontact with water.

2. DESCRIPTION OF RPT PHENOMENON

An RPT is defined as the process that takes place when a liquid rapidly changes phaseto vapour, the large increase in volume (due to the vapour generation) causes a localisedpressure increase which can give rise to an air or waterborne blast wave.

This may occur when a volatile liquid comes into contact with another liquid of ahigher temperature. Energy is transferred from the hotter liquid to the colder volatileliquid. The rate of energy transfer, and hence rate of vapourisation of the volatile liquid,depends on the difference in temperature, the properties of the liquids and the nature ofthe contact, i.e. on the mixing process. These conditions can be such that the energytransfer is so rapid that an RPT can occur.

The nature of RPT phenomena is a complex combination of thermodynamic andhydrodynamic effects, seemingly of a stochastic character [1]. A detailed knowledge of thegoverning physical laws and the magnitude of the parameters involved is necessary tomodel the occurrence and severity of an RPT. Only experimentally verified models of RPT

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probabilities and severities will allow the development of suitable tools for the LNGindustry, for the estimation of RPT hazards.

In some ways a large scale propagating RPT can be regarded as analogous to achemical detonation [2]. A chemical detonation of a solid explosive is caused bycompression and heating of the medium by the passage of a shock-wave. The detonationanalogy for RPTs assumes that the two liquids, water and LNG, are in a premixedgeometry, and that the passage of the shockwave associated with the RPT can releasesufficient energy from vapourisation of the LNG to sustain the propagation of the shock-wave. Presently the specific physical mechanisms involved in an RPT, and especially thelinks between them, are not fully understood.

Many different mixing conditions of LNG and water can be envisaged in which anRPT could be generated. Ranging from situations in which droplets of LNG aresurrounded by the water medium, through stratified layers of LNG floating on water tothe case of LNG droplets entrained into a LNG flow.

For the case of an RPT in an LNG/water system, a model based on the DetonationAnalogy has been developed to predict the energy yields in a given spill situation. Theenergy yields predicted by this model are controlled by phenomena, related tofragmentation and heat transfer effects. In the case of cryogen droplets and water, whenthe shock-wave propagates through the mixture a relative velocity between the dropletsand the host liquid is induced due to the density difference of the two liquids. The relativevelocity causes the liquid droplets to break down into smaller fragments, increasing theinterfacial area and thus the heat-transfer rate from the water to the cryogen.

A qualitative description of a molten metal/water system given by Board and Hall [2]assumes a three phase medium. It addresses the case where the hot liquid is present asdrops surrounded by a colder, volatile liquid. The temperature difference between the twoliquids is high, and because volatility of the cold liquid, the hot liquid drops areencapsulated in vapour pockets. When a strong shock front arrives, the rapid pressureincrease leads to a decreased volume of the vapour pockets and more efficient heattransfer between the two liquids. It also accelerates the phases at different rates, due todensity differences. Sufficiently large relative velocities set up between the phases, willcause the hot liquid drops to break up into smaller fragments, and in this way reachthermal equilibrium with the cold liquid in a short time. The pressure inducedfragmentation, suggested in the Board and Hall model, arises from hydrodynamicinstability of the surface of the hot liquid drops.

In addition to fragmentation, the pressure rise and relative velocity between the speciescauses an increased heat transfer rate. In the case when the initial cryogen release entrainswater droplets into its interior, the heat transfer rate between the hot liquid drops and thesurrounding cryogen, is assumed small due to the insulating effect of the vapour pockets.During the heat transfer and fragmentation at high pressure the state of the cryogenimmediately adjacent to a hot fragment may be high pressure vapour, or liquid, alldependant upon the magnitude of the pressure and the types of liquid. For the small timeconstants and dimensions involved, heat transfer by conduction provides a firstapproximation for estimating the heat transfer coefficient.

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The superheat limit theory is based upon the notion that a cold volatile liquidcontacting a hot liquid will become superheated instead of boiling. In contrast to a hotsolid surface, a liquid surface lacks the microscopic irregularities necessary to promotenucleate boiling. The superheat limit temperature, which is also termed the homogeneousnucleation temperature, is the maximum possible temperature at which a liquid phase mayexist at a given pressure, according to thermodynamic theory [3].

