Influence of structural deterioration over time on the optimal time interval for inspection and maintenance of structures

The influence of the time variation of the structural demand and/or of the structural capacity on the optimal time interval (based on a cost-benefit analysis) for inspection and maintenance of offshore structures is analyzed. Reliability is expressed in terms of the expected number of failures over a time interval by means of closed-form mathematical expressions which consider the structural degradation. The mathematical expressions are incorporated into the cost optimization formulation. Three scenarios are considered: (1) structural demand (for a given intensity) varies in time, while structural capacity remains constant, (2) structural capacity deteriorates over a time interval, while structural demand remains constant, (3) both structural capacity and structural demand vary simultaneously over time. The optimal time interval for inspection and maintenance corresponds to the lowest cost of inspection, repair and failure. The cost optimization is applied to an offshore jacket platform. The damage condition is given by the fatigue crack size at critical joints. It is shown that in order to estimate the optimal time interval for inspection and maintenance of the structure, it is necessary to take into account the variation in time of both its structural capacity and its structural demand (case 3); if one of them were ignored, the optimal time interval could be overestimated.