|Marshall Foss, shown here assisting a bulk carrier in San Francisco Bay, is one of two Foss Maritime tugs testing a system for accurately measuring the strain put on towlines during assist jobs. [Courtesy Foss Maritime]|
For tug masters, it’s perhaps the greatest eternal mystery. Exactly how many jobs can a towline handle before it’s in danger of breaking? How is a line’s integrity impacted by towing large ships through swells and troughs? When should a line be retired?
Until now, there was little empirical analysis available to a tug captain to determine reliable answers to these questions. Two technological developments – both involving electronic tension monitoring – are finally taking some of the guesswork out of safe line handling.
Honeywell International Inc., the maker of the Spectra brand of synthetic towline fiber, has spearheaded an effort to compile tension data constantly on a towline and have the data entered into a computer for inspection. The so-called Line Tension Data Acquisition Project is underway on two Foss Maritime Co. tractor tugs in San Francisco Bay.
In a separate advance, a Markey Machinery Co. device incorporates tension monitoring that triggers a Markey winch to spool out more line automatically when a pre-set maximum desired tension level is reached. The Markey system also involves software that gathers a compilation of line tension.The winches are being used on a pair of tractor tugs that service liquefied natural gas carriers at Elba Island, Ga.
Together, those two developments represent a critical improvement in the ability of tugboats to operate safely, said Capt. Greg Brooks, who runs tug-safety consultant Towing Solutions Inc. In an era when shipyards are producing monster tugs of up to 10,000 horsepower, no longer will captains rely merely on visual inspections and a tally of the number of jobs a towline has performed.
“The ability of boats to tow extreme loads has increased dramatically. There’s a human safety factor, and then there’s the safety factor in becoming separated from the ship that you are assisting in getting into the berth,� Brooks said.
“Some owners are still in the Dark Ages. They’ve been kind of just wandering around in the dark,” he said. “A visual inspection is spotty, at best. If you have a tension meter on your boat, you know what you’re pulling at all times.”
Foss Maritime and its line provider, Puget Sound Rope Corp., have installed the monitoring system on the 6,250-hp sister ships Lynn Marie and Marshall Foss. The 9-inch-diameter Plasma brand towing pendants, which are made of Spectra ultra-high molecular weight polyethylene, are equipped with a roughly 2-inch-long enclosed electronic monitoring device.
That monitor, which can record a bollard-pull measurement as often as 100 times per second, sends signals to a personal computer in the captain’s office. At the end of his shift, the captain makes a few simple entries that describe conditions during the day’s jobs.
Later, when the line is taken out of service, it undergoes laboratory tests to study creep and tension fatigue. Analysts can correlate those results with tension data and the nature of the work that was done over the life of the line.
Over years, the manufacturers and customer will have an ever-growing array of data to support a better retirement forecast. “The issue of line integrity is absolutely paramount,” said Joel Altus, supervisor of vessel rigging at Foss. “The towline is the singular connection between the tug and the vessel. There isn’t anyone who isn’t vitally concerned about it.”
|The Crescent Towing tug Bulldog. The tug escorts tankers at an LNG terminal near Savannah, Ga. [Gregory Walsh]|
Greg Davis, applications development engineer at Honeywell’s Advanced Fiber and Composite division, said the data will help the manufacturers and mariners do their jobs better, more safely and more economically.
“Now we’re really going to understand the history of these ropes and their use,” Davis said. “We’ll be able to learn some of the fault modes. We’ll be able to further refine when we should retire the rope based on the loads we’ve seen. The benefits are pretty obvious, especially for the end user. There are some safety benefits and some financial benefits. It helps them understand the tug performance, as well.”
Up to now, tug operators typically have visually inspected their towlines for obvious abrasions. They routinely retire the lines after a certain amount of service often 1,500 or 2,000 jobs to be on the safe side. Towline assemblies can cost as much as $50,000.
“It’s arbitrary. It may be only half its life and half of its economic utility,” Altus said. “The hope, ultimately, is we’ll have a very precise retirement date.”
Brooks said the Markey winch tension-monitoring system is like a slipping brake device that eases the tension in a line when the level threatens to exceed a safe working load.
“While it’s banging around in a sea state, the system can spool out a little bit of line,” Brooks said.
