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While the evolution of platform and riser technologies, in next-generation drill ships, and in adding subsea processing to production capabilities involve incredibly complex solutions, their successful application continues to rely on ROVs that can keep pace and allow those technologies to be realised.
The basic challenge of physically placing an ROV into the deep water areas necessary to perform its tasks involves doubling the operating depths—from the 2500 metres of traditional WROVs to depths closer to 4000 and even 5000+ metres. One can readily appreciate the increasing size, weight, and powering capacity of an umbilical cable to support this activity.
An umbilical cable for ROVs must meet a range of mechanical and environmental requirements not commonly experienced in other applications. Deepwater temperatures are typically between 0 and 3°C. Also of concern are pressures: the pressure of ocean water exceeds 2900 psi (2000 N/cm2) at 2000 meter’s depth. Adding the long lengths of the cables and the need to withstand twisting and other movements, you can see the tensile loads on the cable are quite challenging.
Packaging the required elements in an umbilical assembly involves protecting them both mechanically and environmentally. At the same time, the design should be no larger or heavier than necessary and should be flexible to accommodate the movement of the ROV. Umbilical cables typically also serve as the main lift cable for ROV. The design challenge is to manage the tradeoffs between achieving a mechanically robust cable and minimizing size and weight.
Given the complexity of the design and the mission critical need for reliability, only a handful of companies have earned the trust of the global ROV community to produce such cables.
Smaller, lighter cables simplify shipboard deployment by reducing the space required for winching equipment and cable storage. Since deepwater umbilical cables are several thousand metres long, a 10 per cent reduction in a cable’s diameter can significantly reduce the required deck space.
A high-performance umbilical cable typically contains multiple types of elements to handle power, control, video, and telemetry. Thus you will find a mixture of larger conductor power cables, twisted-pair and multi-conductor elements, coaxial cable, and fibre-optic cable in various combinations to meet the specific needs of the application.
An umbilical cable is typically constructed in several concentric layers. Fig. 1 shows a type of construction designed by TE Connectivity that combines power, multimode and single-mode fibre-optic cables, and a shielded quad cable in a double-armoured cable.
As you can see this is a complex design and it is important that all elements work together in harmony to provide the required power, data and mechanical criteria. Each layer is defined not by function, but by the diameter of each cable element to maintain concentricity.
Some cables, for example, may have power cables near the core’s centre, while others have them in outer layers. Maintaining concentricity is important both to clean, efficient winching and to achieving rugged flexibility.
Each layer is wrapped with a tape, typically an aluminium/polymer tape, and voids are typically filled with water-blocking materials. Not called out in Fig. 1 are drain wires and fillers. These are added as necessary.
The outer armouring serves as both strength members and core protection; it functions to disengage the cable elements from the overall tensile load placed on the cable.
The cable is 1.670 inches (42.42mm) in diameter. Its weight in water is 2771 pounds per 1000 feet (4123kg/km). It has a working load 35,000 pounds (156kN) and a bend radius of 33 inches (84cm). The bend radius is most important to the diameter of the sheave. Using the cable at smaller bend radii can increase fatigue resistance and shorten the service life.
Given the challenges of creating robust umbilical cables, designers use advanced simulation tools to analyze the mechanical and electrical performance. It is impractical to build multi-kilometre prototypes, so expertise in design and simulation are essential.
Current efforts in umbilical innovation are focusing on the cable’s core, looking for ways to pack more functionality into the same or smaller space. The Rochester Cable group of TE Connectivity, for example, is working with thin-wall insulation and fibre-optic packaging as key to next-generation cables.
Enabling optical communications
Optical fibres are finding increased use in umbilical cables because of the increased bandwidth they offer over long distances. While fibres have high tensile strength to withstand longitudinal pulling, they can be easily broken or damaged if not protected correctly. As a result, fiber-optic cables typically have their own armouring. While aramid yarn—the same strength members common with other fibre-optic cables—are used, more robust designs also use metallic armouring. The high hydrostatic application pressures can increase attenuation in a fibre.
