Underwater Installations

Underwater power transmission cables have been success-fully installed for over 65 years. This experience has included all of the transmission cable types (with the exception of compressed-gas-insulated and superconduct-ing cables) in a wide range of installation environments.

While the longest submarine cables are dc, primarily because of charging-current limitations associated with long ac cables, the majority of submarine cable installa-tions are ac. Chapter 7 provides a review of worldwide submarine cable experience. Submarine cable installa-tions involve a variety of potential hazards during installation, operation, and even during retrieval for repair. They are far more expensive to install than land-based cables and extremely expensive to repair. Thus, careful planning and thorough engineering of underwa-ter cable installations are essential.

Installing cables underwater involves many technical and economic design considerations beyond those in a conventional underground cable installation. Selecting a cable type and designing it specifically for underwater installation involve more than defining the power trans-mission capability and the desired transtrans-mission voltage.

Requirements for installation, operation, effects of marine life and environment, fault location, cable retrieval, and cable repair, as well as the effects of the installation on the environment, are all factors to be Figure 12-32 Bridge crossing with fiberglass duct (courtesy

POWER Engineers, Inc.).

studied and evaluated in the planning stage of an under-water installation. This is necessary to select the correct cable, cable configuration, route, handling equipment, and installation method.

Typical Design Aspects of Submarine Cables Cables for underwater applications are similar in their basic electrical design to conventional underground cables, but they differ in their exterior mechanical design, sheath requirements, and electrical bonding methods. The most significant difference involves the addition of armor, both to protect the cable from exter-nal mechanical damage and to provide mechanical strength to allow for cable installation and retrieval. The cable jacket and sheath design also have a critical role in ensuring the integrity of the cable. A more detailed dis-cussion on submarine cable system design is provided in Chapter 7. The following are key design aspects.

• Spacing

• Mechanical strength and protection

• Sheath/jacket requirements

• Corrosion protection

• Sheath bonding

• Cable losses

Types of Submarine Cable

The cable/utility industry has had successful submarine experience with a variety of cable types, including high-pressure fluid-filled pipe-type cables (HPFF), self-contained fluid-filled cables (SCFF), conventional solid-type mass-impregnated paper-insulated cables, paper-impregnated gas-pressure-assisted cables, and extruded-dielectric insulated cable. Selection of cable type depends on the desired transmission operating voltage and power transfer level, and the depth, length, and profile of the circuit. The alternative cable types are briefly described below.

Fluid-Filled, Fluid-Pressurized Cables

The highest-voltage submarine cables are those that are impregnated and pressurized with dielectric liquid (pipe-type and self-contained fluid-filled). Pipe-(pipe-type and SCFF cables are considered suitable for voltages up to 765 kV. These cables possess three advantages: their high power transfer; their ability to match overhead transmission voltages, which means transformers and other substation equipment could be eliminated at the land /submarine transitions; and the fact that they are self-protecting and provide warning in case of damage to the sheath or pipe. In case of limited sheath damage, the pressurized systems will pump dielectric fluid out at the site of the damage, minimizing water and other con-taminants from entering and traveling up into the cable.

Fluid pressure drop or reduced fluid levels in the

pres-surizing plant tank or reservoir can provide a warning of potential sheath damage. However, the loss of dielectric fluid is an environmental concern. These cable types are only feasible for moderate water depths and short-to-moderate lengths. Flow rate and pressure drop con-straints related to transferring fluid in and out of the cable due to load changes preclude using these cables for really long lengths, generally more than 30 miles (48 km). As the installation depth increases, the cable must be maintained at a higher and higher pressure to prevent the ingress of water should a leak occur, since the specific gravity of the dielectric fluid is somewhat lower than water. The above-surface portion of the cable and its accessories must be designed to withstand the fluid pressures necessary to provide slightly positive internal pressure at the deepest point in the submarine installation. For example, an 80 psi (550 kPa) pressure is needed for the 1312-ft (400-m) depth of the Vancouver Island cables.

High-Pressure Fluid-Filled Pipe-Type Cables

Pipe-type cable for submarine application has the same construction as for land-based cables. Three impreg-nated-paper-insulated conductors are installed in a pipe, which is filled with dielectric fluid and pressurized.

