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Investigation into high-grade mooring chain fatigue resistance

TWI is working with Industrial Members from the offshore oil and gas sector in a joint industry project (JIP) to investigate fatigue in mooring chains. Demand from industry has stimulated the need for test data generated under realistic conditions to gain a detailed understanding of the factors affecting fatigue performance.

Mooring chains are used for deep-water oil and gas developments, for mooring floating production storage, and offloading vessels, buoys, and spars. Larger links and higher strength materials such as R5 grade are being considered, as they can offer higher static strength. The performance of higher steel grade and large diameter chain is not well understood, however, and it is possible that the increased static strength may not result in improved fatigue performance in seawater.

Existing fatigue design recommendations are based mainly on data obtained in the 1990s, on chains with relatively small diameter (mainly 76mm) and low steel grades (R3 and R4). Now TWI will provide updated independent data for higher chain grades (R5) and larger diameters (127mm) with full-scale fatigue tests in seawater.

The TWI team has designed and manufactured a test rig to provide full-scale fatigue endurance data for mooring chains in seawater. The project started in 2012 is expected to be completed in January 2018. This project will critically review existing knowledge, and provide data for high-strength steel, and a larger link diameter. It will establish the primary material properties affecting performance, and provide independent data for higher chain grades and larger diameters. Furthermore, this JIP is also trying to investigate the failure mechanism, to determine whether crack initiation or crack propagation is dominant in total fatigue life.

TWI is working with the American Bureau of Shipping (ABS), BP Exploration Operating Co Ltd, Shell Projects and Technology, and Statoil Petroleum Co Ltd to improve safety and reliability while reducing costs.

For more information, please email contactus@twi.co.uk.

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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