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  1. IDAC
29 October 2009

IDAC Performs Stress and Fatigue Analysis of a Rope Thimble

Noel Village, an independent UK foundry specialising in carbon, stainless and nickel-based alloys, required an evaluation of the structural integrity of a new thimble design and as such commissioned IDAC to conduct some non-linear structural analyses of the thimble design.

The thimble was subjected to the minimum breaking load (MBL), a fatigue load case and a stern roller load case via a synthetic fibre rope. The analyses were conducted as structural static analyses with nonlinear contact behaviour.

The 3D CAD geometry of the thimble assembly for the MBL/fatigue load cases and the stern roller load case were supplied to IDAC in SolidWorks format. The 3D geometry was imported into ANSYS DesignModeler for further geometric modifications.

A short section of the cylindrical rope, after leaving the thimble geometry, was included in the actual model to define the direction of the pull load. Finally the assembly was divided into two identical halves as only a half-symmetric model was needed for the analyses.

This model was used for the MBL and fatigue load cases. It consisted of the thimble (half), the retaining pin, two representative retaining nuts, the shaft and the fibre rope (two connected volumes). Another half-symmetric geometric model with an angled fibre rope was created and used for the stern roller load case.

Nonlinear frictionless contact was generated between the thimble and the fibre rope, and between the thimble and the shaft. Fully bonded contact was specified for the contacts between the retaining pin and the retaining nuts, and between the thimble and the retaining nuts.

The geometry of the assembly was meshed using 3D higher order elements with higher order hexahedral elements being used in the regular shaped bodies within the retaining pin and the retaining nuts and higher order tetrahedral elements being used in the irregular volumes.

When subjected to a rope failure load of 1,905t (total load on full model) the results showed that the thimble would not fail under this load condition despite localised plastic yielding around the top and bottom edges of the pinhole. A larger fillet area than the current value of 5mm would help to alleviate the high stresses in these areas.

The fatigue assessment of the thimble design, using the maximum mean operating current/VIM load of 1,024kips with a range of 592kips (extracted from Tables 2 and 3 in the Technip Specification TOI Doc No 500000-000-RT-3840-0214) demonstrated that the design would survive indefinitely.

Under the stern roller load (107t) all the stresses in the thimble body were significantly below the yield limit of 470MPa. Again, as with the MBL load case, some localised plastic yielding at the contact edges with the shaft may be seen, but an increase in fillet area would reduce the maximum stress in these regions.

Design benefit

An accurate structural analysis of the new thimble design allowed an assessment of the proposed design under the extreme loading conditions. It was found that fillet areas around the pinhole could be increased to alleviate the high stresses that were being seen in that area. Confidence in the design was also gained from the fatigue analysis which showed that the design would last indefinitely when subjected to the specified load spectrum. The whole analysis process enabled significant reductions in the design and development time.

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