For a pool of liquid hydrocarbon mixture boiling on water, a contributory factor inRPT initiation is that the normal boiling point will increase over time due to preferentialevaporation of the most volatile components. Although the temperature differencebetween the hot and the cold liquid will be high enough initially to maintain stable filmboiling and thus prevent the water from freezing, eventually it will reach the point wherefilm boiling breaks down, allowing the liquids to touch. Experimental measurementscarried out in order to measure this temperature reveal that the superheat limittemperature is important. Experiments have shown that the resulting evaporation isexplosive, once the superheat limit temperature is reached [4].

3. RELEASE SCENARIOS

Observations of large scale spills of LNG into sea water, e.g., at China Lake, 1980 [5]and at Lorient, 1983 [6] show that RPTs can occur spontaneously when LNG is mixedwith water. Such mixing, can be caused either by the momentum of the LNG release whenit impacts with the water surface or by water waves breaking up the otherwise stable poolof LNG that spread over the water surface. In these cases, the mixing process seems tohave initiated the RPT event.

It has been shown that the nature of the premixing prior to the initiation stronglyinfluences the RPT propagation. Experiments performed by BG and GdF [6] have shownthat RPTs can occur in or close to the region where the jet impacts the water surface,where cryogen/water mixing is initiated. Controlled premixing experiments performed byGdF have clearly demonstrated a close correlation between the amount and scale of thepremixing and the resulting severity of the RPT produced.

Studies of RPTs in cryogen-water systems have also shown that the composition ofthe LNG is one of the factors that influence the likelihood of RPT occurrence. When LNGcontaining approximately 90 % or more methane is brought into contact with water in agentle way, it will not undergo a spontaneous RPT. However the composition of theboiling LNG will change with time, lowering the methane content, and at these later timesa spontaneous RPT may eventually be possible. Enger and Hartman [7] showed thatvapour explosions did not occur when LNG containing over 95 % methane was spilledonto water, but spontaneous RPTs occurred if the LNG contained less than 40 %methane.

The volume flow rate of the spill has been observed as another significant parameterand it is believed that it influences the RPT occurrence through the effect of increasedinertia confinement and the conditions at the spill-point. Porteous and Reid [8] stress theimportance of the way the LNG is spilled on the water surface, since they observed vapourexplosions when high methane content LNG was impacted onto water. A high release

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velocity may cause the vapour layer that develops around the cryogen droplets to beweakened and thus make closer contact between the cryogen and water possible, resultingin increased heat transfer. In addition to inducing perturbations on the water surface, ahigh volume flow rate results in a thick layer of cryogen on the water surface, which actsas an inertia confinement of the mixture of water and cryogen that may undergo an RPT.

In RPT hazard assessment it is important to be able to specify and quantify theparameters that may directly influence the severity of a given LNG spill. Important releaseparameters are:• Location, diameter and direction of the release• Release flow rate and pressure• Duration of release

Typical release scenarios identified in safety assessments of LNG terminals as referredto by the EN1160 standard can be grouped into two categories:• Small (1-2 m3) instantaneous releases from the transfer arms. This type of release could

occur when unusual movement of an LNG carrier relative to its berth causes the fastrelease mechanism on the transfer arm to activate.

• Large releases (450-600 m3) with volume flow rates of up to 11,000 m3/hr and aduration of a few minutes. This type of release would be the result of a full rupture ofan LNG flowline.

It is envisaged that the release scenarios for any future Floating Production, Storageand Off-loading (FPSO) installations are likely to be of the same nature as for LNGterminals. However, the probability of an RPT occurring, given a particular spill, may beincreased if the location of the FPSO is not as sheltered as an LNG jetty.

4. PREVIOUS RPT INCIDENTS

This section lists RPT incidents which have occurred during the operation of LNGplant and also during experimental programmes which were not intended to produce RPTevents. This list is not intended to be exhaustive but to illustrate the fact that RPTs canoccur in real life situations and not just in experiments specifically developed to study theRPT phenomena

4.1 RPT events during LNG plant operations

Canvey, England, May 1973: RPTs after failure of discharge line.During a normal LNG carrier off-loading operation, a 100mm bursting disc on a

350mm discharge line failed. LNG was released into one of the LNG tank bunds wherewater had collected from recent rain. Three explosions were heard, but the only damagewas a broken window in an adjacent building.

