Fatigue
Technical Bulletin 2016

Since fatigue became an important issue - led by IACS - in the early 1990s, Bureau Veritas has been at the forefront of developments in understanding the subject. The Bureau Veritas Information Note, NI 611, was published in 2016 and takes into account all the latest developments in the field of fatigue, along with experience acquired from joint investment projects and other international forums.

The first paper in this section addresses fatigue issues in membrane-type LNG Carriers. Most of the Bureau Veritas fleet of LNG Carriers has been granted the notation VeriSTAR FAT 40 years, demonstrating that the ships have been assessed taking into account a fatigue life of not less than 40 years, generally in North Atlantic conditions. This paper details the spectral fatigue analysis methodology that was applied, using the specialized software tool, HOMER, including the use of the modal superposition to calculate ship hydro-elastic response in waves. HOMER has been extensively applied, mainly for containerships, LNG Carriers and offshore structures.

The two subsequent papers, written in cooperation with the offshore industry, are about out-of-plane bending (OPB) of mooring chains, reflecting the critical importance of mooring systems within the offshore sector. The OPB effect on the fatigue strength of mooring chains was the subject of a JIP that Bureau Veritas was actively involved in. During the JIP, tests were carried out on mooring chains that have led to the development of methodologies now incorporated into the Rules. Rule note (NR 604) “Fatigue of top chain of mooring lines due to in-plane and out-of-plane bending” was published in October 2014.The next paper, written jointly between the international oil major, Total, and Bureau Veritas, explores the development of an optimized inspection plan approach to life extension of fixed offshore platform jackets. With many platforms now ageing, there is increasing demand to extend their operating life. The paper explains how the results of fatigue assessment are combined with the criticality of the structural members in order to define an inspection plan for in-service jackets, and this is underpinned by real-life examples from the Netherlands.

Finally, the last paper of this section presents a comparison between experimental and numerical simulation of intermittent wetting and measurements on board a ship in service. Intermittent wetting is not properly taken into account by linear spectral fatigue analysis, in particular for the members located in way of the draft of the ship, such as ordinary longitudinal stiffeners. Several advanced intermittent wetting models – linear and non-linear – dedicated to spectral fatigue analysis are presented and compared to experimental measurements. The conclusion of this study will be reflected in updates to our rules and regulations.

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