Subsea Power Cable
From Open Electrical
Subsea cables (or umbilicals) have been in use since the mid-19th century, initially for telegraphic communications (including the pioneering 1858 transatlantic telegraph cable), and later for power transmission.
The development of subsea power cables has been primarily driven by the HV power transmission and offshore oil and gas industries. One of the early major installations was a 138kV gas-filled cable run 25.3km from the British Columbia mainland to Vancouver island (183m water depth) in 1958.
Subsea cables for oil and gas applications are typically composites, comprising multiple components such as HV/LV power cores, fibre optic bundles, hydraulic or instrument air hoses, etc. This article mainly deals with composite HV power and fibre-optic communications cables in offshore oil and gas applications. For this reason, the focus is on solid polymeric insulated cables, and oil-filled, gas-filled and paper insulated cables are not discussed.
Subsea Cable Design
There are many ways to manufacture a cable, and there is no standard design to which all vendors will necessarily adhere. However, composite subsea power and fibre optic cables will more or less be constructed with a combination of the following typical components (listed below from the inside out).
Power conductors are typically high-conductivity stranded annealed copper. Actually this is a requirement of ISO 13628-5.
Conductor Screen (for HV cables)
The conductors are screened with an extruded semi-conducting tape to maintain a uniform electric field and minimise electrostatic stresses.
The conductor insulation for solid polymeric cables are normally thermosetting materials, eg. XLPE or EPR.
Insulation Screen (for HV cables)
The insulation screen is typically an extruded semi-conducting material and has a similar function as the conductor screen (ie. control of the electric field).
Water Blocking Tape
Usually a helically wrapped swelling tape that serves as a longitudinal water and moisture barrier for the conductors.
Conductor Sheath (for HV cables)
A conductive sheath / shield, typically of copper tape or sometimes lead alloy, is used to provide a path for fault and leakage currents (sheaths are earthed at one cable end). Lead sheaths are heavier and potentially more difficult to terminate than copper tape, but generally provide better earth fault capacity and water blocking.
A jacket over each insulated and sheathed conductor, typically of extruded polyethylene, is used to provide an impermeable barrier against water for corrosion protection of the metallic conductor sheath. The anti-corrosion sheath can also be extruded over the binder tape as opposed to over each individual conductor.
The interstices of the sub-bundle (ie. comprising power conductors, fibre optic bundle, hoses, etc) are normally filled with a soft polymer material such as polypropylene string or polyethylene.
The binder tape is applied helically over the sub-bundle (ie. comprising power conductors, fibre optic bundle, hoses, etc) to “maintain stability after laying up of the sub-components” (ISO 13268-5 Clause 9.10).
Note that anti-teredo protection is not shown on the typical cable arrangement diagram above.
Inner Sheath / Bedding
An inner sheath, typically a polymer like polypropylene or polyethylene, is applied over the taped bundle for “mechanical protection, bundle stability and to provide a bedding for the armour” (ISO 13268-5 Clause 9.11). For armoured cables, the inner sheath is a requirement of ISO 13268-5.
Continuously extruded thermoplastic (eg. polyethylene) in lieu of roving is required for dynamic applications.
One or more helically wrapped layers of armour wiring, usually of galvanised steel, provides mechanical protection and substantial weight for bottom stability. Cables that need to be torque balanced or require acceptance of high tensile loading can comprise two layers of contra-helically armouring (ie. wrapped in opposite directions). Contra-helical double wire armour cables cannot be coiled and it is necessary to use either a turntable or a drum.
Corrosion-resistant coatings can be applied on the steel wire armour to improve corrosion resistance characteristics. Stainless steel is not typically used, in part because of the potential for low oxygen levels in water (stainless steel depends on a self-repairing oxide coating for corrosion resistance).
Outer Sheath / Serving
Typically either a continuously extruded polymer sheath (such as polyethylene), or a covering of helically applied string rovings (such as polypropylene yarn).
Continuously extruded thermoplastic (eg. polyethylene) is required for dynamic applications.
Cable Design Considerations
The lay of a cable describes the manner in which the conductors, fibre optic bundles, hoses, tubes, etc in a cable are laid in a helix to form the sub-bundle. The terminology arises from the manufacture of fibre and wire ropes. Right hand lay refers to the strands appearing to turn in a clockwise direction, or like a right-hand thread, when looking at the cable as it points away. Vice versa for left hand lay.
Choice in the direction of lay is important in the drumming operation, and the incorrect choice in both cable lay and drum rotation can lead to torque build-up potentially causing spooling problems and damage to the cable.