Arzew, Algeria, March, 1977: RPTs after valve rupture.Due to the rupture of an aluminium valve several thousand cubic meters of LNG were

released over a 10 hour period. The leakage took place on the ground, near a frozen soiltank, the LNG pool spread onto the sea and several RPTs were observed. Shockwavesand/or projectiles broke a number of adjacent windows.

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Badak, Indonesia, December 1992: LNG Leakage into drainage system.An LNG leak occurred when starting a liquefaction train, it was decided to continue to

operate the train despite the leak. Protective water curtains were operated to reduce theeffects of the vapour cloud produced. About 11 hours after the plant had been startedRPTs occurred in a drainage channel covered by a concrete slab. The drainage channeland concrete slab were broken, adjacent pipework was damaged and some concreteblocks were thrown about 100m. No personnel were injured as the area had beenevacuated because of the leak.

Fos-sur-Mer, France, September 1995: RPT during fire extinguisher demonstration.During a demonstration of using a lorry mounted dry powder extinguisher to

extinguish a 25m2 LNG pool fire an RPT was caused because the contents of a waterpuddle was blown into the LNG pool, by the flow from the extinguisher. A fireball wasproduced doubling the size of the flame for a few seconds. The demonstration wascontinued and the fire was successfully extinguished.

Montoir terminal, France, October 1995: RPT resulting from a vapouriser leak.A leak occurred on the stuffing box of a high pressure at the top of a water falling film

vapouriser unit. Water runs down the outside of the vapouriser tubes into a basin where itis collected to be returned to the river. The leaking LNG was at high pressure (about100bar) and came into contact with water. An RPT occurred followed by a few minorpops; the only damage was to the corrugated plastic structure which surrounded thevapouriser and acted as wind protection.

4.2 RPT events during experimental programmes

Nantes, France, 1971: RPT during Gas Dispersion Tests.During an LNG vapour dispersion test conducted at the GdF test facility at Nantes,

LNG was released onto a 100mm deep layer of water. The LNG was released from a 3m3

tip tank. Several RPTs were observed several seconds after the LNG was released.

As a result of the RPTs the wooden structure intended to retain the water and LNGwas broken, the stainless steel tip tank was bent and some ice was ejected outside thepool.

China Lake, U.S.A., 1980: RPTs during Burro gas dispersion experiments.The Burro tests were carried out by LLNL at China Lake between 1978 and 82 to

study vapour cloud dispersion. The vapour clouds were produced by releasing LNG ontowater. During tests carried out with high LNG flowrates (720 to 1080 m3/h) severe RPTswere produced both on immediate contact between the LNG and water, and at some timeafter the spill. The LNG released during these experiments was of high methane content(>83%). The most severe RPT event produced air overpressures which were estimated tohave a TNT equivalence of 3.5kg.

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Nantes, France, March 1982: RPT during LNG pool fire experiment.During an LNG pool fire suppression test, conducted at the GdF test facility at Nantes,

where a new emulsifier product was under evaluation. The product was unsatisfactory anda heterogeneous mixture of foam and water was produced and poured onto the ignitedLNG. About 11 minutes after ignition the fire had still not been effectively suppressed so itwas decide to stop application of the foam. After a further 6 minutes a violent RPToccurred, the massive increase in vapourisation rate produced a fireball which reached aheight of 40m, about four times the previous flame length. The fireball caused asignificant increase in the radiation produced by the fire and was estimated to have lastedbetween 5 to 10 seconds.

5. PRACTICAL SIGNIFICANCE OF PREVIOUS RESEARCH

The major problem facing safety engineers is the prediction of the risks and hazardsassociated with a given RPT scenario. Although the phenomenon is now betterunderstood (see section 2), the quantification of the severity and pressures generated byRPT still remains approximate. In addition, there is, as yet, no method available toestimate the probability of initiating an RPT for a given release scenario. In this section, anoverview of the methods for RPT prediction are given, built up on the results of thecollaborative research programmes conducted since the early 1980’s (RPT1, RPT2,RPT3) involving BG plc, Gaz de France, Statoil, the Gas Research Institute of Americaand other companies.

