Time-dependent risk assessment of aging ships accounting for general / pit corrosion, fatigue cracking and local denting damage. Discussion

The present paper is a summary of recent research and developments in the areas related to time-dependent risk assessment of aging ship structures, jointly undertaken by the Pusan National University and the American Bureau of Shipping. The age-related structural degradation (i.e., corrosion and fatigue crack) and mechanical damage (i.e., local dent) are considered as types of damage. The mathematical models for predicting corrosion and fatigue cracking damage are developed as a function of ship age. Two sets of time-dependent corrosion wastage models for 34 longitudinal member groups of tankers / FPSOs, and for 23 longitudinal member groups of bulk carriers are developed by statistical analysis of corrosion measurement data. While initial crack characteristics can be analyzed by the fatigue analysis using the S-N curve approach or detected by survey, crack growth behavior can be assessed by the fracture mechanics approach. The crack size prediction model as a function of ship age is presented in the present paper. Extensive theoretical, numerical and experimental studies are in this work carried out to investigate the ultimate strength reduction characteristics of ship panels that are wasted due to general or pit corrosion, or have fatigue cracking and local denting damage. The prime concern is with derivation of relevant design formulations for the ultimate strength of structural members with pitting, cracking or local denting damage. Time-dependent variations of ultimate longitudinal strength reliability/risk of a bulk carrier, a double hull tanker and a ship-type FPSO under vertical bending moments are studied accounting for the effects of general / pit corrosion, fatigue cracking and local denting damage, in both deterministic and probabilistic form. It is aimed at developing more rational repair and maintenance scheme that explicitly takes into account the ultimate longitudinal strength. It is concluded that the insights and methodologies developed from the present study will be very useful for assessing time-dependent risk of aging ships and also establishing damage tolerant design procedures for new ships.

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