Computational fatigue assessment of mooring chains accounting for residual stresses

• Published in: Volume 3A: Structures, Safety and Reliability
• Main Author: Martínez Pérez, Imanol
• Other Authors: Venugopal, Vengatesan, Borthwick, Alistair
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: The University of Edinburgh 2019
• Subjects: mooring chains; fatigue; manufacturing; service life; Dang Van fatigue criterion; critical plane methods; Finite Element Analysis; fatigue assessment method; twisting; Out-of-Plane Bending; fatigue crack initiation; Arctic
• Online Access: http://hdl.handle.net/1842/35912

Mooring chains are used to keep dynamically floating structures on a fixed geographical position within a specified tolerance. Chains for permanent moorings have been traditionally used by the Oil and Gas industry for Floating Production Storage and Offloading (FPSO) and have recently found application in the Offshore Renewable Energy Industry, as for example in mooring floating wind turbines. For both industries, the failure of the mooring can give rise to large accidents with devastating economic losses as well as drastic environmental consequences. During the last decade, the increasing number of mooring incidents has rise to concern among Oil and Gas companies. In most of these incidents, chain links were the root cause, and fatigue the main damage mechanism. This research aims to investigate the fatigue of mooring chains from a global approach. That is, to follow the life cycle of a mooring chain, which is mainly composed of two stages: manufacturing, and service life. The fatigue of mooring chains has been studied using the Dang Van fatigue criterion. Dang Van fatigue criterion and critical plane methods are a set of fatigue criteria that have proven to be accurate, and account for complex phenomena (for example nonproportionality of the loading, mean load effects, among others); however, they have a complex mathematical formulation which involves solving optimization problems, and therefore such methods carry substantial computational overhead if they are applied to an industrial component with complex geometry. The research of this thesis is divided in three main parts. In the first part, different numerical methods are reviewed for solving the optimization problems faced when applying critical plane methods and Dang Van fatigue criterion. The best performing method for applying the Dang Van fatigue criterion is identified. In the second part, the residual stress field after the manufacturing of a chain is predicted by means of Finite Element Analysis (FEA). Relevant manufacturing steps are modelled. A ...