Posted by: towmasters | July 12, 2009

UNCHAINED?: Check Your Tow Rig And Stay Attached To Your Barge.

Diamond Dave, Eddie, Alex and Mike were unchained in Oakland back in ’81, and they were bad-ass, but that just ain’t too cool when it comes to towing barges around. So, just as with your tow wire, you must regularly inspect any stud-link chain that is a part of your towing assembly (or towline, as it is referred to in the regulations). And, for that matter, any bridles, pennants/pendants, shock lines, surge chains, tow(ing) plates, thimbles, shackles, detachable links, open/end links, swivels, chocks and pad eyes. In short, everything that bears a load or is in any way a part of the connection between tug and tow when towing astern. Just because a particular item isn’t a part of the tug’s equipment doesn’t mean you aren’t responsible for it if it fails and a worst-case scenario ensues. Key word: must. At a minimum, monthly inspections have to be done. Refer to 33 CFR § 164.74(a)(3)(iii) if you doubt this.

So where do you find out what the deal is with chain? Once again we go to the U.S. Navy Towing Manual, aka the TOWMAN, for the information we need. Appendix D – Chains & Safety Shackles is where to find all things chain, including the inspection parameters. Sections D-8 (Inspection), D-9 (Types of Wear), and D-10 (Special Precautions), which are on pages D-3 through 5, cover it but the entire appendix is only six pages long so print the whole thing and put it in your tow wire log behind Appendix B (which covers wire inspection). The Baldt Anchor & Chain Co., of Chester, Pa., also has some excellent information available for download, including this slideshow about how their stud-link chain is made. The history of anchor chain is quite interesting in its own right.

And what are you supposed to be looking for? Bent, cracked, corroded, damaged, elongated or worn links, and links with loose or missing studs. Same for the rest of the hardware. So break out the tape measure and the vernier calipers again…..

Any of the following conditions are grounds for getting rid of a particular chain link, section of chain, or other hardware.

  • Length of individual link exceeds 6.15 x the nominal chain diameter.
  • Length over 6 links exceeds 26.65 x the nominal chain diameter.
  • 3 degrees or more of out-of-plane bending.
  • Average of the 2 measured diameters at any point less than 95% of nominal or a diameter in any direction less than 90% of nominal.
  • Missing the stud.
  • Crack at the toe of the stud weld extending into the base material.
  • Excessively loose stud. Consider discarding if: 1. the stud can move more than 1/8-inch (0.125″) axially (lengthwise) or 3/16-inch (0.1875″) laterally in any direction, or 2. a gap of more than 1/8-inch (0.125″) exists between the stud end in a link with a stud welded only on one end. Note: feeler gauges work well for making these measurements.
  • Surface cracks or sharp gouges: attempt to eliminate by light grinding. Discard if the diameter is reduced to less than 90% of nominal after grinding.
  • Cracks detected by magnetic particle inspection in the internal locking area of a detachable link.
  • External surface defects in detachable links: attempt to eliminate by light grinding. Discard if the diameter is reduced to less than 92% of nominal.
  • Excessive wear or deep surface cracks on shackles, open links or swivels: attempt to eliminate by light grinding. Discard if the cross-section area, diameter or critical thickness in any direction is reduced to less than 90% of nominal by wear or grinding.

Chain, if it’s kept off the bottom, will normally wear primarily in the grip area. This is where adjacent links make contact with one another or a connector. Another place to check carefully is where the chain contacts the inside of closed chocks or other fairleads. Don’t forget to look where the detachable links or shackles connect to the towing (fish) plate.

You’ll need the spec/certification sheets for all of this stuff, or measure it yourself when it’s new, so you can have an accurate baseline for your calculations. Without it you’re just guessing. Also, and particularly with chain, there is a tendency towards the if-it-ain’t-broke-don’t-fix-it (or even look at it) mentality. That is why these inspections are mandatory, not optional or discretionary. Historically, inspections left to whim or chance often don’t get done.

I’ll quote directly from the Special Precautions section: “Because chain is generally the most rugged component of the towline system, there is a tendency to become overconfident in its capability and somewhat less rigorous in inspection. Avoid overconfidence when using chain.” That sums it up quite nicely. I would, however, broaden the application of this principle somewhat: avoid overconfidence, generally.

See last month’s WIRED post for wire rope inspection and replacement guidelines.

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Responses

  1. […] are and the specific procedures for checking them please consult the previous posts Wired and Unchained, which are derived from the U.S. Navy Towing Manual. Also see When The Wire Breaks! for information […]


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