Coordinateurs du projet
Context
Marine biofouling can have many harmful effects on offshore structures and therefore poses a challenge for engineers during the design and maintenance phases. Several standardized methods for in-situ inspection and measurement of marine organism characteristics have therefore been developed in order to obtain relevant parameters such as: the percentage coverage of different species, the total percentage coverage, and the weight and thickness of biofouling. One of the obstacles identified in this project is the definition of roughness.
The objective is to propose a new formulation of roughness to MRE system certifiers and thereby move away from offshore oil regulations. The second objective is to provide software for calculating hydrodynamic coefficients based on roughness and wave conditions.
Scientific breakthroughs and innovation
For more than 20 years, a single definition of roughness due to biofouling has been used for rigid species. However, in situ measurements carried out by Nantes University show significant heterogeneity in the size and density of individuals on the surface. Preliminary studies conducted with Total and Ifremer have shown a 20% increase in hydrodynamic forces due to micro-roughness. The innovation consists of redefining roughness to facilitate comparison with in situ measurements, in particular using the Aksi-3D device developed by Nantes University and MAREI (Ireland).
Expected technical and economic impact
- Improve the modeling of fluid-structure interaction coefficients for engineering, in the presence of biofouling.
- To reduce the design/maintenance costs of MRE systems.
- By improving the definition of roughness caused by rigid marine organisms.
Demonstrator
Two testing methods were used to characterize biocolonization: the two SEMREV (Ecole Centrale de Nantes) offshore sites and UN@SEA with the Biocolmar measuring station (University of Nantes). The measurements were carried out using the Aksi3D protocol from the University of Nantes. IFREMER’s test tanks were used to characterize the hydrodynamic effects.
Results
As part of the Lehero-MG project, an in-depth examination of the effect of biofouling on hydrodynamic forces was carried out. The focus was mainly on the influence of biocolonization on the drag force of fixed elements under stable and oscillating currents.
This study highlighted the fact that hydrodynamic forces on cylindrical elements could be significantly modified by introducing all relevant physical parameters of marine fouling. Most previous studies have calculated the drag coefficient solely on the basis of the surface roughness ratio, particularly in the region of the critical Reynolds number. However, this work shows that the dependence of biological fouling solely on surface roughness is subject to major debate. Therefore, although most models depend only on relative surface roughness, CD must be defined based on several parameters.
Thus, a new approach based on the data collected is proposed to calculate the drag coefficient as a function of surface roughness, surface coverage, type of aggregation, and species of biological fouling. A multi-parameter equation has been proposed to estimate the drag coefficient of circular elements covered with biofouling. The proposed numerical models are capable of predicting the effects of biofouling in a more realistic and reliable manner.
Methods were tested to identify the best measure of roughness. Madogram was shown to be the most relevant. It was also demonstrated that a one-centimeter variation in mussel roughness could cause a 10% increase in forces on a realistic anchor configuration tested in a basin.
Perspectives
- Analysis of other species, particularly flexible ones
- Analysis of dynamic effects
- SHM of biocolonization