Jim R. Attwood

Cable Design for Subsea Power Links

he number of subsea cable links T being installed world-wide is rap- idly increasing, driven by the desire to connect to more economic or renewable energy sources. Advances in subsea ca- ble technology, the fast rate of return on investment compared to power station construction, and the ability to import power from hydroelectric or wind gener- ating systems have combined to make the installation of subsea cable links at- tractive.

Subsea cable technology has ad- .+-

vanced to allow the manufacture of both single and three core cables for bulk power transfer between transmission systems in continu- ous lengths up to 100 km. The mechanical and electrical integ- rity of these cable systems is enhanced by the ability to manufacture the cable in long lengths, thereby removing dis- continuities at jointing positions. Handling of long-length ca- bles is complex, requiring manufacturing facilities capable of insulating, sheathing, armoring, storing, and off-loading the ca- ble to be located on the same site. A 100 km continuous length of completed, armored cable, for example, may weigh in excess of 8,000 tons. High standards for quality control and manufac- turing-plant specification are of the utmost importance, particu- larly in continuous processes, such as lead sheathing, which require reliable extrusion over a period of some 30 days.

The installation of subsea cable presents many challenges in the fields of mechanical and marine engineering. The cable must withstand the significant mechanical forces that are generated during installation due to its own weight and the action of tidal currents. The installed cable is also at risk from damage by an- chors, fishing activity, vessel impact, and movement of the cable or seabed terrain. To protect the cable during laying or in service, metallic armor wires are applied in one or two layers to the con- struction as part of the manufacturing process, however, addi- tional protection can be offered by burying the cable in the seabed using a variety of jetting, ploughing, or trenching techniques.

This article describes the choices of cable design that exist for subsea power links and the merits of these various designs. The manufacture and installation of long continuous lengths of subsea power cable will be discussed with reference to some key installations.

This article is part of a series based on presentations made at the panel session on Power Cables in Energy Development in the 21st Century, which was held at the IEEE PES Winter Meeting 2000. J. Attwood is with BICC General.

HVDC Submarine Cables The world’s first HVDC submarine cable connection was in- stalled between the Swedish mainland and the Island of Gotland in 1954. The cable was designed with a nonpressurized mass-impregaated paper dielectric suitable for long route lengths. Since then, considerable experience has been accumu- lated through a number of connections applying principally the same cable design. This cable type is still the only feasible solu- tion for transmission of bulk energy over large submarine dis- tances.

Working voltage and loading capacity per cable has been subject to a continuous development starting from 100 kV and 20 MW in 1954 and reaching and reaching 450 kV and 600 MW in connection with the Baltic Cable project from Sweden to Ger- many in 1994. This development has been made possible by ex- tensive research and development efforts performed by several cable manufacturers, covering all aspects of an HVDC subma- rine connections, i.e., cable design, manufacturing processes, transportation and laying technology, protection, service, and maintenance. A milestone in this field was the Fenno Skan pro- ject between Sweden and Finland, where 400 kV service volt- age and a transmission capacity of 500 MW per cables were achieved for the first time.

Service experience is excellent for HVDC cables designed on the basis of existing CigrC test recommendations provided that the cable system is properly protected from mechanical damage from ship anchorslfishing gear and wear and tear from wave ac- tion and sea currents. Tests on samples of the Konti Skan cables between Sweden and Denmark and the Skagerrak from Norway to Denmark have revealed no aging of the dielectric after 15 years in service. Reference is made to two CigrC reports:

9 “Methods to prevent mechanical damage to submarine ca- bles,” paper no. 21-12, CigrC conference, 1986 session

IEEE Power Engineering Review, September 2000 02 72-1 724/00/$10.0002000 IEEE 13

0 “Reliability of underground and submarine high voltage cables,” paper no. 2-07, CigrC Symposium in Montreal, 1991.

The extensive development work in connection with the Fenno Skan, Cook Strait, and Skagerrak projects demonstrated, however, the need for a revision of the above mentioned test rec- ommendations. On the basis of the experience from these pro- jects, CigrC has revised the existing test recommendations.

This fact was taken into account for the Baltic cable (Swe- den-Germany), which was commissioned in 1994. The testing of this cable system for 450 kV and 600 MW was based upon a special test recommendation worked out by the customer (Baltic cable) and the supplier.

