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Reprint from: Outside Plant Magazine
by Ben Osborne and Win Miller
Since the advent of fiber optic cables, major communications companies have replaced embedded copper cable with fiber optic cables to achieve better quality transmissions and higher transmission speeds. Along with improved quality and efficiency, however, have come some unanticipated problems related to installation and maintenance of fiber optic cables.
In the past few years, some fiber optic cables have mysteriously failed for no apparent reason, returning in most cases to full speed in a short time. Some failures have been a degradation of speed and quality, while in other instances complete failures have occurred. Since these service interruptions tended to happen during winter months, the weather was a prime suspect.
After extensive investigation it has been determined that these failures are the result of water freezing within the fiber optic conduit.
Water in Ducts
Freezing within an innerduct or conduit has been occurring for as long as cables have been put in duct, but fiber optic cable is susceptible to freezing in a way that traditional copper cables are not; fiber optic strands may be bent by the tremendous forces that occur when water crystallizes into ice. These microbends in the fiber optic strands can cause various degrees of signal degradation. Signal strength usually returns to normal when the ice melts.
Driven by intense competition, several companies have installed fiber optic systems in the most economical way possible, with extensive construction along railroad rights-of-way until an obstacle such as a bridge or culvert is encountered. Bridge or culvert crossings of fiber optic cables are usually accomplished by attaching carrier pipes to the sides of the bridge or by laying a carrier pipe on the bridge deck. This is far more economical than placing the cable underground by means of a directional boring device.
Driven by economics, the use of carrier pipes quickly became the industry standard. It was not apparent that these bridge crossings could in fact result in fiber optic transmission speed and quality problems by exposing water-filled ducts to freezing temperatures.
How does water get into innerducts? It gets in by infiltration and condensation. An innerduct is like a garden hose laid out along a railroad track, with occasional access points. Water may infiltrate at these access points and flow down hill, which may result in hundreds of gallons of water being present in the system. As long as the innerduct is buried below the frost line, no freezing will occur. But where a cable is exposed to freezing temperatures, such as at an exposed bridge crossing, ice will form in the duct. Field experience has shown many cases where, upon entry to an innerduct, water has gushed out for several minutes.
Condensation is another possible culprit, especially at exposed bridge crossings where differences between the ambient temperature and the inside innerduct regularly exist. Just as dew forms on grass, or condensation forms on a cold glass, so too, does condensation form inside innerducts and conduits.
Although ambient temperatures in the contiguous forty-eight states may reach -40°F, with a wind chill exceeding -80°F, the temperature inside a buried conduit will not drop below somewhere around 25°F. Fiber optic cable in a conduit exposed to the elements on a bridge crossing will not experience temperatures below the ambient outside temperature; wind chills do not affect unexposed surfaces.
Most fiber optic freeze-ups occur two to three feet within the head wall of a bridge. Freeze-ups are similarly a problem in abandoned pipelines where the pipes are exposed or placed on a cantilever when they cross creeks or rivers.
Upon defining the problem, a major communications company retained C&S Contract Services of Crest Hill, Illinois, to resolve these freeze-up problems. After an investigation that included an inventory of the fiber optic systems, C&S recommended two alternative solutions.
The first solution was the option of rerouting cables by means of horizontal boring at each bridge crossing. This would insure that cables would be below the frost line at all times, and that any water present in the system would not freeze. This is an expensive solution, costing between $200,000 to $1 million per bridge, depending on the location, length, geology and so forth. Given that a typical 100-mile cable route may have 100 to 150 bridge or culvert crossings, this solution could be very expensive.
The second alternative was to apply some type of freeze-proof material that would displace water and fill the void between the cable and the innerduct as these crossings.
C&S determined that such a product did not exist at that time, and contacted American Polywater of Stillwater, Minnesota, a specialty chemical manufacturer with broad experience in underground cable systems. A detailed description of the needed material was developed during discussions between C&S and American Polywater laboratory personnel.
A critical part of the discussions at the beginning of the discussions was the engineering firm's ability to describe the needs of the customer in a way that allowed the chemical firm's lab personnel to focus on specific characteristics of the needed material. While the material development was underway, C&S worked with a fiber optic placement contractor to develop application and insertion methods. As a result of these efforts, the laboratory worked toward creating a material that would: have a freezing point below -40°F; be capable of dissolving ice; be long-lived (preferably having the same expected useful life as the cable); work with applications methods developed, and be environmentally safe.
In a matter of days the American Polywater laboratory created a material called IceFree Antifreeze Gel, which met the necessary requirements for field use. IceFree is made from materials that have been extensively used and have known compatibility with cable.
Essentially similar to aircraft wing de-icing, the idea is to insert a material that will dissolve ice and prevent ice from forming. In addition, the material should combine in a predictable way with any water that finds its way into the innerduct at a later time.