The system is at work on winches aboard Crescent Towing’s Bulldog and Moran Towing Corp.’s Edward J. Moran. Both tugs are 6,500-plus horsepower; each was recently built at Washburn & Doughty shipyard in East Boothbay, Maine. Both are at Elba Island, near Savannah, Ga., performing LNG escorts.
The Amsteel Blue-brand towlines used in the Elba Island project are from Samson Rope and are made with Dyneema high-molecule polyethylene fiber.
“This high technology, including the tension display and data logging, is absolutely essential to this high-level escort work,” said Blaine Dempke, president of Markey Machinery.
Lawrence Ohler III, vice president and port engineer at Crescent Towing, said the line monitoring is the only way the tug and ship captains can truly know how much sea conditions are straining the vital link between their vessels.
“The winch is able to pan out and pan in,” Ohler said. “There’s a bollard pull device built into the winch. We can set the line tension on the winch to pan out when it reaches a certain bollard pull. As far as safety is concerned, it’s very important.”
The software on the Markey system that compiles line tension is a big advance over simply using tension monitors real-time, said Dennis Sherman, manager of technical sales at towline-maker Samson Rope Technologies Inc.
|A Markey DESF-48 100-hp winch aboard the Crescent Towing tug Bulldog. The winch has a computerized system for recording strain on the line. The winch also allows the line to slip when loads become dangerously high. [Courtesy Markey Machinery]|
“The system is being adapted to tugs that are working in offshore terminals, where we do see extreme load levels due to sea states,” Sherman said. “The software is something new added to the system that we have really never seen before to help map it out. It’s really integral.”
Measurement Technology Northwest Inc., which engineered the software on the Foss boats, also designed line-tension monitoring for the hoist cable systems that lift transfer spans at Washington State Ferry stations.
On the Foss tugboats, the captain starts and stops the data log. At the end of the shift, he needs only to enter a few key pieces of information, including the vessel under escort. For spikes in line tension, he uses a simple drop-down menu to enter the type of maneuver, wind speed and other conditions.
Tomas Rezanka, director of the line control instruments program at Measurement Technology Northwest, said the system has been refined to avoid too many time-consuming clerical entries by busy boat masters.
“The tugboat captains have asked us to parse out the data until after a full day’s work,” instead of requiring data entry at intervals during a shift, Rezanka said.
The systems began entering service about a year ago. It didn’t take long for the companies to discover some eye-opening strain levels.
“One of the things that caught our attention is the severity of tension on some of the rope,” said Randy Longerich, president of Puget Sound Rope. “Sometimes when we’ve seen significant seas, we’ve seen some very significant spikes loads. It’s very interesting to see some of the values quantified for the first time. This is graphic evidence.”
Measuring and predicting line retirement isn’t the only benefit for boat crews. “There’s a wealth of information that can be gleaned from this,” Longerich said. “It is potentially quite a teaching tool in the long term. To see graphically what’s being done with the rope, it will allow them to gauge exactly what their actions are doing to the whole system. This also could be used to evaluate new (line) construction.”
Sherman agrees that adjustment may be made to rope designs for towing hawsers and ship-assist lines, based on various applications of line-tension analysis.
“We know that abrasion is a very aggressive failure mechanism. In mooring, it’s more an issue of fiber fatigue,” Sherman said. “You can introduce different coatings to try to enhance abrasion resistance, and you can tweak construction by varying twist levels.”
Foss plans to take advantage of the software’s training capabilities, Altus said.
“When you have a master teaching a mate about a job, the master can show the mate that history and suggest to the mate that maybe at this time another maneuver or letting a little more line outwould have been beneficial,” Altus said.
Brooks, the tug-safety consultant, predicts that the industry will embrace the tension-measuring equipment and new winches for wider use.
“You would hope that they would be on all tractor tugs,” said Brooks, a former tug captain in the petroleum industry with 38 years experience.
Despite the empirical data, the industry will still need to be cautious in projecting line retirement dates. “We’re never going to reduce failures completely,” Longerich said. “These failures are going to happen still, at intervals.”
Brooks noted that the polyethylene is reliable enough that very few accidents are caused by a failure of the line. More accidents are caused by broken cleats, chocks or fairleads, he observed, and those failures continue to deserve the attention of safety innovators.