Underwater Cables Cross Strait and Sound
The installation of high-voltage cables presents a utility with many challenges. These challenges increase when the project involves an ocean crossing and the use of submarine cables. Located on the U.S. Pacific coastline off the state of Washington, the San Juan Islands are supplied electrical power by the Bonneville Power Administration (BPA; Portland, Oregon) via several submarine cables 7 miles (11 km) in length, laid in the open ocean between the islands and the mainland.
Recently, when one of the existing 30-year-old submarine cables failed, BPA decided to replace it. This was considered the preferred investment option, because the existing “wet design” cable had reached it's design life and because of the problems involved in repairing a cable sited in deep water.
San Juan Cable Project History
The four existing BPA cables, installed along a parallel route connecting Fidalgo and Lopez Islands, are each comprised of two submarine cable sections laid across Rosario Strait and Lopez Sound.
The first BPA 25-kV, 300 MCM three-core copper conductor, butyl rubber-insulated submarine cable with bare galvanized armor wires rated 20 MVA was installed in 1951. The first failure of the cable occurred in 1964. Following further fault repairs in 1965 and 1969, the cable in Rosario Strait was abandoned and removed; however, the cable crossing Lopez Sound remained in operation until May 2001.
Cable No. 2, a 35-kV, 500 MCM three-core copper conductor, polyethylene-insulated cable rated 35 MVA, was installed in 1966. The Rosario Strait cable failed in 1994 from an unknown cause. The failed section was not repaired; however, the remaining section in Lopez Sound is still in operation.
Cable No. 3, a 35-kV XLPE-insulated cable similar in design to Cable No. 2, rated 35 MVA, was installed in 1972 to provide additional capacity and improve system reliability. In 1985, polarization cells were added to the installation. Site measurements confirm satisfactory operation of the cathodic protection system. The cable has given fault-free service for more than 30 years.
Cable No. 4, a 115-kV, three-core, low-pressure fluid-filled cable with a single layer of PE-insulated galvanized armor wires and cathodic protection rated 150 MVA, was installed in 1982. Polarization cells were installed in 1985, and this cable has afforded fault-free service for more that 20 years.
Cable Route Description
The cable route starts on the mainland at Fidalgo, followed by an 8-km (5-mile) undersea crossing through Rosario Strait at a maximum depth of 100 m (330 ft). The overhead route crossing Decatur Island is 6.2 km (3.9 miles) in length, and the sea crossing of Lopez Sound terminating at Lopez Island is 3.5 km (2.2 miles) in length at a maximum depth of 30 m (100 ft). At Lopez Island, the local utility, Orcas Power & Light Cooperative, purchases the energy for distribution to several San Juan Islands.
Key characteristics of the two ocean sea crossings include:
Rosario Strait has a granular sediment seabed with a high concentration of cobbles and boulders present in some 250-m (820-ft)-long sections that are subject to high-velocity tidal currents. Correlation is revealed between boulder fields and high-velocity currents (over time, sand and silt having been washed away by tidal currents). Tidal currents of 2.2 m/s (12.5 ft/s), measured at a position equal to 10% of the water depth above the seabed, were recorded during the route survey.
Lopez Sound. Trenchable seabed soil of fine- to medium-grained sediment with a few cobbles and boulders. Some sand migration is expected at the landing positions of the submarine cable.
Environment and Cultural Sites
Early meetings with various federal and state agencies focused on searching for construction methods designed to minimize disturbance to eelgrass beds, which provide vital salmon habitat in these near-shore areas. BPA was asked to consider directional boring to avoid disturbance to eelgrass beds located at all four of the landing sites.
Instead, BPA proposed using a lightweight, underwater, four-wheeled, water-jet trenching machine to install cable through the near shore areas. BPA found success with this method but with less than the required design depth achieved in some areas due to coarse sediments. The eelgrass is naturally restoring itself along the cable route.
BPA also worked closely throughout the project with two of the local Native American tribes. Test excavations were conducted along the shore and inland areas to determine the extent of cultural resources in the project area. These excavations revealed sites potentially rich with cultural material. Because these would be impacted by the project, data-recovery excavations and cultural studies were proposed to mitigate project impacts. Archaeologists spent several months on the island carefully excavating, studying and recovering 54,000 artifacts along with the discovery of a prehistoric house.
Submarine Cable Specification
The San Juan maximum power demand occurs during the winter months. BPA's basic loading specification for the 69-kV cable included a 100 MVA winter rating, a minimum 80 MVA summer rating and a 75% daily load factor. The operating experience of the existing cables, together with BPA's thermal property assessment based on samples taken from the proposed route, were used to determine the cable's technical requirements.
At the design stage of this project (1999), there was a lack of a complete national or international submarine cable specification. For example, the International Cable Committee (ICC) had a Submarine Cable specification No. IEEE P1120/D, but this is just a checklist of factors to consider. CIGRÉ Study Committee 21 published “Recommendations for Tests on Submarine Power Cables (WG21.02),” which included mechanical tests for submarine cables. Being conservative, BPA specified the cable to be in accordance with AEIC Specification (CS7), which requires a minimum insulation thickness of 18 mm (0.70 inches) for a 69-kV system voltage. However, as in practice, 69-kV submarine cable insulation thickness varied, so BPA allowed, as an alternate, a minimum insulation thickness of 12 mm (0.47 inches), provided vendors could supply a cable system with at least a 40-year design lifetime.
