Abstract
Residual stresses in large mooring chains were measured for the first time in this study. Two measurement techniques were employed (neutron diffraction and hole drilling). Elastic-plastic finite element simulation of the proof loading was conducted, and the computed residual stresses were compared to the experimental measurements. Further, the cyclic plasticity of the material was taken into account to investigate residual stress redistribution caused by introduction of corrosion pits and cyclic service loads. A critical damage parameter was employed to estimate fatigue crack initiation life of corroded mooring chains subjected to various service load levels considering the effect of residual stresses.
Highlights
Sources of energy at sea and under the seabed have captured engineers’ attentions from onshore to offshore productions in the past few decades
Two chain links with the same size and material, one only subjected to proof load and no cyclic service loads and the other exposed to service loads as well as the proof load, were selected for the experiment to investigate residual stresses (RS) changes after several years of service
Residual stresses (RS) in offshore mooring chains have been measured for the first time
Summary
Sources of energy at sea and under the seabed have captured engineers’ attentions from onshore to offshore productions in the past few decades. A single line failure increases the load level on the adjacent lines that may cause failure in them resulting in the operation shutdown, pollution, material loss and risking human life. The high rate of failure of offshore mooring chains demands enhanced understanding of the phenomena correlated to the fatigue life of these chains, e.g. residual stresses (RS) and corrosion, to improve their design and enhance their reliability against failure [1,2,3]. To assure the strength capability of mooring chains, offshore industry standards (e.g. see [4,5]) require mooring chains to be proof load (PL) tested before being installed offshore. The offshore industry standards lack the consideration of the effects of PL level and material grade on fatigue life of different size mooring chains.
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