Assessment of aged offshore structures based on current and historical data is a critical issue for the life extension of offshore structures. The leak-before-break (LBB) concept was first developed and applied in the nuclear power industry to ensure the safety of the pressurized components in nuclear power plants. This concept can also be applied as a robust and cost effective tool for structural integrity management of offshore structures that are either partly submerged (jackets, semi-submersible, ships), or contain fluids (pipes, pressure vessels). The core idea of LBB consists of guaranteeing that enough time is available between the moment a crack breaks through the hull or wall, causing a detectable leakage, and the moment when the crack becomes unstable, causing a structural failure. This paper presents a case study of applying the LBB concept for a tether string that is part of the mooring system of an offshore structure. The tether string is a steel pipe containing circumferential defects. The critical through-thickness crack size was calculated with ductile tearing being taken into account. It has been found that the choice of failure criterion has a significant impact on the determination of the critical through-thickness crack size, which might have influence on the verification of leak-before-break. The effect of residual stress on leak-before-break has also been studied.