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Noel Village (Steel Founder) Ltd
Stress & Fatigue Analysis of a Rope Thimble
 
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Company Profile

Noel Village are an independent UK foundry specialising in carbon, stainless and nickel based alloys.

Noel Village has the capability to produce castings in over 200 compositions of carbon, alloy & stainless steels and nickel based alloys. The company provides a complete range of foundry skills from initial design through patternmaking, moulding, casting, inspection to finishing and machining. One of the major strengths is solidification simulation using software to ensure castings of the highest quality and integrity.

Serving most engineering sectors, they have earned a reputation for the production of innovative components in exotic materials, across a spectrum of industries including the offshore and petrochemical sector where high pitting and crevice corrosion resistance in severe environments is needed.

Background

The use of rope thimbles in loops is industry best practice. Where a wire or synthetic fibre rope is terminated with a loop, there is a risk that it will bend too tightly, especially when the loop is connected to a device that spreads the load over a relatively small area. A thimble can be installed inside the loop to preserve the natural shape of the loop, and protect the cable from pinching and scraping on the inside of the loop. The thimble prevents the load from coming into direct contact with the wires.

 

Analysis

Noel Village required an evaluation of the structural integrity of a new Thimble design and as such commissioned CADFEM UK CAE Ltd. to conduct some nonlinear 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 CADFEM UK CAE Ltd. 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. The figures above show both models.

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 the rope failure load (MBL) of 1905 tonnes (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. The figure to the left shows these areas.  A larger fillet area than the current value of 5 mm 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 1024 kips with a range of 592 kips (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.  The figure to the left shows the results from the fatigue tool, a minimum life of 1 x 107 was calculated.

Under the Stern Roller load (107 tonnes) all the stresses in the Thimble body were significantly below the yield limit of 470 MPa.  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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