Statistical theory is developed and used to estimate the risk of total ship loss arising from wave bending in a typical naval frigate and in a large tanker, each with a 20 year world-ranging mission profile. The extreme values and distributions of vertical moments are derived using linear strip theory, Pierson-Moscowitz wave spectra, and order statistics. Recently derived strength analyses for bi-axially compressed and pressure loaded welded steel grillages are used in conjunction with a slightly modified simple beam theory to estimate mean hull strengths for all ductile modes of hull rupture. Strength variances are obtained using conventional error analysis and material fabrication and dimensional properties. Systematic and random errors are considered. Load and strength distributions are combined statistically to estimate risks of failure for the various modes. The total life risks of ship loss are then found for the frigate to be in the range 10 to the minus 3 power to 10 to the minus 5 power, depending upon superstructure effectiveness, and 10 to the minus 8 power for the tanker. These compare optimistically with 10 to the minus 3 power estimated from ship casualty data. The much lower safety for the frigate arises because the conventional static wave balance is appreciably optimistic for small ships, and also the structural efficiency in compression is low. The study demonstrates that risk of failure is a viable and preferred approach to safety, though the results should be used comparatively. Statistically derived characteristic values for loads and strengths seem attractive for use in design, and are certainly preferred to conventional predictions. Calculations are required for a wide range of ships, but it would seem there is scope for appreciable material economy providing steel is selected with care to avoid spontaneous fracture.

  • Supplemental Notes:
    • Presented at North East Coast Institution of Engineers and Shipbuilders, Newcastle upon Tyne, England.
  • Corporate Authors:

    Royal Institution of Naval Architects

    10 Upper Belgrave Street
    ,   United States 
  • Authors:
    • Mansour, A E
  • Publication Date: 1972-11

Media Info

  • Features: References;
  • Pagination: 21 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00041365
  • Record Type: Publication
  • Source Agency: Ship Structure Committee
  • Files: TRIS
  • Created Date: Mar 2 1973 12:00AM