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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.
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 providing 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.
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.
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."
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.
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.
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.
Today these articulated pipes are also utilised for their bend restriction properties, allowing them to be utilised as bend restrictors for the protected cable.
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.
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 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.
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.
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.
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.
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.
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.
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.
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 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.
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.
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).
Dynamic positioning (DP) is a computer-controlled system to automatically maintain a vessel's position and heading by using its own propellers and thrusters. Position reference sensors, combined with wind sensors, motion sensors and gyrocompasses, provide information to the computer pertaining to the vessel's position and the magnitude and direction of environmental forces affecting its position. Examples of vessel types that employ DP include, but are not limited to, ships and semi-submersible mobile offshore drilling units (MODU), oceanographic research vessels, cable layer ships and cruise ships.
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.
Seabed gouging by ice is a process that occurs when floating ice features drift into shallower areas and their keel comes into contact with the seabed. As they keep drifting, they produce long, narrow furrows most often called gouges, or scours. This phenomenon is common in offshore environments where ice is known to exist. Although it also occurs in rivers and lakes, it appears to be better documented from oceans and sea expanses.
A deep foundation is a type of foundation that transfers building loads to the earth farther down from the surface than a shallow foundation does to a subsurface layer or a range of depths. A pile or piling is a vertical structural element of a deep foundation, driven or drilled deep into the ground at the building site.
Subsea is fully submerged ocean equipment, operations or applications, especially when some distance offshore, in deep ocean waters, or on the seabed. The term is frequently used in connection with oceanography, marine or ocean engineering, ocean exploration, remotely operated vehicle (ROVs) autonomous underwater vehicles (AUVs), submarine communications or power cables, seafloor mineral mining, oil and gas, and offshore wind power.
NKT Flexibles is a supplier of flexible pipelines for the offshore and chemical industries based in Denmark. NKT Flexibles was acquired by the Houston, Texas-based company National Oilwell Varco in 2012. The company was formed in 1999 when it split off from NKT Cables. It has production facilities in Kalundborg, Denmark and offices in Brøndby, Denmark. In July 2011, the company announced its second Frame Agreement with Brazil oil company Petrobras. The agreement, worth up to 1.3 bn Euro, includes the establishment of a Brazilian production facility. This facility will be located in the new Acu Superport.
A steel plate shear wall (SPSW) consists of steel infill plates bounded by boundary elements.
Plastic pipe is a tubular section, or hollow cylinder, made of plastic. It is usually, but not necessarily, of circular cross-section, used mainly to convey substances which can flow—liquids and gases (fluids), slurries, powders and masses of small solids. It can also be used for structural applications; hollow pipes are far stiffer per unit weight than solid members.
The Conductor Pipe is a large diameter pipe that is set into the ground to provide the initial stable structural foundation for a borehole or oil well.
Friction stud welding is a solid phase welding technique involving a stud or appurtenance being rotated at high speed while being forced against a substrate, generating heat by friction. The metal surfaces reach a temperature at which they flow plastically under pressure, surface impurities are expelled and a forged weld is formed.
Tube bending is any metal forming processes used to permanently form pipes or tubing. Tube bending may be form-bound or use freeform-bending procedures, and it may use heat supported or cold forming procedures.
A pipelaying ship is a maritime vessel used in the construction of subsea infrastructure. It serves to connect oil production platforms with refineries on shore. To accomplish this goal a typical pipelaying vessel carries a heavy lift crane, used to install pumps and valves, and equipment to lay pipe between subsea structures.
Offshore wind power or offshore wind energy is the use of wind farms constructed in bodies of water, usually in the ocean, to harvest wind energy to generate electricity. Higher wind speeds are available offshore compared to on land, so offshore wind power’s electricity generation is higher per amount of capacity installed, and NIMBY opposition to construction is usually much weaker.
A steel catenary riser (SCR) is a common method of connecting a subsea pipeline to a deepwater floating or fixed oil production platform. SCRs are used to transfer fluids like oil, gas, injection water, etc. between the platforms and the pipelines.
Commercial offshore diving, sometimes shortened to just offshore diving, generally refers to the branch of commercial diving, with divers working in support of the exploration and production sector of the oil and gas industry in places such as the Gulf of Mexico in the United States, the North Sea in the United Kingdom and Norway, and along the coast of Brazil. The work in this area of the industry includes maintenance of oil platforms and the building of underwater structures. In this context "offshore" implies that the diving work is done outside of national boundaries. Technically it also refers to any diving done in the international offshore waters outside of the territorial waters of a state, where national legislation does not apply. Most commercial offshore diving is in the Exclusive Economic Zone of a state, and much of it is outside the territorial waters. Offshore diving beyond the EEZ does also occur, and is often for scientific purposes.
A Vertebrae Bend Restrictor (VBR) is used in the oil and gas industry as part of offshore deep sea drilling operations. It is designed to prevent damage to an umbilical cable from overbending. It offsets the action of applied loads which could kink or buckle the internal conduits of an umbilical, cable, flexible riser pipe or MUX line.
A bend stiffener is a type of cable protection system. They are conically shaped polyurethane mouldings designed to add local stiffness to a riser, flowline, cable or umbilical. They limit the bending stresses and curvature to acceptable levels. They are used in the oil and gas industry as part of offshore deep sea drilling operations.
Offshore geotechnical engineering is a sub-field of geotechnical engineering. It is concerned with foundation design, construction, maintenance and decommissioning for human-made structures in the sea. Oil platforms, artificial islands and submarine pipelines are examples of such structures. The seabed has to be able to withstand the weight of these structures and the applied loads. Geohazards must also be taken into account. The need for offshore developments stems from a gradual depletion of hydrocarbon reserves onshore or near the coastlines, as new fields are being developed at greater distances offshore and in deeper water, with a corresponding adaptation of the offshore site investigations. Today, there are more than 7,000 offshore platforms operating at a water depth up to and exceeding 2000 m. A typical field development extends over tens of square kilometers, and may comprise several fixed structures, infield flowlines with an export pipeline either to the shoreline or connected to a regional trunkline.
A submarine pipeline is a pipeline that is laid on the seabed or below it inside a trench. In some cases, the pipeline is mostly on-land but in places it crosses water expanses, such as small seas, straits and rivers. Submarine pipelines are used primarily to carry oil or gas, but transportation of water is also important. A distinction is sometimes made between a flowline and a pipeline. The former is an intrafield pipeline, in the sense that it is used to connect subsea wellheads, manifolds and the platform within a particular development field. The latter, sometimes referred to as an export pipeline, is used to bring the resource to shore. Sizeable pipeline construction projects need to take into account many factors, such as the offshore ecology, geohazards and environmental loading – they are often undertaken by multidisciplinary, international teams.
The tripod is a type of foundation for offshore wind turbines. The design is strictly guided by the functional requirements of a long lasting predominantly dynamic loaded structure in harsh environment. The tripod is generally more expensive than other types of foundation. However, for large turbines and higher water depth, the cost disadvantage might be compensated when durability is also taken into account.
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