Wednesday, July 28, 2010

Deck Machinery: A Systematic Approach to Selecting Towlines

By Merry Schnell

Getting as much value out of every purchase you make is critical in today’s economy. Rarely does a one-size-fits-all proposition bring the greatest value. Take your high-performance synthetic towlines for example. You want to get the most out of them so it is important to consider each component and situation the line encounters. This includes the line’s specific application, and the condition of the decks and hardware encountered, in addition to what type of mainline, backer line, pendant, chafe protection, and hardware is best suited to your needs. Each component works to compliment the other, creating a towing system. This approach allows you to extend the line’s service life, save time and money, and create a safer environment, among many other benefits.

Abrasion and Twist
Abrasion is a major culprit for the towline. Vessels and equipment that have traditionally used wire rope have often sustained significant damage caused by fishhooks, broken strands, etc., that come in contact with the deck and other equipment. These conditions can damage or significantly reduce the life expectancy of HMPE ropes. However, an owner/operator can take preventative measures to reduce the impact of these issues. All contact points such as fairleads, the inside of winch plates, and surfaces with grooving and rust should be repaired to a smooth and consistent surface.

The addition of chafe protection placed on the areas of surface contact is critical for the ropes longevity. These are sleeves that slide for adjustability or they are fixed by splicing into the line, depending on the construction of the rope.

Twist is often overlooked as a contributing factor in the reduced life an HMPE line. As little twist as four turns per meter can reduce the rope’s strength by as much as 20 percent. The importance of preventing twist in the towline cannot be stressed enough, and prevention is as simple as adding a swivel and strap to the towing system.

Configuring the Towing System
A towing system can be configured in a number of ways. Some operators like a single mainline from tug to vessel. Others will add a pendant to the mainline, while others will add a backer line to the combination.

Pendants connect the mainline to the vessel. They are particularly useful in preserving the mainline because most of the abrasion a towline is exposed to will exist on the vessel under tow and they can be easily replaced once abrasion has ended the pendant’s useful life.

A backer line is attached to the winch and can be used when the flange is narrower than what is required by the synthetic mainline. Backer lines are sometimes used as a sacrificial line so that if a line parts, it is the backer, and the mainline stays intact. Backer lines can also be used to increase grip on the winch if the mainline is too slippery.
Jacketed Lines vs. Nonjacketed Lines

Jacketed lines are commonly used as mainlines worldwide. Jacketed lines are perceived as having a longer service life because of the jacket and core construction. However, some operators find that the jacket ruptures or wears faster than the core, resulting in the need to replace the entire line due to the integrated construction of both units.

For ship-assist applications, a 12-strand line made of a 100 percent HMPE fiber such as Dyneema® protected with 100 percent Dyneema® chafe gear provides both safety and a long service life. Twelve-strand lines protected with chafe gear are stronger than jacketed lines size for size and they are easy to inspect and repair. If damage occurs, only the chafe protection has to be replaced, saving time and significant investment, and they are lighter in weight that jacketed lines. This configuration provides the ultimate in cut and abrasion resistance. The rope maintains its strength and results in long-term value.

Line Installation
After the towing system has been selected and surfaces prepped, the working line must be installed on the winch with significant back tension. The device used to create the tension should have a smooth and consistent surface, and the installation speed or tension applied should not generate excessive heat build-up on the rope.

As the line is wound onto the winch, it should be closely packed to reduce rope “diving” or burying into the layers on the winch. Each layer should be installed in the valleys of the previous layer, which supports each subsequent layer. Never stack the layers on top of each other.

Another method of loading the winch is to cross-wind. This method calls for the rope to be wrapped onto the drum in a similar way a fishing line wraps on a reel.

Inspection Schedules and Residual Strength Testing
Towlines are a long-term investment; taking them for granted can be costly and dangerous. Routine line maintenance, along with a stringent line inspection-and-retirement program is enough to mitigate most failure mechanisms and significantly extend service life. The responsibility of how HMPE towlines are handled and maintained on a daily basis is left to ship owners and operators; however, guiding the proper line selection and educating owners/operators on the importance of preparation belongs to the rope manufacturer. It is essential that crewmembers be trained on rope handling and safety procedures, rope inspection, the use of chafe gear, rope repair and splicing techniques. Only a qualified technician approved by the original rope manufacturer should conduct training, and periodic in-field inspections of the line and associated equipment by the towline manufacturer are encouraged.

Merry Schnell is the marketing communications specialist for Samson and has extensive experience in technical writing and editing in the sciences and engineering.