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Fatigue Design Curves for Steel Mooring Chains in Seawater

Fatigue design curves for high strength steel mooring chains in seawater -TWI Core Research Project  P1135

This case study reports on the outcome of recent fatigue tests on full-scale mooring chains of high steel strength, grade R5 in free corrosion conditions (without protection from corrosion) in seawater.

Overview

Although moorings chains of high strength steel, grade R5 have been used in service in large quantities for many years now, the fatigue design curves are based on the fatigue data of the lower steel grades R3 and R4. Therefore, the fatigue performance of higher strength steel grade R5 chains is not well understood.

Objectives

The objectives of the project were to:

  • Conduct fatigue testing in sea water in free corrosion conditions on full-scale mooring chains of R4 and R5 in order to obtain fatigue endurance data
  • Evaluate the viability of applying the existing fatigue design curves to a high strength steel, grade R5 mooring chain
Figure 1. Full-scale fatigue test set-up in free corrosion conditions in seawater
Figure 1. Full-scale fatigue test set-up in free corrosion conditions in seawater

Approach

A test rig, Figure 1 above, was designed and manufactured for testing mooring chain sections of 76mm link in diameter.  Six full-scale fatigue tests were conducted: three on high strength steel, grade R4 and three on high strength steel, grade R5.  In each test, a chain line of eleven links was loaded under tension into free corrosion conditions in seawater, Figure 1.

When a link failed, it was replaced with a Kenter (temporary) link to enable continuation of the test, until three link failures had been achieved in each test.  Magnetic particle inspection (MPI) was carried out to characterise the arising cracking location, crack size and crack growth.  Fracture surfaces were examined to reveal any features associated with fatigue crack initiation.

The testing programme generated sixteen link failures: eight from grade R4 and eight from grade R5.  Statistical analyses were performed to determine whether the fatigue data associated with R5 was significantly different from that of R4.  The fatigue data obtained in this testing programme was also compared with the mean curve for grades R3 and R4, provided by the earlier Noble Denton (ND) Joint Industry Project on fatigue of mooring chains, and the design curves in the API RP 2SK and DNVGL-OS-E301 standards, Figure 2.

 

Conclusion

Statistical analysis of the fatigue data confirmed that there was no clear effect of free corrosion conditions on performance of the steel grades in seawater, as investigated.  The results confirmed that the existing design curves given in standards API RP 2SK and DNVGL-OS-E301 can be applied to studless chain links of high strength steel grade R5.

Figure 2. Comparison of fatigue endurance between grades R4 and R5.
Figure 2. Comparison of fatigue endurance between grades R4 and R5.
Avatar Yanhui Zhang Consultant – Fatigue Integrity Management

Yanhui has a background in metallurgy, and graduated from the University of Science and Technology Beijing in 1982 with a Bachelor’s degree, before obtaining a PhD from the Open University, UK in 1992. Before joining TWI in 2001, he worked on Ni-based super-alloys as a Postdoctoral Researcher at the University of Cambridge. Yanhui’s expertise includes fatigue design, fatigue and creep life evaluation, engineering critical assessment (ECA), fatigue and creep testing, and failure investigation. He is also highly experienced in establishing relationships between mechanical properties and the microstructure of materials. Yanhui has published over 60 academic papers in journals and at international conferences.

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