Suction anchor foundations serve as a critical anchoring solution for submerged floating tunnel (SFT) cable systems. In marine environments, these foundations must endure not only static loads but also long-term oblique cyclic loading caused by wave excitation, which can result in soil weakening and a reduction in bearing capacity. This study systematically examines the oblique cyclic bearing behavior of SFT suction anchors using a combined experimental and numerical approach. The results demonstrate that (1) the cyclic load ratio initially increases with increasing wave periods, then decreases, before rising again; (2) displacement accumulation at the mooring point occurs rapidly during the initial wave loading cycles, gradually stabilizing as cycling progresses; (3) during foundation failure, tension redistribution displays asymmetric characteristics, with connected cables experiencing load reduction while adjacent cables are subjected to amplified forces; (4) numerical analyses quantify key parametric relationships, revealing that the weakening coefficient (alpha) decreases with increasing loading angle, exhibits a positive correlation with zeta b, and shows a negative correlation with zeta c. These findings advance the understanding of cyclic performance in SFT anchors and offer essential insights for SFT safety evaluations.

The double wellhead system equipped with a suction anchor is subject to alternating loads, such as shear forces and bending moments, which can lead to fatigue failure. Considering the impact of the suction anchor on the subsea wellhead, an equivalent theory has been derived, and a refined equivalent model established to determine the fatigue hot spot moment-stress transfer function. To address the challenges of high computational complexity and convergence difficulties in the dynamic analysis of the refined equivalent model, a simplified theory of global coupling analysis has been derived, and a corresponding simplified subsea wellhead model established. Utilizing the environmental conditions of X gas field in China south sea, the fatigue life of the double wellhead system with suction anchor is evaluated. The results were compared with those of a conventional subsea wellhead, demonstrating that the double wellhead system with suction anchor can significantly improve fatigue life of the subsea wellhead. The fatigue life of high and low pressure welds in conventional subsea wellheads under drilling conditions has been extended from 17.77 years to 1.20 years-186.16 years and 25.62 years, respectively.