Aux Winch Frame FEA

The Alatas Engineering and Design Services based in the UK carried out an aux winch frame FEA in the Irish Sea. The oil operator client requested Alatas UK to their platform near Ireland as a split to the rib plate laminar that occurred to the auxiliary winch base frame for a Liebherr BOS 90/950.

A third-party inspection of the auxiliary winch base frame found one of the rib plates on the cab side with a laminar split throughout its entire thickness, which may lead to further progression and collapse of the aux winch base frame.

Job Scope for Aux Winch Frame FEA

Here is a summary of the work scoped by Alatas Design and Engineering team.

• 3D solid modelling of the auxiliary winch, including base frame with the defect in the reported stiffener.
• Carry out a winch simulation and base frame with the defect simulating for the worst load case scenario.
• Produce a structural assessment report with conclusions and recommendations.

Alatas solution

• Alatas UK’s Engineering and Design team carried out an extensive structural assessment by employing a finite element stress analysis technique to assess the structural strength and integrity of winch base frame in general, and 4 nos. rib plates, in specific, to evaluate an equivalent (Von-Mises) stress, linearised equivalent stress along the fillet weld seems of the rib to main support plate in the presence of the existing laminar split according to the findings of the 3rd party inspection report and to further investigate associated deflection and laminar split rib plate weld forces for suitability of the weld strength.
• The laminar split rib plate was modelled and simulated to demonstrate in-situ condition of the winch base frame. The laminar split rib plate outer edges were used as contacts with the adjacent weld connecting plates to show a fillet weld to determine weld forces in this assessment.
• Numerous numbers of finite element analysis (FEA) simulations were carried out utilising Ansys® Workbench based on different worked out load cases scenarios.
• The team reported the max equivalent (Von-Mises) stress for the laminar split rib plate as 408MPa creating a minute stress concentration hotspot under the maximum static brakeforce. But overall maximum and average equivalent (Von-Mises) stress in the whole winch base frame structure was witnessed less than the permissible equivalent (von-Mises) stress 267 MPa. In addition, this was found within the allowable limit according to Standard for Certification No. 2.22-Lifting Appliances, June 2013.
• As derived from the simulation results, the fillet weld forces around the laminar split rib plate were seen to be well within the force required/mm. The run of the fillet weld was adopted (assumed a 6mm FW) and met the sufficient capacity of the required weld for the laminar split rib plate.
• In conclusion, the winch frame structure with one laminar split rib plate reported in the third-party inspection report was found suitable for use under normal operational loads.

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