Polyamide cables for marine renewable energy applications.
A recent review presented synthetic cable options for marine renewable energy system anchoring (Weller, 2015). For marine renewable energy applications on sites at shallow depths (between 50 and 100 m), in energy environments in the Maritime Public Domain, the search for a low-cost flexible mooring line is essential.
The elongation of the lines being schematically the movements of the platform divided by the length of the lines, a specificity of marine renewable energy platforms is the significant elongation of the mooring lines, more important than those of the offshore oil industry (on sites at deeper depths). Steel, in its elasticity range, limits us to a strain of less than 1%, aramids and HMPE to 4%, polyester to 10% and polyamide to 20% of elongation at break. Thus, polyamide ropes appear to be a potentially attractive technical solution for marine renewable energy platforms.
Polyamide fibres, or « nylon » (the original brand name), can be of different types (PA6, PA66, PA11, PA12,…) even if PA6 is the most commonly used for ropes. Its modulus of ~5 GPa offers a higher elongation than polyester, for an equivalent ultimate tensile strength.
The first key point is the validation of the durability of polyamide ropes. Normative standards exist and provide experimental data on the mechanical and fatigue behaviour of fibres developed for offshore oil and gas. Studies of the fatigue of polyamide ropes are much rarer. Feedback on nylon ropes nowadays exists mainly for hawser and short-term anchoring applications. The service life of these applications does not exceed 1 to 2 years due to the degradation of properties caused by internal abrasion between the rope elements, underlining the importance of fibre coatings. Regarding permanent anchorage based on nylon rope, there is very little data and the durability of nylon for numbers of loading cycles equivalent to those performed on a system anchored for a period of 20 years has not been documented to date.
The second key point is the qualification and simulation of the mechanical behaviour of these lines. Three time scales are to be considered: the response to in-service stresses (in relation to wave and tides), the long-term evolution of the pre-tension stress (impacted by accidental events such as storm), and the evolution at very long times (creep effect during the whole lifespan of the rope). Behavioural models of commercial numerical simulation codes for the response to in-service stresses are very limited with respect to the complex behaviour of polyamide. The approaches determining the long-term evolution of the pre-tension stress are nowadays very empirical and do not allow to optimize the lengths deployed on site. As part of the Interreg MERiFIC project (Marine Energy in Far Peripheral and Island Communities), IFREMER and the University of Exeter have been working on the behaviour of polyamide at sea (Weller, 2014; 2015). They noted a significant change in properties, especially stiffness, after aging. For these three time scales, relevant data and models are currently sorely lacking. In conclusion, despite the results of some partial studies, we note that there is very little information on which to base the design of a polyamide anchor line for a period of 20 years.
General information on nylon fiber and ropes
More precise information is given in each chapter, but this section provides a brief, general introduction to the material and ropes that will be studied in the following chapters. Nylon 6 can be described chemically as a semi-crystalline thermoplastic when in fiber form with the following structure: This fiber is highly hydrophilic but commonly used in synthetic rope marine applications as a low-cost material. The crystallinity ratio is around 40% for Nylon and can be schematically represented as shown in Figure 1-4. To describe these fibre materials a specific textile unit is used for the linear density: the tex, which is equivalent to grams per kilometers. This linear density allows us to compare one size of rope to another. The Nylon yarn used in this study is manufactured by Nexis fibers (reference 1880 f 280) (with a linear mass of 190.5 tex).
One critical property to know is the glass transition temperature (Tg) in the wet and dry states for this material, as when exposed to relative humidity the Tg can be quite close to ambient temperature, as shown below (Figure 1-5) for the same yarn by Humeau (2017). This suggests that the polymer will be in the rubbery state (above the Tg).
Rope vocabulary and specification
In the context of mooring ropes, a Minimal Breaking Load (MBL) is frequently cited; this is the minimum value that causes the cable to break during testing.
In this document, the load value will usually by presented in N/tex (with the tex being a unit of linear mass commonly used in the textile and cable industry), this allow us to avoid defining the section of the rope, and provides a specific stress which can be used to compare two different sizes of rope.
The strain presented in the manuscript will be the logarithmic strain calculated by the following formula: = ln ( 0).
The main difficulties for demonstrating the durability of polyamide ropes and modelling their mechanical behaviour simulation and qualification can be divided in two major scientific and technical issues.
The first issue concerns the complexity of testing the material under realistic environmental conditions (temperature and moisture). Polyamide is a highly hydrophilic material and water absorption drastically alters its glass transition and therefore its mechanical properties. For the targeted applications in the marine environment, this phenomenon is a first-order factor because it means that the operating temperatures will be very close to the glass transition temperature, or even higher. All the non-linear components of behaviour are therefore expressed, which is a major difficulty in modeling. As with fatigue tests, this problem is complicated by the difficulty of transposing characterizations to the scale of the strands for cable characteristics. One of the project’s challenges will therefore be to propose a characterization approach for intermediate scales, then to apply a modeling of the visco-elasto-plastic response at these scales, and finally to validate the proposed approach at the cable scale.
