Improving the tow line – the vital link
In much of the tug industry, rope has been regarded as a commodity – "if it breaks, throw it away and replace it”, has often been the mantra.
Svitzer operates a fleet of more than 430 vessel globally, mostly made up of large high-capacity tugs of 80tbp and above, mid-range tugs of 50 to 70tbp and what it describes as ‘legacy’ tugs used mostly for utility work.
With such a large fleet, a major investment is the various types of cordage, perhaps most important of which is the tow line. For the high-capacity tugs, often handling the largest and highest value tows, ultra-lightweight, high-strength, 100% Dyneema fibre ropes such as Samson’s Saturn-12 will be specified while mid-range tugs often utilise traditional fibre ropes such a polyester, either jacketed or single-braid.
To meet strength requirements these latter ropes are large, heavy and difficult to handle particularly when wet. The braided cover of jacketed ropes protects the core from damage, but problems can occur when the jacket parts, compromising the line’s strength and also where the jacket itself masks damage to the core. Repairing the damage can lead to long splicing sessions taking the tug out of service for as long as a day hence it being cheaper to replace the rope. It is the tow lines of these mid-range tugs that Samson and Svitzer are now striving to improve.
TRIALS ON THE THAMES
Svitzer required a rope that could withstand the strain of standard operations with its mid-range tugs and eliminate the problem of undetected damage but without the expense of high-performance all-HMPE ropes.
The answer was Samson’s Fusion-12 rope. As a hybrid rope it blends the strength and performance of Dyneema HMPE with polyester fibre into a single-braid construction significantly stronger than other polyester ropes but lighter, more flexible and stronger including the ability to spool well without the tendency of polyester to bury itself on the winch drums.
Samson state that on-board inspection is a simple process with on-site splicing taking a fraction of the time required for jacketed constructions. The rope has reduced elongation and better resistance to abrasion compared to traditional polyester lines.
The decision was made to carry out trials with the Fusion-12 rope in Svitzer’s London fleet and Svitzer Warden, one of a pair of Damen ASD 2411 tugs delivered to Svitzer in 2006 was selected as first vessel in the trial. The standard configuration installed comprised a 150m Fusion-12 mainline with a 20m Saturn-12 pendant with chafe protection. Samson was on board for the rope’s installation and to conduct crew training in inspecting, splicing and maintaining the new ropes.
In general, Svitzer’s larger tugs are fitted with Saturn-12 mainlines and pendants protected with chafe gear, each installed with a backer line at the drum with the same system (with Fusion-12) installed on its mid-range tugs. The higher coefficient of friction from the polyester content of Fusion-12 means a backer line can be eliminated.
The field trial plan was to outfit two tugs in the first year and comparing them to historical data for jacketed lines previously in use. Typical retirement for the jacketed lines was 1,200 tows, the jacket being the main cause for replacement, but the Fusion-12 lines have performed 2,500 to 3,000 tows so far and still counting. Since receiving the initial results of the field trial, two more of Svitzer’s London tugs have been retrofitted with two more on schedule after that.
One difference noted by the crews was that the new lines allow more rope on the winch drum compares to polyester rope as Fusion-12 lays more neatly on the drum. If the rope buries it often just pulls out eliminating the need for the tug crew to haul out stoppers to pull out buried turns.
Adrian Hopkins, Svitzer Warden’s master has 30 years tug experience and speaks of the impact of the new rope: “With Fusion-12, you just get that confident feeling.” adding: “There’s no creaking or banging with it (like jacketed lines)—you just feel confident. When the pilot calls for full power, you can put the levers down knowing the rope will hold up.”
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