Subsea Cables

The main umbilical provides surface-to-subsea communication and power transmission, or communication only. Different communication methods are required depending on the range; details are covered in the Ultra-Long Range Communication section. Regardless of the method, the cable must meet the following requirements:

1. Neutral Buoyancy: The cable must float in water. This is typically achieved by using a foamed polyurethane jacket. Considering prolonged exposure to sunlight on deck, anti-aging additives must be incorporated into the jacket material to maintain neutral buoyancy after aging, thereby preventing adverse effects on the ROV's motion.
2. Tensile Strength: The cable must possess adequate tensile capacity. Kevlar filling is typically employed to achieve a tensile rating of 100–300 kg.

When neutral buoyancy and high tensile requirements are difficult to reconcile—for example, in legacy heavy-duty deep-sea work-class ROVs requiring continuous power delivery—the cable's inherent weight makes achieving full-length neutral buoyancy impractical. Forcing full neutral buoyancy would result in excessive cable diameter and an unmanageable minimum bend radius, making deployment impossible. In such cases, the entire cable must be armored to prevent failure under its own weight. Additionally, the section of the cable closest to the ROV (typically 100–200 m) is fitted with external buoyancy modules to maintain neutral buoyancy locally, providing the vehicle with free movement space. Alternative approaches involve deploying repeaters; however, as these are considered legacy technologies, they will not be discussed further here.

The new direction involves battery-powered deep-sea work-class ROVs where the umbilical is reduced to a single thin fiber optic cable dedicated solely to real-time data transmission.