Submarine Cables and Systems

PART 5

Submarine Distribution and Transmission

CHAPTER 42

Submarine Cables and Systems

Submarine cables are used in three basic types of installation:

(a) river or short route crossings which are generally relatively shallow water installations;

(b) major submarine cable installations, coast to coast and island to mainland often laid in deep water and crossing shipping routes and fishing zones; these cables are generally required for bulk power transfer in a high voltage either a.c. or d.c. transmission scheme;

(c) between platforms, platforms and sea-bed modules or between shore and a platform in an offshore oil or gas field; these cables are currently laid in depths up to about 200 m but it is anticipated that much deeper installations will be required in the future.

Submarine cables are usually subject to much more onerous installation and service conditions than equivalent land cables and it is necessary to design each cable to withstand the environmental conditions prevailing on the specific route. Subject to certain restrictions, mass impregnated solid type paper insulated cables, fluid-filled cables, gas-filled cables and polymeric cables are all suitable for submarine power cable installations. Polymeric and thermoplastic insulated cables are used for control and instrumentation applications. There is an increasing tendency to include optical fibre cores, where possible, in power cable constructions, for purposes of control, communication and measurement of cable strain and temperature.

Cables for river crossings

Cable routes for river crossings are generally only a few kilometres long and cross relatively shallow water. The length of cable required can often be delivered to site as a continuous length on a despatch drum. Normal methods of installation include laying the cable from a barge into a pre-cut trench and mounting the drum on jacks on one shore and floating the cable across the river on inflatable bags. Installation of the cable is therefore a relatively simple operation which does not involve excessive bending or tension. The cable design is the most similar to land cable practice of all the different

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622 Electric Cables Handbook

types of submarine cable. However, it is considered prudent to improve the mechanical security of the cable by applying slightly thicker lead and anticorrosion sheaths and to use armour. If the cable is to be laid across the river at the entrance to a port, it is recommended that the cable be buried to a depth of at least I m. A cheaper but less effective alternative is to protect a surface-laid cable by laying bags of concrete around it.

Should the cable be considered liable to damage due to shipping activities, an alternative solution to direct burial of the cable is to entrench a suitable pipe into the river bed and then pull in the selected type of cable.

Requirements for long cable lengths laid in deep water

Any cable to be laid on the sea bed should have the characteristics given below, the relevant importance of each particular characteristic being dependent on the depth of water and length of cable route.

(a) The cable must have a high electrical factor of safety as repair operations are generally expensive and the loss of service before repairs can be completed is often a serious embarrassment to the utility concerned.

(b) The cable should be designed to reduce transmission losses to a minimum, as submarine cable routes are generally long and the operating power losses are therefore significant in the overall economics of the system.

(c) The cable should preferably be supplied in the continuous length necessary to permit a continuous laying operation without the need to insert joints while at sea. Proven designs of flexible joints are available to permit drum lengths of cable to be joined together, either during manufacture or prior to loading the continuous cable length onto the laying vessel.

(d) The cable must withstand, without deterioration, the severe bending under tension, twisting and coiling which may occur during the manufacture and installation programmes.

(e) The cable must also withstand, without significant deformation, the external water pressure at the deepest part of the route.

(f) The cable, and where appropriate the terminal equipment, must be designed to ensure that only a limited length of cable is affected by water ingress if the metal sheath is damaged when in service.

(g) The armour must be sufficiently robust to resist impact damage and severance of the cable if fouled by a ship's anchor or fishing gear.

(h) For deep water installations, the cable must be reasonably torque balanced to avoid uncontrolled twisting as it is lowered to the sea bed.

(i) The weight of the cable in water must be sufficient to inhibit movement on the sea bed under the influence of tidal currents. Movement would cause abrasion and fatigue damage to the cable.

(j) The cable must be adequately protected from all corrosion hazards. (k) All cable components must have adequate flexural fatigue life. (1) All paper insulated and some polymeric insulated cables are required to be

watertight along their complete lengths. Water ingress impairs the electric strength of these cables.

The requirements for cable laid on the sea bed also largely apply to cables which are to be buried. The bending characteristics of the cable as it passes through the burial

Submarine Cables and Systems 623

device may need further consideration, and the friction of the serving against rollers and skid plates has to be taken into account. It is essential that information be provided on the length of the proposed route, the nature and contour of the sea bed, tidal currents, temperatures etc. before a provisional cable design can be prepared for the proposed installation. Sufficient information can often be obtained from Admiralty charts to enable a tentative cable design to be prepared to complete a feasibility study but in most cases it is necessary to carry out a hydrographic survey before the cable design can be finalised.

A.C. cable schemes

Where practical, a.c. submarine cable schemes are the first choice. As in the case of land cable circuits, the use of 3-core cables up to and including 150 kV is preferred to single- core cables provided that they can meet the required rating. 3-core cables also offer savings in both cable and installation costs as only two cables compared with four for a 3-phase scheme need be installed when security of supply is required if one cable is damaged.

If a 3-core submarine cable is damaged externally, e.g. by a ship's anchor or trawling gear, all three cores are liable to be affected. It would therefore be necessary to install two 3-core cables from the outset, preferably separated by 250m or more to obtain reasonable security of supply. In 3-core solid type cable installations (i.e. for circuits up to 33kV rating) single lead type cables (i.e. HSL cables) are sometimes preferred, particularly for deep water installations.

For major a.c. power schemes it will probably be necessary to use single-core cables as 3-core cables will be unable to meet the rating. In this case the cables are spaced far apart so that the risk of more than one cable being damaged in a single incident is minimised. The installation of four single-core cables for one circuit, or one spare cable for two or three circuits, would be expected to provide reasonable assurance of continuity of supply. However, widely spaced single-core magnetically armoured cables give rise to high sheath losses. These losses can be reduced substantially by the use of an outer concentric conductor underneath the armour.

D.C. cable schemes

For major cable installations requiring bulk transfer of large quantities of power, the choice has to be made between an a.c. or a d.c. transmission scheme. The longest submarine transmission schemes are invariably d.c. as their length is not limited by the necessity of supplying the charging current inherent in the a.c. system. Where it is intended to connect two separate power systems, e.g. between different countries, d.c. is again chosen as it is possible to keep the two systems independent , thereby preventing risk of instability. A d.c. system also allows for a greater degree of control of power flowing through the cables. Where the link is relatively short, the a.c. system will be more attractive than the d.c. because of the high capital cost of the converter stations required at both ends of the d.c. route. In a small number of routes it may be possible to position some reactive compensation for the a.c. system on conveniently placed islands, although this solution may affect the economics of the scheme. One additional advantage of a d.c. scheme is that, for major links incorporating single-core cables, only two cables are necessary whereas a minimum of three cables is invariably required for a.c. schemes.


