BOTDR Monitoring of Tensile State in Three-Core Fiber-Optic Composite Submarine Cables with a Three-Layer Mechanical Structure and Dual-Threshold Sensing Model

Submarine cables are critical components for power transmission in offshore wind farms, making their condition monitoring paramount for ensuring operational reliability. Addressing unclear strain transfer and underdeveloped Brillouin optical time-domain reflectometry (BOTDR) sensing models for three-core fiber-optic composite submarine cables, this study investigated a 66 kV cable and clarified a BOTDR monitoring principle based on the three-layer mechanical structure. Using the external optical unit’s average Brillouin shift for temperature compensation, four characteristic parameters (Δvy, Δvp, vm, vF) were analyzed. The results show the optical unit’s tensile strain-induced Brillouin shift exhibits periodic distribution along the cable. The stable average peak vF achieved a correlation coefficient of 0.98 with tensile load Fi. A dual-threshold sensing model was established: no shift response below F0 = 90 kN (7.84% Rated Tensile Strength (RTS)); strong linear correlation between vF and Fi beyond Fm = 110 kN (9.58% RTS) with a tensile sensitivity coefficient of 0.03788 MHz/kN. This study provides key BOTDR technical support for submarine cable tensile monitoring in complex marine environments.