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July 2012 Issue

Nylon Shows Potential for Moorings on Wave Energy Devices
By Stephen Banfield

The properties of fiber-rope hawsers used to moor tankers to single-point moorings (SPM) were studied in the late 1970s and early 1980s. It was found that wet nylon rope had a relatively short cyclic fatigue life. Marine yarn finishes, however, greatly improved nylon rope's fatigue characteristics. SPM hawsers typically have a life of one to two years when left floating in the sea and up to five years when used on the stern of a tandem offtake if stored on a reel. However, due to short fatigue life, nylon cannot be designed for permanent moorings of around a 20-year life.

The properties of fiber ropes for deepwater mooring systems were studied in the 1990s by Tension Technology International Ltd. (Eastbourne, England), or TTI, and others. The industry found that polyester rope was well-suited for deepwater moorings with its moderate stiffness and superior cyclic tension fatigue properties.

One requirement for deepwater oil production platforms is that horizontal movement due to environmental forces is minimized to prevent excessive loads on the drill stem or production riser. These platforms are typically moored waters in 1,000 to 3,000 meters deep. But the horizontal movement must typically be less than 1 or 2 percent of the water depth. Taut leg moorings must be used instead of catenary anchor legs because of weight and other considerations. Polyester rope was found to have good compliance characteristics and is now commonly used in deepwater platform moorings.

The mooring system requirements for wave energy conversion (WEC) systems are unique and different from those for tanker mooring or deepwater oil production platforms. WEC systems are typically moored in less than 100 meters of water. The maximum wave peak-to-trough height might be as high as 25 meters. Thus, in survival mode, the WEC mooring system might have to accommodate horizontal movements that are at least 25 percent of the water depth. TTI has designed WEC moorings in polyester, but this has led to longer ropes and higher loadings than could be achieved with nylon.

The company came up with an idea for a method to redesign nylon rope to increase fatigue life and applied for funding from the Carbon Trust in the U.K. to research the cyclic tension fatigue life of a nylon rope made by Bridon International Ltd. (Doncaster, England), which collaborated on the study. Fatigue testing was conducted on scale subropes at TTI Testing in Wallingford, England, for about a year. Spliced subropes were sprayed constantly to simulate the ocean environment and cycled around a typical wave period of 10 seconds.

The design of splice and eye termination was critical to avoid termination failures and high variance in the data. Some failures were clear of the splice, and some were at the last tuck of the splice. This resulted in very low variance, which is important because too high variance leads to a lack of confidence and a lower fatigue design curve, with the application of -2 standard deviations to derive a lower bound design line. Based on this study, a new design fatigue curve for nylon was developed, showing that the fatigue life significantly improved to a level that made it possible to design nylon for WEC permanent moorings.

A fatigue analysis for a wave energy device was conducted in accordance with American Petroleum Institute specifications for a fatigue histogram of loadings for 30 years. The data from tests conducted on eight-strand plaited and braid line nylon constructions in the 1980s were compared with the more recent nylon data from the fatigue tests for the Bridon-developed subrope in the Carbon Trust-funded study. For the old nylon mooring, there was hardly any life at all available, and, for a 30-year service life, it was overused by 11,400 percent. In contrast, the new nylon data gave a life in excess of 2,000 years.

Although this project was undertaken with the emphasis on reducing energy costs by making more efficient moorings for wave energy devices, the size of wave energy devices now being considered means that nylon ropes have become an enabling, rather than just an alternative, technology for these moorings.

The Carbon Trust is providing funding for further work to gather data required by a mooring system engineer to conduct a detailed design. These next steps include elongation and stiffness data; fatigue testing for a variety of mean loads and load ranges; the applicability of Miner's summation, which calculates fatigue life under variable amplitude loading using constant amplitude fatigue life data; tests on larger ropes to investigate any scale effects; and an analysis of degradation mechanisms.

TTI is presently working on analysis of the nylon's long-term properties and mooring analyses, and the work is expected to be finalized in six months. Of particular interest is the complex temporal nature of the axial stiffness properties of nylon and how to model this behavior in global mooring system response. This is not a new topic and also applies to any polymer.

Offshore wave and tidal energy mooring systems do not yet have any codes or standards specifically written for such systems. TTI is working with Det Norske Veritas (Oslo, Norway) to develop classification rules for the wave energy market in a joint collaboration project funded by the Carbon Trust, along with partners Bridon, AWS Ocean Energy Ltd. (Inverness, Scotland), Koninklijke Ten Cate nv (Almelo, Netherlands), SSE Renewables (Dublin, Ireland), University of Exeter and Promoor Ltd. (Douglas, Isle of Man).

The company is working with clients interested in using nylon on their demonstrator and full-scale devices and hopes to see the first application very soon.

Stephen Banfield is the managing director of Tension Technology International Ltd. in Eastbourne, England. He joined the company shortly after its formation in 1986. Banfield pioneered the application of nylon ropes in permanent moorings for breakwaters, with proven field experience of about seven years, and created the opportunity to expand this knowledge to the renewable market.


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