Ongoing testing over the following months confirmed the predicted compatibility with the cable jacket and innerduct materials. Confidence in the life of the material is based on accelerated aging tests and because products with similar composition have been observed to last in excess of 10 years.
Blocking Ingress and Egress
To keep the total cost of fixing the freezing problem as low as possible, the material would be applied only within the innerduct along the bridge crossings to a point on each side of the bridge where the cable would be below the frost line. A method of confining the product in that location without migration was achieved by installing split duct plugs at each end of the bridge, beyond the frost line. This required excavation at the ends of the carrier pipes in the railroad bed on each side of the bridge for access to the innerduct below the frost line. The innerduct was then opened, IceFree Antifreeze Gel injected, and split duct plugs applied. Cost per bridge using this methodology was about $1,000 to $20,000 per location, including all materials and labor - a substantial saving over the horizontal boring alternative.
The procedure worked well, but, due to railroad safety, railroad traffic downtime, and the hazards of excavating fiber optic cables, an improved method that would not require digging on the railroad bed itself but would rely on access from the carrier pipe was envisioned by C&S: a chemical plug would be used to hold the product in place. An obvious time savings would result if the antifreeze material could be held in place by a chemical duct plug that would not inhibit future cable removal and could be placed in the innerduct along with the antifreeze.
Such a duct plug material would be re-enterable, allow future access to the duct or cable, and would not interfere with cable removal if that was ever required. Collaboration between C&S and American Polywater resulted in the development of IceFree Chemical Duct Block in a matter of days.
The chemical duct plug is a thick, dough-like material that absorbs water and IceFree Antifreeze Gel, and which has an antifreeze character of its own. The plug expands when water contacts it, improving its effectiveness as a barrier. Because it is inserted with the IceFree, both time and money are saved.
Use of the chemical duct plug reduced the cost of installation at each bridge site by about 50 percent, from $500 to $10,000 per location.
From January to August 1992, the product was installed in over 1,200 fiber optic bridge crossings from two to eight years old in Michigan, Wisconsin, Illinois, Indiana, Iowa, Nebraska, Minnesota, South Dakota and Kentucky. During the installation process, 85 percent of the systems were found to have water trapped within the innerduct, proving the widespread nature of the problem. Installations made from January to April were complicated by the presence of ice in many innerducts; 20 percent of all innerducts had to be thawed with steam before IceFree Gel could be inserted.
In addition, 50 percent of the locations had one or more maintenance items such as loose or missing hanger brackets, broken rollers, bad extension joints, or a broken carrier pipe that needed fixing.
Leaving over 1,000 bridges (85 percent of the 1,200 bridges) with trapped water in the innerduct would be like gambling that 1,000 backhoes could excavate buried cable without an accident. The likelihood of multiple cable failures is high whenever cold weather hits, and--with ever-increasing volumes of traffic on fiber networks, and monumental downtime costs--proper maintenance against the cold weather culprit is cheap insurance.
During the past winter, areas treated with the new product experienced no difficulties from freezing. In one particular case a bridge crossing was treated with the antifreeze product on one side and with weep holes and foam duct block on the other side. This bridge froze up at the end of January 1993. Visual inspection confirmed that the freeze up occurred on the side with the weep holes and foam duct block, while the side treated with the new gel did not freeze up.
Drilling weep holes in the carrier pipe has proved unsuccessful because this does not eliminate water in the innerduct, plus elevation differences in the carrier pipe can trap water where it cannot drain out. Weep holes also become a maintenance problem when they become clogged due to freezing.
Foaming the carrier pipe ends or the entire pipe did not solve the problem because water was still trapped within the innerduct.
In another instance, a treated cable had to be removed for a reason unrelated to freezing, and no problems were encountered in removing the cable; the chemical duct block came out in chunks with the cable.
Results from the new technology appear promising, with the probability that many additional installations will occur. Fiber optic lines can now be operated at maximum efficiency regardless of the weather.
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Important Notice: The statements and information here are made in good faith based on tests and observations we believe to be reliable. However, the completeness and accuracy of the information is not guaranteed. Before using, the end-user should conduct whatever evaluations are necessary to determine that the product is suitable for the intended use. The user assumes all risks and liability in connection with such use. The statements contained herein are made in lieu of all warranties, express or implied, including, but not limited to, implied warranties of merchantability and fitness for a particular purpose, which warranties are hereby expressly disclaimed. American Polywater's only obligation shall be to replace such quantity of the product proven to be defective. Except for the replacement remedy, American Polywater shall not be liable for any loss, injury or damage, direct or indirect, arising from the use or the failure to properly use these products, regardless of the legal theory asserted. The foregoing may not be altered except by a written agreement by the officers of American Polywater Corporation.
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