2 research outputs found
Computational fatigue assessment of mooring chains accounting for residual stresses
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