D.C. transmission schemes may consist of a one polarity cable (monopole) carrying full circuit power with sea return, or preferably two cables of positive and negative polarity (bipole) each carrying half circuit power. The magnetic field created by a monopole cable causes compass errors near the cable route which may be unacceptable to the relevant Admiralty authorities as it would create a potential hazard to shipping. ~ The monopole scheme suffers the further disadvantage that if the submarine cable is damaged there would be no transmission capability until the cable was repaired. In a bipole transmission scheme, if only one pole cable is damaged, the remaining cable will continue to carry half circuit power with sea return. The installation of three single-core cables from the outset provides reasonable assurance of full transmission capability even if one cable is damaged.

ELECTRICAL DESIGN FEATURES OF CABLES

Conductors

The conductor design will be influenced by the choice of transmission scheme in which it is required to operate. Long submarine routes are generally d.c. schemes, which allow for the use of concentrically stranded conductors. As most land cables operate on a.c. there will be no difference in the design of conductor used in a.c. submarine cables. Large a.c. conductors will be of the Milliken type.

Copper is generally preferred to aluminium for the conductors of all submarine cables as its use permits a higher current density, thereby reducing the overall diameter of the cable. In the event of the cable being damaged and sea-water entering it, a copper conductor is much more resistant to corrosion than an aluminium conductor.

Details of the design of conductors are given in a later section dealing with specific types of cable.

Insulation thickness

The insulation thickness is designed on the same basis as for land cables and for the higher voltages is usually determined by the lightning impulse test requirement. It is considered prudent, however, to employ slightly lower maximum design stresses for submarine cables than would be adopted for land cables, to compensate for the more severe bending and tension which a submarine cable may need to withstand during the laying operation. The resultant increase in the electric strength of the cable is considered to justify the small increase in cost. For d.c. operation, design stresses up to 32 MV/m have been used for solid type cables and 35MV/m for F F cables (paper and PPL insulated). Therefore both types of cable can be considered for the highest voltage schemes.

All d.c. submarine cables are expected to meet the electrical test requirements specified in the latest CIGRE 'Recommendat ion for tests for power transmission d.c. cables for a rated voltage up to 600 kV' (see chapter 38), following mechanical tests carried out in accordance with the C I G R E 'Recommendations for mechanical tests on submarine cables'. 2

350

3OO

v 250

150

IO0

0 I ! I I

0 5 10 15 20

Effective sheath resistance (ohm, 10-4/m)


Fig. 42.1 Relationship between effective sheath resistance and sheath losses in 630 mm 2 single- core fluid-filled submarine cable

Current ratings

The current rating of submarine cables is mainly dependent on the maximum recommended conductor temperature, the thermal resistivity of the dielectric and the environment in which the cable is laid, and in the case of single-core a.c. cable schemes the axial spacing of the cables on the sea bed and the choice of armouring material.

The thermal resistivity of the environment in which the cables operate is dependent on site conditions and the metlaod of laying. If the cables are laid on the surface of tlae sea bed, thermal values of 0.3 K m / W have sometimes been assumed. If the cable is to be buried in the sea bed, however, values of up to 1.0 are usually used for cables buried no deeper than 2.0 m.

In single-core a.c. submarine cable schemes, the cables are normally widely spaced on the sea bed and the sheath circuits are bonded at both ends. The wide cable spacing increases the sheath circulating current and reduces the current rating of the cables. Whereas the use of aluminium alloy armour permits the highest current density in the phase conductors at minimum cost, the mechanical properties and corrosion susceptibilities of the aluminium alloy are often inadequate to protect the cables from external hazards. Galvanised steel wire armour has excellent mechanical characteristics but when applied to single-core a.c. cables gives rise to eddy currents, hysteresis and circulating currents in the magnetic material.

The current rating of single-core a.c. cables is dependent, inter alia, on the effective resistance of the sheath circuit (i.e. the metallic sheath, armour wires and any reinforcing tapes in parallel). The currently favoured method of reducing the losses and increasing the current rating is to reduce the effective resistance of the sheath circuit. This may be achieved by using an outer concentric conductor which usually consists of a layer or layers of copper wires. This reduces the external magnetic field and permits the use of steel wire armour. In certain cases to achieve a low conductance, the steel wire is replaced with hard drawn copper strips.

Figure 42.1 shows the relationship between the effective sheath circuit resistance and the sheath losses in a typical single-core submarine a.c. cable laid at wide spacing.

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Protection of anticorrosion sheath against voltage transients

On long submarine cable routes it is necessary to take steps to ensure that the voltage appearing across the anticorrosion sheath, due to voltage transients in the transmission circuit, does not approach the electrical breakdown level of the sheathing material. It is standard practice, therefore, to bond the lead sheath and any associated reinforcing tapes electrically to the armour wires at regular intervals along the cable length.

MECHANICAL DESIGN FEATURES OF CABLES

Effect of method of installation

Two alternative techniques are available: laying from large coils formed in the hold of a suitable vessel (fig. 42.2), or laying from a large turntable or drum. The turntable or drum technique is the technically preferred option as the cable has to withstand only bending with no twisting action, apart from tension considerations. Provided that turntables are used in all stages of cable manufacture the angle of lay and tension of application of all helically applied cable components need differ little from those used for land cables.

Winding of the cable into a coil imparts a 360 ° twist in each complete turn of cable in the coil. In a typical case, coiling a cable of 125 mm overall diameter to a minimum eye of 7.5m (60 times the overall diameter) causes a twist of 15 ° per metre, the twist per metre decreasing slightly with each turn coiled outwards. For long cable routes the cable coil may be several metres high when coiled into the holds of the laying vessel.

Fig. 42.2 Coiled lengths of cable awaiting transfer to the laying vessel


Cable is generally coiled clockwise when viewed from above. This causes all components which have been applied with right-hand lay, i.e. clockwise, to tighten and all components applied with left-hand lay to loosen when the cable is coiled down. It follows that at the interface between a right-hand component applied over a left-hand component the former is under abnormal tension while the latter is slack. These conditions are conducive to the formation of creases in the slack components unless the effect is controlled by careful selection of the angle of lay and the application tension of every helically applied component of the cable. Lifting of the cable from the coil immediately prior to laying removes the twist from the cable so that it is in the twist-free condition as it passes outboard from the laying vessel. In a well designed cable every component should be free of creases when uncoiled.

A single wire armoured cable will tolerate coiling provided that the armour wires are applied so that they loosen as the cable is coiled down. It is virtually impossible to coil a cable with a double-wire reverse lay armour as there is no combination of lays in the two layers of armour which will avoid either crushing the cable or creating such interfacial pressure between the two layers of a rmour that it is physically impossible to cause the cable to lay flat in a coil. When site conditions necessitate the use of a double wire reverse lay armoured cable, the only laying technique available for long continuous cable lengths is from a turntable or pipe laying drum barge which obviates the need for coiling. Double wire armoured cable with both layers applied in the same direction can be coiled and handled in a similar manner to single-wire armoured cable.

