Assessing failure probability of coastal structures based on probabilistic representation of sea conditions at the structures’ location

Abstract

In the present paper a thorough probabilistic methodology is presented, aiming at estimating the reliability of coastal structures, such as rubble mound breakwaters during their lifetime, based on the probabilistic representation of load environmental and resistance parameters. One of the innovative points and main objectives of this study is the estimation of the failure probability of a coastal structure based on the long-term wave climate at the structure's location, usually met in intermediate waters, using wave observations or measurements in deeper waters. This task is accomplished by applying a wave propagation statistical model in order that the joint probability density function of all random load parameters be estimated at the structure's location. Moreover, a relation between an event-based extreme value analysis and an analysis on sea-state conditions within storm events is derived in order that both of these two approaches could be compared estimating the same kind of failure probability; an unconditional one. The latter is properly defined here as the percentage of the structure's Lifetime that the structure will be in a failure situation. This unconditional failure probability provides direct information on the time period that the structure will be in a situation of failure (since it considers the total range of loadings and total lifetime), and thus can be incorporated more efficiently in an integrated risk analysis with consideration of social and economic costs. Besides, another specific issue of scientific originality could be considered the investigation on the proper time step denoting the sea state in the sea state analysis applied. In this manner, the actual history and shape of each storm event is taken into consideration. Furthermore, it is shown that these two approaches could be incorporated in the design of a coastal structure. Moreover, two different fully probabilistic methods, Direct Integration Method and Monte Carlo Method, were applied (and compared) by using a combination of variables with zero and non-zero hazard rate, referred here as a combined time-invariant and time-variant analysis. Finally, the effect of considering additional and different types of parameters as random variables on the assessed failure probability of the structure has been investigated. The aforementioned methodology has been applied to a sample of wave and sea level data at the structure's location generated for this purpose. The original wave data were derived from measurements in deeper waters than the structure's location, covering a period of 8 years, obtained from an oceanographic buoy, located in the western Mediterranean off Malaga, Spain. Sea level data were also obtained from a tide gauge, located in Malaga's harbor. Finally, it is shown that the methodology derived from this study could be incorporated into a coastal structure's design process to meet specific safety requirements.