Reliability analysis of Submerged Floating Tunnels

Abstract

A submerged floating tunnel (SFT) can be a promising solution for crossing a deep or wide waterway. This tunnel concept will consist of an immersed tube, either attached with anchor cables to the seabed or attached to pontoons floating on the water surface. The reliability of the tether-stabilized SFT is assessed in this research. A suitable target reliability is determined in order to design a full probabilistic SFT. Subsequently, a calibration of partial factors from Eurocode is performed. The robustness of the structure is also analyzed and improvements are suggested. Important failure mechanisms are defined as yielding and slackening of the tethers, longitudinal failure and transverse shear failure of the tube. A first-order reliability method (FORM) and a Monte Carlo simulation (MC) are performed for the limit state functions of these mechanisms. Design parameters are determined so that a target reliability index of 3.8 is met, because of consistency with Eurocode. Consequently, the design points from FORM are used to calculate partial factors for different loading types. The calculated factors and the general partial factors from Eurocode are compared. Slackening of the tethers proved to be the governing failure mechanism in this analysis. The resistance against slackening depends on the force equilibrium, whereas the resistance of the other mechanisms depends on structural strength. The influence factors from the FORM analysis indicated that permanent loading parameters were dominant, i.e.\ concrete density, water density and tube diameter. It was found that for the strength (STR) mechanisms, the factors from Eurocode result in an overly safe design of the SFT. The calculated partial factors for unfavorable permanent load and variable load are significantly lower than the corresponding general factors from Eurocode. For the equilibrium (EQU) case, Eurocode is not safe to be applied. The general partial factor for the unfavorable permanent loading is insufficient. The robustness of the structure is assessed by considering important scenarios. Excessive leakage has large consequences and will result in global structural failure. However, it has a low probability of occurrence. Mitigating measures are available to prevent failure due to leakage. Furthermore, an SFT will be constructed at a specific location. Wave conditions and geolocation need to be taken into account to reach an optimal design. At a depth of 30 meters, the impact of waves becomes insignificant. Lastly, failure of a single tether should not result in failure of adjacent tethers (i.e. progressive failure). A redundant system can be created by installing more or higher quality tethers. Consequently, when all four tethers of one element fail at the same time, this does not result in longitudinal failure. Overall, it was demonstrated that the reliability requirements of the SFT can be met in the design. Moreover, the design can be optimized by a full probabilistic calibration of partial factors.