Design differences for submarine application may involve size and strength of skid wires, pipe wall thick-ness, and concrete pipe coating. Heavier-walled pipes are often used (0.5-in. or 0.375-in. vs. 0.25-in, wall [12.7-mm or 9.5-mm vs. 6.4-mm wall]), and provide a number of benefits, including increased mechanical strength, greater weight to reduce buoyancy, better abil-ity to withstand a corrosive or electrolysis condition in the event of a coating failure, and reduced probability of burn-through during a fault. A concrete coating is often added to the exterior of the pipe for mechanical and corrosion protection and for negative buoyancy.

The advantage of the pipe-type design is that the pipe provides mechanical protection for the cables and allows the three phases to be installed in close proximity. This minimizes the installation trench size, spoils removal, and construction time, and reduces environmental impacts during construction. Pipe-type cable is not nor-mally considered for long underwater crossings, because it is limited by the mechanical forces required to pull the cables into the pipe. The installed length cannot contain field splices; splices do not fit in the pipe, nor do they have the strength to be pulled into the pipe. In the event of an electrical failure, the cables can be pulled out and replaced from the land ends, with no need to deploy sur-face repair ships or perform at-sea splices. The longest section of submarine pipe-type cable, installed across Long Island Sound near New York City, is 6900-ft (2100-m) long (Bazzi 1982). One of the more ambitious

projects was a 230-kV, 2-mile (3.2-km) crossing of the Baltimore Harbor with five cable circuits and two splices per circuit, which were made on platforms in the harbor (Ruekert and Bien, Jr. 1979).

SCFF Cables

Single-conductor cables of this type utilize a conductor with a hollow core for feeding dielectric fluid to the cable, which is insulated with paper tapes impregnated with a low-viscosity cable fluid. Three-conductor designs, which are not normally utilized above 138 kV, provide ducts in the spaces between conductors for flow of dielectric fluids. The fluid can be pressurized to over 200 psi (1380 kPa) during operation. The insulated con-ductor is covered with a lead-alloy sheath, insulating jacket, bedding, and armor. This design can be installed to depths of 3280 ft (1000 m). The length of the cable depends on dielectric fluid-feeding limitations. With very low-viscosity fluids, large duct size, and high fluid-feed pressures, this cable could be installed in lengths of over 20 miles (32 km).

Impregnated-Paper, Nonpressurized, Solid Cables

This cable type consists of a conductor insulated with paper tapes impregnated with a very viscous dielectric fluid. The insulated conductor is covered with a lead-alloy sheath to prevent water ingress. An insulating jacket, bedding, and armor wire are applied outside the lead sheath. This cable is considered suitable for opera-tion up to 45 kVac and 250 kVdc. Without changes to the conductor and armoring design, this cable is limited to installation in water depths of approximately 1500 ft (500 m) (Bazzi 1982). Its greatest advantage is for use on long submarine cables. It can be installed in very long lengths, because it is nonpressurized, and thus limita-tions regarding fluid-feeding pressures and fluid chan-nel size do not pertain. The disadvantage of the nonpressurized insulation is that the maximum operat-ing voltage is limited, due to ionization limitations, par-ticularly with alternating current. Also, its maximum operating temperature must not exceed approximately 60°C or the impregnant in the cable will drain, causing dielectric failure.

Gas-Filled, Pressurized Cables

This cable design utilizes a hollow-core, copper conduc-tor wrapped with paper insulation that is impregnated with a high-viscosity (jellylike) liquid. The cable is pres-surized with gas. The advantages of this design, com-pared to liquid-pressurized cables, are that the duct size can be very small, thus reducing overall cable size and weight, and extremely long lengths of cable are feasible, since no dielectric liquid-feeding limitations exist.

Installation depth is limited to approximately 1300 ft (400 m), based on gas pressure limitations. Below that depth, the cable could be crushed due to water pressure.

This is only true if gas pressure is lost; the cable must be designed for this contingency. The maximum voltage for this cable is approximately 275 kVac and 250 kVdc (Bazzi 1982).

