Experimental Investigation and Simplified Assessment of Ultimate Pulling Length for Long-Distance Submarine Cable Pulling Using a Ball-Frame and Tensioned Steel Cable Series System

Long-distance pulling of submarine cables inside horizontal directional drilling (HDD) steel casings is often limited by high interfacial resistance, rapid pulling-force accumulation, and poor adaptability to muddy-sandy environments. In addition, conventional outward-extending roller devices are relatively bulky, which increases the required casing diameter and reaming size. To address these issues, this study proposes a compact ball-frame and tensioned steel cable series drag-reduction device. An 18 m local full-scale pulling test system was established using an actual engineering submarine cable, a practical-scale steel casing, and full-scale drag-reduction devices. The effects of pipe curvature, device spacing, terminal reaction force, and in-casing medium conditions on the equivalent friction coefficient were investigated. The results show that the equivalent friction coefficient of the submarine cable–steel casing system is maintained at 0.25–0.36 under most test conditions, which is significantly lower than the commonly adopted value of 0.55 for direct contact. Based on the experimentally identified parameters, a simplified assessment model for ultimate pulling length was established for construction scheme comparison and preliminary capacity estimation. The results indicate that, with the implementation of the tensioned steel cable series system, the ultimate pulling length increases from 431/696 m for direct pulling to 954/1424 m. These results provide valuable technical references for drag-reduction scheme selection and preliminary construction-capacity assessment in HDD landfall sections.