Evaluation parameters and methods for optimal mooring system of submerged floating tunnel

Previous studies on the optimal mooring system of a submerged floating tunnel (SFT) have mostly focused on numerical methods, with a limited number of experimental studies conducted under regular waves. The primary evaluation factors used for identifying the advantages and disadvantages of different mooring systems have been the maximum motion response and mooring tension in the time domain. To evaluate the optimal mooring system of SFT more effectively and comprehensively, this study introduces several new evaluation parameters alongside the maximum motion response and mooring tension.

Physical modeling tests were conducted under regular and irregular waves, focusing on the effect of different mooring systems on the hydrodynamic characteristics of the SFT. Based on the time history of motion responses and mooring tensions obtained from the tests, the advantages and disadvantages of different mooring systems were comprehensively examined based on three evaluation factors: the maximum motion response and its spectral area, the maximum mooring tension and its distribution uniformity, and the frequency of cables reaching a relaxed condition during wave actions.

Results revealed the following: (1) The diagonal-cable systems CM3 (four diagonal cables at either end of the SFT) showed better performance than those of the "diagonal cable + vertical cable" system CM1 (two diagonal cables and one vertical cable at either end of the SFT) and CM2 (two diagonal cables and two vertical cables at either end of the SFT). (2) The natural periods of three mooring systems were observed to follow the order of CM1> CM2> CM3. In the mooring system CM3, the natural periods of surge and heave were only approximately 50% of the mooring system CM1. Meanwhile, the natural period of pitch was significantly reduced to approximately 25% of the mooring system CM1. In addition, the natural period of pitch was significantly shorter than those of surge and heave, indicating that the mooring system CM3 had a strong constraint effect on pitch. (3) The 0th-order moments (spectral areas) of the dynamic response reflect the overall degree of the motion response. The 0th-order moments of the motion response and mooring tension under three mooring systems followed the order of CM1> CM2> CM3. In the mooring system CM3, the spectral areas of surge, heave, and pitch of the SFT were significantly lower than those in the other two mooring systems, and the difference was particularly significant under large waves. (4) The uniformity of mooring tensions is crucial for determining an optimal mooring system. For the three mooring systems, the ratios of the maximum mooring tensions of all cables at either end of the SFT were as follows: CM2 1.0-1.36, CM2 1.0-1.37, and CM3 1.0-1.12. The distribution uniformity of the mooring tensions of all cables in the mooring system CM3 was better than those in the other two mooring systems. (5) The cables in the mooring system CM2 attained a relaxed or nearly relaxed state most frequently, while the mooring system CM3 exhibited the lowest number of such instances.

Compared to the maximum motion response and mooring tension, analyzing the maximum motion response and its spectral area, the maximum mooring tension and its distribution uniformity, and the frequency of cables reaching a relaxed condition provides a more effective and comprehensive method for evaluating the advantages and disadvantages of the mooring system of SFT. These evaluation parameters and methods are crucial for guiding project design.