Pacific Maritime Magazine - Marine Business for the Operations Sector

TOW ROPES

Selecting and protecting your high-performance, high-tech investment

 

Eight-strands are more susceptible to abrasion than 12-strands. Photo courtesy of Samson.

When discussing new technology in the tug industry, propulsion systems, hull forms, and new winch designs often get a lot of attention. But what is arguably the most critical component of the vessel’s mission, the tow rope, isn’t usually in the mix. However, if you ask tug operators which technologies are critical to their success, you’ll hear a different story.

“The most important connection between our customers and us is the towline,” said Capt. Steve Huttman of G&H Towing in 2010. “The performance of that line allows us to successfully service our customers.”

Similarly, Tom Crowley Jr., CEO of Crowley Maritime, said in a 2007 interview that the most significant technological advancement to effect the efficiency of the tug-assist business is synthetic rope. “It is as strong as steel wire, it floats, and it has enabled us to do some amazing things.…The tanker escorting business is built around these ropes, and there’s no way you could do it without them.”

Today’s Tow Line Technology

High-performance synthetic tow ropes made with high modulus polyethylene (HMPE) fiber like Dyneema®, work with a variety of hardware, from the conventional to the newest technologies, while still maintaining their extremely high strength balanced with very light weight. The combination has meant easier, safer line handling, more efficient operations, and longer lasting, more reliable tow lines. But there is more to the equation than just fiber. Modern developments in rope technology are the result of manufacturers partnering with leading tug operators to develop new products that bring the benefits of research and development to the working deck of today’s tugs.

One such rope manufacturer is Samson, headquartered in Ferndale, Washington. While Samson engineers brought the first HMPE ropes to the commercial marine market many years ago, the company continues to advance the industry by partnering with firms such as Crowley Maritime, G&H Towing, Moran Towing, McAllister Towing, Seaspan Ship Management, and Harley Marine, to develop new products that make ship assist and tug operations safer and more efficient—ultimately improving the bottom line.

It’s Not Just a Rope, It’s a System…

We used to talk only about tow lines. But now, with the advances in fiber and coating technology, and the innovations in chafe protection, we talk about towing systems. Think of your tow line as just one part of an entire system critical to the mission of your vessel. Selecting the right towing system is not a “one-size-fits-all” proposition. The system components can include a backer section that attaches to the winch drum, a main line section that is the strength member, the pendant that does most of the day-to-day travel between vessels, and the chafe system that keeps it all protected from the hazards of abrasion. Some operators even include a section that allows greater elongation to mitigate potential shock loads. It all works together to keep the tug’s deck a safe and efficient working environment.

So Many Options

For years, wire rope or polyester were considered standards in the towing industry, in spite of the inherent dangers of working with heavy wire rope and polyester ropes that have low tensile strength, are heavy and tend to absorb water. Today’s high performance fibers, such as Dyneema®, have taken over as the new standard. Beyond fiber, there are other factors to consider: rope construction, and resistance to abrasion, cutting and UV damage. High-performance ropes for towing applications are available in 8-strand and 12-strand braided constructions, and also in jacketed constructions. So what are the differences, or advantages between them?

8-strand constructions have been staples of the towing industry for many years. They are easily spliced, have a firm structure that works well on winch drums, and largely because of their distinctive profile lock well when used on H-bitts. Inspections, both for external and internal problems are simple, but 8-strands are more susceptible to abrasion than 12-strands. They also accept twist, and care should be taken to make sure the direction of turns is alternated when using H-bitts to help avoid twist and the potential for hockling.

12-strand constructions, all things being equal, will generally achieve higher strengths than 8-strands. Their profile is rounder than the square-braided 8-strands, and they are generally more flexible. Splicing is easy, and access to internal strands for inspection is simple and direct. 12-strand ropes generally work well on winches and have better resistance to abrasion.

