Replacing Vintage Subsea Cable

By Contributed Article | August 12, 2014

In this case study about replacing vintage subsea cable to scenic Drummond Island, Kerite’s Peter Ebersold describes how the company overcame the environmental challenges to meet the tight timeline dictated by permit conditions.

When Cloverland Electric Cooperative wanted to replace the aging subsea cable that serves Drummond Island in Michigan’s Upper Peninsula, it turned to experienced utility consultant GRP Engineering Inc. to plan and design the project. As the successful bidder to Cloverland Electric, Kerite Corporation provided the new cable, which was shipped out using a special truck designed for extra-heavy loads. Running cable from the Michigan mainland out to Drummond Island under the tight timeline dictated by permit conditions went very smoothly, despite a few bumps in the road. The successful project ensures that island visitors, residents, and businesses have the electrical power they need for the foreseeable future.

Growing Load Drives Need to Replace Cables

Two subsea electrical cables serve Drummond Island, located at the end of Michigan’s eastern Upper Peninsula. Referred to as the “Gem of the Huron,” and “Michigan’s Ultimate Playground,” the 87,000-acre island is the second largest fresh water island in the nation. It has 150 miles of rugged scenic shoreline, 133 square miles of forested landscape, and 34 inland lakes. The island has a growing number of large resort communities, as well as some full-time residents. It is also home to the Drummond Island Quarry, a major producer of crushed and broken dolomite, which is used in manufacturing steel, glass, paper, and as a soil neutralizer for agricultural applications.

The two subsea cables, one serving mainly residential dwellings and one serving the quarry, were installed in 1975 and 1989 respectively, and had reached the end of their useful life. When the quarry was operating at full capacity, there was insufficient capacity between the cables in the event of a failure, causing the quarry to partially shut down. While the load at the quarry is not anticipated to grow, Cloverland determined that the resort load is likely to increase as older, smaller cabins are turned into larger ones, and modern amenities like air conditioning are added. Cloverland decided to install a new cable that could handle the entire island. The plan was to maintain two circuits: The first would be replaced in 2014, while the sister cable is budgeted for completion in 2017.

Cloverland called on GRP Engineering, a Michigan-based power utility consulting firm, to handle the first cable replacement project. The firm, which has more than a decade of experience with municipal and rural electricity cooperatives in Michigan, handled initial planning, cost estimates, permitting and design, as well as installation oversight and startup.

GRP Engineering developed the bid specifications, and the project was bid out from termination top to termination top. Kerite was the successful bidder for the subsea cable, offering the lowest price as well as the most experience. With more than a hundred years of know-how in providing subsea cable, Kerite is also the only US manufacturer of EPR subsea cable. Kerite’s ethylene-propylene-rubber (EPR) insulation formula enables its subsea cables to operate in direct contact with water, without the need for an impervious lead sheet or asphalt coating. The cable can be covered with individually jacketed steel armor wires for mechanical protection and ease of installation pulling. During the bidding process, Kerite responded to Cloverland’s questions about thermal issues, providing a report with engineering test data to confirm the cable’s ampacity, showing that the cable is good for 340 amps. Kerite used a subcontractor to dig trenches and install the cable (Figure 1).

Cable trenching

Cable trenching

Coordinating with Cloverland on permitting and budgeting, GRP Engineering was responsible for oversight of manufacturing and installation done by Kerite and its subcontractors to ensure everything was done in accordance with the permit. For example, the island’s location in a sensitive environmental area meant a joint permit was required, incorporating separate guidelines from both the Michigan Department of Environmental Quality and the US Army Corps of Engineers. Fish spawning was an important issue at the time of the year during which the project was scheduled, and the permit schedule accommodated that concern.

The project required 7,465 feet of 28 kilovolt (KV) cable; Kerite provided the full length in one continuous piece of cable with no splices. The line is currently being operated at 7.2/12.5KV, and Cloverland will potentially be switching to a 14.4/24.9KV operating voltage. There are three single-phase conductors, each 350KCM (1000 circular mils), and a full uninsulated neutral at 350 KCM. Also included is a 24-count fiber optics cable that Cloverland is using for communications with its substation and motor-operated switches on the island. Spare fibers are being leased out to the island’s high-speed Internet service.

The design called for installing the cable 20 feet below the low water datum at a depth of six feet. Once the cable is beyond 20 feet, it is transitioned out and then lays on the bottomland of the DeTour Passage. Typical cables are installed 15 feet below the low water datum at a depth of four feet, but with this location on the St. Mary’s River, the deeper installation protects the cable from boats, ice, or other objects that might puncture the cable.

Tricky Shipping But Installation Goes Smoothly

The reel holding the continuous piece of cable weighed 171,000 pounds. Kerite used its onsite rail siding and special lifting equipment at its Seymour, Conn., manufacturing plant to place the reel onto a freight truck specially equipped to handle the extra weight. The truck had a larger-than-standard number of axles and a trailer that sits very low to the ground so it can clear most bridges. The vehicle was subject to special permits and was delayed a few days when New York State did not have troopers available to escort the truck in accordance with that state’s requirements. Because the truck was not permitted to go over the Mackinaw Bridge, when it arrived at Rogers City, Mich., the reel was loaded onto a barge for the final part of the journey.

Figure 2 shows photos of the cable reel. On the left, the reel is being loaded onto the barge in Rogers City and on the right, the reel is ready for use.

Cable reel

The cable arrived safe and sound, and the installation moved forward. The original plan was to pull the cable from the mainland side at DeTour village to the Drummond side. However, due to a rather strong southwest wind, GRP Engineering made the decision to pull from the Drummond side to the DeTour side. Figure 3 shows the cable installation. The left shows cable payout from DeTour, and the right shows cable floating to DeTour.

Cable installation

To protect the aforementioned fish spawning, the project was originally scheduled for completion by October 1. Due to shipping issues, GRP Engineering worked with the MIDEQ to get a brief extension. Weather is a key concern for a project like this, but all went smoothly and the entire project was completed in eight days. The cable is now energized and carrying the quarry load on the cable.

Peter Ebersold works for Kerite Company.

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