There are different methods of estimating the energy yield of RPTs. These variousmethods will be described briefly, stating the underlying assumptions for each method.

It is now commonly understood that the severity of an RPT is directly related to theproperties of the mixing zone between the cryogen and the water (volume, composition,mixture). Thus, two calculations will be necessary before assessing RPT overpressureeffects, the potential cryogen/water mixture characteristics and the energy yield if initiationoccurs.

5.1 Estimating of the Cryogen/Water Mixing Volume Under a Spill

Different theoretical and experimental approaches were carried out to estimate thevolume of the mixing zone produced by a spill of cryogen, onto water.

Experimental Correlations. Experimental correlations were obtained on a laboratoryscale for the mixing volume of a liquid nitrogen jet impacting a pool of water, and on amedium scale for an LNG jet released vertically downwards onto a water surface [9]. Thefollowing relation between the volume of the mixing zone and the spill characteristics wasobtained at small scale with liquid nitrogen :

Vm = (π/3) × tan2(a/2) × H3 where H = CH × U × D

with CH (sm-1) empirical coefficient, U (ms-1) the liquid nitrogen discharge velocity, D(m) the diameter of the discharge pipe, H (m) the depth of penetration of the liquid

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nitrogen cone, and a the experimental angle of the mixing cone. This correlation which isgiven for a mean value of H subjected to large time variations (up to 60%) gives excessivemixture volume when extrapolated for large scale continuous spill.

Computer Prediction. Many models [10,11,12] for the assessment of thecharacteristics of the mixing zone, were proposed in the nuclear industry for the finemodelling of sodium/water interactions. All of these models solve unsteady equations ofconservation of mass, momentum and energy, based on a multi-phase approach withvarious levels of sophistication. The different phases (water, LNG, LNG vapour, air) areassumed to form a continuous medium.

The CHYMES model [11] was adapted within the RPT3 programme, to model theinteraction of a cryogen liquid with water in a cylindrical geometry. This model describesthe formation of cryogen/water mixing zones following the impact of a jet of cryogenvertically downwards onto a water surface. Estimation of the secondary mixing zones(induced by breaking waves, interaction of the resulting stratified LNG/water interfacewith obstacles) was not addressed. For a given spill scenario, the resulting CRYOMIXcomputer code provides an unsteady and localised description of the mixing zone (ratio ofthe different phases) which may be used to obtain precise estimation of the reactivemixture volume, to remedy the inadequacy of experimental correlations. The reactivevolume can thereafter be used in a general estimation of the recoverable energy.

5.2 Estimating Energy Yield

5.2.1 Experimental Approach at Large ScaleThe early 1980s large scale tests were perform by a group of organisations including

GdF and BG at a test facility at Lorient [13], that allowed releases of LNG onto the sea.These tests, performed within the RPT1 research programme, had the largest volumes (1to 9 m3) and LNG flowrate (average flow up to 2300m3h-1) test as yet conducted.During this test programme RPTs were generated with TNT equivalents of up to 4.2 kg.These RPTs occurred within the well mixed LNG/water zones either in the vicinity of thespill point or in the region of breaking waves. These tests are reasonably representative ofa release scenario corresponding to rupture of a pressurised unloading arm. They are lessrepresentative of scenario such as rupture of a transfer line, because in this case, thespillage conditions (larger, long duration flow) are very much different from the Lorienttests. No experiment has yet been conducted which fully reproduces possible industrialscale releases.

5.2.2 Experimental Approach at Small ScaleIn small scale experiments [14] conducted at the GdF test site at Nantes, propagating

RPTs were initiated (typically 2 meters propagation) with controlled premixed conditions.Because these experiments were conducted at relatively small scale, measurements ofoverpressure could be made within the RPT reaction zone so that the maximumoverpressures generated could be measured. Magnitude and overpressure time historiesare essential for evaluating the load on the structures in contact either with the reactivecryogen/water mixture or with the air or the water close to an RPT.

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The results obtained from these small scale experiments conducted with awater/nitrogen mixture can be summarised as:

• Propagation velocities of up to 300 ms-1 were observed in the reactive mixture• Very high overpressures were measured in the reactive mixture.• The duration of the pressure pulse inside the reactive medium is in the range of 1 ms,

with a rise time of approximately 0.1ms.

