PhD – Biofouling and fatigue damages of mooring lines

On 11 December at 2:00 PM (CET), an online for a PhD defence on the impact of biocolonisation on the fatigue damage of floating wind turbine mooring lines in a perspective of in-service monitoring of the structures will be held. This work was carried out as part of the MHM-EMR project.

The monitoring of floating wind turbine mooring lines with the aim of assessing their fatigue damage all along their lifetime is a scientific and industrial challenge. In this thesis, we opt for strategies relying on numerical models that simulate tension or stress in mooring lines. The main issue is the updating of uncertain intrinsic parameters, such as hydrodynamic coefficients,mass per unit length, anchor position or mooring length that influence mooring lines tension. It leads us to investigate the influence of bio-colonisation, also called marine growth, on fatigue damage of mooring lines.

The thesis aims to answer the following question: Does biocolonisation stochastic process, by changing mooring lines structural loading, significantly propagate its uncertainties to fatigue damage assessment of mooring lines? The building of a bio-colonisation spatial distribution model along mooring lines is the first original contribution, based on data reviewed from literature and new experimental campaigns at sea. Equipped with this spatial model, the contribution of bio-colonisation in mooring lines fatigue damage is assessed on a numerical model of a 10 MW semi-submersible floating wind turbine, moored with catenary chains. Thus, the provided answer is contextually linked to the case study. Bio-colonisation is shown to have a significantly low influence on catenary mooring lines fatigue damage. The global uncertainty on fatigue damage due to bio-colonisation variations is one hundred times smaller than the one due to environment variations (waves, wind and current). However this conclusion cannot be extended to all mooring systems and the thesis provides a replicable study methodology that bases on a preliminary physical understanding of mooring lines response.

The last part is the second original contribution. It focuses on the updating of bio-colonisation mass from monitoring. This method relies on the specification of environmental conditions, said to be qualifying, which favour the mass update. The method practical implementation is studied and discussed for the unfavourable case of catenary moorings, thanks to propagation uncertainty through a dedicated meta-model of the numerical model.

Composition of the jury

  • Rapporteurs:
    • John Dalsgaard SØRENSEN, Professor, Aalborg University
    • Jimmy MURPHY, Professor, University College Cork
  • Reviewers:
    • Alaa CHATEAUNEUF Professor, Université Clermont Auvergne
    • Violette HARNOIS , Engineer, Principia
    • Jean-Christophe GILLOTEAUX, Research Engineer, Ecole Centrale de Nantes
    • Thomas SOULARD, Research Engineer, Ecole Centrale de Nantes
  • Thesis director and co-director:
    • Franck SCHOEFS, Professor, Université de Nantes
    • Pascal CASARI, Professor, Université de Nantes

Access to the defense will be done using the following connection link:

Photo credits: France Energies Marines and Naval Energies

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