Fatigue is a critical failure mechanisms of catenary type mooring lines since failure in one or more lines causes drift of the vessel and thereby jeopardizes the safe flow of oil and gas through subsea installations and riser pipes. The fatigue strength of chains is influenced by several factors, the main ones being the number of stress cycles, mean stress, corrosion state and length of the mooring lines. In order to predict the reliability in the residual service live of the mooring line the effect of these factors must be understood and integrated in a corresponding probabilistic model representation of the fatigue phenomenon. Since the validity of Miner’s rule is also important for the prediction of life under variable amplitude loading a wave spectrum was developed and tests were carried out under service loading.
This paper presents results from tests on R4 high strength steel chains that had been in service from 9 to 20 years in the North Sea. The main objectives of the tests were to determine remaining life and the correlation between the state of corrosion damage and life. A considerable effort was made to determine surface damage criteria in terms of damage categories from 3D scans that were tied to fatigue life.
The results are expected to provide input to future revisions of fatigue design guidance for mooring chains, in particular changes to the design S-N curves in DNV GL-OS-E301 Position Mooring.