Effects of Twist on Chain Strength and Fatigue Performance: Small Scale Test Results

Mooring chain can twist in response to torque generated in the adjacent elements of a mooring, or even be inadvertently installed in a twisted condition. This paper explores the effect of initial twist on the static strength and fatigue life of studless chain. It reports test results for 16mm studless chain (with similar proportions as the much larger chains now employed in hydrocarbon production vessels) with known levels of initial twist subject to axial tensile loading. Twist levels of up to 24° per link were examined. The results were generally better than had been expected. There was little scatter in the results, giving confidence in the test procedures. The effect of initial twist on static strength was modest, with a maximum strength loss of about 4% at 18° twist per link. The effects of initial twist on fatigue life were also small. Indeed, some twist seemed to improve fatigue performance, perhaps because the contact between links was displaced from the region of high residual (compressive and tensile) stress resulting from proof loading during manufacture. While the results of these small scale tests suggest that twist levels up to 24° per link were not detrimental, other practical reasons remain for continuing to minimize twist in mooring chain. Furthermore, caution should be applied in extrapolating these results to predict the effect of twist in full scale offshore mooring chains.