Offshore LNG Transfer

ABSTRACT An offshore transfer system for low-temperature liquids, which allows tankers to be loaded with LNG even under extreme weather conditions, has been developed. The essential part of this new installation is the insulated hose, which allows the transfer of liquid natural gas for example. The stresses, thermodynamic and hydrodynamic effects on the hose system were comprehensively examined. Furthermore, the movement evelope of a ship connected by a hose to an offshore loading system, was determined. Also described are the coupling and disconnecting procedures for the hoses during normal operations and under emergency conditions. The various possibilities of line connections between a loading buoy and an LNG platform by a pipe bridge or underwater lines are demonstrated. INTRODUCTION The constant shortage of energy and rising energy prices are encouraging the development of new energy resources which so far have not been utilized. Lately studies have been made as to the offshore liquefaction of natural gas. Investigations in this field show that an offshore liquefier is an economic alternative to' gas pipeline connections to shore in cases of longer distances between the coast and the gas field and in cases of marginal gas and oil fields with specific problems of the non-utilizable associated gas. The offshore liquefier has been developed to purify and liquefy natural gas in a process plant mounted on a platform. The liquefied gas at -162°C (111 K, -324 °F), i.e. about 1/600 of the volume of the former gaseous aggregate state, can be stored under atmospheric pressure on the platform or in separate offshore storage tanks until discharging. The transfer of liquids from offshore storage tanks to tankers is nothing new. Crude oil is pumped into tankers at many offshore installations. For this procedure hoses are used which convey the crude oil. In most cases these hoses connect the loading buoy with the tanker. The hoses can either float on the water surface or, as is also practiced, be suspended between the boom of a mooring buoy and the tanker bow. These hoses allow unrestricted moving of the tanker at the buoy during the loading procedure even under severe weather conditions. Adoption of this proven system for the transfer of low-temperature liquids from a buoy to a tanker is recommended. There is, however, the problem that the existing transfer hoses cannot be used for the transfer of these low-temperature media. Therefore, besides the special offshore LNG loading system introduced by this paper, many different offshore LNG loading systems (see Fig. 1) have been developed.(1) (2) (3). One system for example uses the well-known swivel joints of ductil steel which are also used in onshore plants for the LNG loading. Another system consists of a combination of inflexible pipes with a vertical slewable loading arm and special hoses for the transport of the low-temperature media. Hoses, pipes and swivel joints used for the transfer of the liquefied gas must be of a material which avoids embrittlement but still provides sufficient strength.