Working from winches, the 12-strand construction really shines. 12-strands have a round cross section with the firmness dictated by braiding angles. The ropes pack and layer well on the winch drum yet are flexible and easy to handle. But not all 12-strands are created equally. The rope designer has several choices: braiding helix angles, fiber content, pre- and post-braiding treatments, and proprietary coatings to help protect the finished rope all contribute to performance. Extending the helix angle of the braid results in a stronger finished rope, but one that is easily snagged and subject to strand pulls. Heat setting can affect the initial strength of the rope, but testing has shown that the cost is a reduction in service life and a lower resistance to tension fatigue. Good rope design should be the result of carefully weighed compromises that allow for maximum strength, ease of handling, abrasion resistance, and longest possible service life.

Jacketed constructions typically consist of a load-bearing core, usually of 12-strand braided construction, covered with a non-loadbearing braided jacket to protect the strength member from abrasion and damage. Since the core of a jacketed rope is protected, they are typically perceived to have a longer service life. However, in practice, the cover typically wears or breaks down much faster than the core. The result is often a rope that is retired early because the jacket is no longer providing the needed protection to the core.

Partnering to Fight Abrasion

As mentioned before, a systematic approach is required to achieve an optimal towing system.

Regardless of the rope’s construction, abrasion is the natural enemy of high-performance ropes. When the pendant goes through the chock to the assisted vessel, all control over surface preparation and condition is lost. Proper chafe gear becomes a necessary partner in the system.

For many years, chafe protection was relegated to materials such as inverted fire hose, ballistic nylon, and polyester fabrics—all good materials that served their purpose, but were sometimes overwhelmed by the conditions to which they were exposed. When the tow rope sees multiple deployments and retrievals per day, the need for specially designed chafe gear with more robust specifications becomes critical.

Chafe gear has benefitted from advances in high-performance fiber technology as well. Since cut and abrasion resistance are both inherent characteristics of HMPE fiber, it makes sense to exploit these properties when it comes to developing chafe gear. In this application—protecting the strength member from damage, a low coefficient of friction is both a feature and a benefit. Open weave style chafe protection such as Samson’s Saturn Dynalene, made from 100% Dyneema® protected with an abrasion and friction reducing proprietary coating, is easily spliced into the main line or pendant where needed, but allows the rope to be fully inspected both for external conditions and internal abrasion without being removed. Chafe gear protects the surface of the rope from external abrasion and reduces internal abrasion by minimizing the relative movement of internal fibers and surface fibers. In the long run, chafe with Dyneema® lasts longer than the polyester and nylon alternatives, and is easily changed out by splicing a new section on to replace the worn out section. Best of all, it protects the critical strength member from both external and internal abrasion.

DC Gard is a close-weave product from Samson that can be spliced onto the rope for protection. Essentially a separate jacket or cover, it must be removed for inspections, but, by fully covering the strength member, it provides the ultimate in protection from seriously compromised hardware or surface conditions.

The Newest in Abrasion Mitigation Is Not a Chafe Product

When Samson engineers set out to find new ways to improve abrasion resistance, they wanted to look beyond traditional chafe protection products. They developed a new, proprietary coating applied at the yarn level that testing confirmed helped reduce internal abrasion significantly. The next step was to get out of the lab and test it in the real world. Long-term customer G&H Towing Co. of Galveston, Texas, committed 11 tractor tugs for the two-year duration of the field trials. Altogether, 35 HMPE ropes from two manufacturers and more than 27,000 combined pulls contributed to the database. Typical test results showed the new rope—Saturn-12—retained 95 percent of its original breaking strength after 909 assists and more than seven months in service. In addition, rope failures were reduced to roughly 25 percent of the levels recorded prior to the trials.

Similar results were experienced by other tug operators in the U.S. and in Australia.

How to Spot Abrasion Damage and Learn From It

External abrasion is easy to spot –fuzzy areas of broken filaments and, eventually, worn strands are apparent—but the rope needs to be checked for internal abrasion as well. Poorly maintained deck hardware that the rope passes over or through can cause the surface fibers to move relative to the internal fibers of the rope as it abrades against the compromised surface. This relative movement generates heat and causes degradation of the internal fibers, shortening the rope’s service life dramatically. Inspect for internal abrasion periodically by opening up the strands and observing the internal yarns. A reduction in the size of the yarns compared to others in the same rope, or powdered fibers are indications of internal abrasion.

