High-Strength Mooring Lines Being Tested for Offshore Wind Installations
Date: October 8, 2012
Glosten Associates Tests Innovative High-Strength Mooring Lines for Deepwater Offshore Wind Installations
Note: Excerpt from the U.S. DOE Wind Program Newsletter, Third Quarter 2012 edition. Glosten Associates is a participant in a group led by Michigan Tech and including GVSU MAREC that is seeking federal funding for demonstration project involving a floating offshore wind platform.
Funded by the U.S. Department of Energy's Wind Program, Glosten Associates recently completed testing on innovative high-strength, high-stiffness synthetic-fiber ropes being proposed for use in deepwater offshore wind power installations. Glosten Associates and its partners tested six sub-rope specimens to confirm the ropes' strength, elongation, and stiffness properties. The innovative rope design was developed by Samson Rope Technologies for Glosten's PelaStar floating wind turbine tension-leg platform (TLP).
Tension-leg platforms have been deployed for decades by the offshore oil and gas industry and represent one proposed solution for deploying deepwater offshore wind technology. The TLPs use vertical high-tension mooring lines known as tendons to minimize vertical platform motions. Traditional TLPs use tendons comprised of tubular steel with universal-joint connectors at the upper and lower ends. These tubular steel tendons are robust but costly and difficult to deploy. The new synthetic rope tendons comprise several sub-ropes, jacketed together, to form a rope tendon. These rope tendons can be spooled on the deck of a barge and deployed at a lower cost than the traditional systems.
Synthetic-fiber ropes are a well-known technology, both on land and offshore. They have been used in applications such as heavy lifting, military, high-performance racing sailing yachts, and offshore catenary mooring systems. Several factors combine to make synthetic-fiber ropes an attractive option for TLPs. Recent advancements in material science have produced rope fibers that are resistant to creep and fatigue—key concerns for offshore wind turbine TLPs. New termination methods also enable the manufacture of rope tendons to precisely specified lengths. Finally, the cost of rope terminations and connections to the floating platform and seabed anchors are lower than those used in traditional steel tendons.
In the recent testing, sub-rope test specimens were subjected to static and dynamic loading. In the laboratory, loads were applied to the test specimens by a programmable hydraulic tensioning machine while the ropes were continuously sprayed with water to simulate offshore conditions. Static tests verified the strength and quasi-static elongation-stiffness properties of the rope. Dynamic testing verified the effects of aging on the rope's properties and measured the dynamic elongation-stiffness properties of the rope, which change as a function of strain rate or the combination of load frequency and amplitude. The dynamic testing also provided a limited verification of the ropes' resistance to fatigue damage. The test results indicate that the ropes perform as expected and provide the initial data needed to proceed with synthetic-fiber rope tendon design for floating offshore wind TLPs.
Image courtesy of Pelastar/Glosten Associates.
Glosten Associates is a naval architecture and marine engineering consultancy based in Seattle, Washington. Technical support, preliminary testing, and sub-rope test specimens were provided by Samson Rope Technologies. Independent rope testing was conducted by Tension Member Technology Laboratories in Huntington Beach, California.