Issue 78

A. Sancak et alii, Fracture and Structural Integrity, 78 (2026) 186-206; DOI: 10.3221/IGF-ESIS.78.11

I NTRODUCTION

M

ooring and anchoring chains used in offshore platforms, floating energy systems and ship anchoring applications are exposed to multiple degradation mechanisms under marine service conditions, including seawater corrosion, variable wave- and current-induced loading, impact effects, inter-link contact/wear and fatigue. Since these chains are among the primary load-bearing components that maintain the position of offshore and floating structures, their degradation directly affects operational safety and structural integrity. Early studies demonstrated the importance of static and fatigue performance in chain links and connecting links [1], while subsequent investigations emphasized corrosion loss in working chains continuously immersed in seawater [2]. Inter-link wear was also shown to contribute to long-term material loss in mooring chains [3]. In addition, material-development studies for offshore mooring chains have reported that corrosion degradation in service may exceed design allowances and have discussed high-manganese steel as a potential alternative material for mooring-chain applications [4,5]. More recent studies have highlighted the importance of numerical modelling strategy in predicting the stress response of mooring chains [6], as well as the mechanical performance of welded chain links intended for mooring applications [7]. Much of the existing research has examined the mechanical response of chain links through stress analysis, stress concentration evaluation, fatigue assessment, residual stress characterization and fracture-mechanics-based approaches. Stress concentration factors in studless mooring chain links operating in fairleads were investigated under tensile loading conditions [8]. Boundary element, finite element and other numerical approaches have been widely used to evaluate stress distribution, operating stress states and crack-related parameters such as stress intensity factors in mooring chain links [9– 11]. Fatigue-related studies have also shown that complex loading conditions, such as out-of-plane bending, and residual stresses can significantly affect fatigue behaviour and fatigue life [12,13]. These studies provide valuable information on the macroscopic mechanical response and damage tolerance of chain links. However, they mainly address stress state, fatigue life or crack propagation, rather than the degradation of impact toughness after controlled seawater corrosion exposure. Seawater corrosion is one of the most critical environmental factors limiting the service life of steel chains operating in marine environments. Corrosion loss in low-alloy steel chains continuously immersed in seawater has been estimated by considering relevant environmental parameters [2]. In addition to general corrosion, contact and wear between chain links may lead to additional material removal [3]. Localized corrosion in mooring-chain steel exposed to seawater has also been investigated, showing that degradation in these components should not be considered solely as uniform section loss [14]. The use of seawater in addition to substitute seawater is important because coastal seawater may exhibit hydrographic, seasonal and biological variability that cannot be fully represented by a standardized substitute solution. The Strait of İ stanbul and the Sea of Marmara are characterized by complex water exchange and ecological variability, which may influence the physicochemical characteristics of seawater collected from coastal regions [15,16]. Related studies on marine structural steels indicate that corrosive seawater environments can alter mechanical performance and corrosion-fatigue behaviour. For example, corrosion behaviour prior to crack initiation was examined in E690 steel fatigued in simulated seawater [17], the mechanical properties of Q690 high-strength steel were evaluated after exposure to a marine corrosive environment [18], and corrosion-fatigue crack growth in low-alloy steels for offshore engineering structures was investigated under controlled seawater-related conditions [19]. Seawater-related degradation is also influenced by environmental parameters. For example, dissolved oxygen content in seawater has been reported to increase the stress corrosion susceptibility of pipeline steel through the formation of corrosion pits and microcracks [20]. Environmentally assisted degradation in corrosive media is therefore not controlled only by material strength, but also by the interaction between local damage, loading condition and exposure environment [21]. Mechanical-property studies on mooring-chain steels and related marine structural materials have mostly addressed tensile behaviour, fatigue, fracture toughness, welded joints, chemical composition and heat-treatment effects. The effect of post weld heat treatment on the mechanical and microstructural properties of extra-high-strength steel weld metals for mooring equipment has been investigated [22]. The influence of cooling conditions and chemical composition on high-strength steel weld metals used in mooring components has also been examined [23,24]. Welded joints in high-strength cast steel suitable for mooring components were evaluated in terms of their mechanical performance [25], and arc-welded chain links were studied with respect to mechanical properties and fatigue performance [7]. Toughness properties of friction hydro-pillar processed offshore mooring-chain steel have been assessed using Charpy V-notch and crack-tip opening displacement tests [26]. The fracture toughness of mooring-chain link material under cathodic protection in a seawater-related environment has also been examined, with attention to hydrogen-assisted degradation [27]. Charpy impact testing has further been used to evaluate toughness degradation and ductile-to-brittle transition behaviour in steels, including aged weld metals [28] and high-strength low-alloy steels used in ship construction [29]. Corrosion damage has been shown to reduce the fatigue resistance of structural steels when corrosion pits act as local stress raisers and promote early crack initiation [30].

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