Resistance of cable to water pressure

The ability of a submarine cable to withstand the external water pressure is determined by the internal pressure of pressure-assisted cables (i.e. fluid- or gas-filled cables), the hardness and coefficient of expansion of non-pressure-assisted cables and the thickness and composition of any metal sheath and metal tapes external to the sheath. The external pressure in sea-water increases by approximately 0.1 bar per metre depth. Distortion of a conventionally designed circular non-pressurised cable does not usually occur at depths less than about 150 m. At greater depths the metal sheath is liable to suffer distortion following the first heating cycle unless the cable is specially reinforced against the external water pressure. Figure 42.3 shows the cross-section of a solid type cable which withstood an external pressure of 27.5 bar at ambient temperature but which became misshapen when cooling to ambient temperature following a heating cycle to 60°C. Thermal expansion of the impregnant had distended the lead alloy sheath. On cooling the external pressure did not contract the sheath uniformly, causing it to become distorted. An adjacent sample of cable withstood an external pressure of 65 bar and repeated heating and cooling when a steel tape 1.5 mm thick was lapped over the lead sheath.

Because of the high coefficient of expansion of XLPE insulation, it may be necessary to restrict the temperature rise and hence the current loading when lead-sheathed XLPE cables are laid in deep water, so as to avoid excessive distension of the lead sheath.

The efficacy of any metal tapes applied external to the lead sheath to reinforce the cable against the external water pressure is proport ional to t3n/d 3 where t is the thickness of one tape, d is the pitch circle diameter of the tapes when applied to the cable and n is the number of tapes applied. A single tape of I mm thickness will therefore provide as much resistance to the external water pressure as eight tapes each 0 .5mm thickness.


Fig. 42.3 Deformation of solid type PILS cable resulting from a heating cycle to 60°C with hydrostatic pressure of 27.5 bar

Development tests have shown that optimum results are achieved if a polyethylene sheath is applied over the lead sheath followed by one or more layers of reinforcing tape.

In the case of FF cables, the internal pressure of the impregnant will increase as the laying depth increases. However because the density of the impregnant is lower than that of the sea-water a differential pressure will exist which increases with depth by approximately 0.16 bar per metre, the exact value depending on the actual densities of the impregnant and the sea-water. The reinforcement provides some resistance to deformation but cables to be installed in deep water must be designed to withstand this external pressure. This aspect is covered in the CIGRE test recommendations. 2 Gas pressure cables are usually laid without pressurisation and therefore experience the full external pressure of the water during laying.

Choice of lead alloy sheath

The lead alloy sheath of a submarine cable has a very onerous duty imposed upon it compared with that of land cables. It will probably be subjected to strains and vibrations when the cable is laid, when in service if the cable is not buried and due to the motion of the ship if the cable has to be recovered for repair. Tests have shown that lead alloys E (0.4% Sn and 0.2% Sb) and F3 (0.15% As, 0.1% Bi, 0.1% Sn) have adequate fatigue life for this duty) Submarine cables have also given satisfactory service, however, when sheathed with other alloys, e.g. ½B (0.45% Sb) and ½C (0.2% Sn, 0.1% Cd).


Choice of armouring

Submarine cables need to be armoured to withstand the highest tensile loading likely to be encountered when laying and the residual tension left in the cable after laying (typically 1-3 tonnes) and to provide reasonable resistance to impact and abrasion damage from trailing ships' anchors or fishing gear. The armour must be resistant to corrosion as failure of individual wires in service may cause kinks to form followed by electrical failure of the cable. Galvanised steel wires meet the mechanical and anticorrosion requirements at lowest material cost but, as described earlier, the use of magnetic armour on single-core cables may cause an unacceptable reduction in the current rating of the cable.

Resistance to impact and abrasion damage is improved by applying double wire armour. If the cables are to be coiled when transported, it is necessary to apply both layers with the same direction of lay. As explained in the following section, this may not be acceptable for cables to be laid in deep water. Additionally, the added protection may only be considered necessary over part of the cable route such as the shore ends. This raises manufacturing difficulties, e.g. in order to produce a cable with acceptable handling and bending performance it is necessary to 'let in' individual armours by welding them to adjacent wires over a distance of a few metres in the transition between single and double wire armour.

When the economics of steel wire armoured a.c. single-core cable schemes is unfavourable, hard drawn copper wires or strips can be used. 4 Aluminium alloy armour wires were applied to the 420 kV a.c. submarine cables laid between Denmark and Sweden. 5 However, there are published reports of chemical and electrolytic corrosion failure of the aluminium alloy armour wires on the cables laid between Connecticut and Long Island. 6

Torque balance in cable construction

When a single wire armoured cable is suspended from the bow sheave of the laying vessel, a 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 sea bed. The twisting action therefore tends to concentrate in the suspended cable between the bow sheave and the sea bed. The problems become more severe with increasing immersed weight per unit length and increasing depth of laying. Figure 42.4 shows a submarine cable which has developed a kink due to lack of torque balance.

The twisting action can be greatly reduced by applying a second reverse layer of armour wires which under tensile loading conditions produces an approximately equal and opposite torque to that of the inner layer of wires. This construction is now generally used for cables to be installed in water depths greater than about 500 m. The twisting action can be reduced to acceptable levels for cables to be installed at lower depths by the application of metal anti-twist tapes below a single layer of armour wires to produce a counter torque, as shown in fig. 42.5. The requirements may be expressed mathematically.


Fig. 42.4 Recovered sample of submarine cable containing kink due to lack of torque balance

The torque produced by the armour is

Tra sin 0 (kg m)

and the torque produced by the anti-twist tapes is

Trat sin q~ (kg m)

where T = tension in cable (kg) 0 = angle of armour wires to cable axis ~b = angle of anti-twist tapes ra = pitch circle radius of armour wires (m)

r a t = pitch circle radius of anti-twist tapes (m)

The torque is balanced when

Tra sin 0 = Trat sin ~b


j~ ~ e '~ Wire armour

nti-twist tape

Fig. 42.5 Torque balance in single-core cable by using single wire armour and anti-twist tapes

i.e. when

ra sin ~b = rat sin 4

Although it is difficult to achieve complete torque balance with a single layer of armour and anti-twist tapes, the solution is economically attractive for installation in depths up to about 200m as the anti-twist tapes make a significant contribution to reinforcing the cable against internal and hydrostatic pressures.

Avoidance of cable movement on sea bed

Any cable which is subject to movement across the sea bed due to tidal currents is liable to premature failure due to abrasion damage. It is therefore important that the submerged weight of the cable is adequate to resist the maximum tidal sea-bed currents expected, even under storm conditions. The resistance to movement is proport ional to the square root of the coefficient of friction of the cable with respect to the sea bed multiplied by the W/D ratio of the cable, where W is the submerged weight of the cable and D is the overall diameter. The coefficient of friction may be as low as 0.2 if the cable is laid across smooth rock but a value of 0.5 is typical of a cable laid on sand or shingle.

