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Microcracks can rapidly grow and develop in high-strength steels used in offshore structures. It is important to render these microcracks harmless to ensure the safety and reliability of offshore structures. Here, the dependence of the aspect ratio (As) of the maximum depth of harmless crack (a hlm ) was evaluated under three different conditions considering the threshold stress intensity factor (ΔK th ) and residual stress of offshore structural steel F690. The threshold stress intensity factor and fatigue limit of fatigue crack propagation, dependent on crack dimensions, were evaluated using Ando's equation, which considers the plastic behavior of fatigue and the stress ratio. a hlm  by peening was analyzed using the relationship between ΔK th  obtained by Ando's equation and ΔK th  obtained by the sum of applied stress and residual stress. The plate specimen had a width 2 W = 12 mm and thickness t = 20 mm, and four value of As were considered: 1.0, 0.6, 0.3, and 0.1. The a hlm  was larger as the compressive residual stress distribution increased. Additionally, an increase in the values of As and ΔK th(l)  led to a larger a hlm . With a safety factor (N) of 2.0, the long-term safety and reliability of structures constructed using F690 can be secured with needle peening. It is necessary to apply a more sensitive non-destructive inspection technique as a non-destructive inspection method for crack detection could not be used to observe fatigue cracks that reduced the fatigue limit of smooth specimens by 50% in the three types of residual stresses considered. The usefulness of non-destructive inspection and non-damaging techniques was reviewed based on the relationship between a hlm , a NDI  (minimum crack depth detectable in non-destructive inspection), a cr N  (crack depth that reduces the fatigue limit to 1/N), and As.

Reliability Improvement of Offshore Structural Steel F690 Using Surface Crack Nondamaging Technology