Secondly the search for a law of cyclic (fatigue) and creep responses and performances of a polyamide cable, which is an essential part of the project, requires the characterization of behaviour at long times and at ultra-low speeds. Indeed, the high viscosity of these materials cannot be relaxed after one hour of creep testing, nor can they be removed by a slow deformation rate (10-5 s-1). However, this behaviour over long periods of time is the main part of the stress of this type of material (Bles, 2009) and according to (Davies, 2000), and in the case of polyester cables, the most difficult part to model, the remainder being close to a linear viscoelasticity.
Testing machines for wet synthetic ropes
In order to test synthetic mooring ropes, specialized test machines must be used. These machines are quite rare and expensive. A review from the MERiFIC project (a program to develop marine energy in the North of France and UK) gives some of them in Europe (Davies, 2012); for example, there are only about a dozen test machines of 100 ton capacity of greater suitable for rope testing in Europe. Indeed, testing synthetic mooring ropes, especially polyamide ones, requires very long piston strokes. The strokes of classical hydraulic test machines are limited to a few hundred millimeters. Due to the splices required, the length of synthetic textile rope specimens are about 200 times their diameter. The combination of elongations up to 20% with this length-to-diameter ratio makes the classic hydraulic test machines useless for this type of ropes.
These tests are generally performed horizontally, without water for most installations, or with water spraying for laboratories specialized in mooring rope like IFREMER and only a few have the possibility to test ropes fully submerged. Testing rope fully submerged is more complex, time consuming and more costly as it either needs a box containing a lot of water with watertight joints, or a one-time use special cover around the rope to maintain it fully wetted.
Polyamide and water
The service environment of the ropes under consideration here (for a marine environment), associated with the highly hydrophilic nature of polyamides, raises acute questions of water absorption and the resulting consequences on the mechanical properties and durability.
A first question concerns water diffusion in polyamide, into order to evaluate the absorption and desorption kinetics, the possible gradients induced and the saturation contents. For polyamides, these studies have been conducted for a long time (Puffr, 1967; Lim, 1999; Reuvers, 2012; Colin, 2014; Broudin, 2015a; 2015b), and show that the diffusivity of water in the material can be evaluated correctly.
Water sorption in polyamide induces a very significant change in mechanical properties (Puffr, 1967; Flory, 1988; Launay, 2013), which is called the « plasticizing effect » (Merdas, 2002), and in fatigue resistance (Kenney, 1985; Bernasconi, 2007). Huntley (2016) shows that after wetting of 1 hour, values of modulus decrease significantly (50-80%) and are totally stabilized in 12 hours. In the work carried out by (Launay, 2013), the authors highlight the changes in the behavior of polyamide when the temperature of the material deviates from its glass transition temperature. But as presented in the first chapter, the glass transition of the material when it is fully saturated in water is below 0°C (Humeau, 2017) meaning that for our experiments at around 10 to 20°C, we are on the rubbery plateau and quite far from this value so the result should not be too much affected by the temperature (the value of the laboratory temperature has been marked for each test).
Modeling the behavior of semi-crystalline thermoplastic
The study of the mechanical response of a semi-crystalline thermoplastic such as polyamide is a complex problem which is the subject of various approaches in the literature. A first option is a multi-scale approach (molecular, macromolecular, supramolecular scales) (Spathis, 1998; Van Dommelen, 2003; Parenteau, 2009). Nevertheless, in view of the complexity of such a problem (Fond, 2002), many authors prefer to work at a larger scale by considering the material as macroscopically homogeneous with possibly the integration of microstructural variables (Détrez, 2008; Boisot, 2009; Parenteau, 2009; Regrain, 2009). In the framework of the study of thermoplastic composites, it is the latter type of approach that has been developed and deepened in recent years by the partners of the project (Launay, 2011; 2013; Bles, 2000; 2002), by proposing a rich behavior model that can be used in fatigue design, associated with a robust identification process. Different phenomenological models have also been proposed for the modelling of thermoplastic fibers (Northolt,1995; Baltussen, 2001; 2003; 2004; Chailleux, 2003; 2005).
Table of contents :
1.1 Context of synthetic fibre rope mooring lines.
1.2 Polyamide cables for marine renewable energy applications.
1.3 Problem statement
1.4 General information on nylon fiber and ropes
2. Constitutive response and behavior law
2.4. Mechanical tests on a 4-ton polyamide wet sub-rope
2.5. Investigation of the mechanical constitutive characteristics
2.6. Constitutive law
2.7. Identification method
2.8. Validation of the constitutive law
3. Long term creep behavior
3.3. Creep tests on yarn
3.4. Long term creep device
3.5. Long term creep tests
3.6. Comparison between the results on Yarns and on 4-ton ropes
3.7. Comparison with the multi-creep test
4.2. Yarn on Yarn abrasion tests
4.3. Fatigue tests
4.4. Yarn and rope failure mechanisms
5. An attempt to use a heat build-up measurement protocol to evaluate the fatigue properties of ropes rapidly
5.2. Evaluating the cyclic dissipated energy from thermal measurement
5.3. Heat build-up tests on 4T sub rope
5.1. Heat build-up tests on 8T sub rope
5.2. Comparison between the 4T and 8T sub-ropes
6. Conclusions and further work
6.2. Further work