Although in many cases the cable may ultimately be silted over, so inhibiting cable movement, it would be unwise to rely on this occurring uniformly along the complete length of the cable. Therefore, standard practice when designing a submarine cable is to calculate the water velocity likely to cause movement. Should this velocity be less than the maximum expected tidal current at the sea bed, including the shore approaches where tidal currents may be the highest, it would be necessary to increase the weight of the cable. For lead sheathed cables an increase in the weight of the cable is achieved at minimum cost by increasing the thickness of the sheath. For non-metallic sheathed cables it may be necessary to apply lead tapes or to insert lead fillers in the cable to attain an adequate weight-to-diameter ratio.

Mechanical performance of submarine cables

All a.c. and d.c. submarine cables are required to comply with the requirements of the CIGRE 'Recommendations for mechanical tests on submarine cables'. 2 This


specification requires that the cable, including flexible joints, should be subjected to tension tests, bending under tension, external pressure withstand tests and, in the case of pressure-assisted cables, an internal pressure withstand test. If the cable is to be coiled, either during manufacture or for transporting to site, all the tests listed above are preceded by a coiling test. The cable is required to withstand a voltage test following the mechanical tests. The cable should be free of damage or untoward features when subjected to visual examination following the mechanical and electrical tests.

P R O T E C T I O N AGAINST C O R R O S I O N

The service life of a submarine cable will be dependent, inter alia, on the efficacy of the anticorrosion protection, as sea-water is an aggressive environment in which to operate. Should the metal sheath of a non-pressure-assisted cable suffer corrosion damage, water would enter the cable and a voltage failure would ultimately occur. In the case of internal pressure-assisted cable, i.e. gas- and fluid-filled cables, a leak in the pressure- retaining sheath would cause the pressure alarms to operate. Small leaks can be tolerated for a short time by maintaining the gas or fluid feed until it is convenient to undertake cable repairs. Corrosion or failure of the sheath reinforcing tapes would cause the sheath to burst. This would necessitate taking the cable out of service and undertaking emergency action to avoid water entering the cable.

Lead alloy sheaths are preferred to aluminium sheaths for submarine cables because of the vastly superior resistance of lead to corrosion when immersed in sea-water. I f an aluminium sheath is used, any defect or damage to the anticorrosion sheath would be liable to cause rapid corrosion failure of the metal sheath.

Extruded polyethylene provides the most impermeable barrier to moisture ingress and is used practically universally for the anticorrosion sheath. It is applied directly above the lead alloy sheath or above the reinforcing tapes in the case of pressure- assisted cables. If protection against teredo attack is considered necessary, one or two layers of brass or copper tapes are applied over the polyethylene sheath.

The provision of an anticorrosion sheath external to the armour wires of an a.c. submarine cable is generally unnecessary and technically undesirable. In all but the simplest cable installations it would be impossible to guarantee that any extruded thermoplastic or elastomeric oversheath would be undamaged during the cable laying operation, particularly if laying over rocks. Should the oversheath survive the laying operation without damage, it would still be subject to damage while in service from marine borers, sometimes by fish and invariably by shipping activity. If the oversheath were damaged, some of the current in the cable sheath/armour circuit would flow to the sea at the point of damage to the oversheath, causing electrolytic corrosion of the armour wires.

If the cable is served overall with textile tapes instead of an extruded oversheath, the armour wires are uniformly in contact with the sea-water along the complete cable route and the resultant distributed electrolytic action on the armour wires is very small. Some manufacturers recommend that each individual armour wire should be sheathed with a thermoplastic anticorrosion covering. Severe problems have sometimes been experienced when this solution was adopted on single-core a.c. cables because the corrosion by-products formed following corrosion of one damaged wire caused corrosion damage to adjacent but previously undamaged wires. The use of individually


sheathed armour wires reduces the number of armour wires which can be applied to the cable, renders the cable subject to damage between adjacent wires and reduces the ultimate tensile strength of the cable.

Experience has shown that the zinc coating of galvanised steel armour wires may deteriorate after a few years' immersion, thus displaying small areas of non-protected steel. Slight corrosion pitting has been observed in such areas, but in a typical 138 kV a.c. single-core cable installation the diameter of individual armour wires merely decreased from 5.89 to 5.86 mm after 24 years' service.

Overall finishes

When laying a submarine cable it is necessary to brake the cable as it is paid out over the bow or stern skid in order to maintain control of the disposition of the cable on the sea bed (chapter 43). The amount of compression applied to the cable for braking purposes is dependent on the coefficient of friction between the finish of the cable and the braking gear. Apart from the disadvantages of a plastic oversheath on an a.c. cable, to which reference has been made, plastic materials have a low coefficient of friction which necessitates undesirably high pressure on the cable to attain the required braking effort, particularly for deep water installations. For these reasons a bitumen coated jute or polypropylene string serving is preferred, both of which have a high friction coefficient with respect to the braking gear. A string serving also has the advantage of being sufficiently elastic to permit an increase in the pitch circle diameter of the armour wires resultant on coiling the cable without adverse effect on the serving. If a plastic oversheath were applied and the bursting force of the armour wires or other mechanical incident caused rupture of the plastic oversheath, bird-caging of the armour wires would be expected at that position when coiling the cable.

SOLID TYPE MASS I M P R E G N A T E D CABLES

Solid type mass impregnated cables have been used for submarine installations up to 34.5kV a.c. or 450kV d.c.

A.C. cables

These are virtually all of the 3-core construction. The 3-core HSL type of cable is generally preferred to the 3-core H type cable, particularly for deep water installations as the construction is inherently more resistant to external water pressure.

The conductors of 3-core HSL and H type cables are normally circular, of compacted construction. The paper insulation, conductor and core screens are applied in a similar manner to that of comparable land cables except that if the cable is to be coiled down, either during manufacture or for transport to site, it may be necessary to modify the paper lapping tensions to attain satisfactory coiling characteristics. The cables are usually impregnated with a viscous fluid compound as this results in more complete filling of the cable than a non-draining compound. This helps to restrict the passage of water along the cable in the case of damage in service. The individual impregnated cores for HSL cable, or laid-up cores for H type cable, are sheathed with a lead alloy and served with an anticorrosion sheath. The three served cores for HSL cables are laid up


and padded circular with bitumen impregnated jute or polypropylene string fillers. Multiple lengths of non-armoured cable are jointed together prior to armouring into a continuous length. After armouring and serving, the cable is coiled down or wound onto a turntable to await pre-despatch tests and transfer to the laying vessel.

D.C. cables

The development of the mass-impregnated solid type cable for operation at high d.c. stresses has led to the possibility of high power long length d.c. connections. The important advantage of the solid type d.c. cable over the pressure assisted types is that as there is no requirement to maintain pressure in the cable, there is no limit on the length of cable that can be used. The practical maximum length for a FF cable is about 60 km. The Baltic connection between Sweden and Germany consists of a solid type cable operating at a voltage of 450 kV with a length of 250 km. v Proposals have been made for an Iceland-UK connection which would have a route length of over 1000km. s There are now numerous submarine interconnections between the power systems of Europe.

