Net Scantlings and Structural Reliability

Vessel structures deteriorate over time. These ageing effects mean that ship strength and structural reliability diminish over time. In the past this was implicitly taken into account in the rules of classification societies. Today’s assessment of ship structures is based upon the use of first principles. This presupposes a realistic description of the deteriorated structure as well. Present rules mainly do this by reducing scantlings of plates and stiffeners by an assumed amount of uniformly distributed corrosion. Some experimental, phenomenological and analytic results are presented that illustrate failure behaviour of local and global ship structures quite different from that using net scanlings only. Typical aspects that play a role are non-uniform degradation, such as groove corrosion. Energy absorption may be the governing instead of ultimate strength as is normally assumed. It is shown that assessment of the reliability of aged ship structures requires additional information and analysis different from that normally performed today.