The Worst Submarine Cable in the World
Fox Islands Electric Cooperative, Inc. and their customers went through 15 years of power interruptions and outages as a result of 45 faults on what was probably the worst performing submarine cable system in the world.
In 1976 four single phase submarine cables were laid between Rockport, Maine and the islands of North Haven and Vinalhaven. The 10.4 miles of 34.5 kV circuit replaced what had been notoriously unreliable, on-island, diesel generation, which by 1973 the locals referred to as "hour power"; one hour on, one hour off. The cables were installed for Fox Islands Electric Cooperative, Inc., formed in 1974 to take over the investor owned utility which had run the diesel generators.
In October 1983, seven years after the installation of the cables, an article in RE Magazine stated "today the bad years are being forgotten". It should have added "knock on wood". Unknown to the Cooperative, the cables had been laid over a number of areas of rugged bottom, the worst of which was a 50 foot high outcrop with near vertical walls on both sides.
The cables were 2" in diameter with 1/0 copper conductors, EPR insulation and #4 BWG galvanized armor wires each with a 0.04" polyethylene jacket. They were laid on the seabed in up to 250' of water.
In March of 1990, after 13 years in service, the first fault occurred in the area of the outcrop. That was followed by faults #2 and #3 in 1991, #4 in 1992 and #5 in 1993. During the repair of those faults a procedure was developed by the Cooperative, Prock Marine Ltd. of Rockland, Mr. Dave Honey from Swans Island Electric, Dan Conner, Bob Dube and Russ White from Central Maine Power and Greg Canders of Canders Diving Services in Bangor. It was a procedure that would be followed more times than any of those involved could have imagined.
Repairs presented a number of problems for a small utility with less than 2000 customers and limited resources. Sending divers or an ROV down to locate and fasten a line to the cable could cost a significant portion of the utility's annual budget. Because the precise location of the cables was not known, anchoring a barge in the area of the fault was considered too dangerous. The procedure developed was:
1. A crane barge with two tugs would pick up the cable in shallow water and begin underrunning. The term underrunning refers to a barge or ship moving along the route, picking up cable at the bow and laying it back down over the stern.
2. A thumper (10 kV jolts at 15 second intervals) was necessary to help locate the fault, given the precision of the as-laid information, the limits of the navigation equipment on board and much of the cable being covered in marine growth or mud.
3. Once the fault was on board, the barge moved ahead another 200' and was held in position by fastening ropes to the cable at the bow and the stern (i.e. using the power cable as an anchor). The cable was cut and spliced to the spare reel. The splices were hand wrapped and encased in protective steel tubes.
4. The barge then backed up about 400' where a second splice was installed and the cable was lowered to the bottom. The intent of splicing 200' from the fault was to, hopefully, be beyond any water ingress in the conductor. (It was later concluded that the conductors were full of water and future repairs were often made using one splice.)
The author was contacted in 1997. After 4 years without a problem, fault #6 occurred and Dave Honey had retired. Dave is an old friend and when I called, he told me working on these cable repairs would be like having an annuity. The next 8 years and 40 faults proved him right.
Fault statistics for power cables are difficult to obtain however CIGRE (an international association of large utilities) did a reliability study of 137 submarine cables in the 1980's and found an average occurrence rate of .32 faults per year per 100 circuit km (.5 faults per year per 100 circuit miles). At 15 faults per year per 100 miles, the Fox Islands Electric cables probably carry the dubious distinction of being the worst performing submarine cable system in the world. It should be noted that the faults were almost exclusively the result of external forces and the cables themselves had preformed extremely well both electrically and physically.
On my first repair we came to a location where two cables had become tangled. The tangle had come to within 80' of the surface and a diver went down and tied a buoy line ahead of the "snarl". The cable was then placed back on the seabed. The buoy line was picked up and the cable raised using a one point lift.
Anytime a cable is lifted it is subjected to abuse. Underrunning increases that abuse. That one point lift was not the worst, and certainly not the last time, the cables would be severely abused. Working with barges and cranes there will always be things that go wrong. Throw in the currents and weather on Penobscot Bay and there will be times when you do what you have to do and pray.
The performance of the cables was quite remarkable. In the manufacturing, transportation and installation of submarine cables, suppliers and installers are extremely diligent in maintaining quality and in ensuring the cables are not overloaded. During the repairs these cables were:
i.) jammed between the sheaves and frames,
ii.) bent well beyond the minimum bending radius,
iii.) dropped from the barge to the seabed,
iv.) jolted by the crane and
v.) under tensile loads so high there was no noticeable deflection over the 80' span between sheaves.
Each of the locations where any of the above occurred was recorded. Amazingly, until 2004, and fault #35, none of these locations was the site of a future fault.
After 9 faults, an assessment to determine the cause began. Samples from four of the next seven failure locations were sent for laboratory inspection and testing. All four indicated that there was no significant deterioration of the insulation or other components and that the cause was probably external. In addition to the laboratory testing, the locations of the first 20 faults were placed on a profile of the route and the external inspection reports of each fault were reviewed. The assessment concluded:
a. The primary cause of the faults was abrasion at locations where the cables were resting on hard points and being moved by the tidal currents.
b. Some of the faults occurred at locations where the cables were suspended and hooked by anchors or fishing gear.
c. The conductors were not strand filled and were probably filled with salt water, from shore to shore, as were the interfaces on either side of the shielding tape.
d. Faults caused by damage during earlier repairs were unlikely, but a possibility at 3 locations.
The last item was of particular concern. Each time a fault occurred there was the realization that it could be the result of abuse during a previous trip. A review of the locations and the inspection of each fault indicated that this was highly unlikely for most of the failures. This held true until fault #35. Of the final 11 faults, 5 were probably the result of damage caused during previous repairs.
The existing cables would not be fully amortized until 2012 but it was obvious to the Cooperative that the faults would continue and a new interconnection was required. The problem was the $6 million estimate for a new cable which would avoid the hard bottom and be buried for additional protection. After three years of work they were able to obtain a combination of grants and loans that allowed the project to proceed.
In the meantime the problems continued. The fourth cable was a spare and usually allowed the utility to come back on line shortly after each trip. On seven occasions however two cables were out of service at the same time and the islands were reduced to single phase power. In September of 2004 with three faulted cables there was no power available for the best part of a day. Two of the cables were put back in service but the third was beyond repair. In the last 12 months of operation there were 14 faults. On March 04, 2005, with a new three phase cable on its way to the site, the 45th and final repair was completed.
The new 3 phase, XLPE cable was supplied and installed by Caldwell Marine International, LLC of Toms River, New Jersey. In order to utilize the same landing points but avoid the hard bottom, the existing cables had to be crossed at four separate locations. To bury the new cable required cutting the old cables at each of the crossings. On April 15, 2005 the cables were cut at two locations on the mainland side. On April 18 and 19 they were cut at two locations on the island side. It felt almost like insulting them, to destroy so completely the cables I had worked on for eight years. The label, worst submarine cable system in the world, might fit but the emphasis should be on system, not cable.
On April 22, 2005 the new cable was energized. There have been no problems to date. Knock on wood.
About the author:
Wally MacDonald, P.Eng is from Charlottetown, Prince Edward Island and has over 30 years experience in the installation, inspection, maintenance and repair of submarine cables. He can be reached at wmacdonald@marencoengineering.com