Scientific objective

→ A methodology to quantify, when possible, parameters of bio-colonisation all along the lifetime in order to reduce uncertainties on long term fatigue damages (before a scheduled storm, for example).

A Monitoring Methodology of Bio-Colonisation on Mooring Lines of Floating Wind Turbines

DECUREY Benjamin, MELL Ludovic, SCHOEFS FranckResearch Institute of Civil Engineering and Mechanics, UMR 6183 CNRS/ECN/UN, Nantes,

France

PARTNERSHIP‘This work has benefited from state aid managed by the French National Research Agency under the ‘Investments for the Future’ program with the reference ANR-10-IEED-0006-19 / MHM-EMR.’

INDUSTRIAL ISSUE

METHODOLOGY

Global View of a Reliability Analysis

CONCLUSION & OPENING

Qualifying Sea State (i) :  Environmental parameters

Identification by solving an inverse problem

Constant physical observations- Mussels Colonisation

1) Dominant invading specie on mooring lines in the first 30 meters.2) Decreasing with depth.3) Self-organising in bulbs, presenting peaks and deeps.4) Quickly, after some months, the colonisation is axisymmetric.

Thickness (t) Parameters* random values, bounded based on expert advices and observations during design phase.** fixed, based on observations during design phase.

→ A distribution of bulb lengths (**).→ A correlation between peaks and deeps (**).→ A 2-parameters (*) decreasing tendency for peaks.→ A variance σ (*) for peaks around that tendency.→ A correlation length lc for peaks (*).

Density and Roughness

→ Fixed density (d), based on seasonal arguments.→ Fixed mean roughness (k) for a multilayered colonisation of mussels.

Function of a mooring system : Handling relative positioning and/or stability of the floating wind turbine during all its life-time (25 years or more).

© reneweconomy.com

Decommissioning

Commissioning

Storm(Risk of failure ?)

Corro sion & W

ear ing.

Fatig ue due

to w

av es, curre nt,

wind

an d b io-

colon isati on.

Bio-colonisation: Aggregate of marine organisms (seaweed, sponges, mussels, oysters, barnacles, anemones, corals…) on offshore industrial structures.

- reduction of minimum tension.- reduction of buoyancy.- shifting of natural frequencies towards larger periods.- increase of effective tension’s variance.

Decrease of mooring lines lifetime + Increase of uncertainties on damages.

Modifcaton of hydrodynamic and mass loads.

Damages accumulation in a mooring line during its lifetime

0,88

cm

D0

t k

ρ

How to collect data to build a local ‘a priori’ model ? 

During design phase, on mooring lines of weather buoy or cardinal buoy.

A qualifying Sea State ? 

Sensitivity of tension response of mooring lines to a significant change in bio-colonisation. Example : in a calm  sea state (low wave height, low wind and current velocities).

(αi)An ‘a priori’ stochastic model

of bio-colonisationBc=Bc(σ , lc ,μexp , d , k ,CD ,Cm)

A metamodel (i)T=f i(Bc ,αi)

Measured tension+

Measured environmental parameters (α̂i)

(T̂ )B̂c=f i−1(T̂ , α̂i)

Metamodel

Numerical model built during design phase. It could be improved with measures from inspections.

σ

pdf

σ1 σ2

1σ2−σ1

priorposterior

NB1 : Same for other random  values of bio-colonisation model and hydrodynamic parameters.

NB2 : Requirement of only one or  few sensors along the mooring line to reduce uncertainties on ‘bio-parameters’.

Environment (waves, wind, current) history

Mooring Line Features

A stochastic model of bio-colonisation

Tension, inclination, torsion

histories

Experimental plan

Estimation of long-term cumulative

damage

History of Full WF and LF Dynamic

Response of Mooring Lines

Estimation of damage distribution

moments due to one sea state

Reliability analysis based

on both FLS and ULS

Using a FEM or spring/mass model

Thanks to this mechanical engineering approach → Decrease of uncertainties on temporal fatigue damages due to bio-colonisation.Final purpose :  Is this decrease economically valuable ? 

→ Complex reliability analysis (number of random variable > 20)→ Expensive finite element computations of the structure

Wind

Waves

Different components (floater,  mooring lines, turbine...)

Multi-scale reliability

Approaches to tackle challenging simulations : - High performance parallel computing- discretization error estimation → bounds on Pf

Strategy to reduce computational cost of reliability analysis

Biocolmar (R) at UN-SEA-SMS test siteUN/Biolittoral (c)