BPA specified a minimum conductor area of 400 mm
BPA contracted a thorough survey of the seabed, conditions of the ocean waters at the surface and below, and the weather and shipping movements across the route in advance of preparing the complete performance specification for the manufacturing and installation of the cable. Nexans (Alcatel when the contract was awarded) was able to meet BPA's demanding specification for a cable manufactured using modern processes that minimized voids, contaminants and depressions, and maintained conductor centricity that contained the least number of factory joints and none within the underwater sections.
Nexans' Cable and Accessories
The three-core composite cable has a 40-year design lifetime. The thermal design, normal, emergency and short-circuit ratings based on BPA design data are:
Maximum air temperature (summer/winter): 35°C/15°C (95°F/59°F)
Maximum ground temperature (summer/winter): 18°C/9°C (64°F/48°F)
Thermal resistivity (seabed soil): 0.7 K m/W
Thermal resistivity (onshore soil): 0.8 K m/W
Burial depth: 0.5 to 1.0 m (1.55 to 3.1 ft)
BIL: 350 kV
Load at winter season: 840 A
Daily load factor: 75%
Design fault current and duration: 12 kA/30 cycles (60 Hz).
Because the quality of the cable, especially with XLPE insulation, is dependent on the cleanliness of the materials used and the manufacturing process, particular attention was given to Nexans' quality program. Parsons Brinckerhoff Quality Services (U.K.) was contracted to audit and witness the factory testing of the cable sections when BPA personnel were not available to perform the task.
Transport and Installation
In 2001, the 69-kV three-core submarine cable (including fiber-optic), manufactured by Nexans-Norway, was spooled on-board a turntable located on the purpose-built cable laying vessel Havila Skagerrak. This ship is equipped with all the necessary power tools, dynamic positioning system and remotely operated vehicle (ROV), plus instruments to control touch down and to modify the cable laying in the boulder fields if the cable suspension noted was deemed unacceptable.
The Nexans contract included responsibility for deciding the actual cable route based on the survey, performance and installation requirements. To avoid crossing existing cables and to limit the impact on eelgrass areas, the routes selected that offered sufficient space for the new cable were between cables No. 3 and 4 across Rosario Strait and between cables No. 2 and 4 across Lopez Sound. Provision was made to afford mechanical protection (burial) on minor sections in the splash zone to a water depth of 10 m (39 ft) in sea and on land for the Rosario Strait and along the entire cable route in the sea and on land for Lopez Sound.
The installation of the new cable on the seabed had to allow for varied seabed conditions, tidal currents and weather conditions during the laying operation. All obstacles were avoided, and the cable was laid in smooth large radii curves and burial depths of between 0.5 to 1.0 m (1.55 to 3.1 ft) were achieved by trenching to provide the mechanical protection where specified.
Normally, submarine cable is laid on the seabed following the contour with continuous touchdown. However, free spans can occur due to changes in elevation and the presence of boulders. The suspended cable or free span could suffer damage due to anchor drag/fishing gear and high local sea velocity.
To monitor these situations, BPA specified that a ROV monitor the cable touchdown to ensure it was not placed on large rocks or had long unsupported span lengths. Free span lengths of less than 5 m (16.4 ft) were specified for this project, and videotape of the touchdown for the complete route required by BPA confirmed the recorded free span lengths were no greater than 3 m (9.9 ft).
After laying Cable No. 5, the decommissioned cables, No. 1 in Lopez Sound and No. 2 in Rosario Strait, were recovered and scrapped.
Nexans' pre-installation studies and analysis based on sea current velocity measurements and the properties of the submarine cable were used to determine vortex-induced vibration, strain cycles and cable element fatigue. These were contributing factors for the successful completion of this submarine cable-laying project.
Cable and Installation Testing
Nexans conducted factory tests according to AIEE Paper 57-660, appropriate IEC standards (IEC 60840) and CIGRÉ's “Electra No. 189 Recommendation for testing long AC submarine cables with extruded insulation for system voltage above 30 to 150 kV.”
During the manufacturing process, Nexans subjected each cable core length to a 150-kV, 30-minute test before laying up the three power cable cores. Short samples were also subjected to partial discharge tests. Following manufacture, each phase of the cable was subjected to a 90-kV test voltage before loading on the cable-laying vessel. Samples taken from each cable length were subject to partial-discharge tests together with void size count checks.
BPA conducted the following site tests on the installed cable system:
Megger test to verify the integrity of the system
OTDR test on the fiber-optic core to provide a footprint for future comparison.
Hot Collar test performed on each termination to check for gross problems and to set a benchmark for each termination.
24-hour Soak test with acoustic monitoring of the termination before the system was commissioned.
Conclusion
The installation of submarine cables can present many challenges, but with good planning, surveying the route in detail, dealing with a reputable cable manufacturer, and having sufficient time and finance with provision for the unexpected, it can be a success.
Mark Korsness, a senior project manager for the Transmission Business Line of Bonneville Power Administration, managed the San Juan Area Submarine Cable Project from inception to energization. Korsness has been with BPA since 1991, working as a project manager of several important transmission projects. He has a BA degree in Geology and a BSCE degree. He is a licensed professional engineer.
Leslie Kelleher is the environmental resource requirements coordinator in the Environment, Fish and Wildlife Division of BPA, which she joined in 1991. Kelleher has a BA degree in biology and an MA degree in secondary education and environmental science.
Lincoln Koga has worked for Bonneville Power Administration since 1974, first in substation design and then in substation specifications. Currently, Koga is manager of the Substation Specifications and Design Section and he is a member of the IEEE Insulated Conductors Committee. Koga has a bachelor's of engineering science degree.
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