Installation of Submarine Cables Underwater technology has made great progress over the last decade as a consequence of offshore drilling and production ac- tivities, particularly in the North Sea. Underwater electronic navigation systems, underwater remote operated vehicles (ROVs), computer-controlled dynamic positioning systems for laying vessel navigation, etc. have made possible cable laying concepts that one could only dream of at the time of the installa- tion of first two cables across the Skagerrak Sea. Precision lay- ing of cables avoiding obstacles on the seabed, installation of a cable at a defined distance from an underwater construction (for instance, a pipeline), crossing of a pipeline over a bridge etc. represent today no problem for the experienced crew on board the advanced cable laying vessel, the Norwegian C/S Skagerrak.

The cable laying system also includes methods and means for offshore jointing of manufactured lengths in case of long cable lengths exceeding the loading capacity of the laying ship and per- formance of offshore repairs in case of failures on the cable system. Offshore jointing has been successfully performed in connection with repair operations on the existing Skagerrak cables and during installation of the Fenno Skan cable and the Baltic cable.

Regarding the present state-of-the-art installation technol- ogy, reference is made to the Cigr6 report no. 21-202 presented at the 1990 Paris conference, “A survey of installation and re- pair techniques presently in use for submarine cables.”

The navigation of the ship can be performed by means of an integration of satellite-based Global Position System (GPS), dy- namic position system of the vessel according to well-proven procedures and underwater navigation systems controlling un- derwater ROVs. The need for underwater touchdown control of the laying process will depend on seabed conditions, which will have to be studied in detail in a separate route survey.

In case of failure on the cable, methods and equipment for the performance of repairs are available. On depths below 100 m, a specialized vessel like the C/S Skagerrak will have to be utilized due to the complexity of the operation. The risk for failures in deep waters is, however, according to statistics, very low. With regard to laying depths, the 550 m experienced in the Skagerrak crossing represents no limit. Tests clearly indicate that modern laying technology ensures safe handling even down to more than 2,000 m.

Protection of Submarine Cables Failure statistics on submarine cables show that the most fre- quent cause of failure is external damage caused by anchors and heavy fishing tools. Failures of internal origin are not very prob- able. As an example, the only failures that have occurred on the two cross-wire armored Skagerrak cables installed in 1976/1977 have been caused by external mechanical impact

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from heavy equipment (beam trawlers, anchors, and a towing weight between a tug and a barge). No failures have occurred on sections where the cables have been buried into the seabed. This experience is typical for other projects according to interua- tional statistics.

Consequently, submarine cables should be protected in areas where the risk for mechanical impact is high. An efficient method is to bury a cable into the seabed.

An effective burial system (CAPJET) has been developed in Norway based upon tluidization of seabed materials by means of water jets. The machine follows the cable positioned on the seabed and produces a trench into which the cable sinks. Typical burial depth is 50 cm to 1.5 m. In areas with hard seabed condi- tions, other protection methods have to be applied, for instance, covering the cable with overburden (rock dumping) from a sur- face vessel.

The CAPJET burying system has demonstrated its capabili- ties through a number of projects in Norwegian waters and on the export market. More than 2,500 km of cables and pipes have been buried up to date. In connection with the Skagerrak 3 cable, the CAPJET machine successfully performed the burial down to a water depth of 550 m.

New Challenges in the Field of HVDC Submarine Cable Systems With reference to what has been stated above, it was clear that the cable technology represented no hindrance when the plans to connect Norway with the European Continent matured in the nineties. Three agreements between were initially entered, two between Norway and Germany and one between Norway and The Netherlands. two 600 MW HVDC connections to Ger- many, and one 600 MW connection to The Netherlands aiming at construction and commissioning in first half of first decade of the new century. One of the agreements between Norway and Germany was cancelled in 1999 for commercial reasons.

The agreements were based on the economics in exchanging energy between the two different systems, the hydropower-based Norwegian system with its high degree of efficiency in supplying peak power and the thermal power based European system which is more cost-efficient at constant load. This means that the connections will make it possible to utilize the ability of the Norwegian system to supply cost-efficient peak power to Ger- many/The Netherlands in periods with high demand and to im- port energy to Norway during low demand periods on the continent. Thereby, energy in the form of water could be saved in the Norwegian water reservoirs in an import situation. The Norwegian system will consequently act as a large pump stor- age plant.

During the planning period for the two projects, it became evident that environmental issues would play an important role in the concession processes and that such issues also would have a great impact on the selected technical solutions. Sea electrodes were not accepted, due to the possible seawater pollution caused by electrolysis of seawater, due to the claimed effect from elec- tric field in the sea on marine life, and due to the risk of corro- sion on underground/submarine steel structures. Further, external magnetic field from the cable system should be negligi- ble in order to avoid any effect on marine life and on magnetic compasses.

Consequently, the technical solutions for the two connec- tions from Norway were chosen in order to satisfy the environ- mental requirements:

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IEEE Power Engineering Review, September 2000