Tow winches used in a marine environment are subject to harsh, arduous operating conditions. Typically they are designed to withstand high shock loading – significantly higher than the operational design load – due to the potential of snatch/snag loads on the towed body. They are also designed to withstand fatigue failure since they subject to cyclic inertial loads, and fully reversed loading rotating drum loading.
A client of Armech Solutions was developing a tow winch as part of a launch and recovery system (LARS) for a marine environment. They had a stringent weight requirement and decided to design the tow winch in aluminium. They usually manufacture structural-steel equipment and were unsure how to proceed with assessing the fatigue life of an aluminium structure. The client decided to engage Armech Solutions to perform a fatigue analysis.
Step one in a study of this type is to frame the problem. Armech proposed using Eurocode EN 1999-1-3 Fatigue of aluminium structures, as a guideline for the analysis. Then working hand-in-hand with the client design team, defined a conservative – but realistic – duty cycle and set of load cases.
The next step is to determine the areas of interest. Armech ran multiple static structural analyses to simulate the various operating states (main tow cable at the drive end and main tow cable at the non-drive end). These preliminary analyses identified the regions of high-stress magnitude. Most of these regions were at welded interfaces, due to reentry stress raisers.
Identifying the regions of interest allows the mesh to be refined only where high stress is expected, maximising computational efficiency. Armech then analysed these areas in detail and extrapolated the maximum and minimum principal stress using the hotspot method, to mitigate FEA singularities.
We then applied the Eurocode method to the FEA data to obtain the fatigue life of the tow winch. The Eurocode method defines S-N curves for various weld details and load cases. It then calls for application of the Miner’s rule to determine the cumulative fatigue damage and assessment of this factor against the required fatigue life.
The project was a resounding success for the client. The analysis identified some potential failure points that were subsequently modified. These possible failure points, if undetected, could have resulted in crack initiation, and even failure of the tow winch in service.
The client design team were so satisfied with the method used that they have added it to their arsenal of design tools, and adopted it as their standard method for fatigue assessment.
An efficient, systematic plan and excellent collaboration between both parties allowed completion of the project in under two weeks. As a result of this manufacturing started on schedule, with everyone having full confidence in the structural integrity of the design.