Model of spills and fires from LNG and oil tankers

A comprehensive model for predicting the dynamics of spills from LNG and oil product tankers is constructed from fluid mechanics principles and empirical properties of oil and LNG spills on water. The analysis utilizes the significant tanker hold and discharge flow area dimensions to specify the cargo liquid outflow history and the ensuing pool characteristics, including the establishment of a pool fire. The pool fire area, duration, and heat release rate are determined as functions of the tanker cargo variables. Examples of an LNG and gasoline spill show that for likely discharge flow areas these spills may be regarded as instantaneous, simplifying the evaluation of risk consequences.     

Keys to modeling LNG spills on water

Although no LNG ship has experienced a loss of containment in over 40 years of shipping, it is important for risk management planning to understand the predicted consequences of a spill. A key parameter in assessing the impact of an LNG spill is the pool size. LNG spills onto water generally result in larger pools than land spills because they are unconfined. Modeling of LNG spills onto water is much more difficult than for land spills because the phenomena are more complex and the experimental basis is more limited.

The most prevalent practice in predicting pool sizes is to treat the release as instantaneous or constant-rate, and to calculate the pool size using an empirical evaporation or burn rate. The evaporation or burn rate is particularly difficult to estimate for LNG spills on water, because the available data are so limited, scattered, and difficult to extrapolate to the large releases of interest.

A more effective modeling of possible spills of LNG onto water calculates, rather than estimating, the evaporation or burn rate. The keys to this approach are to:

• Use rigorous multicomponent physical properties.

• Use a time-varying analysis of spill and evaporation.

• Use a material and energy balance approach.

• Estimate the heat transfer from water to LNG in a way that reflects the turbulence.

These keys are explained and demonstrated by predictions of a model that incorporates these features. The major challenges are describing the effects of the LNG–water turbulence and the heat transfer from the pool fire to the underlying LNG pool. The model includes a fundamentally based framework for these terms, and the current formulation is based on some of the largest tests to-date. The heat transfer coefficient between the water and LNG is obtained by applying a “turbulence factor” to the value from correlations for quiescent film and transition boiling. The turbulence factor is based on two of the largest unignited tests on water to-date. The heat transfer from the fire to the pool is based on the burning rate for the largest pool fire test on land to-date.     

Model of large pool fires

A two zone entrainment model of pool fires is proposed to depict the fluid flow and flame properties of the fire. Consisting of combustion and plume zones, it provides a consistent scheme for developing non-dimensional scaling parameters for correlating and extrapolating pool fire visible flame length, flame tilt, surface emissive power, and fuel evaporation rate. The model is extended to include grey gas thermal radiation from soot particles in the flame zone, accounting for emission and absorption in both optically thin and thick regions. A model of convective heat transfer from the combustion zone to the liquid fuel pool, and from a water substrate to cryogenic fuel pools spreading on water, provides evaporation rates for both adiabatic and non-adiabatic fires. The model is tested against field measurements of large scale pool fires, principally of LNG, and is generally in agreement with experimental values of all variables.     

The spreading and evaporation of LNG- and burning LNG-spills on water

This paper contains the results of a theoretical investigation into the evaporation and spilling of LNG and the burning of LNG on open water and on a confined water surface.The spreading and evaporation of LNG spilled on open water are calculated and compared with experimental reaults. As little is known about the evaporation of LNG on a confined water surface a model has been derived which describes the evaporation including the forming of an ice layer. The models derived for the spreading and evaporation of LNG on open water and on a confined water surface are also used to calculate the spreading and evaporation of burning LNG-spills. The heat radiation from the flames into the pool has been calculated from experimental data from LNG-fires on land.It is concluded that the results of this investigation concerning the evaporation of LNG on water agree well with the available experimental data, but that the calculated results for burning LNG can only be considered as a rough estimate.     

An LNG release, transport, and fate model system for marine spills

LNGMAP, a fully integrated, geographic information based modular system, has been developed to predict the fate and transport of marine spills of LNG. The model is organized as a discrete set of linked algorithms that represent the processes (time dependent release rate, spreading, transport on the water surface, evaporation from the water surface, transport and dispersion in the atmosphere, and, if ignited, burning and associated radiated heat fields) affecting LNG once it is released into the environment. A particle-based approach is employed in which discrete masses of LNG released from the source are modeled as individual masses of LNG or spillets. The model is designed to predict the gas mass balance as a function of time and to display the spatial and temporal evolution of the gas (and radiated energy field).

