Risk Analysis of the Vertical Offshore Breakwater at Gela, Italy

A reliability method (FOSM) has been used to assess the probability of sliding failure of the 33 years-old island caisson breakwater sheltering an oil terminal at Gela (Italy). The vertically composite structure is placed in just 13 m depth upon a weak silty seabed. It has experienced settlements of about 1 m, toe scour around 2 m and degradation of the slender reinforced superstructure, especially damaged after a severe storm on 24 November 1991. On the basis of new detailed wave climate studies and field investigations a modern probabilistic analysis has been performed to verify the actual risk of failure, particularly against sliding over the rubble mound foundation due to wave loads. A new formulation of the failure function as been derived introducing new expressions for the pressure coefficients in the Goda formula. A detailed sensitivity analysis of the failure probability with the caisson geometry has also been pewrformed.