Low-cycle fatigue assessment of mooring chains based on the critical plane approach

Abstract

The integrity of mooring chains is important to the safety of a range of offshore platforms for the exploitation of natural resources, including oil, gas, wind, and wave energy sources. However, mooring line failures have been occurring earlier than they are designed for with a high number of these failures occurring due to fatigue. The failures occur in the first three years, when operating in harsh sea environments, indicating that the failure mechanism could be attributed to low-cycle fatigue, where the fatigue hotspot is sensitive to the mean load and there is plastic strain accumulation from the multiaxial stressstrain responses of the material, leading to cyclic plastic damage accumulation. However, the traditional SN approach, suggested by mooring standards, does not consider these effects, and it is proposed that the lack of this consideration under low-cycle fatigue conditions is the reason for the current non-conservative fatigue assessments of mooring chains. Therefore, this thesis aims to develop a fatigue approach, based on a critical plane multiaxial fatigue criterion. This method is capable of accounting for the mean load and cyclic plasticity effects on the fatigue damage, which governs the low-cycle fatigue of mooring chains. This allows for an investigation into how fatigue damage prediction changes with a more accurate representation of the low-cycle fatigue regime under stochastic tension loading in service for intact mooring chains. A finite element model with explicit solver is developed and shown to be robust for modelling representative stresses in mooring chains when compared with other finite element models available in the literature. Using the determined finite element model, the critical plane approach is developed based on the multiaxial stress-strain conditions at the hotspot and this is correlated with the fatigue life derived from tests taken from the mooring standards. Subsequently, the developed critical plane approach is compared with the traditional SN approach using a mooring line simulation of an FPSO as a case study. The result found that the critical plane approach predicts significantly higher proportions of low-cycle fatigue than the traditional SN approach in the mooring line loading. This is because the mean load effect is detrimental when it is above 20% of the minimum breaking load, shown to make up a large proportion of the loads in high sea-states. However, the effect of the cyclic plasticity is limited as the mechanisms are only applied when the tension load exceeds 70% of the minimum breaking load due to the strain hardening from the proof load application during the first cycle. The traditional SN approach is shown to be conservative when the mean load is below 20% of the minimum breaking load, but after this, it is non-conservative, which is observed in harsh environments where the majority of mooring chain failures were witnessed. The low-cycle fatigue analysis in this study explains why current fatigue assessment in mooring chain applications is non-conservative and demonstrates that the critical plane approach should be accounted for in mooring line design.

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Identifiers

ORCID for Gilang Muhammad Gemilang: orcid.org/0000-0001-9641-9495

ORCID for Adam Sobey: orcid.org/0000-0001-6880-8338

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