To tether, or not to tether?

A Brazilian remotely operated vehicle (ROV) operator recently approached JDR with the concept of building an optimum-cost work-class ROV system to service a South American client base. The company saw deepwater ROV systems as requiring large capital investment with substantial ongoing operational costs and considered reliability a critical issue beyond 6,560-ft (2,000-m) mean water depths. Equipping their in-house-built vehicles was viewed, therefore, as a serious undertaking.
Going for a free-swimming ROV and eliminating the tethered management system was extremely attractive, as this not only would reduce cost but also eliminate several subsea components that affect overall system reliability. A major drawback in free-swimming ROVs, however, is the huge umbilical cable required. This absorbs much of the vehicle's power, limiting its operational window. A detailed study of umbilical-ROV combinations showed that a free-swimming ROV was feasible for deep water with a pure "power and optic" cable.
ROV design
Off-the-shelf packages were used as far as possible for the vehicle design. An existing 100-hp hydraulic system was selected from a UK company to provide just enough power to the ROV while minimizing the power conductors in the umbilical. Crucially, the vicious circle of bigger cable, more drag, more power and bigger cable was to be avoided at all costs.
Navigation reliability was improved by equipping the ROV with two attitude, heading and reference system "engines." These guarantee performance even in local areas of hard magnetic anomaly.
The command and control system was developed from scratch. Based on a standard PLC platform, all data and signal information is converted to optical format and transmitted through fibers in the umbilical. Sufficient channel count and bandwidth has been included in the cable to allow future system expansion for intelligent sensors and tools as required.
The resulting ROV, umbilical and handling system delivers a 30% reduction in cost with increased reliability. The ROV and umbilical design evolved together to achieve an optimal system.
Key goals for the umbilical development are:
• minimize diameter and hence drag on the ROV;
• maximize reliability for deepwater operation; and
• minimize material and manufacturing cost.
The first goal was addressed by drawing up all potential cable variations. Each design was analyzed and assessed. Eliminating spare components and replacing data and telemetry lines with optical fibers achieved the optimum cable diameter. The result: a power and optic umbilical.
Understanding the effects on ROV umbilicals and components in deepwater applications of 9,840 ft (3,000 m) allowed the second objective to be met. Optical fibers are susceptible to high attenuation when locally pressured or stretched; this gives two possible approaches in cable design. The first is to "ruggedize" the fiber so that it can withstand the pressure and strain. The second is to isolate the fiber from the environment.
Ruggedized fibers use a "tight buffer" of plastic to distribute pressure evenly onto the glass fiber, while reinforcement typically is achieved with aramid fiber or steel armor. As a result, the component is quite large. This is not a problem when only one or two channels are required, but when all telemetry and data lines are switched to optics, the channel count rapidly reaches 10 or 12 optics. The diameter and cost goals therefore are compromised.
"Loose tube" fibers use a plastic or metal tube to isolate the fibers from the environment and are able to carry more than one fiber per tube. When the tube is cabled into a helix, the fiber is able to move inside the tube to relieve any strain. Plastics have a relatively low yield stress, making them unsuitable for pressures above 2,900 psi at 6,560-ft (2,000-m) water depth, while stainless steel tubes can withstand up to 14,500 psi at 32,800 ft (10,000 m) before collapse.
It commonly is assumed the pressure rating for the fiber is based on the operating depth; however, in an ROV umbilical, the electro-optic cable core also experiences radial compression from the steel armor wires under tension. This is highest with the cable fully deployed and at the overboarding sheave wheel. This has been verified by bend flex fatigue testing over sheave wheels where plastic tube fibers fail and steel tube fibers survive. Steel tube fibers have been proven in operation to 19,680-ft (6,000-m) water depths.
As the umbilical operates as a dynamic cable, a special coating is applied to the tubes, enabling slip within the cable. This ensures stresses on the steel tube are minimized and cable life is increased.
The third goal was achieved by optimizing the number of power cores and fiber optic units so that the cable could be assembled in two layers. The steel armor is designed using a nonlinear core crush algorithm to ensure torsional stability and elongation compatibility.
Designing the ROV for the umbilical as well as the umbilical for the ROV has achieved all the goals set out. Operational performance will be reviewed following sea trials.

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