Abstract The integrity of mooring chains is essential to the safety of a range of offshore platforms. However, mooring line failures are occurring earlier than their design lives, with a high number of these failures occurring due to fatigue. Early in the fatigue life of the component fatigue initiation processes occur, where the fatigue hotspot is sensitive to the mean load and there is plastic strain accumulation from the multiaxial stress-strain responses of the material, leading to cyclic plastic damage accumulation. The traditional SN approach suggested by mooring standards does not consider these effects, and it is proposed that this lack of consideration under low-cycle fatigue conditions is the reason for the current non-conservative fatigue assessments of mooring chains. This paper aims to develop a fatigue approach based on a critical plane multiaxial fatigue criterion for mooring chains that can consider the damage-induced by the cyclic plasticity and the mean load effect, to investigate the importance of incorporating low-cycle fatigue into the mooring chain life prediction. To develop the critical plane approach, the multiaxial stress-strain states are extracted for the critical plane at the fatigue hotspot from a finite element model of a mooring chain. This is then correlated with a fatigue life prediction provided by conventional fatigue design data. It uses a simulation of an FPSO as a case study to demonstrate the importance of low cycle fatigue, which shows that the mean load effect is significant in reducing the fatigue life for mooring chain applications, while the effect of fatigue damage-induced cyclic plasticity is limited. The fatigue damage accumulation predicted by the critical plane approach is significantly higher than that of the traditional SN approach and should be accounted for in mooring line design.

中文翻译：

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海上系泊链在服役载荷下的低周疲劳评估

摘要 系泊链的完整性对一系列海上平台的安全至关重要。然而，系泊缆故障发生的时间早于其设计寿命，其中大量故障是由于疲劳造成的。在部件疲劳起始过程的疲劳寿命早期，疲劳热点对平均载荷很敏感，并且材料的多轴应力应变响应会产生塑性应变累积，导致循环塑性损伤累积。系泊标准建议的传统 SN 方法没有考虑这些影响，并提出在低周疲劳条件下缺乏考虑是当前对系泊链进行非保守疲劳评估的原因。本文旨在开发一种基于临界平面多轴疲劳准则的系泊链疲劳方法，可以考虑循环塑性和平均载荷效应引起的损伤，研究将低周疲劳纳入系泊链的重要性。寿命预测。为了开发临界平面方法，从系泊链的有限元模型中提取疲劳热点处临界平面的多轴应力-应变状态。然后将其与传统疲劳设计数据提供的疲劳寿命预测相关联。它使用 FPSO 的模拟作为案例研究来证明低周疲劳的重要性，这表明平均负载效应在降低系泊链应用的疲劳寿命方面很重要，而疲劳损伤引起的循环塑性的影响是有限的。临界平面方法预测的疲劳损伤累积明显高于传统的 SN 方法，应在系泊线设计中加以考虑。