A New Experimental Study on the Spreading of Liquid Nitrogen Over Water

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Abstract An experimental apparatus was designed and constructed to study the behavior of a cryogenic liquid pool spreading over water, for the purpose of improving the current LNG pool spreading models. The experiment allows the rate of evaporation to be determined directly unlike most past experiments. The apparatus also allows the cryogen-water interface to be directly observed and for the pool thickness distribution to be quantified along with its turbulence intensity. This paper describes the apparatus, and presents the evaporization rates obtained from a series of tests involving liquid nitrogen spills on water. The results shed light on the physical behavior of cryogenic pool-water interaction, and identify shortcomings of some assumptions made in current pool spreading models. Similar tests using LNG are planned, to obtain quantitative data for the pool spreading models. Background and Motivation The LNG industry is continuing to enjoy a period of great activity, which started a few years ago and has resulted in well over 100 new LNG terminals being proposed (and some already built) around the world. This has been accompanied by a rapid growth in the number of LNG carriers being ordered and built - according to industry sources, approximately 300 LNG carriers will be in operation by the end of 2008 [LNG Express 2008]. In addition to LNG cargo deliveries from a liquefaction terminal to a regasification terminal, recent technological developments have made ship-to-ship transfers of LNG a concrete possibility - either in order to offload an LNG carrier to a moored floating storage and regasification unit (FSRU) or, to transfer product from offshore liquefaction facilities. The industry has upheld an exemplary safety standard relative to the marine transportation of LNG, with only a very limited number of safety incidents of any kind and no major accidents with loss of cargo in over 40 years since LNG commerce began in 1964. The growth in marine LNG traffic, the increasing size of new generation Q-max and Q-flex LNG tankers and the development of numerous new terminals have raised the need to consider and predict the hazards associated with a large-scale release of LNG onto the water surface, from a vessel at sea or near a terminal. In the context of risk analysis, the typical sequence of events for an LNG spill on water that needs to be considered is as follows:The LNG carrier's double hull is perforated, as a result of an accidental (e.g., collision/allision) or intentional event (e.g., terrorist attack), and one or more LNG tanks are breached;The LNG in the tank flows out of the tank through the hole;The LNG flow spills onto the water surface;Due to the lower density of LNG, the spill forms a pool on the water surface. The pool spreads on the water surface, as more LNG flows out of the breach;Heat transfer from the water (and, to a lesser extent, from the air) vaporizes the LNG and forms a dense, cold vapor cloud;If a viable ignition source is present in proximity of the LNG pool the LNG vapor cloud ignites and forms a pool fire, which burns until the fuel is consumed;If the LNG vapor cloud does not encounter a viable ignition source in proximity of the pool, it migrates downwind. The vapor cloud progressively warms up and dissipates as it mixes with the warmer ambient air and absorbs heat from the environment. If the vapor cloud encounters a viable ignition source away from the pool, while the gas concentration in the cloud is still at or above the lower flammable limit (LFL = 5% molar fraction of methane in air), a flash fire occurs. The flash fire may then burn back to the LNG pool, in which case a pool fire ensues.