The 20-Hurricane Test: New Knowledge on Deepwater Polyester Mooring Integrity

Abstract

In a recent U.S. Minerals Management Service (MMS) sponsored project exploring the “value of polyester insert recovery and testing,” we found that: (1) the only major cause of rope structural integrity loss was third-party damage to the mooring system components, and that only in-situ inspection of the mooring ropes by ROV video (not insert recovery/testing) can effectively discover this damage; (2) because of the demonstrated structural integrity of polyester ropes, the major concerns of rope creep and fatigue (or cyclic wear) damage have been proven to be inconsequential; and (3) based on a risk/benefit analysis, insert recovery/testing has been determined to have no benefit in reducing the risk of normal operations. As a result, regulatory requirements have been relaxed on this issue. We recommended that a polyester rope cycling (fatigue) test based on being subjected to 20 hurricanes (one per year for 20 years) with the strength of Hurricane Katrina should be adopted as a “benchmark” for qualification testing of different designs and brands of polyester rope. Based on the very first 20-hurricane tests with twisted, three-strand, unjacketed subropes being subjected to 20,000 cycles of loads from 30% to 60% of average breaking strength, we found a reduction in breaking strength of only 6%. Now, for DeepStar, we are expanding the testing to lower mean loads and different brands of rope/subrope. A recent finding is that the lower the mean load for cycling (with the same cycling range), the greater the cyclic wear damage to the subrope, and the lower the residual rope strength. This result is contrary to that found for steel wire rope and chain fatigue. Our paper provides details of our 20-hurricane testing of various polyester subrope designs, and describes continuing development of a new strain-based hypothesis for estimating the remaining life performance of a polyester rope, based on a prediction of major hurricane exposure and the design life of the mooring system.