Cable protection system

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A cable protection system (CPS) protects subsea power cables against various factors that negatively impact on the cable lifetime, normally used when entering an offshore structure. When a subsea power cable is laid, there is an area where the cable can be subjected to increased dynamic forces, which the cable is not necessarily designed to survive over the lifetime of the installation.

Submarine power cable

A submarine power cable is a transmission cable for carrying electric power below the surface of the water. These are called "submarine" because they usually carry electric power beneath salt water but it is also possible to use submarine power cables beneath fresh water. Examples of the latter exist that connect the mainland with large islands in the St. Lawrence River.

Offshore construction installation of structures and facilities in a marine environment

Offshore construction is the installation of structures and facilities in a marine environment, usually for the production and transmission of electricity, oil, gas and other resources. It is also called maritime engineering.

Contents

Cable protection systems are used to allow the specification, and thus cost, of a subsea power cable to be reduced, by removing the need to include additional armoring of the cable. The resulting cables can be produced more cheaply, whilst still prividing the 20 years + lifetime required.

Offshore windfarm developers in particular have adopted the use of Cable protection systems due to the dynamic area where the cable comes from the seabed and enters the monopile/J-tube. This is in part due to the potential for localised scouring to occur near the structure.

Seabed The bottom of the ocean

The seabed is the bottom of the ocean.

Bridge scour

Bridge scour is the removal of sediment such as sand and gravel from around bridge abutments or piers. Scour, caused by swiftly moving water, can scoop out scour holes, compromising the integrity of a structure.

A CPS generally consists of three sections, a Centraliser or Monopile interface, a protection system for the dynamic area, and a protection system for the static area.

The installation of J-Tubes for offshore renewable monopiles was viewed as a costly approach, and a 'latching' type of cable protection system which penetrates the outer wall of the monopile, via a specifically designed angled aperture enables the simplification of monopile design, and removes the need for additional works post pile driving which usually involved the use of divers. This approach is becoming the industry standard in monopile design, assisting developers to reduce their costs for construction.

History

Articulated half-pipe Cable protections systems have traditionally been used for the protection of cables at shore landings, and other areas where cable damage could be envisaged, and burial was not practical. Patents for variations of articulated pipe cable protections date back to 1929. The system was described as a cable armor shield

"adapted to protect the cable from damage and wear occasioned by rubbing on rocks, contacting with ships, anchors or other objects, and has for its object to provide a practical flexible armor shield of this class which can be readily applied to the cable at any point along its length." [1]

From their outset cable protection systems were designed to be simple, effective, and easy to assemble. The systems consisted of a series of half shells which had a convex flange at one end and a larger socket flange at the other allowing the sections to form a flexible universal joint connection between them. Due to the intended use of heavy cast or forged metals they also had the added advantage of increasing the weight of the cable being installed, thus reducing movement on the seabed.

Forging manufacturing process involving the shaping of metal

Forging is a manufacturing process involving the shaping of metal using localized compressive forces. The blows are delivered with a hammer or a die. Forging is often classified according to the temperature at which it is performed: cold forging, warm forging, or hot forging. For the latter two, the metal is heated, usually in a forge. Forged parts can range in weight from less than a kilogram to hundreds of metric tons. Forging has been done by smiths for millennia; the traditional products were kitchenware, hardware, hand tools, edged weapons, cymbals, and jewellery. Since the Industrial Revolution, forged parts are widely used in mechanisms and machines wherever a component requires high strength; such forgings usually require further processing to achieve a finished part. Today, forging is a major worldwide industry.

Over the years innovations have occurred improving the articulation of the joints with modern articulated pipes being more akin to ball-joints, and some manufacturers providing 'boltless' articulated pipes, thus saving assembly time. [2] [3] [4]

Ball and socket joint

The ball and socket joint is a type of synovial joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. The distal bone is capable of motion around an indefinite number of axes, which have one common center. It enables the bone to move in many places.

Changes in the metallurgy have also happened, leading to most half shell articulated pipe now being made from ductile iron, due to its improved strength and elasticity characteristics. [5]

Metallurgy domain of materials science that studies the physical and chemical behavior of metals

Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are called alloys. A special type of alloy was invented in 1995, when Taiwanese scientists invented the world's first high-entropy alloys of metals that can withstand the highest temperatures and pressures for use in industrial and technological applications such as state of the art race cars, spacecraft, submarines, nuclear reactors, jet airplanes, nuclear weapons, long range hypersonic missiles and many other areas of technology. Metallurgy is used to separate metals from their ore. Metallurgy is also the technology of metals: the way in which science is applied to the production of metals, and the engineering of metal components for usage in products for consumers and manufacturers. The production of metals involves the processing of ores to extract the metal they contain, and the mixture of metals, sometimes with other elements, to produce alloys. Metallurgy is distinguished from the craft of metalworking, although metalworking relies on metallurgy, as medicine relies on medical science, for technical advancement. The science of metallurgy is subdivided into chemical metallurgy and physical metallurgy.

Ductile iron type of cast iron

Ductile iron, also known as ductile cast iron, nodular cast iron, spheroidal graphite iron, spheroidal graphite cast iron and SG iron, is a type of graphite-rich cast iron discovered in 1943 by Keith Millis. While most varieties of cast iron are weak in tension and brittle, ductile iron has much more impact and fatigue resistance, due to its nodular graphite inclusions.

Today these articulated pipes are also utilised for their bend restriction properties, allowing them to be utilised as bend restrictors for the protected cable.

