Fatigue of improved polyamide mooring ropes for floating wind turbines
Introduction
As Offshore Energy Converter systems such as floating wind turbines approach commercial development, there is a need for mooring lines in shallow water (between 50 and 100 m). Since these applications aim at harnessing marine energies, the systems are located in exposed areas, where dynamic loading is significant. In addition, the inherent moderate to low water depth results in a globally stiff behaviour of the mooring system when the standard components for deep-water floating oil and gas platforms (Weller et al., 2015a) are used (steel chains or polyester rope). The system integrity will be maintained by damping the dynamic loadings, requiring the use of more stretchable fibre ropes than polyester in order to remain within limited floater offsets to protect the export cable.
Polyamide 6 (nylon) fibres are a good candidate for these applications because of experience from previous use in other marine applications, low price and high breaking strain (up to 25%). It is first instructive to examine the behaviour of individual nylon fibres. Bunsell developed a single fibre tensile test machine (Bunsell et al., 1971), which was used in several fatigue test programmes on nylon (Bunsell and Hearle, 1971; Ramirez et al., 2006; Colomban et al., 2006), polyester (Le Clerc et al., 2007; Lechat et al., 2006) and higher stiffness (Lafitte and Bunsell, 1982; Davies et al., 2010) fibres. The nylon used in those studies was polyamide 6.6, but a comparison with polyester showed that the tensile fatigue mechanisms are similar (Bunsell, 2009). Both fibres show distinctive failure morphology; fatigue cracks originate at the surface or just below it at inclusions, and run within the fibre in the axial direction producing long characteristic fatigue cracks. In terms of fatigue lifetime, one approach is to define a median number of cycles to failure (the lifetime at which 50% of the samples break, at a given load range). Typically 25–30 fibres are tested for each median value and tests are run dry. Table 1 shows some published values for nylon 66 and polyester fibres obtained in the same laboratory. Overall, the PET lifetimes appear longer, but the fibres tested in the two studies were not identical and this is reflected in differences between the two sets of data. For individual test series such as those with a load range 0–75%, the median lifetimes for PET in (Le Clerc, 2004) and nylon in (Herrera Ramirez, 2004) are very similar. It should also be underlined that any comparison between nylon and polyester is complicated by the much lower stiffness of the former. All the tests reported in those fibre studies were run under load control; if strain control had been applied the nylon results would probably exceed those for polyester. Finally, it should be noted that it is necessary to apply very large load ranges to obtain failures, well above the usual service loads, indicating that the intrinsic fatigue strength of both these fibres is high.
When ropes rather than single fibres are considered, studies on braided nylon 6 ropes in the 1980's (Kenney et al., 1985; Mandell, 1987) have shown relatively poor fatigue performance compared to polyester, which provided little incentive for developing nylon mooring ropes. Indeed, nylon single point moorings used offshore are typically changed every one or two years. A study published by OCIMF (the Oil Companies International Marine Forum) in 1982 presented results from tests on small and large nylon rope hawsers, both new and after service. These revealed that a semi-log relationship of the type:N = eA(100−L)
Could be used to represent the data, with N the cycles to failure, A an empirical parameter and L the maximum load as a percentage of break load. The empirical parameter A was found to be in the range 0.14–0.16 for ropes tested during that study. For a wet rope this corresponded to only a few hundred cycles at 50% of the break load.
Several other studies in the 1980's and 1990's generated fatigue data on braided nylon ropes. For example, Seo et al. (1997) examined wear and fatigue of nylon and polyester fibres and ropes. They showed yarn-on-yarn (YoY) abrasion data which indicated that while nylon 66 fibres showed better durability than polyester when dry, when tests were run wet the nylon 66 showed significant reduction in wear resistance whereas polyester did not. They concluded that comparisons of YoY wear between nylon and polyester will vary significantly with the loading condition. Nylons are superior at very high tensions whereas at low tensions polyester lifetimes exceed those of nylon. There is a widely perceived idea that polyester ropes have superior fatigue resistance to nylon ropes. Quite recently, however, Ridge et al. (2010; Banfield and Ridge, 2017) have shown significantly improved fatigue results for twisted nylon ropes; those authors qualified these ropes as ideal for wave energy convertor moorings and Flory et al. (2016) also discussed these applications. Two major differences compared to the earlier fatigue work were the use of twisted ropes with long lay lengths and the application of improved fibre coatings. Both can help to reduce internal abrasion, which was shown by Mandell to be a major failure mechanism during low-load, high-cycle fatigue (Mandell, 1987). At higher loads, creep rupture is the main concern, and this depends on the polymer structure rather than the fibre interactions.
There are few other relevant and recent results for these materials, but Weller et al. (2015b) examined a 44 mm diameter parallel twisted strand nylon 6 rope that had been tested at sea for 18 months on a wave energy buoy and showed the importance of loading history and operating conditions on nylon rope performance.
