Concrete For Marine Transport And Storage Of Cryogenic Liquids
Authors: Marshall Alex L. 
Published: December 2007

There is an important international trade in liquefied gases, particularly petroleum and natural gas (LPG and LNG) and exploitation of off-shore resources is increasing. The low temperatures of the products require special measures for their transport and storage. Prestressed concrete has been used successfully for land storage and its characteristics make it potentially valuable for marine applications. Several schemes for concrete gas carriers and floating storage facilities have been proposed. However, the future is complicated, not just by technical factors but by reluctance to innovate among owners and operators and political, environmental, safety and legislative considerations.

Introduction

Cryogenics can often be regarded as an esoteric study but if it can be used to solve challenging problems in the demanding environment of the sea by employing one of our most familiar, if not prosaic materials, then we have the essence of good engineering.

Storage and transport of cryogenic liquids is by no means new, of course - liquid nitrogen and oxygen tankers are now a familiar sight on our roads. There are many substances with a boiling point lower than normal ambient temperature which point lower than normal ambient temperature which results in their being stored either under pressure or at atmospheric pressure and a temperature below their boiling point (or at above-atmospheric pressure at a temperature between boiling point pressure at a temperature between boiling point and ambient). Obviously, such substances vary in commercial significance but although others are critical to different processes, the two currently most important economically are probably liquefied petroleum gas (LPG) and liquefied natural gas (LNG) petroleum gas (LPG) and liquefied natural gas (LNG)In the words of an OECD report, Gas is the cleanest and most flexible natural fuel but it is more difficult to store or to transport overseas than liquid and solid fuels...... The usual method today is to transport it by pipeline or in liquefied form, as LNG, if it has to be transported overseas ....(where gas occurs off-shore) an economic solution would be to liquefy it on platforms and move it by LNG tankers. The advantages of such bulk storage and transport are that the liquefied gases are light (the specific gravities of methane, propane and butane lie between 0.40 and 0.60) and propane and butane lie between 0.40 and 0.60) and liquefaction by refrigeration reduces their volume dramatically (methane by a factor of about 600 and propane by about 300). The major disadvantages propane by about 300). The major disadvantages result from the low temperatures at which they must Be carried (methane -165 degrees C).

Before discussing the technical aspects of the subject a brief word about the market for gas carriers and structures might be appropriate. Without going into the complexity of market predictions it does appear that there will be growth predictions it does appear that there will be growth in demand for gas. What is in doubt is the rate of growth which is governed by political and environmental factors as much as by energy needs. World shipping problems and changes in, for example, IMCO recommendations (formulated for steel vessels, we should remember) also complicate the picture. It also seems reasonable to expect increased development of off-shore processing and storage capabilities, particularly using buoyant structures, as we move into deeper waters or exploit more marginal resources. Consequently it is advisable to consider seriously the viability of employing a material in common use, particularly if it might well have advantages over other materials.

A ship is a particular type of marine structure. Whether considering marine transport or storage, therefore, it may be useful to look briefly at 'conventional' gas carrier construction before going on to discuss relevant experience with concrete and possible developments offshore. possible developments offshore.