Rope Handling and Inspection

When working with high-performance synthetic fiber ropes, it should become second nature for everyone handling the rope to visually inspect it every time it is retrieved. These ropes are serious tools and capital investments that need proper care and maintenance to continue to provide safe and efficient operations. As the rope comes aboard and is replaced on the winch, look for abrasion and cut or pulled strands. Variations in the rope’s diameter could indicate shock loading or broken internal strands. Inspect for discolored sections or areas where compression has stiffened the fibers. Some of these conditions can be repaired; others are cause for considering retirement of the rope. Check with the manufacturer for inspection guidelines and retirement criteria based on field use and the conditions observed during operation.

Other elements of the inspection program include good surface maintenance of all deck hardware and installation of proper chafe gear to help mitigate surface abrasion. Deck hardware should be regularly inspected and maintained. For maximum working life, these ropes require smooth surfaces free of rust or scarring from wire ropes. Winch drums and drum flanges, the bullnose, or H-bitts, and any cleats that will be securing ropes or chocks the rope passes through, should be smooth and prepared to the rope manufacturer’s surface specifications. Often, the bullnose and sometimes the H-bitts are clad in stainless steel—a super smooth surface that resists corrosion and requires less maintenance than typical painted surfaces in the marine environment.

Establishing regular inspections and break-testing both on working lines and retired ones help determine appropriate customized retirement criteria. A partnership with the rope supplier should result in a program that will provide a constant source of information which will be invaluable for proper line management. This has been the case for G&H which as a result of its partnership with Samson has developed standard operating procedures that include:

Deck and hardware preparation to mitigate abrasion.

Crew training in splicing, handling, and inspection.

Twist management program.

Scheduled line inspections and end-for-ending.

Residual strength testing to determine retirement criteria.

The Effect of Twist: Unbalanced Loads and Weakened Ropes

As the rope is retrieved, look for any twist in the line. Braided ropes are designed to be non-rotational. That is, half the strands are twisted to the left with an equal number of strands twisted to the right. The result is a rope that, when under load, should not rotate. Try attaching a weight to the end of a 3-strand twisted rope. Lift the weight by the rope. The weight will rotate. This is because a twisted rope wants to untwist when loaded.

Even a small amount of twist in a braided line will create an imbalance in the load on each of the strands in a non-rotational rope. Testing shows that as few as four instances of twist per meter can reduce the rope’s strength by up to 30 percent. To avoid introducing twist into a line, always use a braided messenger line with a swivel within a few feet of the attachment to the pendant. The swivel will isolate the pendant from any rotation in the messenger line as it is passed from vessel to vessel.

Checking for twist is relatively easy. On 12-strand ropes, the braid pattern is such that each row of crowns in the rope follows straight down the length of the rope. Some manufacturers paint a line down the length of the rope to make visual inspection for twist simple.

If a twist is found, pull out a length of rope longer than the twisted section from the winch and, working from the winch towards the working end, rotate the rope in the opposite direction until the twist is removed.

Working from H-bitts can introduce twist into the rope and promote hockling. To help eliminate the twist that’s introduced, alternate the sides of the H-bitts from job to job. The objective is to wrap the horns of the bitt in the opposite direction, countering the twist introduced in the previous use.

Attention to Detail Pays Off On the Bottom Line

Inspections, both for external and internal problems are necessary, but simple. Photo courtesy of Samson.

Every time the rope deployed, give it a visual inspection as it goes over the side; on retrieval, take another look. Pay particular attention to the part that has passed through the other vessel’s chocks. Check for twist. Look at the surfaces it is passing over—do they need to be faired?

And, most importantly, when in doubt, check with the manufacturer. They should have any information you need on the proper usage and maintenance of their products from surface preparation requirements, to determining retirement criteria, and day-to-day handling techniques to help this critical system function at its best and safest for the longest possible time.

Jeff Smith is a principal at SmithWalker Design in Seattle and Charlotte Wells is the Director of Marketing at Samson.

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