Effect of bending on steel wire rope sling breaking load: Modelling and experimental insights
Introduction
In any construction business, including subsea, lowering and lifting structures and loads is a daily work performed at construction sites and, if offshore, onboard specialised vessels. In such operations, whether it is a general (routine) or engineered lift, slings are an indispensable part of any lifting (or lowering) arrangement. Although high-performance fibre slings are being introduced in the offshore practice, slings made of steel wire ropes are still the most commonly used components for rigging.
A steel wire rope sling is a rope with a relatively short length and specific end terminations on each side. Slings used in offshore operations can be rather big as shown in Fig. 1. This particular sling will be loaded axially when in operation. However, the same sling can be used in a bending arrangement in other operations, where the breaking load of the rope will be reduced. In order to avoid sling failure, the reduction of the breaking load due to bending needs to be considered during the design of the lift.
The breaking load reduction of a bent rope is prescribed by the codes and regulations, e.g. [1], [2], where it depends only on the ratio, where is the rope diameter and is the diameter of the body over which the rope is bent (pin, shackle, pipe etc.), see Fig. 2. It should be noted that per definition of sling bending, D is not exactly two times the bending radius, which differs from the definition commonly used for ropes. A steel wire rope is made of wires helically twisted to strands and the outer strands helically twisted around the central strand (core). Because of its geometry, the wire rope is flexible and when bent, the strands and wires can easily move along each other when the applied load is small. However, when the load is steadily increased the motion becomes limited due to increased friction force between the strands and wires. This makes the behaviour of steel wire ropes in bending nonlinear. In addition, considering that the ropes have many different combinations of geometric configurations, it is questionable to accept that the load bearing capacity reduction due to bending depends only on a single parameter, which according to the current standards, is the D/d ratio.
The main goal of the current research is to study the effect of the bending over small radii on the breaking load reduction of the wire rope slings of different diameters and configurations, in other words, the sling bending strength reduction. The term “bending strength” is used further in the paper to reflect the breaking load of the slings statically loaded in bending, and is not to be confused with other loading conditions, e.g. bending fatigue. To reach the goal, analytical and experimental studies have been performed. An experimental study, in its large part, has been presented earlier at the OIPEEC conference [3]. Here, more information on full-scale rope test setup is given, namely its modification for unrestricted rope ovalisation and additional insights from the tests such as the fracture type of individual wires is discussed. The modelling of the breaking force as presented in [3] is rather brief and could use further explanation. The current study can be considered broader than only an investigation into the sling breaking load due to bending since more general insight in steel wire rope behaviour under bending is discussed. To the best of our knowledge, this is the first ever attempt to model the bending of the rope over the radii much smaller than that of the sheaves, for which usually the bending models are developed.
Ropes used in the current study, from which the slings were prepared for testing, are of two types (6 × 25F-IWRC and 6 × 36WS-IWRC) and three different nominal diameters (14, 20 and 77 mm). Table 1 shows the properties of the ropes. Because all of them have an independent wire rope core (IWRC), the core type is omitted here from ropes designation in Table 1.
Section snippets
Historical note on modelling of wire rope behaviour
A model developed and described in this paper is based on the vast experience of different researchers and their contribution to wire rope modelling. The most important aspects in model development history throughout the years are briefly described here (authors do not pursue a goal to present a comprehensive literature review and refer a reader to relevant articles, for example, [4], [29], [30]).
With the growing popularity of steel wire ropes in the 20th century also came the interest in their
Testing
Two series of break tests have been performed for slings in straight (control test) and bent configuration. The rope properties used for slings preparation are shown in Table 1. Regarding the rope diameter, the tests were defined as small-scale (14- and 20-mm diameter ropes) and full-scale (77-mm diameter rope). Tests in bending mode were performed with D/d ratios equal to 1, 1.5 and 2 for small-scale slings and 1.5, 2 and 2.5 for the full-scale slings. The number of samples used in the control
Breaking load for straight rope
Modelling and test results for the break load of the rope/sling in straight configuration under axial tensile loading are shown in Table 2, in comparison with the Minimum Breaking Load (MBL) guaranteed by the rope manufacturer. Actual Breaking Load (ABL) is higher than MBL as expected. The computed breaking load is higher than ABL values. The difference with average experimental data, % diff., is in the range of (1.4…4.67) %, which means that the model works very well for straight rope under
Conclusions
Analytical modelling and an experimental study have been performed to gain insight into the factors affecting the load bearing limit of a steel wire rope sling due to bending, or bending strength (breaking load) reduction. Different models available in literature were evaluated and the approaches by Feyrer [1], Papailiou [2] and Wiek [3] were chosen as the basis to build the analytical model used in the current study. The experiments were conducted on small- and full-scale sling samples for
Declaration of Competing Interest
None.
Acknowledgements
The authors would like to acknowledge Peter Meijers, Prof. Frans Bijlaard, Dr. Max Hendriks and Roland Abspoel of TU Delft for the supervision of then MSc students, Hristo Ivanov and Bart de Jong. Great thanks are going to the members of staff of the Stevin II laboratory at TU Delft for the help provided for experimental work, especially to Peter de Vries, who assisted in the design and construction of the rig that was used to conduct the sling tests. Allseas Fabrication B.V. is acknowledged
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Cited by (0)
- 1
Current address: National Renewable Energy Laboratory, Golden, CO, USA.
- 2
Current address: TNO, Department of Structural Dynamics, Leeghwaterstraat 44, 2628 CA Delft, the Netherlands.