Polyester fibre ropes have been used for mooring deep-water offshore platforms for many years now (De Pellegrin, 1999; Bugg et al., 2004; Haslum et al., 2005) and extensive fatigue testing has shown that their fatigue performance is better than equivalent steel components (Banfield et al., 2000). The idea of using nylon ropes for long-term mooring lines is relatively new as there is little available knowledge on the durability behaviour of such structures. Indeed, as Pham et al. (2019) conclude in a very recent study “a comprehensive study on the critical fatigue damage mechanisms of nylon should be the topic of future work.” It is therefore essential to generate further data on polyamide mooring ropes, in order to validate their use for this application. It is also of interest to see how these improved nylon rope constructions compare with the polyester ropes currently in use offshore.
The goal of the present study is to provide a better understanding of the mechanical behaviour and the fatigue damage of twisted polyamide ropes compared to other mooring line options, in order to help design floating wind turbine mooring lines. First, the influence of fibre coating is evaluated by yarn-on-yarn abrasion tests. Then results from quasi-static and cyclic tests on rope samples are shown. Finally, failure mechanisms are described and discussed.
Section snippets
Fibres and ropes
Polyamide 6 (PA6), often described as Nylon 6, in the form of yarns and ropes provided by Bexco (Hamme, Belgium) have been investigated in this study. The yarns used are supplied by Nexis fibers with a linear weight of 188 tex (g/km), provided with and without a proprietary coating. The nylon rope samples used in this series of tests were specially manufactured for the research project. The rope is a 6-m-long (pin-to-pin) three-stranded rope of outer diameter around 15 mm for a nominal break
Yarn on yarn abrasion tests
Yarn on yarn abrasion tests were performed up to failure, which occurred in the inter-wrapped section of the yarn in all cases reported on Fig. 3. On this curve, each point represents 3 to 8 test repeats at each applied stress level, defined in grams/tex, depending on the scatter. The error bars represent the minimum and maximum numbers of cycles found at each load. The dashed lines show a linear best fit on the semi-log scale. The continuous red line corresponds to values in a Cordage
Conclusion
The results from yarn-on-yarn loading abrasion tests show a significant increase in abrasion resistance by the addition of a specially developed coating. Furthermore, the long lay length rope construction enables these polyamide ropes to exhibit very good fatigue performance, similar to that demonstrated recently for another rope product by Ridge et al. (2010; Banfield and Ridge, 2017; Flory et al., 2016).
However, it is also shown here that the fibre coating conditions can significantly affect
Funding
This work was supported by the FEM/ANR POLYAMOOR project (ANR-10-IEED-0006-16). This is led by France Energies Marines, with partners Naval Energies, Bureau Veritas, Bexco, Ensta Bretagne and IFREMER.
CRediT authorship contribution statement
Yoan Chevillotte: Investigation. Yann Marco: Supervision. Guilhem Bles: Supervision. Karel Devos: Conceptualization. Mathieu Keryer: Resources. Maël Arhant: Validation. Peter Davies: Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the contributions of Nicolas Lacotte and Nicolas Gayet of IFREMER to the rope tests and SEM examination respectively.
References (33)
- et al.
Micro-Raman study of the fatigue and fracture behaviour of single PA66 fibres: comparison with single PET and PP fibres
Eng. Fract. Mech.
(2006) - et al.
Creep of nylon-6,6 during concurrent moisture changes
Polymer
(1980) - et al.
Effect of water absorption on the plastic deformation behavior of nylon 6
Eur. Polym. J.
(2009) - et al.
Dynamic modeling of nylon mooring lines for a floating wind turbine
Appl. Ocean Res.
(2019) - et al.
Synthetic mooring ropes for marine renewable energy applications
Renew. Energy
(2015) - et al.
Synthetic rope responses in the context of load history: the influence of aging
Ocean. Eng.
(2015) Standard Test Method for Wet and Dry Yarn-On-Yarn Abrasion Resistance
(2007)- et al.
Fatigue Curves for Polyester Moorings – A State of the Art Review
OTC
(2000) - et al.
Fatigue durability of nylon rope for permanent mooring design
- et al.
Mad Dog Project: Regulatory Approval Process for the New Technology of Synthetic (Polyester) Moorings in the Gulf of Mexico
OTC
(2004)
An apparatus for fatigue-testing of fibres
J. Phys. E: Sci. Instrum.
A mechanism of fatigue failure in nylon fibres
J. Mater. Sci.
Tensile fatigue of thermoplastic fibres, chapter 11
Tensile fatigue behaviour of PBO fibres
J. Mater. Sci. Lett.
Manmade fiber ropes in deepwater mooring applications
OTC
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