Low-cycle fatigue assessment of offshore mooring chains under service loading

doi:10.1016/j.marstruc.2020.102892

Marine Structures

Volume 76, March 2021, 102892

https://doi.org/10.1016/j.marstruc.2020.102892 Get rights and content

Highlights

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The critical plane approach is correlated with conventional fatigue design data.
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How fatigue damage changes against stochastic mooring line loading is investigated.
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Mean load effects are significant in reducing the fatigue life of mooring chains.
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Importance of low-cycle fatigue contributions in mooring failures is shown.

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.

Section snippets

Current design of mooring chains against fatigue

There are many natural energy resources offshore, including oil, gas, wind and wave. As a result, floating offshore platforms are widely used for the exploitation of these energy sources [1,2]. Continuous operation of floating offshore platforms relies on the integrity of mooring chains to keep these platforms in position against adverse environments over design lives of 20-years, or longer. However, mooring failures have occurred at a critically high rate in recent decades, with the potential

Development of a representative chain model

A model of a mooring line is developed using the IACS [30] standardised dimensions for an offshore studless chain with a diameter, D, of 76 mm. The total length, L, is equal to 6D; the total width, W, is 3.35D and the curved section radius, R, is 0.675D, as illustrated in Fig. 1. The analysis is performed at two local hotspots, determined at the critical fatigue locations where the majority of fatigue failures have been found when under tension loading and are indicated in Fig. 1 as the Crown

Material models

In order to determine how the material selection affects the fatigue predictions, material models representative of the monotonic and cyclic mechanical properties are considered, and a comparison is made between them. The R4 steel grade, which complies with DNVGL-OS-E302 [34], is considered, as it is the most prevalent material grade currently in use for mooring chains [8]. A monotonic material model is developed from mechanical properties derived from tensile tests conducted by Rampi et al. [35

Calibration of the critical plane fatigue assessment

Smith et al. [38,39] proposed a critical plane multiaxial fatigue method, where fatigue damage is assessed directly in terms of local strains and stresses under progressive loading cycles. The method is described by Eq. (1), $S W T = σ_{n, max} \frac{Δ ε_{n}}{2},$ where SWT is the damage parameter, σ_n,max is the maximum normal stress and Δε_n is the normal strain range perpendicular to the critical plane during one cycle. The SWT damage parameter represents the real physical properties of the stress-strain function at

Mooring chain loading

In order to simulate the fatigue experienced by the mooring chains in service, the tension-spectrum, representing the fatigue loading, is simulated in a mooring line simulation of an FPSO as a case study using the Flexcom package [41]. The case study simulates a mooring system of an FPSO at a water depth of 810 m in a specific sea-state adopted from Gao and Moan [42] for short-term analysis. The sea-state is characterised by the significant wave height, H_s = 6.25 m, a spectral peak period, T_p

Discussion

The fatigue damage accumulation of the critical plane approach is significantly higher relative to the traditional SN approach, 75% on average for all of the material models used in the case study. This is mainly because the mean load effects are incorporated in the critical plane approach. No cyclic plasticity occurs as the mechanisms are only applied when the tension load exceeds 70% MBL due to the strain hardening effect from the proof load during the first cycle, and therefore the

Conclusion

This paper investigates how fatigue damage prediction changes with a more accurate representation of the low-cycle fatigue regime, using a strain-based critical plane approach, under stochastic tension loading in service for intact mooring chains. 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. The fatigue damage accumulation predicted by the

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors gratefully acknowledge the funding by the Indonesia Endowment Fund for Education (LPDP) and the support of the Lloyd's Register Foundation (LRF). The authors also acknowledge the support of the University of Southampton for access to its IRIDIS5 High-Performance Computing Facility.

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Citation Excerpt :
Fatigue properties of mooring lines are often predicted using the S–N curves. Gemilang et al. (Gemilang et al., 2021/03), however, argued that the S–N approach, which is stressed-based, and suitable for high-cycle fatigue regime (where plastic deformation is either small or negligible) as outlined in DVNGL- OS-E301 ("L--E301 Po, 2015) does not consider the fatigue component initiation process that occurs in the early fatigue life where the hotspot is sensitive to the mean load or cyclic plastic degradation of the material on fatigue damage when subjected to low cycle fatigue loading. This, according to the authors result in the accumulation of plastic strain from the multiaxial stress-strain responses of the material leading to cyclic plastic damage accumulation.
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View full text

Low-cycle fatigue assessment of offshore mooring chains under service loading

Highlights

Abstract

Section snippets

Current design of mooring chains against fatigue

Development of a representative chain model

Material models

Calibration of the critical plane fatigue assessment

Mooring chain loading

Discussion

Conclusion

Declaration of competing interest

Acknowledgement

Mar Struct

Offshore Arctic Eng

Ocean Eng

Eng Fract Mech

Int J Fatig

Surf Coating Technol

Int J Fatig

Int J Fatig

Int J Fatig

Int J Fatig

Int J Fatig

Int J Fatig

Procedia Eng.

Int J Fatig

Int J Plast

Mar Struct

Int J Fatig

Int J Fatig

Ocean Eng

Ocean Eng

Mar Struct

Industry survey of past failures, pre-emptive replacements and reported degradation for mooring systems of floating production units

A historical review on integrity issues of permanent mooring systems

Application of lessons learned from field experience to design, installation and maintenance of FPS moorings

Lessons learned from Norwegian mooring line failures 2010–2013

Offshore Arctic Eng