Keywords: ring-stiffened conical shells, nonlinear, general instability, external hydrostatic pressure, submarines, ANSYS.

The paper presents the experimental results for fifteen ring-stiffened circular steel conical shells, which failed by non-linear general instability. The results of these investigations were compared with various theoretical analyses, including an ANSYS [1] eigen buckling analysis and another ANSYS analysis; which involved a step-by-step method until collapse; where both material & geometrical nonlinearity were considered; for the first time. The investigation also involved an analysis using BS5500 (PD 5500) [2], together with the method of Ross [3]. The ANSYS eigen buckling analysis tended to overestimate the predicted buckling pressures; whereas the ANSYS nonlinear results compared favourably with the experimental results. The PD5500 analysis was very time consuming and tended to grossly underestimate the experimental buckling pressures and in some cases, overestimate them. In contrast to PD5500 and ANSYS, the design charts of Ross were the easiest of all these methods to use and generally only slightly underestimated the experimental collapse pressures. The ANSYS analyses, however, gave some excellent graphical displays.

It is currently estimated that by the year 2030 the world's cumulative demand of primary energy will increase by over 40%, based on 2007 usage figures [4]. Moreover, despite the promotion of alternative fuels, fossil fuels will continue to account for more than three quarters of fuel consumption growth. At present, the exploration of both land and shallow water is advanced, with oil and natural gas estimated to be reaching peak output. Therefore, the search for additional oil and gas supplies is crucial for ensuring worldwide energy security and economic stability. It is for this reason that the world's new oil and gas reserves are moving from shallow waters into deeper waters. Deep water exploration has become the key focus of the energy industries strategic outlook, with proven reserves of approximately 10,000 billion tonnes of methane hydrates [5], together with very large reserves of oil. However, oil and gas production in the deep sea face greater challenges arising from the increasing depths and higher pressures. This results in inseparable high costs and advanced technical requirements, with the relative cost of drilling land, shallow water and deep water, being approximately 1:10:100. It is therefore imperative that the technological advancements required to meet the challenges associated with such depths are not only practical and cost effective, but capable and safe. It is for this reason, that the current dated methods of design and testing require modernisation to aid the development of future technologies without the compromise of accuracy, reliability and safety.

Ultimately it is true to say that most of the resources under the sea remain unknown to mankind. With more humans to having laid a foot on the moon than to have explored the oceans' bottoms, there are likely to be limitless opportunities waiting to be found.

1

ANSYS Inc., Version 11, Canonsburg, PA., USA, 2008/2009.

2

BS5500 (PD5500), "Specification for Unfired Fusion Welded Pressure Vessels", 2009.

3

C.T.F. Ross, "Pressure Vessels: External Pressure Technology", Horwood Publishing Ltd, Chichester, UK, 2001.

4

World Energy Outlook Report, International Energy Agency, 2009.

5

G.R. Dickens, C.K. Paull, P. Wallace, the ODP LEG 164 Scientific Party, "Direct Measurement of In-situ Quantities in a Large Gas-Hydrate Reservoir", Nature, 385, 30th January, 1997. doi:10.1038/385426a0

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