LNGMAP has been validated by comparisons to predictions of models developed by ABS Consulting and Sandia for time dependent point releases from a draining tank, with and without burning. Simulations were in excellent agreement with those performed by ABS Consulting and consistent with Sandia's steady state results.

To illustrate the model predictive capability for realistic emergency scenarios, simulations were performed for a tanker entering Block Island Sound. Three hypothetical cases were studied: the first assumes the vessel continues on course after the spill starts, the second that the vessel stops as soon as practical after the release begins (3 min), and the third that the vessel grounds at the closest site practical. The model shows that the areas of the surface pool and the incident thermal radiation field (with burning) are minimized and dispersed vapor cloud area (without burning) maximized if the vessel continues on course. For this case the surface pool area, with burning, is substantially smaller than for the without burning case because of the higher mass loss rate from the surface pool due to burning. Since the vessel speed substantially exceeds the spill spreading rate, both the thermal radiation fields and surface pool trail the vessel. The relative directions and speeds of the wind and vessel movement govern the orientation of the dispersed plume.

If the vessel stops, the areas of the surface pool and incident radiation field (with burning) are maximized and the dispersed cloud area (without burning) minimized. The longer the delay in stopping the vessel, the smaller the peak values are for the pool area and the size of the thermal radiation field. Once the vessel stops, the spill pool is adjacent to the vessel and moving down current. The thermal radiation field is oriented similarly. These results may be particularly useful in contingency planning for underway vessels.     

Coupling dynamic blow down and pool evaporation model for LNG

Treating the dynamic effects of accidental discharges of liquefied natural gas (LNG) is important for realistic predictions of pool radius. Two phenomena have important influence on pool spread dynamics, time-varying discharge (blow down) and pool ignition. Time-varying discharge occurs because a punctured LNG tanker or storage tank drains with a decreasing liquid head and decreasing head-space pressure. Pool ignition increases the evaporation rate of a pool and consequently decreases the ultimate pool area. This paper describes an approach to treat these phenomena in a dynamic pool evaporation model.

The pool evaporation model developed here has two separate regimes. Early in the spill, momentum forces dominate and the pool spreads independently of pool evaporation rate and the corresponding heat transfer rate. After the average pool depth drops below a minimum value, momentum forces are largely dissipated and the thin edges of the pool completely evaporate, so pool area is established by the heat transfer rate. The maximum extent of a burning pool is predicted to be significantly less than that of an unignited pool because the duration of the first regime is reduced by higher heat transfer rates. The maximum extent of an LNG pool is predicted to be larger upon accounting for blow down compared with using a constant average discharge rate. However, the maximum pool extent occurs only momentarily before retreating.     

Modeling the release, spreading, and burning of LNG, LPG, and gasoline on water

Current interest in the shipment of liquefied natural gas (LNG) has renewed the debate about the safety of shipping large volumes of flammable fuels. The size of a spreading pool following a release of LNG from an LNG tank ship has been the subject of numerous papers and studies dating back to the mid-1970s. Several papers have presented idealized views of how the LNG would be released and spread across a quiescent water surface. There is a considerable amount of publicly available material describing these idealized releases, but little discussion of how other flammable fuels would behave if released from similar sized ships. The purpose of this paper is to determine whether the models currently available from the United States Federal Energy Regulatory Commission (FERC) can be used to simulate the release, spreading, vaporization, and pool fire impacts for materials other than LNG, and if so, identify which material-specific parameters are required.

The review of the basic equations and principles in FERC's LNG release, spreading, and burning models did not reveal a critical fault that would prevent their use in evaluating the consequences of other flammable fluid releases. With the correct physical data, the models can be used with the same level of confidence for materials such as LPG and gasoline as they are for LNG.     

United states regulations for siting LNG terminals: problems and potential

The regulations being applied to liquefied natural gas (LNG) import terminal siting in the United States are reviewed. There are no requirements for exclusion zones to protect the public from LNG spills onto water. Serious problems with current practices used to determine exclusion zones on the land-based part of the facility are identified. Many of the questions that are considered relate to the use of computational fluid dynamic (CFD) models, which appear to offer the best potential for realistic modeling to determine vapor cloud exclusion zones that result from LNG spills into impounded areas with or without dispersion in the presence of other obstacles to the wind flow. Failure to use CFD models, which are already approved by the regulation, and continued use of practices which have been demonstrated to be in error, raises important questions of credibility as well as denies the applicant full use of scientific tools that are available to optimize the design of such facilities so as to best provide for safety of the public.     

