Effects of Current Conditions and Mooring-Line Arrangements on Mooring Forces of Immersed Tube Segments

During the floating transportation and installation of immersed tube tunnels, the mechanical stability of the mooring system is a key factor governing construction safety. Current conditions and mooring-line arrangements are two major factors controlling the magnitude and distribution of mooring-line tensions. However, their coupled effects and the applicability of different calculation methods remain insufficiently understood. Based on a large-cross-section immersed tube tunnel project, this study employed a semi-empirical method specified in the Chinese Code for Loads on Port Engineering and a three-dimensional floating-body-mooring numerical model established in ANSYS AQWA. Multiple combinations of current velocity and direction were considered to compare the mooring-line responses under five typical arrangements corresponding to different construction stages. The effects of current conditions and mooring configurations on the magnitude, distribution, and transfer of mooring-line loads were systematically examined. The results show that current velocity is the dominant factor controlling the magnitude of mooring-line tensions, which generally increase with the square of the current velocity. Changes in current direction directly alter the principal load-bearing mooring-line group, and a current velocity of 1.5 m/s during the falling tide represents the most unfavorable current condition throughout the construction process. The mooring-line arrangement governs the spatial distribution and concentration of the line loads. During the floating transportation and mooring stages with multi-line constraints, the maximum mooring-line tensions calculated using China’s code-based method are 18.5–25.2% higher than those obtained from the numerical simulations, indicating relatively conservative predictions. In contrast, during stages with weakened constraints, such as line release and positioning in the foundation trench, the numerical model captures more pronounced local load concentration, yielding maximum tensions 19.4–28.8% higher than those predicted by the code-based method. Across all operating conditions, the maximum mooring-line tensions calculated by the code-based method and numerical model are 924 and 750 kN, respectively. This study clarifies the coupled effects of current conditions and mooring-line arrangements on mooring-load transfer and identifies the applicable scenarios of the two calculation methods. The findings provide a quantitative basis for calculation-method selection, mooring-force assessment, and construction-safety management during immersed tube tunnel installation.