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Title: Computational fatigue assessment of mooring chains accounting for residual stresses
Author: Martínez Pérez, Imanol
ISNI:       0000 0004 7969 2686
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2019
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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 qualitative validation using data from the literature is presented. Finally, using the numerical method identified in the first part for applying Dang Van fatigue criterion, and the residual stress prediction derived in the second part, the computational fatigue assessment of mooring chains is performed. Two different loading modes have been studied, tension and twisting. The first one is the nominal loading mode; however current standards do not account for the effect of the mean load. The influence of mean load is assessed, and a simplified fatigue assessment method implementing Dang Van fatigue criterion is proposed. The accuracy of the proposed method is proven by comparing the predictions with full scale fatigue testing carried out in sea water at TWI Ltd as part of a Joint Industry Program (JIP). The second loading mode (twisting) is not fully accounted for in the standards; the fatigue analysis predicts cracks at locations that do not correspond with fatigue breakage locations under tension loading or Out-of-Plane Bending (OPB). The predicted fatigue crack initiation locations match very well with cracks found in chains recovered from the field after more than 15 years in service.
Supervisor: Venugopal, Vengatesan ; Borthwick, Alistair Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: 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