D.C. cables are all single-core construction. The conductors of d.c. cables are circular and of high occupancy. A high occupancy may be achieved by stranding layers of segments around a central circular rod. An occupancy of 96% is claimed for the ±270 kV d.c. cross channel interconnection. 9 High occupancy in the conductor has the advantage of presenting excellent resistance to water penetration in the event of cable damage especially when used in conjunction with a viscous impregnating compound. Although the construction of the single-core solid cable is intrinsically very resistant to external pressure it is necessary to control the sheath movement due to load cycles. This is done by limiting the maximum conductor temperature to 50°C.

The cables are insulated on high precision lapping machines similar to those used for FF cables. To minimise the number of flexible joints required, the cables are insulated and impregnated in the longest lengths possible. Some cable manufacturers have special drying and impregnating vessels capable of processing 40 or 50km lengths of cable. This process requires the core to be wound onto a turntable within the drying vessel as it leaves the insulating machine. Flexible joints may be inserted between lengths of core usually at the lead or anticorrosion sheath stage in order to create even longer lengths. This type of joint may be used without any reduction in cable electrical or mechanical strength which makes the maximum length attainable limited only by the size of the storage space or the capacity of the laying vessel.

The lead sheathing, anticorrosion sheathing, armouring and serving processes differ little from standard production techniques for land cable. However, the lead sheathing and plastic sheathing of very long cable lengths necessitate continuous processes, often lasting several days, and require the highest standards of extruder performance to achieve satisfactory product quality.

Water penetration through damaged sheath

If the lead sheath of a single-core or 3-core HSL paper insulated solid type submarine cable is damaged, or the cable is severed, the sea-water causes the paper insulation to swell rapidly, so forming a partial blockage to further water ingress. Further cable beyond the blockage, however, may be affected by moisture owing to the vacuous


conditions in the cooling cable and by wick action in the insulating papers. Because of its high occupancy, the conductor has excellent resistance to water penetration.

The interstitial fillers in 3-core H type cable form a low resistance path for water to enter the cable following damage to the sheath. It may be necessary to replace a substantial amount of cable to eliminate moisture should this type of cable be damaged after laying.

FLUID-FILLED CABLES

Fluid-filled paper insulated cables are suitable for use up to the highest voltages and they were the type of cable chosen for the 525 kV a.c. link between British Columbia mainland and Vancouver Island installed in 1984.1° Fluid-filled cables have the advantage of requiring the smallest insulation thickness for a given voltage so that it is possible in some cases to use a 3-core cable where three single-core cables would otherwise have been necessary. This presents advantages from a manufacturing and installation point of view as well as making the installation less prone to damage when in service.

FF cables are suitable for d.c. operation. However hydraulic considerations limit the maximum circuit length to about 50km for paper cables. This is too short for many applications and therefore this type of cable is not widely used for d.c. submarine connections. A recent development is the application of fluid-filled PPL cable for a 500 kV d.c. 50.7 km circuit in Japan. ~l PPL offers the possibility of reduced insulation thickness, lower charging current for a.c. schemes, and of permitting longer circuits to be considered before hydraulic limitations become important.

Hydraulic design

It is an essential feature of any fluid-filled submarine cable installation that, if the pressure-retaining sheath is damaged, the internal fluid pressure under leak conditions should exceed the external water pressure at that position, so that water does not enter the cable. The worst possible condition would be for the cable to be severed near one terminal during a period when the cable was carrying full load current, as the rate of cooling would then be at a maximum. Whereas under normal operating conditions the fluid reservoirs at each end of the route effectively feed fluid to the mid-point of the route, if the cable is severed near one terminal the seaward length of cable would be dependent on the fluid reservoir at the remote end of the route. This reservoir would be required to feed fluid at a sufficient rate to compensate for the thermal contraction of the fluid within the complete cable length and also to maintain a positive pressure with respect to the external water pressure where the cable was severed. The fluid feed length would therefore be nearly twice the normal length and the pressure drop nearly four times that which would occur if the cable was severed at the mid-point. It is therefore necessary to incorporate fluid channels within the cable having sufficiently low hydraulic impedance that a positive pressure can be maintained under fluid leak conditions, without the need to operate the cable at an excessive internal pressure.

The calculation of the hydraulic parameters of the cable system is similar to that detailed in chapter 30 for fluid-filled cables installed on land, but for submarine cable installations it may be necessary to calculate many alternative solutions to obtain the optimum size of the fluid duct(s) and the feed pressure of the fluid pumps. There are


practical limits to the size of the fluid channels which can be incorporated in a cable and this limits the maximum length of route over which a fluid-filled cable can be operated to about 50 km. On long cable routes and in deep water installations fluid-filled cables are required to operate at higher internal pressure than a conventional land cable and it is therefore necessary to apply additional reinforcing tapes to enable the lead alloy sheath to withstand the internal fluid pressure.

Further details regarding calculation of fluid pressure transients may be obtained by reference to the C I G R E report 'Transient pressure variations in submarine cables of the oil-filled type', t2 This document also explains how to allow for pressure transients encountered during the laying operation.

Figure 42.6 shows the internal fluid pressures in a typical 1000 m m 2 275 kV single-core cable operating to a maximum conductor temperature of 80°C on a cable route having a maximum depth of 90 m. The cable concerned has a fluid duct of 23 m m bore. The graph shows the variation in maximum fluid pressure along the cable route when the cable is heating, the minimum pressure when cooling and the transient pressures if the cable is severed near one end of the route following a period o f full current loading. I t will be noted that, in order to maintain an internal fluid pressure of I bar at the severed end of the cable, it is necessary to operate the fluid pumps at 11 bar, the pressure drop along the complete route from the remote fluid pump being approximately 10 bar. The graph also shows that the cable would need to be reinforced to withstand an internal fluid pressure of 30 bar.

35 ̧

5 Cable severed I " , , ,~h e n cooling

o' ib . . . . . . . . . . . .

~00 I . . . . . . . . 2.5 5 7.5 ~0 ~2.5 ~5 ~7.5 20

~en~h (~m)

Fig. 4~.~ Variation in fluid pressure in 1000mm ~ 275kV fluid-filled single-core cable under operational and fault conditions

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It is sometimes necessary to restrict the temperature rise of fluid-filled submarine cables to reduce the fluid pressure transients to acceptable levels. This necessitates the use of larger conductors than would otherwise be justified by the current loading requirement.

3-core cables

3-core fluid-filled submarine cables can be manufactured for voltages up to 150 kV a.c. subject to certain restrictions on the maximum cross-sectional area of the conductors. As with all types of 3-core cable the capacity of the laying-up machine limits the maximum length which can be manufactured without the need to insert flexible joints in at least the conductors and insulation. Alternatively, drum lengths of completed cable may have flexible joints inserted between drum lengths before they are installed.

