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Feature Article

Determining Tensile Strengths of Large-Diameter, Synthetic Fiber Rope
Tests of 12-by-12-Strand Rope Validate Design Theories of Braided High-Modulus Synthetic-Fiber Constructions for Heavy-Lift Applications

By Michael C. Greenwood
Technical Sales Manager
Randy Longerich
Cortland Co. Inc.
Anacortes, Washington

168-millimeter-diameter Cortland Plasma rope sling grommets, rated at 2,831 tonnes, for London Array Wind Farm project installation.
In 2010, Cortland Co. Inc. (Anacortes, Washington) was awarded an order to design and build synthetic fiber rope heavy-lift slings for installation at the London Array Phase 1, an offshore wind farm with 175 turbines located approximately 20 kilometers from England's Kent and Essex coasts in the outer Thames Estuary. Sling strengths required more than 1,890 tonnes in an eye-and-eye vertical lift. Fiber slings were selected for performance advantages over steel-wire rope, such as flexibility, less damage to surfaces and lighter weight, that decrease rigging time and personnel required while increasing safety.

Three different strengths were needed, two of which greatly exceeded Cortland's largest ropes, and the company lacked verified technical data in those large-size ranges. Cortland's largest high-modulus polyethylene (HMPE) Plasma 12-by-12-strand rope size had a breaking strength of approximately 1,140 tonnes when tested in an eye-and-eye sling configuration. The maximum breaking loads required for the two largest new slings were more than 50 percent greater, meaning technical data on the design of these new ropes would be needed to verify the 12-by-12 construction and its translational strength efficiency.

High-Modulus Synthetic Ropes for Marine Applications
High-modulus synthetic-fiber ropes have been commercially available for more than 40 years, starting with Kevlar aramid. Kevlar and other aramid fibers, such as Technora, HMPE Spectra and Dyneema, and Vectran liquid crystal polymer (LCP) fibers are utilized in high-performance ropes.

Synthetic-fiber rope constructions made with these materials can equal or exceed the tensile strengths of steel-wire ropes of the same size, and HMPE fibers in particular offer significant weight savings (up to one-seventh the weight of steel for comparable strengths). The global use of lightweight large-diameter braided high-modulus synthetic-fiber ropes in traditional marine wire rope applications has grown significantly, especially over the last 20-plus years. Applications include deepwater lifting winch lines, heavy-lift slings, seismic towed arrays, vessel mooring lines and tug working lines. These ropes are now providing designers and end users with performance and project solutions that wire rope cannot match.

The acceptance of these ropes has been slowed by capital-expenditure product pricing being significantly higher than traditional synthetics (e.g., nylon or polyester) and wire. There is also a lack of understanding and acceptance that the lightweight properties of high-modulus ropes can be justified based on a proven service-life track record. Until recently, there also existed a combined lack of technical support material and understanding of large-diameter synthetics, including rope strength efficiencies, shape retention, diameter concerns, abrasion and cut resistance, elongation properties and bending efficiency. By comparison, the wire rope industry has effectively compiled a vast store of performance and engineering data on its products, which are essential for use in the design of systems requiring performance guarantees and a high degree of accuracy.

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Michael C. Greenwood has 36 years of experience in the U.S. rope industry and is the co-holder of two rope design patents. He is a graduate of the University of California, Santa Barbara, with bachelor's degrees in political science and history. He lives in California with his wife and has three grown sons.

Randy Longerich is the president of Cortland Puget Sound Rope, which he joined in 2000. He is a 1969 graduate of the U.S. Merchant Marine Academy at Kings Point, New York, with a bachelor's in marine engineering. He has more than 33 years of experience in the rope and cordage industry.

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