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MAROFF-2-Maritim virksomhet og offsh-2

Predicting the risk of rapid phase-transition events in LNG spills

Alternative title: Beregning av risikoen for eksplosiv faseovergang ved LNG-utslipp

Awarded: NOK 12.3 mill.

The project Predict-RPT has sought to improve our understanding of the phenomenon of rapid phase transition (RPT). This is becoming an important safety issue as the transport and utilization of liquefied natural gas (LNG) is increasing in a Norwegian, European and Global context. It is often necessary to transfer LNG in loading arms and lines directly above the sea. Examples include refuelling of LNG-driven ships and passenger ferries and loading and un-loading of LNG-carriers at liquefaction plants and LNG terminals. During these operations, LNG may accidentally be spilt onto the sea. The water will be at least 160 degrees C warmer than the LNG. When RPT is triggered, the LNG is quickly heated due to the large temperature difference. The quick heating rate causes the LNG to overheat, and in some cases the degree of overheat in the LNG will be so large that it can no longer exist as a liquid. At this point, large volumes of the LNG may instantaneously vaporize, thereby creating a local overpressure. The expansion that follows is a physical explosion that may cause harm to personnel and equipment. Although historically the number of RPT accidents has been relatively low, large-scale experiments have demonstrated that RPT explosions can occur during LNG spills under industrially relevant conditions. The increased use of LNG, for example in commercial transport vessels, can lead to more incidents as well as a higher risk of injuries and loss of life. It is therefore important to understand the RPT phenomenon during LNG spills on water. In Predict-RPT, we focus on the evolution of an LNG spill from initiation to superheating conditions, on the physical mechanisms that trigger the RPT event, and on the subsequent rapid phase transition. We have established a simulation framework for delayed RPTs that combines state-of-the-art CFD, heat-transfer models and thermodynamics, utilizing accurate thermodynamic data for hydrocarbon mixtures from a SINTEFs in-house library. As a part of the project, the framework has been given three major improvements; a new, simplified, model for spreading of LNG on water has been derived, the suitable approximation for the superheat limit of the LNG has been identified, and a best-fit heat transfer model has been selected. The superheat limit is an important parameter for predicting the overpressure caused by the rapid evaporation. In Predict-RPT, we have shown that for single- and multi-component fluids, the classic nucleation theory provides estimates for the superheat limit of LNG that agree well with experimental data. This is incorporated in our model for predicting the potential energy yield in an RPT event. The simplified CFD model for the spreading of LNG on water has been validated by comparisons both with a more complex two-layer pool spreading model and with experimental results from the literature. Several sets of heat transfer correlations have been evaluated and the set with the best agreement with existing experimental data has been implemented into the framework. A simplified tool for assessment of the risk and consequences of LNG RPT has been developed through a combined usage of the modelling framework and theory (https://predictrpt.herokuapp.com/). The tool evaluates key parameters such as risk zone for RPT, mass and energy of RPT in risk zone, and duration until risk state is reached. It is able to predict how these parameters depend upon spilled volume of LNG. The tool can also predict the overpressure in the case of a rapid phase transition event. The advances made in the project are documented in eight published journal papers, of which six stems from the work undertaken by the PhD. A final paper which presents the modelling framework and the simplified model for assessing RPT events from LNG spills is ready for submission to journal. Project results have also been presented at SIGTTO events in Oslo and Houston. The PhD student (NTNU) defended his thesis in March 2019. The thesis included a 130-page monograph summarizing the research, as well as six peer reviewed journal articles that have all been accepted and published since. The PhD student has also presented results at three academic conferences. The most significant academic work was performed while staying in Chicago as a Fulbright Scholar at Northwestern University, working with Professor Stephen H. Davis, a highly renowned expert in fluid mechanics. The most novel result was published in Journal of Fluid Mechanics, arguably the world?s most prestigious journal in the field. His work includes investigations of the stability of the film isolating LNG droplet from direct contact with water, the development of a theoretical prediction of the Leidenfrost temperature, the derivation of non-equilibrium evaporation models from kinetic theory and a simplified model for triggering of delayed RPT and consequent vapour explosion.

Vi har styrket vår kompetanse på rask faseovergang som risikofaktor ved søl av kryogene væsker på sjø, med fokus på flytende naturgass. Et risikovurderingsverktøy med detaljerte beregninger er etablert, samt et verktøy for forenklet risikovurdering (https://predictrpt.herokuapp.com/). Prosjektets resultater er dokumentert i ni vitenskapelige artikler. Prosjektets ferdigutdannede PhD (NTNU) er hovedforfatter for seks av disse. Flere av artiklene ble til i et ny-etablert samarbeid med Prof. Stephen Davis ved Northwestern University i Chicago, en ekspert innen fluidmekanikk. Basert på ny kunnskap i prosjektet har vi etablert en aktivitet på rask faseovergang for flytende hydrogen i et nytt KPN prosjekt støttet av Norges Forskningsråd. Hydrogen har potensiale som utslippsfri energibærer i et nullutslipps energisystem. For norsk maritim industri medfører dette et potensiale for verdiskaping gjennom utvikling av maritime fartøy for drevet av og/eller for eksport av flytende hydrogen.

Due to the expected increase in maritime production, transportation and use of liquefied natural gas (LNG) in Norway, the safety aspect of these activities will become increasingly important. One of the main safety concerns of LNG operations is Rapid Phase Transition (RPT). When spilled onto water, LNG is observed to suddenly, and seemingly at random, explosively vaporize in large quantities at once. For the maritime sector, the greatest risk of LNG spills onto water is believed to occur when it is necessary to transfer LNG across bodies of water, e.g. while refueling LNG-driven ships or loading/unloading LNG carriers, and when vessels experience hull breach due to collisions. RPT events may cause direct harm to equipment and personnel, increase unwanted gas dispersion, as well as initiate large-scale spill accidents through cascading containment failures. This project will enable better risk quantification of large scale accidents caused by LNG spills onto water. This will be achieved by filling knowledge gaps and developing new predictive models regarding the RPT phenomenon, which may be used to provide recommendations and safety guidelines for the industry. The project will be carried out in collaboration with international experts and a strong industry consortium. It will contain both experimental and theoretical/modelling tasks, both aimed at filling the knowledge gaps in the current understanding of LNG RPT. Each of these tasks will educate a PhD candidate in order to strengthen the national knowledge base on the subject. R&D challenges in the experimental task include designing experiments for properly isolating variables of interest, being able to observe the phenomenon clearly and at sufficiently small timescales, and analyzing the data to yield accurate and useful correlations. R&D challenges in the modelling task include modelling the relatively poorly understood triggering phase of RPT, and combining existing and new sub-models into a complete tool.

Publications from Cristin

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Funding scheme:

MAROFF-2-Maritim virksomhet og offsh-2