Flexible joints are technically acceptable in fluid-filled cables. Comprehensive laboratory testing by several cable manufacturers has shown that the electrical characteristics of a well designed flexible joint can be as good as the machine-made cable and the mechanical performance completely adequate. Flexible joints have additionally given satisfactory service in 150 kV a.c. fluid-filled cables for many years.

The design and construction of the conductors, insulation and screens is similar to that for land cables except that the angle of lay and application tension of the paper tapes may need to be modified if the cable is to be coiled down either during manufacture or on the laying vessel. When laying-up the cores, fluid ducts are usually included in the interstitial spaces between the cores. The maximum bore of the ducts which can be accommodated within the spaces available may impose a restriction on the length of route over which a 3-core cable can be laid.

The laid-up cable is generally sheathed with a lead alloy and reinforced against the internal fluid pressure by two or more layers of metal tape. Tin-bronze and non- magnetic steel tapes have been used for reinforcement. As the transient fluid pressure in submarine cables is often much greater than is normally permitted in land cables, the total thickness of reinforcement is correspondingly increased. The finished cable is either coiled down or wound onto a drum or turntable for pre-despatch testing prior to loading onto the cable laying vessel.

Fluid feed tanks are connected to the cable ends immediately after sheathing and a positive fluid flow is maintained from the cable ends on every occasion that the cable is cut, so that no air or moisture can enter the cable. The fluid tanks remain connected to the cable throughout all subsequent manufacturing and laying operations.

Single-core cables

Single-core fluid-filled cables are not subject to the same restrictions on maximum unjointed manufacturing lengths as 3-core cables are, because the maximum lengths attainable are only dependent on the size of the impregnating and drying vessels available. Some cable manufacturers are equipped with very large vessels capable of drying and impregnating 40-50 km lengths of core.

Single-core cables are required to have a central fluid duct in the conductor and the maximum length which can be installed is limited by the impedance to fluid flow which this duct presents. Significant improvements can be made by increasing the bore of the duct as its impedance varies with the fourth power of the radius. This improvement has


to be balanced against the impact of having to increase the diameter o f the cable. Additionally it may be possible to employ a low viscosity impregnant. The single-core conductor may be stranded around a helically wound steel duct or alternatively be formed of interlocking segments which form a self-supporting fluid channel with shaped wires stranded over it. If the cable is to be coiled at any stage during manufacture or installation, special attention needs to be given to the length and direction of wire lays to achieve satisfactory coiling characteristics.

All manufacturing processes are carried out on conventional plant, apart from the use of turntables or coiling down processes, if the length of cable required exceeds the capacity of the largest drums which can be handled. I f flexible joints are to be made prior to despatch, these are normally inserted after lead sheathing or after any subsequent process, as convenient. Fluid pressure tanks are connected to the cable after lead sheathing and at all subsequent stages of manufacture.

The sheath reinforcing tapes are non-ferrous for a.c. cables but steel tapes offer the cheapest reinforcement for d.c. cables. The anticorrosion sheath, armour and servings generally comply with the details previously quoted.

Fluid pressure supply systems

For short length fluid-filled submarine cable installations, conventional fluid pressure tanks and appropriate alarm equipment, as used for land cables, are often adequate for maintaining the fluid pressure within the cable. On longer cable routes it is often necessary to provide pumping plant and fluid storage tanks at both ends of the route because of the much larger fluid flow and high pressure. The pumps are normally duplicated and alarm circuits are provided to signal any malfunction of the pumping equipment. As these pumps are usually energised from the public electricity supply it may be necessary to install auxiliary power generators to ensure the security of the fluid-filled cables in the event of a failure of the electricity supply.

Prevention of water penetration through a damaged sheath

If the lead sheath of a fluid-filled cable is perforated when in service, the internal pressure causes fluid to flow out so that water does not enter the cable. The fluid tanks or pumps at the cable terminals replace the fluid lost and operate alarms to indicate an abnormal rate of fluid flow. In the event of a major fluid leak the standard procedure is then for the cable to be taken out of service and a restrictor actuated in the hydraulic circuit to limit the fluid feed to the minimum necessary to ensure a positive fluid leak to the sea at all states of the tide. Ideally, the fluid leak should be located at an early stage, the cable cut and lifted and the ends checked for moisture and cut back if necessary. End caps would be fitted and the capped ends lowered to the sea bed with marker buoys attached. Experience has shown that after removing the lengths of cable damaged during the lifting operation the remaining cable is generally free of moisture and suitable for jointing to the new piece of cable required to link the cut ends.

GAS-FILLED CABLE

Cable of the pre-impregnated paper gas-filled type has many advantages from a manufacturing and system design point of view. As manufacture of this type of cable


requires no large tank in which to impregnate the core, the possible length of joint- free cable is entirely dependent on the capacity of the laying-up machine or, for single-core cables, the amount of storage space available. The general construction of the conductor, screens and insulation follows solid cable practice except that for the longest lengths of single-core cable a gas duct is required in the centre of the conductor.

The metal sheath is of lead alloy which is heavily reinforced with steel tapes followed by a conventional anticorrosion sheath, beddings, a rmour and serving.

The static pressure in the gas-filled cable system is maintained higher than the external water pressure to prevent ingress of moisture should the cable be damaged. The minimum design operating pressure is 14bar but in one installation the pressure had to be raised to 28 bar to fulfil this condition. ~3 This places an onerous duty on the cable reinforcement and the accessories. Once the cable is charged with gas there is no longitudinal gas flow within the cable which means that there is not the same pressure restriction on cable length inherent in fluid-filled systems, but gas feed and alarm equipment is still required at the cable ends.

Factory flexible joints have been installed into gas-filled cables and have given satisfactory service for many years.

PO LY MER I C INSULATED CABLES

Design limitations

Polymeric insulated submarine cables are in increasing demand for use in offshore oil fields and inter-island links up to a voltage of about 33 kV. Considerable experience in land cable installations in many parts of the world has indicated that the service life of XLPE cables may be limited unless steps are taken to prevent moisture ingress into the insulation. It is therefore necessary to provide an impermeable moisture barrier over the insulation, and for submarine installations this is best achieved by the application of a lead alloy sheath.

The immersed weight-to-diameter ratio of non-sheathed polymeric submarine cables is generally low and the cable is therefore liable to movement over the sea bed when subjected to even modest tidal currents. The application of a lead sheath of appropriate thickness inhibits cable movement, apart from ensuring that the core insulation is maintained in a dry condition.

Submarine cables insulated with EPR require no metal sheath and are installed as a 'wet' construction. This type of cable is suitable for use up to 33 kV providing selection is made of a special formulation of EPR and of a low insulation design stress. Some 3-core designs incorporate optical fibres or other communication cables in the interstices.

To date, polymeric cables have not generally been used for submarine connections above 66kV. This is because they are generally larger than the equivalent pressure- assisted cables and the concern about the ability to water block the cable adequately in the event of sheath damage. Polymeric insulation is not chosen for d.c. use. The electric strength is reduced by the generation and concentration of trapped space charge near to the conductor and insulation screens.