A review of large-scale LNG spills: experiments and modeling

The prediction of the possible hazards associated with the storage and transportation of liquefied natural gas (LNG) by ship has motivated a substantial number of experimental and analytical studies. This paper reviews the experimental and analytical work performed to date on large-scale spills of LNG. Specifically, experiments on the dispersion of LNG, as well as experiments of LNG fires from spills on water and land are reviewed. Explosion, pool boiling, and rapid phase transition (RPT) explosion studies are described and discussed, as well as models used to predict dispersion and thermal hazard distances. Although there have been significant advances in understanding the behavior of LNG spills, technical knowledge gaps to improve hazard prediction are identified. Some of these gaps can be addressed with current modeling and testing capabilities. A discussion of the state of knowledge and recommendations to further improve the understanding of the behavior of LNG spills on water is provided.     

Large hydrocarbon fuel pool fires: physical characteristics and thermal emission variations with height

In a recent paper [P.K. Raj, Large LNG fire thermal radiation-modeling issues and hazard criteria revisited, Process Safety Progr., 24 (3) (2005)] it was shown that large, turbulent fires on hydrocarbon liquid pools display several characteristics including, pulsating burning, production of smoke, and reduced thermal radiation, with increasing size. In this paper, a semi-empirical mathematical model is proposed which considers several of these important fire characteristics. Also included in this paper are the experimental results for the variation of the fire radiance from bottom to top of the fire (and their statistical distribution) from the largest land spill LNG pool fire test conducted to date. The purpose of the model described in this paper is to predict the variation of thermal radiation output along the fire plume and to estimate the overall thermal emission from the fire as a function its size taking into consideration the smoke effects. The model utilizes experimentally measured data for different parameters and uses correlations developed from laboratory and field tests with different fuels. The fire dynamics and combustion of the fuel are modeled using known entrainment and combustion efficiency parameter values. The mean emissive power data from field tests are compared with model predictions. Model results for the average emissive powers of large, hypothetical LNG fires are indicated.     

Assessment of the effects of release variables on the consequences of LNG spillage onto water using FERC models

Liquefied natural gas (LNG) release, spread, evaporation, and dispersion processes are illustrated using the Federal Energy Regulatory Commission models in this paper. The spillage consequences are dependent upon the tank conditions, release scenarios, and the environmental conditions. The effects of the contributing variables, including the tank configuration, breach hole size, ullage pressure, wind speed and stability class, and surface roughness, on the consequence of LNG spillage onto water are evaluated using the models. The sensitivities of the consequences to those variables are discussed.     

Investigations into the spreading and evaporation of LNG spilled on water

This paper contains the results of a theoretical investigation into the evaporation and spilling of LNG on open water and on a confined water surface. Spreading and evaporation are calculated and compared with experimental results. As little is known about the evaporation of LNG on a confined water surface a model has been derived which describes the evaporation including the formation of an ice layer. It is concluded that results agree well with the available experimental data.     

大型常压LNG贮罐的国产化探讨

大型常压液化天然气贮罐的开发和建造正逐步实现国产化,开发初期的问题值得大家共同探讨。文章对LNG的性质、国内外LNG大型贮罐现状、贮罐结构形式、贮罐流程组织、附件的选择及标准的遵循等方面进行了阐述。     

LNG fires: a review of experimental results, models and hazard prediction challenges

A number of experimental investigations of LNG fires (of sizes 35 m diameter and smaller) were undertaken, world wide, during the 1970s and 1980s to study their physical and radiative characteristics. This paper reviews the published data from several of these tests including from the largest test to date, the 35 m, Montoir tests.

Also reviewed in this paper is the state of the art in modeling LNG pool and vapor fires, including thermal radiation hazard modeling. The review is limited to considering the integral and semi-empirical models (solid flame and point source); CFD models are not reviewed. Several aspects of modeling LNG fires are reviewed including, the physical characteristics, such as the (visible) fire size and shape, tilt and drag in windy conditions, smoke production, radiant thermal output, etc., and the consideration of experimental data in the models. Comparisons of model results with experimental data are indicated and current deficiencies in modeling are discussed.

The requirements in the US and European regulations related to LNG fire hazard assessment are reviewed, in brief, in the light of model inaccuracies, criteria for hazards to people and structures, and the effects of mitigating circumstances. The paper identifies: (i) critical parameters for which there exist no data, (ii) uncertainties and unknowns in modeling and (iii) deficiencies and gaps in current regulatory recipes for predicting hazards.     

