This paper presents an approach for calculating the time-variant reliability of floating structures under corrosion deterioration. Based on in situ corrosion measurement data in the South China Sea, a corrosion prediction model is established. Ultimate strength under discrete stochastic extreme load events for the large floating structures is chosen as the time-dependent limit state. An effective method for time-variant reliability assessment subjected to corrosion deterioration is established based on conditional probability theory. The estimation of instantaneous failure probability for a typical floating structure is calculated by the first-order reliability method. Results show that severe corrosion deterioration degrades ultimate strength and reduces time-variant reliability over time.
Time-variant reliability attracts attention from industry when it comes with corrosion and fatigue in offshore floating structures. With the long-term serviceability of large floating structures, general corrosion may occur in stiffened plates and cause degradation of longitudinal or local strength. Therefore, reliability assessment of floating structures with corrosion deterioration is essential during the long-term serviceability. The progressive strength degradation has a detrimental impact on long-term safety and reliability of floating marine structures (Abbas and Shafiee, 2020). Some literature identifies corrosion as one of main failure modes, which can cause loss of reliability (Guo et al., 2008), especially for very large floating structures (VLFS), which consist of several modules, located far away from harbors. Because of the distances and sea environment involved, VLFS cannot be maintained on schedule, and the corrosion degradation needs to be considered in evaluating their service lifetime.
In general, corrosion can have several subcategories: general, pitting, crevice, and galvanic. General and pitting corrosion commonly occur in marine structures. The corrosion rate measurement is used as a method to calculate corrosion depth when the corrosion is uniform. The corrosion mainly leads to thickness reduction of plates and results in reduction of the hull girder section modulus of floating structures (Guedes Soares and Garbatov, 1999).