Design considerations

Cable protection systems are predominantly designed to protect the system from damage throughout the lifetime of the cable caused by fatigue, overbending of the cable, and to provide protection of the cable until it reaches an area of burial.

Design life

The cable protection system will be designed to provide protection for a specific lifetime, the 'design life' of the system, which may vary dependent upon the conditions encountered.

Overbending of cable

Overbending of the cable occurs when the cable is bent in a radius of less than the minimum bending radius defined by the manufacturer. Although the cable may initially survive the overbending, this can lead to subsequent fatigue within the cable ultimately leading to cable failure. The CPS selected should maintain a radius which is greater than the specified minimum bend radius.

Fatigue of CPS/cable within

Subsea cable protection systems can encounter wear due to movement, and general changes in composition due to being submerged for a prolongued period of time, such as corrosion or changes in polymer based compounds. Consideration should be given to the induced effects on the CPS resulting from the dynamic elements in the environment. Simple changes such as changes in temperature, current or salinity can result in changes in the ability of the CPS to offer protection for the life of the cable. It is advisable to carefully assess the potential effects of movement of the CPS, relating to the dynamic abilities of the cable. The CPS may withstand the worst conditions seen over a 100yr period, but would the cable inside the CPS survive these movements. In some instances, such as shore ends for fibre optic cables where rocky outcrops are present, dynamic influences can be reduced by securing the articulated pipe to the seabed rock, thus reducing the degree of movement remaining.

Some manufacturers have performed independent empirical testing to provide a simulated 25yr life cycle of the dynamic forces applicable to their product in order to provide customers with improved confidence in the survivability of the system. [3]

Another cause for failure of subsea power cables is caused by overheating, which can occur where a cable is contained within a CPS without adequate ability to dissipate the heat produced by the cable. These lead to early fatigue of the cable insulation, necessitating the replacement of the cable.

Subsea cable incidents account for around 77% of the total global cost of wind farm losses. Since 2007 this percentage, which has varied between 70% and 80%, is statistically reported year after year. [3]

Seabed stability

Seabed stability is an important factor associated with cable protection systems. Should the cable protection system be too buoyant, it is less likely to remain in contact with the seabed, thus the CPS is more likely to require additional remedial stability measures, such as installation of concrete mattresses, rockbags, or rockdumping.

Suspension strength

When a CPS is being installed to interface with a monopile structure, there is likely to be seabed scouring to some degree. Should the scouring become excessive, the CPS may be suspended within a scour hole, and needs to be capable of supporting its own weight, and that of the cable within. Failure to sustain this loading scenario will lead to failure of the CPS, which will in turn allow the forces to act upon the cable within, ultimately leading to cable damage.

Installation

Within the renewables market in particular, installation of CPS's are preferred to be completely diverless, as this reduces the developers cost, and removes risk to human life through diving in a hazardous area.

Removal/Reinstallation

A final consideration for CPS is that of removal of the cable should a failure occur. Some designs require diver intervention to recover the cable with the CPS. Due consideration should also be given to the removal of a CPS should the CPS itself fail. The costs associated with CPS replacement during the operational lifetime of an offshore wind farm are not insignificant, as the cable will most likely require repair/replacement as part of the process.

Bend restrictors

External video
Nuvola apps kaboodle.svg An example of these polymer bend restrictors
Nuvola apps kaboodle.svg One supplier with metal half shells for the static zone and polymer based bend restrictor
Nuvola apps kaboodle.svg Another example of a polymer and metal system

Various innovative systems have been developed to provide restriction of bending, including ductile iron articulated pipe, and polymer or metal based vertebrae systems. Vertebrae bend restrictors are available in both metal and polymer based forms. Some cable protection systems include a polymer based vertebrae system which restricts the bend radius to a maximum of a few degrees per segment. These systems are lighter than their metal equivalents and often cheaper to produce but must be carefully assessed for longevity in the proposed application. Due to the use of polymers these systems tend to be of a larger diameter than their metal counterparts, which presents a larger surface area for drag induced forces caused by currents.

Bend stiffeners

Bend stiffeners are conically shaped polymer mouldings designed to add local stiffness to the product contained within, limiting bending stresses and curvature to acceptable levels. Bend stiffeners are generally suitable for water depths of 35 metres or less, and their suitability is highly dependent on currents and seabed conditions at site. Extreme care must be taken when selecting a stiffener, especially relating to the lifespan of the system as these themselves can become fatigued/fragile. As the stiffness of these products are dependent upon the nature of the plastic used, careful testing and QA of plastics should be carefully considered as flaws introduced during material manufacture, processing, machining and molding. [3]

Other systems

Various other polymer based systems have been developed which provide a flexible 'tube' which can be attached to the structure in advance of the cable being installed, although these are relatively new to the industry, and considered by some as unproven.

Applicable Standards

Although there are no specific standards for cable protections systems, DNVGL-RP-0360 Subsea power cables in shallow water includes a section on Cable Protection at the interface to a structure (Section 4.7).

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References

  1. O, Hoeftmann Alexander (inventor) (September 8, 1931). "Cable shield - US1822624 A". Google patents. Retrieved 2017-03-15.
  2. "Vos Prodect". www.vos-prodect.com. Retrieved 2017-03-15.
  3. 1 2 3 4 "CPNL Engineering | cable protection solutions". CPNL Engineering | cable protection solutions. Retrieved 2017-03-15.
  4. "Protectorshell Articulated / Split pipe". www.protectorshell.com. Retrieved 2017-03-15.
  5. "Ductile Iron Data - Section 3 - Part 1". www.ductile.org. Retrieved 2017-03-15.