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CIVIL AND STRUCTURAL ENGINEERING COMPUTING: 2001
Edited by: B.H.V. Topping
Pressure Vessels under External Pressure
Department of Mechanical Engineering, University of Portsmouth, United Kingdom
C.T.F. Ross, "Pressure Vessels under External Pressure", in B.H.V. Topping, (Editor), "Civil and Structural Engineering Computing: 2001", Saxe-Coburg Publications, Stirlingshire, UK, Chapter 14, pp 357-386, 2001. doi:10.4203/csets.5.14
Keywords: submarines, buckling, vibration, shells, cylinders, toroids.
This paper describes how to design submarine structures so that they can descend to great depths. Now some three-quarters of the Earth's surface is covered by water, much of it unexplored. Indeed the surface area of the Earth covered by water is over ten times larger than the surface area of the moon! It is believed that the sea is as deep as 11.52km (7.16 miles); a depth that is some 30% larger than the height of Mount Everest!
The paper commences with a brief description of the history of submarines. The paper then describes the different modes of failures of submarine hulls and how these modes of failure can be combated.
The present author believes that for man's survival into the 21st Century, it will be necessary for man to exploit the oceans for fossil fuels, minerals, precious metals and other resources.
Additionally, the great depths of the oceans should prove of considerable military value; even more than the surface of the moon. For example, in the 'star wars' venture, it is likely that large missile launching platforms will appear on the oceans' bottoms. These undersea platforms may be preferred to surface-launchers as neither radar nor heat seeking missiles work underwater. Additionally, undersea platforms can crawl along the bottom of the ocean, thus changing their positions without visual observation. As there is no light at these great depths, spy satellites will not be of any use in detecting their positions.
With reference to commercial applications, Dr. Gerald Dickens of the University of Michigan has discovered large beds of methane, in the form of hydrates, lying on the oceans' bottoms at these great depths. He has found that off the east coast of the USA, there are 30 such beds and that in one bed alone, there is enough methane to satisfy the USA's need for 105 years, based on their rate of gas consumption in 1996. Such pressure vessels, whether for commercial or military use, will have to be designed with special materials and novel shapes to dive to these great depths.
The paper shows that as a pressure vessel dives deeper into the oceans, it is necessary to increase the wall thickness of the vessel to resist the increasing water pressure. If the pressure vessel is made from a conventional material such as high tensile steel, the wall thickness becomes so large that the vessel sinks, as it has no reserve buoyancy. The paper shows other structural materials can be used which have superior strength : weight ratios than that of high-tensile steel.
The paper also shows that if composites are used, it may be easier to manufacture them in the form of corrugated vessels in preference to conventional ring-stiffened ones. Such a vessel was invented by Ross  in 1987. The paper shows other novel pressure hulls, including a tube-stiffened vessel, invented by Ross and Harris [2,3]. In this case the tubes can be wrapped circumferentially around the internal surface of the axisymmetric vessel, so that it stiffens the vessel. If these tubes are put into tension initially, by internally pressurising them, they can further resist the effects of external water pressure on the pressure hull. If the tubes are filled with oxygen and hydrogen, then these gases can be used to power a fuel cell, so that an almost silent means of propulsion can be used.
The paper also considers the dome ends of a submarine pressure hull. The paper shows that by inverting the dome ends , so that they are concave to the effects of pressure, they are structurally superior to conventional dome ends. These new dome ends are not only thinner than conventional dome ends, but they need not be constructed as precisely as conventional domes. These two facts means that the new dome cups can be manufactured more cheaply than the conventional dome caps. Ring-stiffened prolate domes [3,5] are also considered; these domes utilise the internal space of a submarine more efficiently.
The paper also considers structural vibrations under external water pressure. The paper shows that as the external water pressure is increased, the resonant frequencies decrease with increasing pressure and that as the static buckling pressure is approached, the circumferential modes of vibration become very similar to the buckling modes of vibration. This phenomenon leads to the alarming observation that a form of dynamic buckling can occur at a pressure much lower than the value of the static buckling pressure.
The paper shows that if composites are used as the pressure hulls of submarines, the hulls can become more difficult to detect via sonar. The paper provides both experimental and computational results to justify the author's theories. Both conclusions and references are given.
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