Cable design and manufacture

The thickness of the screens and insulation applied to polymeric cables up to 30 kV rating should not be less than the appropriate value quoted in IEC 502, although, as


with all submarine cables, a slight increase in the insulation thickness may be justified to enhance the factor of safety. The thickness of insulation for cables above 30 kV is subject to agreement between the customer and the cable manufacturer, there being as yet no internationally agreed standard for the insulation thicknesses for the higher transmission voltages.

The economics of the insulating procedure for polymeric cores favour the product ion of the longest continuous core lengths which can be accommodated on a turntable or core drum. However, the need to regularly clean the insulation extruders limits the length that can be continuously extruded, thus introducing factory joints of a more complex moulded type. As with all types of 3-core cable the capacity of the laying-up machine limits the maximum length of cable which can be manufactured without inserting flexible joints.

When a lead sheath is applied over a single- or 3-core cable the external protection normally comprises a polyethylene anticorrosion sheath, armour bedding, one or two layers of armour wires and a bituminised textile serving overall.

HV routine testing of polymeric cables and factory joints is more problematic, as d.c. testing is regarded as less effective for polymeric insulation and introduces the risk of stress increase by space charge generation.

Effects of damage to lead sheath

Polymeric materials have a relatively high coefficient of expansion which may cause a permanent distension of the lead sheath after an XLPE or PE insulated cable has been on load. Should the lead sheath be damaged after it has been distended, water would enter the annulus between the core(s) and the inside of the lead sheath. The penetration can be minimised by the use of water-swellable tapes over the cable insulation. I f the damage is sufficiently severe to reach the conductor, then water penetration can be inhibited by filling the conductor with a blocking compound. While these techniques have been developed for land cables, in the case of submarine cables the external water pressures are much higher.

P ILOT, C O N T R O L AND C O M P O S I T E CABLES

There is an increasing demand in offshore oil and gas fields for submarine pilot, control and composite cables which are laid between platforms or between a platform and a sea- bed unit for the remote control and monitoring of equipment. In each case the cable needs to be designed for the specific project, the number and type of cores varying considerably. Generally there are requirements for some pilot cores, speech circuits or TV cores together sometimes with optical fibres, power cores and hydraulic hoses within the same cable envelope. The cable needs to be designed and manufactured to have a high factor of safety, as the failure of a control cable may necessitate closing down an oil well with the consequent loss of production until the cable is repaired or replaced.

PE or sometimes XLPE is favoured for the insulation of speech circuits, PE for TV cores and either PE, XLPE or EPR for pilot and power cores. Flexible hydraulic hoses normally comprise a polyester elastomer inner core which is reinforced with aramid fibre and sheathed with a polyester elastomer jacket.


The individual cable pairs, cores and hoses are laid up with a relatively short lay to reduce the risk that these components are subjected to a significant strain when the cable is under tension. The optical fibre is generally laid up near the centre of the assembly to give maximum protection against damage. Fillers are normally laid up with the cable components to achieve a compact circular shaped cable. Providing there is no severe restriction on the mutual capacitance of the speech pairs it is considered advantageous to inject a thixotropic compound into the remaining interstices of the laid-up cable to restrict water penetration in the event of sheath damage after installation. In most cases a polyethylene sheath is applied over the laid-up cores followed by the appropriate armour and serving, the finish being generally similar to that applied to lead sheathed submarine power cables.

If the cable is required to hang in a catenary from a platform to the sea bed, it is essential that the cable be torque balanced. In these circumstances it is sometimes advantageous to lay up the cable around a central tensile member and to dispense with the external armour wires. If the cable is to be laid on the sea bed it may be necessary to apply a lead sheath to attain an adequate weight to diameter ratio to inhibit movement on the sea bed under the influence of tidal currents. Alternatively, in order to avoid the need for a lead sheath, it may be possible to bury the cable using a ' remote operated vehicle'. Such vehicles are usually equipped with sonar or TV systems and bury the cable using a plough technique.

FLEXIBLE J O I N T S FOR CABLES

Techniques have been developed for constructing flexible joints in all types of cable commonly used in submarine cable installations. Most existing major submarine cable circuits contain one or more flexible joints. These joints have generally given trouble- free service. Individual cable manufacturers have developed different techniques for constructing flexible joints but all are expected to meet the mechanical test requirements

. . . . . . . . . . . . . . .

" . - i . . . . . - , ,

eeee

F.

Fig. 42.7 Construction sheath stage

A. Lead sheath G. Conductor joint

B. Dielectric screen H. Hand appl ied screen

C. Stepped pencil I. Re-applied screen over conductor joint

D. Conductor screen J. Lead sleeve

E. Paper & terylene built up to a diameter of K. Soldered joint lead burn L. Lead burn joint Hand applied paper tapes

of flexible joint in single-core solid type submarine cable up to the lead


recommended by CIGRE Committee 21-06. 2 Figure 42.7 shows a typical design of flexible joint in a solid type paper insulated submarine cable.

Conductor connection

Submarine cables are generally laid with a certain amount of tension in the system. This is either unavoidable because of the length of cable suspended underneath the laying ship or is deliberate in order to prevent kinking. The tensile strength of the conductor connection is therefore of paramount importance. Two alternative methods are used for the jointing of conductors. The individual wires of the conductor may be butt-brazed in a staggered configuration so that the conductor retains its original diameter and flexibility. Alternatively, the multiwire conductor may be butt-brazed to form a solid joint which complies with the original diameter but restricts the flexibility o f the conductor over a short distance.

Paper insulation and semiconductive paper screens

In all types of paper insulated cable the conductor screen and insulation are profiled on both sides of the joint in the conductor. The length of the stepped profile is designed to ensure a controlled electrical stress when the cable is energised. The insulation is then reconstituted over the jointed conductor, pre-impregnated paper tapes being applied by hand or machine under very carefully controlled conditions often in a humidity- controlled atmosphere. Each paper tape is accurately terminated on the steps of the profiles of the two cables being jointed. The re-application by hand can be a fairly lengthy process and it is possible to design a joint with shorter profiles which are quicker to re-insulate but it is then necessary to restrict the number of bends to which the joints will be subjected. This extra speed in completing a joint is very useful where a cable is being repaired at sea.

The outside diameter of the joint is determined by service performance and electrical test requirements applicable to the system. The use of a flush brazed conductor connection and graded paper tape insulation enables the diameter of the dielectric to be kept to a minimum. Sometimes it is possible to insulate the joint to the same diameter as the adjacent core but usually the joint is insulated to a greater diameter. This diameter, however, is rarely greater than the lead sheath of the cable and allows the lead sleeve to fit closely over the insulation and cable sheath. This enhances the mechanical strength of the joint and is convenient for the process of sealing the sleeve to the sheath.