Extruded-dielectric Cables

This cable type has been used commercially for land installations at 230, 345, and 400 kV, and short sections of cable are presently operating under test at 500 kV. It consists of a conductor insulated with extruded solid dielectric, such as cross-linked polyethylene or ethylene-propylene rubber (up to 138 kV). The advantage of this construction is that the dielectric is truly self-contained.

Without the need for internal pressurization, a heavy wall sheath is not required, further reducing the cable size and weight. Length is also not limited by fluid-feeding limitations. However, the dielectric performance of cross-linked polyethylene is significantly weakened in the presence of even microscopic amounts of water.

Thus, although there is no internal pressurization, a watertight metallic sheath is still critical for cross-linked polyethylene cables. The major disadvantage is the basic performance of the insulation. Unlike laminar impreg-nated-paper dielectrics, extruded dielectrics are not for-giving of defects in the insulation. One contaminant or void can cause an eventual breakdown of the cable.

Thus, if extruded insulation is to be used for a subma-rine transmission cable, great care should be taken to ensure that manufacturing quality and factory testing are performed using state-of-the-art materials, equip-ment, and techniques.

Splicing Submarine Cables

The designs of splices for submarine cables differ from those for land-based cables due to the mechanical requirements associated with installation and retrieval, as well as operation under high water pressures. The required depth of installation and the method to be used during installation are critical. For example, installa-tions where the splice must pass over a capstan ten-sioner, through a linear cable tenten-sioner, or over a moderately sized overboarding sheave, often place severe design constraints on the maximum diameter and length of a splice. Therefore, SCFF and extruded-dielec-tric submarine cables are usually supplied in a single length, with smaller “factory splices” being used to join factory lengths of conductor. In three-conductor sub-marine cable, these factory splices are usually staggered over a length of 25–50 ft (7.6–15.2 m) to keep overall dimensions to a minimum. Shallow-to-moderate water depths, where the splice can be lowered off the side of the cable installation vessel, would not necessarily restrict the splice size in the same way, nor would the mechanical stresses be the same.

The electrical design of a splice for submarine cable is no different from that of a land-based cable in terms of maximum allowable radial and tangential electrical stresses. Requirements of the cable-handling equipment tend to be primary factors in the configuration of the splice. For example, limitations of the cable-tensioning equipment make it desirable for the submarine splice to be the same or close to the diameter of the cable.

Whereas splices for land application typically have a m a x i m u m i n s u l a t i o n o u t e r d i a m e t e r ( O D ) o f 1.5–2.0 times the cable outer diameter, splices for deep-water application are often designed as a “reconstruc-tion” of the cable insulation, matching the cable insula-tion outer diameter. Thus, the length of splice tapers, the dielectric tapes used, and the method of application are defined in large part by the desired splice OD.

The mechanical design of the splice is also determined primarily by how it is to be lowered to the sea bottom.

Splices that must pass through or over a cable-tension-ing device are designed to be “on size” with the cable.

These cable splices often use armoring similar to that of the cable. The armor wires from one side of the splice are hand-applied to the splice area and individually welded to the cable armor to effectively reconstruct the cable.

Cable splices to be installed by lowering over the side are often installed in a submarine splice casing. Making the splice is similar to that for a land-based cable, including installing a lead sleeve over the complete splice. The cas-ing has clamps on both sides, both to hold the cable armor wires and to take the mechanical load from the armor wires. After the splice is made and installed inside the casing (lead sleeve and all), the casing is filled with semi-hard insulating compound to provide further assurance that the splice is mechanically protected and water ingress is prevented.

Generally, it is desirable to minimize or eliminate alto-gether the use of splices in submarine cables, because a splice adds a site of potential weakness in the electrical insulation and mechanical structure. Further, if splices are to be made at sea, exposure time to possible storms and shipboard failures is obviously increased. Installa-tion and handling are made more complex depending on the splice design. Submarine cable circuits are more reliable if they do not contain splices, at least not in the lead sheath, the reinforcement, or the armor. Most major underwater cables have been produced splice-free in long lengths stretching from shore to shore. Where necessary, splices have been laid successfully in water of moderate depth such as the English Channel (Gazzana-Priaroggia and Mascio 1973).