It has not been possible to validate these results with more realistic scale experiments.

5.2.3 Methods Based on Released Specific EnergiesMethods based on specific energies (energy released per unit volume or mass of

cryogen within the mixed cryogen/water zone) can only be used for estimating the severityof an RPT in the zones located immediately below the LNG spill or for a mixing zone dueto a breaking wave, for which the characteristics are known. They cannot be appliedwithout knowledge of the extent of the premixing zone, the calculation methods for whichwere previously described.

Two methods based on this concept of specific energy can be used :

Hicks & Menzies model. This model gives the thermodynamical upper limit of therecoverable expansion work in a water/cryogen mixture. According to this theoreticalmodel, the maximum mechanical energy which can be recovered is 480 kJkg-1 ofmethane, for a methane/water mixture.

Shell Experimental Correlations. During the first studies regarding RPT, anexperimental value of the specific energy was measured by Shell during small scaleexperiments. The values obtained were 5.6 kJ/litre of LNG and up to 7.4 kJ/litre ofNitrogen in the mixture zone. There is no guarantee that this value is an upper limit forlarger scale releases. However, it appears to be consistent with the maximum explosionenergies encountered during the large scale Lorient tests.

5.2.4 Method based on a 1D Detailed Propagation Approach.Calculations of RPT propagation rely essentially on the local characteristics of the

mixed zone. Based on a two-dimensional description of the mixed zone provided by thepreviously mentioned CRYOMIX code, a calculation of the propagation can beconsidered either in a one-dimensional or in a two-dimensional geometry. In the first case,the 2D characteristics of the mixed zone need to be homogenised in order to arrive at aone-dimensional description. In the second case, a simple link between CRYOMIX resultsand the initial conditions of the propagation code is needed.

The propagation model developed under RPT3 program is based on works for thenuclear industry [15,16], and in particular on the CUL-DE-SAC code which Gaz deFrance adapted to water/cryogen mixtures. This code is currently operational for a one-dimensional plane geometry.

It is expected that this code will be used either to compute a global equivalent energyor to describe the evolution of the pressure wave at the boundaries of the reactive

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medium. This pressure wave will then be used as the source term for propagation in inertmedia (air or water) surrounding the reactive medium.

A first industrial application of this code can be achieved with simplifying assumptions.The different water/LNG mixing zones resulting from the spillage, the spreading or thebreaking waves, can be characterised by the maximum distance along which an RPT canpropagate. Using this geometrical variable representative of the mixing zone, the available1D modelling will give results less pessimistic than the global thermodynamical approachof Hicks and Menzies. Moreover, more pessimistic values of the mixing zone parameters(water/LNG premixing ratio, void fraction, ...) can be used in order to keep a conservativemodelling approach to predicting hazards.

The 1D approach is the first step in the development of models to predict the severityof RPTs; improving and extending the mathematical modelling techniques will improve thepredictions produced. Any models produced should be validated against experimentconducted in geometries representative of full scale release scenarios.

6. CONSEQUENCES OF POSSIBLE RPT EVENTS

In section 5 the various techniques which can be used to estimate the severity of anRPT event were described. The consequences of an RPT event depend on the severity ofthe event and the vulnerability of surrounding plant and structure the main potential fordamage from an RPT is from the resulting overpressures generated. There is the potentialfor RPTs to generate high overpressures in the reaction zone, but they are generally ofshort duration. This initial pressure field produces different effects in the air and water:

Water Overpressures. High peak overpressures can be generated in the water close toan RPT. Overpressures of the order of tens of bar have been recorded in the water closeto an RPT event but these overpressure pulses are generally of short duration (<10ms).These overpressures have the potential to damage adjacent structures, but as the wateroverpressures produced are attenuated rapidly with distance, the damaging effects wouldbe highly localised.

Air Overpressures. The airbourne blast waves produced are generally of longerduration but lower magnitude, typically in the large scale experiments [6] overpressures ofless than 1 bar were recorded, these pressures would only be damaging to less robuststructures close to an RPT event but could cause injury to personnel in the immediatevicinity.

In addition to the air and waterbourne blast waves an RPT could also produce a highvelocity water jet which could have damaging effects. There is also a potential forprojectiles to be produced, particularly ice that is formed at the cryogen water interface,although significant ice formation would tend to preclude the occurrence of RPTs.

