Monitoring Strategy of Bio-Colonisation on Mooring Systems based on a Qualifying Sea State: Sensing Network Efficiency using Conditional Entropy Metric
Résumé
Bio-colonisation is shown to affect the ageing of materials and the behaviour of offshore structures and consequently their resistance to extreme events or fatigue life time. It was recognized in the 70's that bio-colonisation might change the hydrodynamic loading due mainly to screen and drag effects. Mooring systems and umbilicals belong to a family of components sensitive to bio-colonisation in general: abrasion of ropes by shells, change of dynamic behaviour due to shape and roughness modifications, added mass. However, this stochastic process with time and space is still unknown and knowledge through inspections and monitoring is mandatory. This paper is presenting a first spatial model of bio-colonisation thickness along a mooring line and a method for updating this previous model with sensors. Our method shows that in calm sea state (with low wave height, with low wind and current velocities), the monitoring of mooring lines tension can help to assess and reduce uncertainty on thickness spatial distribution of bio-colonisation. This paper deals with a simple assessment of bio-colonisation on a catenary mooring line by accounting for monitoring of tide level, floater's buoyancy and local tension at some points along the mooring line. A first analysis will be performed in order to estimate influence of distance between sensors using conditional entropy metric.
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