Dyneema

At the seminar last week I got to meet the west coast sales rep for Samson rope. It was my nerdiest dream come true; finally I could ask every question I ever had about the lines we use at work – and I have many. The rope most tugs use for ship or barge work is made of what’s called HMPE, or high molecular polyethylene. Up close it’s thousands of fibers as fine as spider silk, only stronger, twisted into the yarns that are grouped together to comprise each strand in the line (usually 12 to make a round 12-strand plait). These ropes – several inches in diameter – test at hundreds of thousands of pounds of breaking strength; if I remember correctly, many of our retired ship lines have gone back to the Samson factory in Ferndale, WA, for testing and broken at just over 500,000 pounds. The factory has a machine that can pull-test a rope at forces up to 1.13 million pounds.

The industry name for the material in our ship lines is “dyneema”, and this fiber is used in what is probably Samson’s best-known product, Amsteel blue. It’s also in the lines we use on our tractor tugs here in Valdez to assist ships, a product called Saturn 12. Over beers last week, I learned that Saturn 12 and Amsteel blue are actually just the names of the substances used to coat the dyneema fibers to protect the lines and prolong their working life. Saturn 12 is orange and Amsteel is, well, blue. On the conventionals in Valdez we use another type of line called Quantum 12. I learned that spectra and plasma are a different product altogether: spectra is the fiber and plasma is the name of the finished product which is sold by Puget Sound Rope, not Samson (I have wondered about this for literally years). Plasma is usually purple and we used mainly plasma lines on all our ship assist tugs at Foss.

I learned about pick angle, or the angle at which fibers in a braided rope cross each other. A rope with a low pick angle is more tightly braided and abrasion-resistant, but not as strong as a line with a higher pick angle, which has more linear strength but because of the looseness and exposure of the fibers is more prone to abrasion and wear. You never want to tie a knot in this rope because the angles in a knot exceed the efficient bend limits in the fibers and weaken them tremendously. You also don’t want the rope to twist; twists in a rope create lateral stress on the material which reduces its linear strength.

I learned that they anchor drill rigs with synthetic lines because the wire or anchor chain it would require to hold a rig on station in thousands of feet of water (in a place like the Gulf of Mexico) would break under the force of its own weight. HMPE is not only as strong or stronger than anchor chain, it is also considerably lighter and much more flexible.

At any rate, many if not most of these ropes are stronger than steel and will rip a bitt off a ship if the tonnage limit is exceeded. I managed to get my hands on a strand of dyneema from a retired ship line off the Tan’erliq last month and have yet to decide what exactly to do with it all. I hear they make excellent vehicle tow ropes. I also plan to take the factory tour as soon as I can make it up to Ferndale, something I’ve wanted to do for a long time.

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