5. The Towline

The towline, being the connection between the ship and escort tug, is of primary importance to operational safety. Therefore, it cannot be considered separately, and a holistic approach becomes necessary nowadays. In recent years, the winch power, as Griffin ^[18] depicts, has been multiplied sixfold on average over the past 20 years as compared to a doubling of propulsive power in the same time, and towline break strength should be designed accordingly due to the ever-increasing size of assisted ships as the requirements for escort towing forces are higher. Thus, advances in towline technology needed to go hand in hand with new winch technology in order to improve the safety performance of the whole system. This is the main reason why the industry has seen a nearly universal acceptance of high modulus polyethylene (HMPE) synthetic ropes, also known as ultra-high molecular-weight polyethylene (UHMWPE) or high performance polyethylene (HPPE), used successfully in escort towing since the mid-1990s ^{[9][16][18][19]}.

HMPE lines have very little elongation or stretch under load, and if the winch performance can achieve a dampening effect to control the high tensions generated in dynamic environments, this low-inertia characteristic will give a better load control, which helps to work more safely in these conditions. However, regarding this lack of elongation property, some challenges should be overcome, as their use can lead to snap load effects in the towline of an escort tug working in these harsh conditions greater than in a higher elongation rope as polyester (Figure 4) and they can fail without warning. It is the reason why this type of rope should have the highest possible strength and working load factor ^[20]. Thus, to cope with those characteristics, two different strategies can be applied in escort towing:

Figure 4. Rope properties, strength versus diameter of different lines as per Samson Rope manufacturer. Drawing: Authors.

• As new high performance winches can achieve a dampening effect to control the high tensions generated in dynamic environments, the low-inertia characteristics of HMPE synthetic ropes that have very little elongation or stretch under load give them a better load control, which helps to work more safely. In this case, the best option is the use of an HMPE rope as the total tow rope (monolithic).
• In compliance with the Class Rules, the use of a less sophisticated winch (in case the tug owner does not embrace the new high performance winch concept) to limit slack line events and snap loads by increasing the elasticity of the towline with a higher elongation one such as polyester can, in comparison, attenuate the higher frequency energy better. Alternatively, an HMPE line connected with a stretcher (a short line of around 20 m in length with higher elasticity—typically polyester) to accommodate the tug motions with relatively little variation in the towline tension can be used. As it was traditionally made, these two options offer additional elasticity at the expense of having a higher risk of towline failure than with the previous strategy.

Obviously, if an escort tug is equipped with a high dynamic winch, to use an HMPE rope without a stretcher is the best option (though some tug operators use it). The reason is that as a stretcher stores much more energy when peak loads are generated, it is released again when the tug moves toward the tanker afterwards in such a way that if elasticity is too high, the tension cannot be controlled and the tug can be catapulted ^[15].

Although there are small differences among various ropes manufactured depending on their fibers—mainly Spectra^® (Honeywell, New York, NY, USA) or Dyneema^® (DSM Dyneema B.V., Geleen, The Netherlands)—and how rope manufacturers feature these fibers in their ropes, they have a similar strength to wire rope at the same sizes, but only around 1/6th of the weight, which simplifies their handling. It also floats (enabling tug operations to be much simpler and safer). However, it should be taken into account that if jacketed, they can have a higher density and may sink, depending on the rope’s diameter and the material the jacket is made of ^[21] (p. 9).

Abrasion is the ability to withstand wear and rupture due to motion against other fibers or rope components within the structure of the towline itself—internal—or by contact with external surfaces—external. As detailed lab simulations and field observations confirmed, it is the dominating factor and most serious threat to the integrity of HMPE ropes used in escort towing, affecting their service life ^[9][19]. Mitigating it in such a potentially abrasive and demanding environment and protection of the primary working parts of the lines is vital to extend their service life. This is mainly achieved by:

• Proper deck hardware: To minimize unnecessary abrasion damage, all surfaces in contact with the towline should be smooth (it is recommended to be kept at a maximum roughness of 250 microinches, μ”). Typically, on board the tug, this is achieved by well-designed stainless steel staple and bitts being highly polished (Figure 11) and with a generous bend radius—ideally, the cap rail should be made of stainless steel as well ^[19]. As the towline leads from the winch drum to the staple and then directly to the tanker, a point of fatigue is generated at the staple, which is exacerbated by the back-and-forth tug motions, which is the reason why water spray cooling is recommendable to dissipate heat due to the friction there ^[4] (Figure 5).

Figure 5. Wide staple of Escort Voith Water Tractor Tug “Tenax” with water spray cooling for escort towing with a high modulus polyethylene (HMPE) towline. The contact surfaces of both the staple and all around the cap rail are made of a highly polished stainless steel. Photo: Authors.

However, the main source of abrasion leading to failure in the towline, an Achilles’ heel as Arie ^[22] metaphorically describes, is when it comes into contact with poor fittings (chocks and bitts) on the tanker ^[9] (p. 5). A good example to follow in order to tackle this drawback is the tanker–escort tug system implemented in Prince William Sound, where tanker fittings should be designed according to the towline force capability of escort tugs deployed there ^[9][19]. To minimize this potential damage and extend the life of the main towline, a sacrificial pennant—sometimes referred to as pendant or forerunner (a short length of HMPE rope with an eye spliced at both ends)—of the same minimum breaking strength (MBS) is used to endure it ^[20][23]. The connection between the main towline and the pendant bights is usually made by a “cow hitch” (Figure 6) or by a “spectacle splice” (eye-to-eye connection) ^[24] (pp. 14–15).

Figure 6. Cow hitch connecting HMPE towline to a sacrificial pendant. Drawing: Authors.

• Proper chafe protection: The localized abrasion points on the line should be protected by strategic positioning of chafe gear at the appropriate locations (to protect from both internal and external abrasion).

• Coating technologies developed by manufacturers: Manufacturers are developing coatings at yarn level to reduce internal abrasion enhancing rope wear and snag resistance.

As the coefficient of friction (COF) (or μ) of HMPE ropes is low, to prevent them from slipping on the drum winch, a backer polyester rope of a smaller diameter with a higher COF and that is long enough to cover the core first layer of the drum (sometimes referred to as the “safety layer”) is helpful to mitigate this risk. It should be of sufficient diameter to fit into the bitter end of the winch drum and be spooled under as much tension as possible (to minimize the burying of these working ends of the rope). As the load is largely dissipated in friction while it migrates towards the drum core from the first external wraps, it is not necessary for the backer rope to have the same MBS as the HMPE working line ^[9][20].

To compensate for the loss of grip due to its low COF, a HMPE towline requires around twice as many wraps in the drum winch than a polyester one to achieve the same holding power. However, on an equal strength by strength basis, its diameter is approximately half, so the drum width can remain the same ^[25].

Even in relatively moderate sea states, Maritime Research Institute Netherlands (MARIN) found out that, excluding slack line events, towline force can be triple the tug’s bollard pull (BP). To prevent an HMPE towline from breaking in dynamic conditions (according to Allan ^[9], in extreme sea states, the dynamic loads can exceed ten times the BP), an MBS up to six times the BP is commonplace as a working load factor in order to provide greater safety and to extend the ropes’ service life ^[5].