The influence of ice formation on vaporization of LNG on water surfaces

The spillage of LNG on water surfaces can lead, under certain circumstances, to a decrease in the surface temperature of water and subsequent freezing. A model for heat transfer from water to LNG is proposed and used to calculate the surface temperature of water and examine its influence on the vaporization rate of LNG. For this purpose LNG was modeled based on the properties of pure methane. It was concluded that when LNG spills on a confined, shallow-water surface the surface temperature of water will decrease rapidly leading to ice formation. The formation of an ice layer, that will continue to grow for the duration of the spill, will have a profound effect upon the vaporization rate. The decreasing surface temperature of ice will decrease the temperature differential between LNG and ice that drives the heat transfer and will lead to a change of the boiling regime. The overall effect would be that the vaporization flux would first decrease during the film boiling; followed by an increase during the transition boiling and a steady decrease during the nucleate boiling.     

Performance of the cold transport system utilizing evaporation-freezing phenomena with a cold trap

This paper dealt with the performance of a novel cold transport system for the waste cold from the gasification process of Liquefied Natural Gas (LNG), which has been proposed by one of the authors. The system consists of an evaporator, a cold trap, and a pipeline between them, and the LNG cold poured into the cold trap is transported to the evaporator as a flow of low-pressure water vapor. In this study, cold transport rate of a small-scale system was experimentally examined under various conditions, and the efficiency of cold transport was discussed. The results clearly showed that, as suggested by a theoretical relation based on the pressure drop due to vapor flow in the pipeline, cold transport rate is affected both by the pressure difference between the evaporator and cold trap, and by the length, diameter and friction factor of the pipeline. However it was also shown that the transport rate is hardly influenced by the temperature of pipeline wall. Based on the theoretical relation and the experimental results obtained herein, a guideline for designing the cold transport system was derived.     

Potential for BLEVE associated with marine LNG vessel fires

Recent LNG marine shipping hazard studies have discounted BLEVE hazards associated with LNG vessels. This exclusion of a potential major hazard event has been queried, particularly since a recent LNG truck BLEVE-like event in Spain. This paper reviews the physical factors associated with the Spanish LNG truck event and accepts that this had features of a classical BLEVE event and that there is no inherent property of LNG excluding BLEVE-like events, although US LNG trucks would be safer due to design features. Marine LNG vessels have differently designed tanks and it is demonstrated that the combination of physical barriers makes direct thermal input to the LNG inner tank more limited than hypothesized by some, but if it occurs these tanks cannot rise to a pressure sufficient to cause a large flash of liquid and consequent BLEVE event of a scale hypothesized in the literature.     

Hydroelastic analysis of insulation containment of LNG carrier by global–local approach

The insulation containment of liquefied natural gas (LNG) carriers is a large‐sized elastic structure made of various metallic and composite materials of complex structural composition to protect the heat invasion and to sustain the hydrodynamic pressure. The goal of the present paper is to present a global–local numerical approach to effectively and accurately compute the local hydroelastic response of a local containment region of interest. The global sloshing flow and hydrodynamic pressure fields of interior LNG are computed by assuming the flexible containment as a rigid container. On the other hand, the local hydroelastic response of the insulation containment is obtained by solving only the local hydroelastic model in which the complex and flexible insulation structure is fully considered and the global analysis results are used as the initial and boundary conditions. The interior incompressible inviscid LNG flow is solved by the first‐order Euler finite volume method, whereas the structural dynamic deformation is solved by the explicit finite element method. The LNG flow and the containment deformation are coupled by the Euler–Lagrange coupling scheme. Copyright © 2008 John Wiley & Sons, Ltd.     

Experimental investigations of ice slurry flows in horizontal pipe based on monopropylene glycol

In this paper, the pressure drop behavior of ice slurry based on MPG-water in a circular horizontal tube is experimentally investigated. The secondary fluid was prepared by mixing MPG and water to obtain initial MPG concentration varying from 5% to 24%. The pressure drop tests were conducted to cover laminar flow with ice mass fraction varying from 5% to 25% depending on test conditions. Results from test reveal that the ice slurry behaves as non-Newtonian: thickening flow (n > 1) or shear thinning flow (n < 1) and sometimes as Newtonian flow (n ≈ 1). The moving bed is observed in particular flow conditions.The experimental results for viscosities were compared to the analytical results. In addition, experimental results of the Darcy friction factor were compared to Poiseuille model who gives good agreement with experimental results.Furthermore, for transport purposes, it has been shown that 11% initial MPG concentration gives the best results.