Report calls for more capable winches on Prince William Sound escort tugsFeb 27, 2013 03:50 PM
Photos courtesy Prince William Sound Regional Citizens’ Advisory Council
Nanuq, one of the two cycloidal-drive enhanced tractor tugs that serve as escort tugs for tankers in Prince William Sound.
The winches used by tugs escorting oil tankers in Prince William Sound, Alaska, are not state of the art and should be replaced, according to a study written by the naval architect Robert Allan.
Allan recommended installing new constant-tension winches to replace the current escort winches on all five tugs used in escort operations. The new winches would have the capability for automatic tension control, a winch technology that did not exist 14 years ago when the tugs were built.
Crowley Maritime operates the five tugs through a contract with the Ship Escort/Response Vessel System (SERVS), which is part of the Alyeska Pipeline Service Company.
“The escort towing systems on the SERVS tugboats have fallen behind the ever-improving industry standard which has evolved in the past decade or so,” Allan wrote in his August 2012 study.
Allan’s report was done for the Prince William Sound Regional Citizens’ Advisory Council, which advocates for environmentally safe operations of the Valdez Marine Terminal and the tankers that transport oil. The council approved Allan’s report at its September meeting.
Attentive, one of the three ASD escort tugs.
By federal law, every full oil tanker traveling through Prince William Sound must be escorted by two tugs. SERVS has two enhanced tractor tugs (ETT) and three prevention and response tugs (PRT). The two ETT tugs are the cycloidal-drive Nanuq and Tanerliq. The three PRT tugs are the ASD tugs Alert, Aware and Attentive. There are six other twin-screw conventional tugs, but the advisory council focused on the primary escort tugs.
Mark Swanson, executive director of the Prince William Sound advisory council, said the current winches are not adequate for the job. “If these vessels were limited to transit in calm waters and calm seas — if there was never a risk of a rescue in rough waters — I would say that this is adequate. But this is Alaska,” he said.
Andres Morales, director of SERVS, said the current winches are up to the job. He said that the components, and the towing system, were designed for a much higher standard than normal for the industry. “It was designed very conservatively,” Morales said. “The desire was never to come close to the operating limits. It is overbuilt to what was needed.”
The five SERVS tugs were state of the art in 1998 when built, Allan wrote in his study. And the towing gear on the five tugs is of very high quality. “However, the absence of a render-recover type winch on the ETT class VSP tractor tugs is considered a fairly significant deficiency in comparison to escort-rated tugs being built in say the past five years,” Allan wrote. This is particularly true of escort tugs operating in high sea states, according to Allan.
The escort/hawser winch on the ETT tugs is the Markey DYSDS-62 double-drum hawser winch. The escort/hawser winch on the PRT tugs is the Markey type DYS-52/WYW-20 combination single-drum hawser winch/windlass. These winches are both hydraulically driven.
The primary escort tugs are the ETT class. Only the ETT tugs are designed to perform indirect towing maneuvers, which is when the tugs go out to a position where the towline is at a 90° angle from the centerline.
Although the PRT tugs do not do any indirect escort towing, they are used in escort mode using the small bow winches and are used to apply direct pull, Allan wrote.
Allan’s report includes a list of shortcomings of the deck equipment on the tugs. The main escort winch on the ETT tugs is not a constant-tension winch, which means the full towing load is carried on the drum brake. The forward winch on the PRT tugs is a limited constant-tension winch, so the full towing load must be carried on a mechanical brake. In addition, the escort winches on both classes of tugs cannot reduce tension when tension exceeds 50 percent of towline breaking strength. And neither model of the escort winches on the two types of boats has a level-wind system that would reduce line jamming and damage, Allan wrote.
Crowley has addressed the concerns raised in Allan’s report in other ways, Morales said. To prevent line dive, the line is manually wrapped onto the drum, under tension, to provide tight layers, he said. A Kevlar blanket is also wrapped across the entire drum, at the lowest layer of line. More layers of line are wrapped over the blanket on the drum, he said.
