CHALLENGES IN LIFETIME PREDICTION OF STEEL WIRE ROPES WORKING IN MARINE RISER TENSIONER APPLICATIONS
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Summary
Steel wire ropes are a major part of offshore drilling equipment. Especially the Marine Riser Tensioner (MRT) ropes undergo heavy fatigue loading to keep the riser of a drill ship or drill rig in position. Monitoring the ropes in an offshore environment is not as easy as in land-based crane applications, so lifetime calculation tools are used widely to estimate the time until discard. The so-called Ton-Cycle calculations accumulate the product of the bending cycles and the applied load, which is rather constant during MRT operations. When the product (ton x cycles) reaches a certain threshold, the ropes are considered as discard and must be changed. For steady operations this calculation scheme can be considered sufficient to give guidance for rope exchanges, but variations of the operational boundary conditions and the level of applied rope force might lead to unsafe operations. This paper will point out the relevant factors that can reduce the accuracy of the commonly used ton-cycle calculations like rope force, water tide and wave height. It will give guidelines on how common issues can be avoided and operations can be made safer. The Feyrer Method will be used to predict the influence of rope force, and analyses of the rope bending cycles at different locations of the rope will show the influence of wave height and tide. Forensic analysis of discarded ropes is shown to provide practical evidence for the need to improve the ton-cycle procedure.
Author(s): C. Ruffing, H. Khalid, E. Maher, J. Karedan, A. Jeays
