Nylon fibre rope moorings for wave energy converters
The wave energy market has traditionally employed mooring concepts adopted from the mature technology of the oil and gas industry. However, this approach may not always be appropriate to satisfy the particular requirements of Wave Energy Converter (WEC) mooring systems, especially the need for very compliant moorings. Specific challenges are: the huge 1st order motion induced forces generated by storm waves in shallow water; the potential for slack-snap loading on a mooring; and, the fatigue performance of the mooring components. This paper describes the benefits of employing high compliance fibre rope in mooring systems of less than 100 m water depth. Nylon fibre ropes are a very attractive proposition because of their low modulus and cost but have previously been thought to have a poor wet fatigue performance. A specially designed long-lay-length Superline subrope has been tested both for compliance and for wet fatigue performance. The rope compliance is about twice that of parallel-subrope construction polyester rope. The fatigue performance is less than that of parallel-subrope polyester rope but is comparable to that of conventional construction polyester rope and to that of the chain and steel wire ropes which must also be used as mooring components. The high compliance of the nylon subrope construction together with the more than adequate fatigue performance demonstrated in this study qualify such nylon ropes as ideal for WEC mooring systems.
Fatigue durability of nylon rope for permanent mooring design
Over the last nine years three major research studies have led to the development of a new design of nylon rope suitable for permanent moorings with design lives in the region of at least 20 to 30 years. Prior to this research nylon rope designs had a fatigue performance which was only adequate for temporary moorings typically lasting six months to a few years. This paper reviews two pivotal very long duration tests (10 and 20 million cycles) which were inspected for damage following completion of the load cycling, before measuring the residual breaking strength. In addition to these very long duration tests which were undertaken with load ranges more representative of wave loading, to account for all the ranges experienced through life it is necessary to be able to calculate any strength loss. Hence, a series of “partial life” fatigue tests was conducted to determine the strength loss for a series of load ranges. In these tests rope samples were cycled to a representative number of cycles which the rope would experience in service before stopping the test to assess the accumulated damage by means of a series of rope yarn tests. The residual breaking strength data is presented for critical layers through the cross-section which clearly identifies how yarns in different positions in the rope are exposed to varying degrees of inter-strand abrasion. A preliminary strength loss relationship was developed which could then be used to calculate strength loss for the fatigue spectrum based on an example mooring analysis of a WEC device. This work was successfully undertaken under the umbrella of UK Government funded Research and Development test programmes driven by consultancy firm Tension Technology International (TTI), as part of a team with rope-maker, certification body and wave energy converter (WEC) developer. The results of these projects have created a new technology enabling improved mooring systems with the additional benefit of significant cost reduction for the renewables industry. This technology can also be applied to any offshore mooring system where first order motions create high peak line loads.
Avoiding fiber axial compression fatigue in the design and use of tension members
Fiber axial compression fatigue is a unique form of damage which can occur in ropes, cables and other tension members. It can occur during cyclic tension loading, when some fibers go into compression as overall tension on the rope is relaxed. It can also occur under other circumstances, such as when the rope is bent or rotated. In most forms of fatigue, the mechanism is progressive, meaning it increases in intensity as fatigue damage develops. A familiar example is crack propagation fatigue during tension cyclic loading of metals. But the mechanism in fiber axial compression fatigue is regressive, meaning it decreases in intensity as fatigue damage develops. Axial compression fatigue only takes place in those fibers which become slack and go into compression. As this occurs when tension on the rope is relaxed, further axial compression fatigue is arrested because additional fibers do not become slack after those initially slack fibers fail. Thus axial compression fatigue usually does not progress to complete failure of the rope. However, if a significant number of fibers fail due to axial compression fatigue, tension in the remaining fibers increases, and the overall rope strength decreases. Fiber axial compression fatigue can therefore increases the tendency for a rope or cable to fail due to other causes, including the more common forms of fatigue.