Pitting and uniform corrosion effects on ultimate strength of a bulk carrier

Abstract

In general, ship structural integrity assessments consist of prescribed procedures given by the rules of classification societies, and if necessary, direct calculations. Direct calculations mostly include longitudinal strength analyses and buckling calculations for various service loadings and structural members. Resulting stress is compared to the allowable stress, which is also rule-prescribed and acts as a share of the yield stress of the material. This means that the structure is evaluated upon its elastic response. Moreover, ship structural assessments consider as-built scantlings, not diminished by ageing. Generally, extreme or once-in-a-lifetime loadings are neglected. They can include overloading scenarios that lead to the ship structural collapse or the loss of the ship’s carrying capacity, i.e., ultimate strength. Ultimate strength of the ship is represented by the maximum (ultimate) bending moment the structure can withstand. Its evaluation is important to determine the ship’s structural safety level with respect to its collapse. Therefore, this paper delivers the ultimate strength assessment for a typical bulk carrier having 180 m in length. For this purpose, an incremental-iterative progressivecollapse analysis (PCA) is used. In the first stage, ultimate strength of a bulk carrier is determined using as-built scantlings. In the next, ageing effects are considered within PCA by modeling scenarios that include both pitting and uniform corrosion. Pitting corrosion is defined by pitting intensity degree (DOP) and corrosion intensity degree (DOC), whilst the uniform corrosion is defined by thickness reduction. Results show that corrosion effects can significantly diminish the ultimate strength of the ship. This means that, in the design phase, structural assessments cannot be based just on as-built scantlings, but also might include ageing effects, as ships are designed to be reliable on the long-term.