Dynamic Stability of Tunnel Elements During Immersion and Transportation

Abstract

Immersed tunnel elements are prefabricated in the construction yard and then transported to the construction site to be integrated into the tunnel. During the transportation and immersion, the floating tunnel element is connected via cables to the immersion barges/pontoons. In this study, the hydrodynamical behavior of a floating tunnel element during construction has been modeled. The motional characteristics and stability of a floating tunnel element and twin barges during the installation are investigated.This study aimed to investigate, how the stability of the system can be improved to make it possible to immerse a large number of tunnel elements in relatively short span of time in offshore conditions. The hydrodynamical behavior of the system and the related forces in different construction stages are analyzed. The focus of the study was on the determination of the influence of the pontoon configuration on the systems stability, the related forces, and operability. For this study, Fehmrnbelt Fixed Link tunnel project has been taken as a case study. The calculations are performed for two different pontoon configurations, namely: - Catamaran (conventionally applied pontoon for the immersion of tunnel elements) - Semi-submersible (platform used in offshore industry)In the analysis, first, the main dimensions of the two pontoons are determined. The pontoons form the main piece of the immersion equipment. Subsequently, during the transport, the forces and moments on the floating element are evaluated. Two main type hydraulic external forces have been taken into consideration in the model, namely current and wave force. For different positions along transport route, the forces and moments are calculated for finite water depth and different values of flow velocity.From the analysis, it appears that the wave-induced motions of a floating tunnel element are negligible. The relatively small waves are not able to bring the massive tunnel element into motions. The lowest natural periods of the floating tunnel element appears for roll degree of freedom, and it is about 8s. The significant current forces and moments occur during the fitting out. The stability of a floating element during transport is primary determined by the towing velocity.During immersion, the systems stability due to waves and current forces is being analyzed. To assess the stability of the system for different current conditions, the vortex shedding periods are calculated. Then the natural frequencies of the system have been evaluated for different pontoon configurations and tunnel element length.It appears that an immersion system with Catamaran pontoons seems to be less sensitive to vortex shedding period in contrast to the Semi-submersible barge. A Semi-submersible barge can only be used in 80% of current conditions in Fehmarnbelt. A Catamaran pontoon can be applied in 95% of the occurring current conditions.Also, during the immersion, the wave-induced motions of the tunnel element can be ignored, provided that the wavelengths and wave periods are not too large. On the other hand, the relatively light pontoons are sensitive to wave loadings. The motions of the barges are prevented by the element, which leads to significant force fluctuations in suspension cables. In the analysis, the coupling between surge, sway, heave, roll, pitch and yaw degrees of freedom are considered. The calculations are performed for the first order responses that are valid in relatively low wave heights. Hydrodynamic effects caused by the nonlinearities are disregarded in the calculations.The pontoons were considered as a hybrid structure. That means, concerning the horizontal degrees of freedom the pontoon structure is regarded as it is compliant and it behaves like a floating structure. While concerning the vertical degrees of freedom, it is stiff and resembles as a fixed structure, and it is not allowed to float freely. The contribution of the mooring lines to the first order response is considered of minor importance, and it is disregarded. Numerical studies are conducted to compare the dynamical behavior of the Catamaran pontoon with that of the Semi-submersible. The results of this study reveal that:  - The contribution of the first order wave force to the pontoons motions in soft degrees (surge, sway, and yaw) is limited - The motions in stiff degrees of freedom (heave, pitch, and roll) are normative for the immersion operation. When the tunnel element is immersed in wave conditions T>5 s and H > 1 m, then there is a significant danger that one of the suspension cables will break when applying a Catamaran pontoon. If a Semi-submersible barge is used, then the tunnel element can be immersed in wave conditions T < 6.5 s and H < 1.8 m. - Semi-submersible pontoon has larger natural frequencies than Catamaran pontoon. The natural periods of the Semi-submersible barge are approximately a factor 1.4 larger than the natural periods of the Catamaran pontoon. - A Semi-submersible pontoon is more sensitive to the force fluctuation in the suspension cables than the Catamaran pontoon concerning the static stability and floating capacity. Especially the floating capacity became problematic if the force fluctuations become large and therefore the pontoon can be pulled under water. - The heave motions mainly affect the force fluctuations in suspension cables, and they can be considered as normative. - Both barges are sensitive to the increasing wave height and period. However, the effect on Catamaran pontoon is larger. In total, a Semi-submersible barge has favorable operability in the waves and current conditions in Fehmarnbelt. Therefore, if the workability is the primary objective, then it is better to apply a Semi-submersible pontoon. Then in 77% of environmental conditions, the tunnel elements can be immersed.