TE Connectivity offers three different approaches:
* Fibre in Steel Tube (FIST), which places the fibre in a solid stainless-steel tube to protect against hydrostatic pressures, high temperature effects and corrosive environments. FIST packaging is a loose-tube design, which can accommodate several fibres loosely held within the tube and encapsulated in gel. Because the fibres ‘float' within the tube, the length of the fibre is slightly longer than the tube to ensure low strain. FIST technology is the simplest and lowest cost approach. It maintains low strain on the fibre by decoupling stress on the tube from that on the fibre. If the cable stretches during installation or use, the excess fibre can accommodate the stretching without being strained. Loose tube designs also are very forgiving of extreme temperature excursions, but are less suited to the most rugged applications, such as extreme depths and extreme cable lengths.
FIST also offers high density packaging of multiple fibre in the tube and, of the three options, is the easiest to terminate.
* Steel-Light armouring, which uses strands of precisely sized plow steel concentrically arranged around the fibre buffer to protect the fiber from breakage.
* Electro-Light armouring, which is similar to STEEL-LIGHT armouring but uses copper in place of steel. The copper can also be used for power to allow composite cables to be designed with a smaller outside diameter.
Steel-Light and Electro-Light fibre elements are both tight buffered approaches to packaging. Tight buffering, while requiring more careful manufacturing, provides better performance in highly dynamic applications and is the most rugged choice. Steel-Light armouring is the most rugged, designed to withstand hydrostatic pressures of 10,000psi.
Both Steel-Light and Electro-Light fibres have very small diameters, allowing them to be fit into interstices in the cable design. With some of newer small-diameter umbilical cables using thin-wall copper conductors, such spaces may not be available. FIST may be a better choice to minimise cable diameter in such cases.
Picking the correct one of these three options means you will have solved many of your telemetry issues. It is a matter of weighing the tradeoffs required in ruggedness for a given application against cost, convenience, and cable size. Umbilical cable suppliers have the experience to guide you in the best choice for your applications.
More power in the core
As the capabilities of the ROV increase, the power needs of the ROV also increase. There are two ways to increase the power-handling capabilities in the core of the umbilical cable. First, you can use conductors of larger cross section. This will, however, increase the diameter of the cable. The second approach is to use thin-wall insulation in place of standard-wall cable.
TE Connectivity, for example, uses cross-linked polyethylene (XLPE) as insulation on power conductors. Thin-wall insulation can achieve cable diameters that are in the range of 30 per cent smaller than comparable standard-wall products.
While conventional wisdom dictates that thicker insulation is used as power handling increases, new materials and new processing methods have overturned such wisdom.
Thin-wall technology is well established in the military and aerospace industry, which offers many of the same demands for rugged performance as deepwater applications. Thin-wall insulation has excellent abrasion resistance, excellent thermal stability over a wide temperature range, and electrical properties required for power-carrying applications.
With thin-wall-insulated wires, it becomes possible to add additional conductors—and thereby provide more power to the ROV—to the umbilical without increasing its size. Fig. 3 shows an example in which eight thin-wall conductors fit into the same space as seven standard-wall conductors.
Umbilical runs deep
As the need increases for deepwater ROVs to support research and oil and gas exploration and production, umbilical cables are supporting the needs for robust performance. New technologies in insulation and optical packaging allow umbilical cables to supply more power to the ROV and to support the increasingly sophisticated capabilities. This translates into more capable deepwater devices with a wider range connected by a compact cable delivering more power and more data-handling capabilities.
David J Harris, Global Product Director, Wire & Cable Marine/Offshore, TE Connectivity. www.te.com/adm. Article contributors: Mark Casselton, Product Manager - Marine & Offshore Cables, TE Connectivity – Aerospace, Defense & Marine and Sage Wadke, Global Director, Marketing & Business Development, TE Connectivity – Aerospace, Defense & Marine.
Fig. 1. A typical umbilical cable is an armoured assembly containing signal conductors, power conductors, and optical fibres. Source: TE Connectivity
Fig. 2. Innovative optical packaging makes application of fiber optics easier. Source: TE Connectivity
Fig. 3. Thin-wall insulation technology allows more power and functionality to be packed into the same space. Source: TE Connectivity