For mooring chains of offshore floating production units, API (American Petroleum Institute) recommends the use of its TxN fatigue curve considering the MBL (Minimum Breaking Load) of an ORQ (Oil Rig Quality) chain even if the chain has a higher grade. This curve has been used in mooring system design of offshore floating production units since the draft edition of API Recommended Practice for Design, Analysis and Maintenance of Catenary Mooring for Floating Production Systems in May 89 and several fatigue tests have been done by petroleum industries, chain manufacturers and research centers. Those fatigue tests show that the use of the MBL of an ORQ chain for higher grades is a conservative assumption. This paper will present an overview of the fatigue curves of materials for mooring lines: stud and studless chains, steel wire ropes and polyester fiber ropes. This overview is based on recent tests, rules and published papers.

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