Use of statistical methods and imperfection databank to determine the buckling strength for imperfect cylinders
Master thesis
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https://hdl.handle.net/11250/3032572Utgivelsesdato
2022Metadata
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Sammendrag
Buckling for thin metal shells are not an easy failure mode to design structures for. The large variety ofgeometries, material properties, and uncertainties has med standards and best practice for designingthin metal shells very conservative with their safety factor. The design is usually based on the theoreticalstrength for buckling, and then, a knockdown factor, is multiplied with the strength in order to achievea strong enough structure. For many geometries, this knockdown factor reaches a value of 0.2, thereforereducing the design strength to one fifth of the theoretical strength. Equinor is planning to create asubsea shuttle tanker (SST) in order to transport liquids to and from subsea wells. The outer hull ofthe SST will not be subjected to large external pressures dew to cargo tanks inside and a flooded inside.However, the shell will be very large (100 meters long and 17 meters in diameter), and must be designto withstand some pressure. This thesis will not look at the effect of stiffeners, as the main goal is toestablish a method to design a safe structure that is lighter and cheaper. Using existing studies done onsurface imperfections and physical tests (a study done on small nickel cylinders in the 1970’s known asthe A-shells), this thesis has put together both Final Element Analysis (FEA) and statistical analysis inorder to achieve a knockdown factor that is larger than what is standard and best practice today. Manyrealisations of cylinders with different imperfections are simulated and then the result is fitted to aprobability distribution. The best distribution found was the distribution in the Estimation of ExtremeValues by the Average Conditional Exceedance Rate (ACER) Method. With this method, the knockdownfactor could be increased from 0.238 to 0.823 for the A7-shell and from 0.497 to 0.691 for the SST.