HYDRATE CARRIER OFFLOADING USING SLURRY-Lossing av hydratbåter ved bruk av slurry Ulf Vegard Jensen May 1996 Abstract The Norwegian University of Science and Technology and Aker Engineering are cooperating on a project concerning gas transportation in hydrate form. This project is based on large scale transportation of natural gas over long distances as frozen hydrate in large carriers. A carrier has already been designed by Aker, but the offloading procedures proposed are based on on-board melting of the hydrate or the use of mechanical offloading equipment. The mechanical alternative is too expensive and complex, and on-board melting makes it impossible to store the hydrate on shore and the gas has to be compressed all the way from atmospheric to delivery pressure. Hydrate carrier offloading using a slurry of hydrate and condensate is a new alternative that makes high pressure melting and on shore storage of the hydrate possible. If slurry is used the offloading of the carrier can be done through a buoy placed off the coast, an advantage under special circumstances. This report s about hydrate carrier offloading using slurry, the procedure, the equipment and the economical aspects. The idea is to flood the porous hydrate cargo with cold condensate (-10degC), and then pump the hydrate-condensate mixture out of the carrier using ejectors and slurry pumps. Condensate is used because it does not destroy the hydrate, and it makes it possible to bring both the hydrate and the condensate produced offshore to land in the same carrier. It has to be cold to ensure that the hydrate remains stable. This report uses different particle sizes and porosities to find the permeability of the hydrate cargo. The Darcy equation is then used with the different permeability values to find the pressures necessary to flood a tank with condensate in 4 hours, which is the available offloading time. The results show that flooding is possible for hydrate permeabilities greater than 1,5 D with a maximum pressure in the bottom of the tank of approximately 10 bar. Equipment necessary in the offloading process are ejectors, pumps, hydrate -condensate separators and a condensate storage tank. The equipment proposed is simple and the technology is known. The economical study also shows that the slurry concept has a price that is less than half of the mechanical concept. The conclusion is that offloading a hydrate carrier using slurry is possible to do, it is economically favourable to other alternatives and it serves the needs of the process. Introduction Natural gas playes an important role as an energy source. Several countries benefit from the production and the export of natural gas. The export is mainly done by using pipelines which connects reservoir and consumer in an unbroken chain. Soon, the fields to be developed will be smaller, and the distances to the consumers will be larger. The result of this is that pipelines not can be used due to the high costs involved. One alternative that exists is to transport the gas as LNG (Liquified Natural Gas), but this is costly and the risk involved is high. So other alternatives are investigated. The Norwegian University of Science and Technology has developed a new method. This method is based on transporting the gas as hydrate in tankers. The method is called NGH (Natural Gas Hydrate), and is according to Børrehaug and Gudmundsson (1996) cheaper than LNG. Another advantage is that the gas is transported under safe conditions. Accidents will not cause great explosions, and the enviromental risks are small. This method may make it financially possible to develop fields situated in remote areas, such as the Snøhvit field in the Barents Sea. Transportation of the hydrate must be done in as simple carriers as possible. Large volume and little equipment is ideal. A hydrate carrier has already been designed. The design is based on on-board melting, a solution that makes it impossible to store the hydrate, and the gas has to be compressed all the way from atmospheric to delivery pressure. A more cost effective and simple method was wanted. Professor Gudmundsson thought of another solution based on pumping the hydrate out mixed with condensate. In other words, as a slurry. This solution will make it possible to store the hydrate on shore, the equipment will not be complex and the condensate from the field can be shipped ashore together with the hydrate. This project work considers the slurry offloading system on a future hydrate carrier. The report will show that it is possible to make a pumpable mixture by flooding and dilution of the hydrate. It will also be shown that it is possible to pump this mixture out of the carrier as fast as necessary. Equipment and design suggestions together with an approximate economic evaluation of the slurry concept will be made. Conclusions 1. This report has demonstrated that hydrate carrier offloding using slurry is possible and has practical advantages compared to previous offloading procedures. The hydrate can be stored on shore and high pressure melting is possible. 2. Flooding calculations show that it is possible to flood the tank with a given condensate rate and acceptable pressures if the permeability is greater than 1,5~10^-12 m^2 (1,5 D}. 3. The equipment used is simple with few moving parts. It is also possible to use commercially available equipment to handle the offloading. This ensures reliability and small development costs if the slurry concept is used. 4. Buoy offloading is an alternative to a large scale port under special conditions, especially if the port costs are high. 5. A rough cost estimate show a cost of 83 million NOK for the slurry offloading equipment in each carrier. The mechanical offloading equipment would cost approximately 202 million NOK per carrier. This also indicates that the slurry concept is favourable compared to previous offloading procedures. References A. Børrehaug and J.S. Gudmundsson, (1996). Gas Transportation in Hydrate Form. EUROGAS 96, Trondheim, Norway, 7 pages. Yoshiyuki Matsuno and Katsuhiko Shibata, (1970). Research on Salt Slurry Carrier Equipped with Hydraulic Unloading Apparatus. Mitsubishi Heavy Industries, Ltd.,Technical Review, September 1970, page 68 - 80. Dr G. S. C. Wang, (1981). Coal shiploading - the future to 2000. Coal-Trade Transportation and Handling, Volume I, 1981, page 215 - 224. Aker (1996). Natural Gas Hydrates for large Scale, long Distance Gas transportation. A feasibility study. A report for the NGH Project Group. L.P. Dake, (1978). 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