In the case of an RPT event occurring during a loading or unloading of a LNG carrierthe structures which would be vulnerable to the underwater overpressures generatedwould be the LNG carrier and the jetty. Jetties are generally very robust structuressupported from the seabed by a number of members. Due to the localised nature of an

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RPT event it is unlikely that more than a few of these members would be damaged so thatan RPT is unlikely to impair the integrity of a jetty.

In the case of the LNG carrier it may be possible that the localised very highoverpressures could rupture the outer hull which could have significant consequences. Ifthe damage to a LNG carrier also extended to the LNG tanks within the carrier there ispotential for escalation of the event. In the case of an LNG carrier operation involving anFPSO type installation the comments on the possible effects on the LNG carrier wouldalso apply to the FPSO.

7. SUMMARY

The paper provides a brief description of the RPT phenomenon the hazards associatedwith an RPT. It is based on the results of considerable collaborative research. The factorsaffecting the occurrence, severity and consequences of RPT events have been discussed,with information provided on the various techniques which exist for estimating theseverity of RPT events.

It has been demonstrated that RPT events can occur when LNG is spilt onto a watersurface and as such they should be considered along with other the other hazardsassociated with the operation of import/export facilities or FPSOs.

8. REFERENCES CITED

1. Sainson J., Baradel C., Roulleau H., Leblond J., Makim V., Rapid PhaseTransitions of Cryogenic Liquid Boiling on Water Surface, ProceedingsEurotherm Seminar 14, Belgium, 1990.

2. Board, S.J., Hall, R.W., Hall, R.S., Detonation of Fuel Coolant Explosions,Nature, Vol. 254, March 27, p.319-321, 1975.

3. Carey, V.P., Liquid-Vapor Phase-Change Phenomena, Chapter 5, p.127-167Hemisphere Publ. Co., 1992, ISBN 0-89116-836-2.

4. Shepherd, J.E., Sturtevant, B., Rapid Evaporation at the Superheat Limit,J.Fluid Mechanics, Vol.121, p.379-402, 1982.

5. Reid, R.C., Rapid Phase Transitions from Liquid to Vapor, Adv. Chem. Eng.Vol. 12, p.105-208, 1983.

6. A record of measurements made during a series of experiments to studyLNG/water RPTs, carried out jointly by BG, GdF and Shell research. BritishGas, January 1985.

7. Enger, T., Hartman, D., Rapid phase transformation during LNG spillage onwater. Shell pipe line Corp. R&D lab.

8. Porteous, W.M., Reid, R.C., Light Hydrocarbon Vapor Explosions, ChemicalEngineering Progress, Vol.72, p.83-89, 1976.

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9. Dahlsveen, J., Experimental study of premixing by a cryogen jet injected intowater, Statoil, September 1995.

10. Berthoud G., Valette M., Calculations of the premixing phase of an FCI withthe TRIO code, Proc. CSNI Specialists Meeting on Fuel-Coolant Interactions,pp. 27-36, January 1993

11. Fletcher D.F., Thyagaraja A., 1991, The CHYMES mixing model, Progress inNucl. Energy. Vol 26, No. 1, pp.31-61, 1991.

12. Theofanous T.G., 1995, Multiphase flows in nuclear reactor severe accidents,Advances in multiphase flow, pp. 573-599, 1995.

13. Salvadori A., LeDiraison J.C., Neldelka D., Contribution to the Study of theBehaviour of LNG Spilled onto the Sea, LNG7 Conference, 1983.

14. Sainson J., Gabillard M., Williams T., Propagation of Vapour Explosions in aStratified Geometry, Experiments with Liquid Nitrogen and Water, CSNISpecialists Meeting, 1993.

15. Fletcher D.F.,1993, Propagation investigations using the CULDESAC model,CSNI FCI Specialist Meeting, 5-8 Jan., SANTA-BARBARA, USA,1993.

16. Berthoud G., Brayer C., 1995, An attempt to model stratified thermalexplosions with the multidimensional, multicomponent code MC3D. Proc. ofInt. Conf. on multiphase Flow ‘95 - Kyoto, Japan, April 3-7 1995.