There are three different types of splices to be consid-ered: flexible factory splices, field splices (at sea), and repair splices. Flexible factory splices are provided to allow the maximum length of cable to be installed with-out the need for splicing at sea. Factory splices are supe-rior to field splices in that their armor can still be applied continuously, splice free, and they are made in highly controlled conditions with trained factory technicians.

At-sea splices present greater risks, in that the splicing must be accomplished on the vessel in varying weather and sea conditions, and at-sea splices involve the recon-struction of conductor, insulation, shields, jackets, and armor. Extra cable is needed to allow the cable to be brought up to the vessel, resulting in a “U-bend” in the cable. While splicing is performed in a temperature- and humidity-controlled enclosure, the cable-tensioning equipment and vessel-positioning systems must be capa-ble of holding the cacapa-bles steady during the splicing oper-ations. Heavy seas and severe deck motions can obviously affect the quality of the splicing or, at least, extend splicing time. In very heavy seas, it is sometimes necessary to cut the cable free to protect the cable vessel and its crew. At-sea splices should be completed as quickly as possible to minimize exposure to bad weather and sea conditions; if possible, they should be avoided entirely.

Even if a submarine cable is designed and installed with-out splices, a repair splice design must be available in case of damage to a cable in mid-run, unless replace-ment of the total length of submarine cable, shore to shore, is economically and logistically feasible. There have been cases where the submarine cable has been damaged during the initial installation, in one case by the anchor of the installation vessel. Thus, the repair splice design should be completed before the installation begins, and a suitable number of repair kits should be available at the time of installation.

Underwater Installation Environment

Designing an underwater crossing requires consider-ation of a number of factors related to the installconsider-ation environment. These factors affect the selection of the type of cable, detailed cable construction, overall cable transmission system configuration (i.e., how many cir-cuits, how many spare phases, etc.), and the route of the cable crossing. Obviously, length of the cable and maxi-mum water depth are key technical issues, but addi-tional factors must also be evaluated.

Underwater Profile and Bottom Topography

The underwater profile and bottom topography can be significant factors in selecting the type of cable, the route, and the method of installation. A mild, uniform

elevation change for a cable crossing is generally desir-able. Securing cables on steep embankments usually pre-sents some difficulties, and once installed, they could be subject to problems such as impregnant migration with solid paper cables, or creepage of the insulation. Cables on steep embankments are also more prone to damage from submarine landslides. Many methods have been used to secure cables on steep embankments—including concreting or grouting in place, anchoring, snaking, etc., all of which appear to have worked, but require periodic inspection. The submarine cable design engi-neer should select the most appropriate method based on site conditions.

A rough, erratic bottom makes cable installation diffi-cult and time consuming, and therefore expensive. If the cable is to be laid without being embedded, it is impor-tant for the cable to lay flat on the bottom. If the bot-tom is such that lengths of the cable are unsupported and underwater currents are present, these sections of the cable could be subject to damage due to cable move-ment. Damage due to vibration from underwater cur-rents (called strumming) can include abrasion, chafing, and sheath fatigue. If the cables are to be embedded, a rough bottom makes the job of trenching and placing the cables far more difficult. Some waterways have sig-nificant numbers of man-made obstacles such as sunken barges, anchor lines, etc.

Thus, bottom soundings along potential routes should be taken to obtain accurate profile information. If the bottom conditions are not uniform along the route, more detailed bottom information should be obtained.

Serious problems with bottom conditions can warrant the consideration of alternate routes and, at the least, dramatically increase the cost for installation.

Surface and Subsurface Currents

Surface currents must be evaluated in determining the method and equipment to be used to install the subma-rine cable or retrieve it. Fast surface currents might require special anchoring or active propulsion systems for positioning a cable ship or barge, or might preclude

Surface currents must be evaluated in determining the method and equipment to be used to install the subma-rine cable or retrieve it. Fast surface currents might require special anchoring or active propulsion systems for positioning a cable ship or barge, or might preclude