What happens when LNG is released underwater? Does a LNG pool form on the water surface? What are the attributes of the vapor released?

Transcription

1 What happens when LNG is released underwater? Does a LNG pool form on the water surface? What are the attributes of the vapor released? by Paper Presented at 2009 International Symposium Mary Kay O Connor Center October 27-28, 2009 College Station, Texas 1

2 Potential underwater LNG release Releases from scenarios LNG vessels caused by accidents or intentional acts Underwater pipelines Water submerged LNG powered road vehicles (accidents on bridges) 2

3 LNG vessel release classifications 3

4 LNG vessel release classifications (cont d) 4

5 LNG Safety Studies Most of the efforts to date (both field tests and modeling) have been on LNG behavior on water Pool formation is assumed in all safety assessments LNG-water interactions are never considered other than water as a heat source for LNG pool evaporation This may lead to incomplete or erroneous conclusions on type and extent of hazards to the public 5

6 Current knowledge of LNG-water interaction Very scant data in the literature on LNGwater interaction US Bureau of Mines field test in is the only test Liquid cryogen-water interactions have been studied (small scale) by other industries 6

7 A field test by US Bureau of Mines 7

8 Water release into a molten metal Source: Sibamoto, et al., [10], Copyright 2002, Nuclear Technology. Reproduced with permission from the American Nuclear Society. Cavity formation, water accumulation, rapid evaporation and liquid shattering in a water jet plunging into a molten metal 8

9 LNG underwater release model Model developed as a part of submission to FERC (Available on FERC website under Docket # CP ) [ then Click on Appendix Q, the report is on pages 43 to 97] Pressurized LNG release from a LNG-pipeline, submerged at different depths of water is modeled Considers different release rates, depths of release, size (diameter of jet), temperature of water, etc. 9

10 Maximum droplet diameter (d p ) 10

11 Findings from literature Instantaneous release of 7.7 kg of LNG at 4.6 m depth resulted in buoyant gas release at the water surface. No pool formation LN2 jets released at velocities 10 to 30 m/s in small scale tests showed complete evaporation in distances of centimeter to one meter and N2 gas emanating from water surface at water temperature Tests with water injection into molten metal indicate complete shattering of jet into droplets in 10 to 15 jet diameters. Cold vapor bubbles of 1 cm or less in diameter heat up to ambient liquid temperature within 0.3 to 1 m distance (10 to 15 jet nozzle diameter) rising at respective terminal velocities. 11

12 Underwater LNG-jet release model Assumptions Jet shattering (to liquid droplets) occurs within 10 jet diameters Liquid droplets of all sizes smaller than a critical value are formed. Liquid droplets evaporate ( film boiling ) forming gas bubbles of sizes less than a critical size. Gas bubbles rise at their laminar terminal velocities and are heated by water with an overall heat transfer coefficient of 100 W/m 2 K. 12

13 Modeling LNG release underwater from a pipeline Schematic representation of evaporation of liquid droplets, formation of vapor bubbles, heat transfer from water to bubbles and bubble rise in the water column 13

14 LNG underwater release model - Results Yellow bars indicate the distance of rise in which vapor bubble temperature = 99.5% of water temperature Rise distances in the water column in which the temperature of the largest LNG vapor bubble formed is equal to 99.5 % of the water body temperature, for different depths and diameters of LNG jet release 14

15 Model conclusions LNG release underwater results in its rapid shattering into small droplets of liquid in distances that are of order of magnitude tens of centimeters to a meter. The size of the largest liquid droplet formed due to shattering is in the 1 mm to 2 mm range. The complete evaporation of the liquid droplets occurs in vertical water column distance of the order 1 m to 2 m. The heating of the largest of the (cold) vapor bubbles to near water temperature occurs in vertical distances in water of the order of 1 m. to 1.5 m. 15

16 Model conclusions (cont d) Most underwater LNG releases will NOT result in the formation of a floating LNG pool on the water surface. The temperature of vapor emanating from the water surface, for LNG releases at depths greater than 3 m, will be close to that of water. The vapor released from water surface will be natural gas at water temperature and, hence, will be buoyant. Ignition of the gas released at water surface may result in either a limited size gas fire (such as from a warm natural gas pipeline release from underwater) or a fire ball, depending upon rate and quantity of gas release. The characteristics of these fires will be different from that of a LNG liquid pool fire on water surface. The hazard distances may, potentially, be smaller than from a LNG pool fire. 16

17 Thank you for your attention and interest Questions? 17