Cables that are subject to movement on the seabed as a result of tidal currents face the prospect of abrasion damage, and premature failure. The submerged cable must have sufficient weight to resist the maximum tidal seabed currents expected, even under extreme storm conditions.
Calculations for bottom stability are typically performed to DNV RP E305, “On-Bottom Stability Design of Submarine Pipelines”, and require metocean data from the prospective installation site.
“When a single wire armoured cable is suspended from the bow sheave of the laying vessel, a high proportion of the tensile load is carried by the helically applied armour wires. This loading produces a torque in the armour wires which, unless appropriate precautions are taken in the design of the cable, tends to cause the cable to twist so that the lay of the armour wires straightens towards the axis of the cable, and thereby transfers strain to the core(s).
The twisting action cannot pass backwards through the brakes of the cable laying gear to the cable yet to be laid, nor forward to the cable already laid on the seabed. The twisting action therefore tends to concentrate in the suspended cable between the bow sheave and the seabed. The problems become more severe with increasing immersed weight per unit length and increasing depth of laying.
The twisting action can be nullified by applying a second layer of armour wires, which under tensile loading conditions produces an equal and opposite torque to that of the inner layer of wires.” .
Jacketed and Free-Flooded Designs
In a jacketed design, an extruded sheath over the sub-bundle is used to form a pressure-restricted barrier preventing water penetration into the sub-bundle / core. The jacket wall needs to be of sufficient thickness to withstand the water pressure when immersed.
In a free-flooded design, water is free to migrate into the interstices of the sub-bundle and fill the internal voids of the cable. In a free-flooded design, the individual sub-components (eg. conductors, fibre-optics, etc) will require suitable water blocking tapes and jackets, which will likely increase weight and diameter. Also, subsea termination of a free-flooded cable may be more difficult.
For cables that are intended to be installed in J-tubes, consideration should be taken regarding the J-tube diameter, radius of curvature and lead-in angle. The radius of the J-tube needs to be below the minimum bending radii specified by the cable manufacturer. A smaller lead-in angle will aid in installation and pull-in. The diameter of the J-tube needs to accommodate the cross-sectional area of the cable as it is pulled through.
Fibre Optic Design
Like the power conductors, there are several ways to design a fibre optic bundle. Two common methods for packaging up fibres are as follows:
Slotted Core Design
The slotted core design consists of an extruded cylindrical slotted core, with the optical fibres set into the helical slots. The fibres are usually encapsulated in gel for support and to prevent longitudinal water propagation if the cable is severed.
Binder tape is helically wrapped around the core for protection and support. A metallic sheath provides protection against water and gas, and an extruded polymer oversheath provides further mechanical protection and a measure of corrosion protection.
In a tube design, the fibres are encapsulated in gel (for water ingress protection) inside a stainless steel tube (for mechanical and strain protection). The tube is usually armoured, with an extruded polymer oversheath applied over the armour.
There are a number of stages involved in the installation of a subsea cable, from site surveying to termination. ISO 13628-5 Section 15 has a good outline of the requirements for the general cable installation operation. Refer also to the paper by Hosseini et al , which gives a detailed account of a subsea cable installation in the South Pars gas field (Persian Gulf).
A site survey is conducted pre-installation, with consideration to the following (largely summarised from ISO 13628-5):
- Surveillance of the planned cable lay route (using a side-scan sonar or ROV)
- Bathymetric sub-bottom profiler and side-scan sonar survey of the proposed route
- Confirmation of the position of seabed obstructions (pipelines, cable and other structures)
- Identifying the location of any debris along the proposed corridor and removing the debris (if possible) before installation
- Identification of any deviations from the proposed route
- Survey the host facilities, including the J-tubes / I-tubes and the area for topside termination
- Deployment of temporary installation aids as necessary
- Deployment of transponders or beacons at critical positions (eg. subsea crossings)
- Longitudinal profile, seabed conditions and water depth along the proposed route length
Cable Laying Vessel
The cable laying vessel employed for composite subsea cable laying operations are selected to suit the application. In general, there is little crossover between cable laying vessels designed for handling subsea telecommunications cables and power cables. Features to consider when selecting a cable laying vessel (from Electric Cables Handbook ):
- Suitable hold dimensions for the storage of cable coils, drums or a turntable
- Suitability of the deck layout for fitting cable handling equipment
- Overall dimensions including minimum operating draft
- Adequate manoeuvrability
- Navigational and communications equipment and facilities for fitting additional items as necessary
- Power supplies available for additional equipment
- Accommodation and messing facilities and space for fitting temporary additions if necessary
- Age and general condition of the vessel such that specially constructed items may be of use for future operations
- Charter terms and conditions
ISO 13628-5 Section 15.2 outlines the requirements for the cable laying vessel and equipment, which shall include:
- Communications facilities
- Navigation and position systems
- Lay chutes, of a size that will avoid infringement of the minimum bend radius of the umbilical
- Conveyor systems to move the umbilical without the presence of uncontrolled spans or the possibility of the umbilical coming into contact with surfaces other than those of the handling and storage systems
- Cable engines
- Powered / unpowered sheaves
- Trenching / burial equipment
- ROV spread
- Diving spread
- Tension-measuring equipment to continuously monitor and record the tension to which the umbilical is subjected (plus alarms).
