Ifsttar PhD subject

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Title : Centrifuge modelling of the installation mode of offshore wind turbine monopiles

Main host Laboratory - Referent Advisor   -

Director of the main host Laboratory   -

PhD Speciality géotechnique

Axis of the performance contract 2 - COP2017 - More efficient and resilient infrastructure

Main location Nantes

Doctoral affiliation ECOLE CENTRALE NANTES

PhD school Sciences de l'Ingénierie et des Systèmes (SIS)

Planned PhD supervisor BLANC Matthieu  -  Université Gustave Eiffel  -  GERS - CG

Planned PhD co-supervisor THOREL Luc  -  Université Gustave Eiffel  -  GERS - CG

Planned financing Contrat doctoral  - Ifsttar

Abstract

Offshore wind turbines are widely founded on large-diameter monopiles, often installed through impact driving. If it is acknowledged that the method of installation should be considered in the design, the effect on the horizontal response is still neither well understood nor quantified.
The horizontal behaviour of impact-driven monopiles in saturated dense Fontainebleau sand is studied using centrifuge modelling. New devices, including monopile driver at 100g, have been developed and implemented. The experimental parameters on the lateral response of monopiles concern the installation mode, the length-to-diameter ratio L/D, the hammering blow rate and the fluid viscosity. The performance of the design methods (standards and PISA method) for laterally loaded monopiles in sand are then evaluated after developing a one-dimensional finite-element model.
The main findings are: (a) the effect of impact driving is pronounced and should accounted for in the design especially, for monopiles with smaller L/D and higher thickness-to-diameter ratio, (b) the monopile behaves from pure rotation to combined rotation-flexure as L/D enhanced from 3 to 5, inducing a less pronounced linear rotation-deflection response at ground-level, (c) the increase of pore fluid pressure during the installation process influences the lateral response of monopile in dense sand but not in a pronounced way, (d) the experimental p-y curves obtained from the mpact-driven installation remained smoother than those predicted by using offshore standards, but are similar to them, (e) the PISA design method captures reasonably well the overall response of laterally loaded centrifuged impact-driven monopiles, even if it is less accurate for the local behaviour.

Keywords : Impact-driven monopile, lateral response, centrifuge modelling, saturated dense sand

Main host Laboratory - Referent Advisor	-
Director of the main host Laboratory	-
PhD Speciality	géotechnique
Axis of the performance contract	2 - COP2017 - More efficient and resilient infrastructure
Main location	Nantes
Doctoral affiliation	ECOLE CENTRALE NANTES
PhD school	Sciences de l'Ingénierie et des Systèmes (SIS)
Planned PhD supervisor	BLANC Matthieu - Université Gustave Eiffel - GERS - CG
Planned PhD co-supervisor	THOREL Luc - Université Gustave Eiffel - GERS - CG
Planned financing	Contrat doctoral - Ifsttar
Abstract Offshore wind turbines are widely founded on large-diameter monopiles, often installed through impact driving. If it is acknowledged that the method of installation should be considered in the design, the effect on the horizontal response is still neither well understood nor quantified. The horizontal behaviour of impact-driven monopiles in saturated dense Fontainebleau sand is studied using centrifuge modelling. New devices, including monopile driver at 100g, have been developed and implemented. The experimental parameters on the lateral response of monopiles concern the installation mode, the length-to-diameter ratio L/D, the hammering blow rate and the fluid viscosity. The performance of the design methods (standards and PISA method) for laterally loaded monopiles in sand are then evaluated after developing a one-dimensional finite-element model. The main findings are: (a) the effect of impact driving is pronounced and should accounted for in the design especially, for monopiles with smaller L/D and higher thickness-to-diameter ratio, (b) the monopile behaves from pure rotation to combined rotation-flexure as L/D enhanced from 3 to 5, inducing a less pronounced linear rotation-deflection response at ground-level, (c) the increase of pore fluid pressure during the installation process influences the lateral response of monopile in dense sand but not in a pronounced way, (d) the experimental p-y curves obtained from the mpact-driven installation remained smoother than those predicted by using offshore standards, but are similar to them, (e) the PISA design method captures reasonably well the overall response of laterally loaded centrifuged impact-driven monopiles, even if it is less accurate for the local behaviour.
Keywords :	Impact-driven monopile, lateral response, centrifuge modelling, saturated dense sand

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