“That spreads the tension on the line and it keeps the line from diving,” he said. Since these changes, the tether has not been sucked down to the drum or blanket level, he said.
Line dive is caused when the line is wound on the drum under low or zero tension, and results in the line biting down on the other line wraps, according to Allan’s report. It causes excessive line wear.
On a broader level, SERVS constantly analyzes the towing system as a whole. “Sometimes you can change a component and cause more risk in the entire system,” he said. Adding a level-wind system adds unknown risk — “you are adding another mechanical element which might break or create a hard point,” he said.
Allan noted that there have been towline failures. “Among the information reviewed were several reports of towline failures or similar incidents affecting vessel availability, primarily in 2003 and 2004,” according to the report. He cited the following incidents: Aware’s towline parting in heavy weather on Dec. 21, 2003; and Aware’s towline parting on June 22, 2004.
“We’ve learned from those incidents,” Morales said. “Crowley has made changes in the operating parameters and how they use these winches.”
However, towing operators interviewed for Allan’s report said that the majority of towing gear failures on the tugs in Prince William Sound have been the result of poor fittings on the escorted ship.
Capt. Gregory Brooks, president of Towing Solutions in Spring Hill, Fla., said the escort towing system has to be able to respond “when the worst possible thing happens in the worst possible place under the worst possible conditions.” Brooks said he has done consulting work for Crowley in the past.
Brooks noted that the Allan report mentioned a winch issue that could lead to the towline parting in the worst conditions. “For an escort tug to have doubts as to whether it will stay connected in the upper limits of allowable environmental conditions, if we have any doubts as to whether or not the line will part, we should act on that,” he said.
There are limits on the conditions in which tankers and their escorts are allowed to operate in Prince William Sound, said Morales. At the Hinchinbrook Entrance, where Prince William Sound opens up into the Gulf of Alaska, transit is closed when winds reach 45 knots or there is a 15-foot sea state.
Brooks said that the Crowley crews on the escort tugs are well trained and do a very good job. And he said that the escort system is probably the best in the United States. When asked if the winches are adequate for the task, Brooks said yes. However, they should be replaced, he said. “In Prince William Sound we have an escort system in place so that it (the Exxon Valdez oil spill) never happens again,” he said. “Technology has given us another aspect to make sure it never happens again.”
Capt. Nathaniel F. Leonard, former director of Crowley’s Valdez operations, praised Crowley’s training program and emergency drills. Leonard now owns Little River Marine Consultants in East Boothbay, Maine. “If you didn’t have a good, fundamental training program, constant-tension winches would be absolutely paramount in the system,” he said. “If you have a good training program, then you are not as readily exposed to issues with heavy weather because you have trained for that weather with the equipment you have.” He said that going to constant-tension winches would add a greater sense of security, but the current equipment is very good and the towing system is sound.
Cmdr. B.J. Hawkins, commanding officer of U.S. Coast Guard Marine Safety Unit Valdez, said, “There is no requirement for the winches to be replaced.” The current winches, he continued, “have been proven adequate during regular towing exercises with laden tankers under a variety of sea conditions.”
The winches are just one part of the escort system that includes “regulatory and industry limitations addressing weather and safe speed,” he said. “By limiting tanker movements based on a variety of factors, including weather and speed of advance, we mitigate the likelihood that the tugs will need to operate in extreme conditions.”
A narrow focus on the winches does not take into account the overall system, which includes the propulsion plant and the professional crew, which is used to safely escort laden tankers through the sound, said Hawkins.
Winch replacement would be expensive. Allan wrote that it would cost between $1.5 and $2 million per vessel to replace the entire escort winch, including changing the generators.
Another issue is the lack of data about the indirect steering force capability of the ETT tugs and similar data for the maximum towline force for the PRT tugs. So Allan recommends that indirect towing tests or a fluid dynamics computer analysis be done for all five tugs before designing new constant-tensions winches.