- Length measuring system
- Departure angle measuring equipment to continuously monitor the angle at which the umbilical leaves the vessel (plus alarms)
- Umbilical functional testing equipment
- Installation aids
- Device to cut the umbilical, and holding clamps, in case of emergency
The cable laying operation is typically scheduled during an adequate window of predicted favourable weather conditions so that the full cable lay can be completed in one uninterrupted operation.
ISO 13628-5 Section 15.7 describes the mechanical handling requirements for the main cable lay, which is to avoid:
- Introduction of excessive slack in the vicinity of the touch-down position, by virtue of low tension/large departure angle, to preclude the possibility of loop formation
- Infringing the minimum bend radius at the touch-down point
- Introduction of large rates of twist into the umbilical, to reduce the probability of loop formation and bird-caging
- Application of excess tension, which may overstress the umbilical
- Flexing the umbilical, close to the overboarding point, where catenary loads are at their maximum, and at the touch-down point for extended periods to exclude the likelihood of fatigue failures of the umbilical structure
Laying tension, cable length, and departure angle are monitored and controlled throughout the laying operation. Touch-down positions are visually monitored by ROV to verify that the cable is being laid within the proposed corridor.
What not to do - below is a video of a subsea cable reel going out of control:
There may be a need for subsea cables to cross seabed obstructions, especially in areas that are congested with subsea pipelines and other cables. Methods for subsea crossings of seabed obstructions include:
- Concrete mattresses to support cable over obstructions
- Use of protective cable sleeves (such as Uraduct) over the obstruction
J-Tube / I-Tube Pull-in Operations
The requirements for the installation of the subsea cable at the host facility through a J-tube / I-tube are described in ISO 13628-5 Section 15.4. In summary, the process is typically as follows:
- Preparatory work, including review of installation calculations, pigging of the J-tube / I-tube and installation of the messenger wire in the J-tube / I-tube
- Recovery of the messenger wire at the host facility and cable lay vessel
- Overboarding of cable pull-in head and vessel positioning to enable entry of cable into the J-tube / I-tube bellmouth at correct angle
- Pulling-in of cable through J-tube and I-tube to relevant deck level
- Securing of cable on J-tube / I-tube, either with a permanent hang-off or a temporary fastening arrangement (employed when time is critical and testing / cable lay is to proceed without additional delays)
- Sealing of J-tube / I-tube (optional) for corrosion protection, along with chemical protection such as chemical inhibitors, biocides and oxygen scavengers.
The termination of the cable to a topsides termination panel, or via an in-line splice, can be completed at any time after the cable is secured with a permanent hang-off arrangement.
The following standards are relevant for the design and installation of subsea cables:
- ISO 13628-5, “Petroleum and natural gas industries – Design and operation of subsea production systems – Part 5: Subsea umbilicals”
- DNV RP E305, “On-Bottom Stability Design of Submarine Pipelines”.
- IEC 60502, “Power Cables with Extruded Insulation and Their Accessories for Rated Voltages from 1 kV (Um = 1,2 kV) up to 30 kV (Um = 36 kV) - Part 2: Cables for Rated Voltages from 6 kV (Um = 7,2 kV) and up to 30 kV (Um = 36 kV)”
- CIGRE Electra No. 68 “Recommendations for Mechanical Tests on Submarine Cables”
- IEEE STD 1120, "IEEE Guide for the Planning, Design, Installation and Repair of Submarine Power Cable Systems"
- McAllister, D. “Electric Cables Handbook”, Granada Publishing, 1982
- Hosseini. M.K.A., Ramezzani, M. T. and Banae, M., “Submarine Cable Installation between Production Platform and Satellite Wellhead Platform of South Pars Gas Field – Phase 1 in the Persian Gulf”, OCEANS ’04, IEEE, November 2004
- Attwood, J.R., “Cable Design for Subsea Power Links”, IEEE Power Engineering Review, September 2000