Computer aided evaluation of the reliability of a breakwater design

Abstract

The risk~analysis offers a method to determine the probability that an important function of a structure is no longer fulfilled (failure or malfunctioning). This probability should be selected having in mind the minimization of the generalized cost of the structure and the maximization of its utility (Minimax). Comparison of the contribution of failure mechanisms to the total probability of failure of a design leads the way to rational improvement of a design and to further research directed towards the improvement of weak elements in the design. In a complex structure like a rubble mound breakwater with a crest wall, many events (accidents) can be distinguished which may lead directly or indirectly to failure of the structure. Hydraulic and geotechnical stability as well as material failure by fracture or loss of position, must be considered. Failure may start in the toe, near the berm, on the slope, at the crest or on the inside slope, and it may progress into total failure with time (accident sequence). Failure of the structure should be carefully defined. The choice of the definition will predispose the meaning of the calculated probability of failure. The more precise the definition the clearer the physical meaning and the better the insight presented through the failure probabilities. A clear and detailed definition can be achieved in a fault tree, which in a logical sequence states the possible causes leading to eventual failure. The CIAO project group limited the scope of work in this report to the following activities: * reviewing the reliability theory and risk analysis * indicating the simulation models available for the various failure mechanisms performing a case~study for a rubble mound breakwater, applying the risk~analysis to relevant failure mechanisms and calculating the probabilities of failure using a fault tree approach * evaluating results of the case~study. The underlying theory of the risk~analysis is only presented in outline. Much work is presently done to further develop applications [7,9,10], In essence it depends on the reader to which degree of sophistication he wants to formulate failure. The project group elaborated an existing design following the probability theory. It would ofcourse have been better to evaluate design alternatives, but this was not possible within the time available. The case study is the substantial part of the work of the project group. Again, it is only an illustration of how the risk~analysis can be applied, leaving the reader free to simplify or extend the method. The geometry of the rubble mound breakwater used in the case study and its composition are shown in Figure 1.2.1. Actually, it is an existing breakwater, the available information of which has been F.C. de Weger BV, missing data necessary for the study have been fancied as reasonably as possible. Therefore, the outcome of the study cannot be conceived in absolute sense reflecting the reliability of the existing breakwater