For solid type cables it is usual to baste the reconstituted insulation with impregnating compound at regular intervals during the insulating process. When making flexible joints in fluid-filled cables two alternative techniques are available for preventing air or moisture from entering the cable during the jointing operation. The joint may be made under a continuous flow of fluid fed from pressure tanks located at the remote ends of the cables being jointed, or the cable may be frozen on each side of the joint and the papers applied without a fluid feed. In both cases the joint is required to be impregnated with fluid after the re-insulation process. This is achieved either via special fluid fittings in the lead sleeve which will need to be sealed afterwards or by impregnating using a temporary sleeve and then swaging down a lead sleeve over the insulation with continuous fluid flow.


Polymeric insulation

The insulation of PE, XLPE and EPR cores is pencilled down to a smooth controlled profile using special tools. The cable core is then reconstituted with special insulating and screening tapes which can also be applied with specially designed machines. Application by machine ensures even tension and correct geometric shape. Tempora ry binder tapes are applied to the reconstituted joint to compress the hand applied tapes while the joint is pressurised and heated under carefully controlled conditions to bond the PE tapes and to vulcanise the insulation in the case of XLPE or E P R cores.

An alternative procedure is to use an injection moulding machine for all types of core.

Lead sheaths

Prior to the start of jointing lead sheathed cables, a lead sleeve is passed over the lead sheath of one of the cable lengths being jointed. After completing the application of the core insulation and screens, the lead sheath is positioned over the core joint and progressively swaged down until it is a close fit over the insulated core. I f the diameter of the reconstituted core insulation is equal to that of the original core, the lead sleeve is swaged down and cut to length to permit butt joints to be made to the cut ends of the lead sheath of the cables being jointed. However, if excess insulation has been applied over the joint, the lead sleeve is swaged down to be a close fit over the original lead sheath.

Table 42.1 Major submarine transmission interconnections

Installation Site Voltage Cable type Maximum Length date (kV) depth (m) (km)

1954 Gotland 100 d.c. MI 160 100 1 9 5 6 Vancouver 132 a.c. GF 183 25 1 9 6 5 Sardinia-Corsica 200 d.c. MI 500 119 1965 Cook Strait 250 d.c. GF 245 40 1 9 6 5 Sweden-Denmark 285 d.c. MI 80 64 1 9 6 9 Vancouver 300 d.c. MI 200 27 1 9 7 3 Sweden-Denmark 400 a.c. FF 37 7 1 9 7 6 Skagerrak 260 d.c. MI 570 125 1 9 8 0 Hokkaido-Honshu 250 d.c. FF 300 42 1 9 8 4 Vancouver 525 a.c. FF 400 30 1 9 8 5 Jersey-France 90 a.c. FF 25 27 1 9 8 6 England-France 266 d.c. MI 55 50 1 9 8 9 Finland-Sweden 400 d.c. MI 117 200 1 9 9 3 Skagerrak 350 d.c. MI 540 125 1994 Baltic 450 d.c. MI 30 265 1 9 9 5 Denmark-Germany 400 d.c. FFF 21 45

In progress Spain-Morocco 400 a.c. FF 610 27

MI Mass impregnated solid type FF Fluid-filled GF Gas-filled FFF Flat fluid-filled


A fusion technique known as lead burning is the conventional method of sealing the lead sleeve to the cable sheath and lead burns have given excellent service over many years. The technique requires highly skilled operators and is very time consuming. More modern methods based on a TIG welding technique are under development.

Other components

Metal tapes such as reinforcing tapes or anti-teredo tapes are jointed by brazing or welding. Polymeric anticorrosion sheaths are reconstituted using polyolefin heat- shrinkable sleeves passed over one of the cable cores prior to jointing.

Armour wires in joints between drum lengths of completed cable or in repair joints may be jointed by welding or by the use of threaded turnbuckles. Both these methods give more than adequate mechanical strength. For factory-made joints it is possible to pass the joint through the armouring machine and armour the cable and joint as one continuous process. This solution is adopted where continuous lengths of cable are specifically required.

MAJOR TRANSMISSION INTERCONNECTIONS

There is a growing number of major high power submarine interconnections. Table 42.1 gives details of some of the more important installations.

REFERENCES

(1) Buseman, F. (1963) 'The magnetic compass errors caused by d.c. single-core sea cables'. ERA Report No. B/T 116.

(2) CIGRE Study Committee 21, Working Group 06 (1980) 'Recommendations for mechanical tests on submarine cables'. Electra (68), 31-36.

(3) Anelli, P., Donazzi, F. and Lawson, W. C. (1988) 'The fatigue life of alloy E as a sheathing material for submarine power cables'. IEEE Trans. PD-3 (1).

(4) Dominguez Miguel, M., Ruiz Urbieta, M., Benchekroun, A., EloKindi, O. and Gallango Faraco, E. (1994) 'Technical requirements for the submarine electrical interconnection Spain-Morocco'. Paris: CIGRE Paper No. 21-205.

(5) Gazzana Prioroggia, P. and Maschio, G. (1973) 'Continuous long length a.c. and d.c. submarine h.v. power cables'. IEEE Trans. PAS-92 (5), 1744-1749.

(6) Chamberland, D. M., Margolin, S. and Shelley, M. (1979) 'The Long Island Sound submarine cable interconnection'. 7th IEEE Transmission and Distribution Conf.

(7) Ekenstierna, B. and Nyman, A. (1993) 'Baltic HVDC interconnection'. New Zealand: CIGRE SC 14 Colloquium Paper No. 63.

(8) Guonason, E. and Henje, J. (1993) 'A 550MW HVDC submarine cable link: Iceland-UK-Continental Europe'. lEE Conf. on Power Cables and Accessories 10 k V-500 k V, pp. 220-224.

(9) Arkell, C. A., Ball, E. H., Hacke, K. J. H., Waterhouse, N. H. and Yates, J. B. (1986) 'Design and installation of the U.K. part of the 270kV d.c. cable connection between England and France, including reliability aspects'. Paris: CIGRE Paper No. 21-02.


(10) Foxhall, R. G., Bjorlow Larsen, K. and Bazzi, G. (1984) 'Design, manufacture and installation of a 525 kV alternating current submarine cable link from mainland Canada to Vancouver Island'. Paris: CIGRE Paper No. 21-04.

(11) Fujimori, A., Janaka, T., Takashima, H., Imajo, T., Hata, R., Tanabe, T., Yoshida, S. and Kakihana, T. (1995) 'Development of 500 kV d.c. PPLP-insulated oil-filled submarine cable'. IEEE/PES Summer Meeting, July 23-27, 1995.

(12) CIGRE Study Committee 21, Working Group 02 (1983) 'Transient pressure variations in submarine cables of the oil-filled type'. Electra (89), 23-29.

(13) Crabtree, I. M. and O'Brian, M. T. (1986) 'Performance of the Cook Strait +250kV d.c. submarine cables, 1964-1985'. Paris: CIGRE Paper No. 21-01.

Documents

Submarine Cables and Systems