Étude numérique des pieux d'éolienne offshore soumis à la traction en milieu sableux

International audienceL'étude numérique d'une fondation offshore en sol sableux de type pieu soumise à un effort vertical en traction est présentée. La méthode des éléments finit est utilisée. Les résultats numériques sont comparés aux résultats expérimentaux obtenus en centrifugeuse géotechnique (des pieux battus à 1xg ont été testés à 100xg). La fondation est représentée à travers un modèle en deux-dimensions axisymétrique, des outils et des hypothèses du domaine de l'ingénierie sont utilisés. Deux conditions d'état de contrainte initiale dans le sol sont analysées. Le premier cas, simulant un pieu moulé, considère une distribution géostatique des contraintes initiales. Le second cas prend en compte la modification de l'état des contraintes initiales dues au fonçage du pieu, au travers de la méthode CPT ICP-05. La comparaison de ces résultats met en valeur l'importance de la prise en compte de l'état des contraintes initiales après la mise en place d'un pieu pour la détermination de sa capacité portante

Changes in carbon structure distribution and nanostructure of functionalized biochars

Energetic vectors produced from the thermochemical conversion of biomass and waste is considered as environmental-friendly energy. The syngas composed of CO and H2 at convenient ratio, could be used for various subsequent uses. However the industrial development is hold back by the production of unwanted by-products: tars and biochar/ash (aromatic hydrocarbons and solid residues respectively) which decrease the global efficiency yield and require very costly treatments. Some recent studies investigated potential applications to use these biochars for catalytic or sorbent applications, such as catalytic tar cracking or pollutants removal [1]. Biochars are cheap candidates and their functionalization is an opportunity for further development as an inexpensive catalyst from renewable resources. Please click on the file below for full content of the abstract

Étude numérique des pieux d'éolienne offshore soumis à la traction en milieu sableux

National audienceThe response of an offshore pile embedded in sand under tension loading is studied. An axisymmetric two-dimensional finite element model is built using common engineering tools. Results are compared to experimental data obtained through geotechnical centrifuge tests (during the tests piles were driven at 1×g and tested at 100×g). Two models with different initial state of stresses in the soil are analysed. First, a geostatic case where radial stresses around the pile are a linear function of the soil weight, this hypotheses is used to represent a cast-in-place pile. Second, an initial state of stress is calculated and introduced in the model. In this case, stresses are calculated using the CPT method ICP-05, this method allows to take into account the effects in the stress field due to a driven pile. Comparing the results from both cases with the experimental data allows to emphasize the role of the initial state of stress to calculate the load carrying capacity of a pile foundation. MOTS-CLÉS : Pieu, offshore , traction, capacité portante, CPT, ICP-05.L'étude numérique d'une fondation offshore en sol sableux de type pieu soumise à un effort vertical en traction est présentée. La méthode des éléments finit est utilisée. Les résultats numériques sont comparés aux résultats expérimentaux obtenus en centrifugeuse géotechnique (des pieux battus à 1×g ont été testés à 100×g). La fondation est représentée à travers un modèle en deux-dimensions axisymétrique, des outils et des hypothèses du domaine de l'ingénierie sont utilisés. Deux conditions d'état de contrainte initiale dans le sol sont analysées. Le premier cas, simulant un pieu moulé, considère une distribution géostatique des contraintes initiales. Le second cas prend en compte la modification de l'état des contraintes initiales dues au fonçage du pieu, au travers de la méthode CPT ICP-05. La comparaison de ces résultats met en valeur l'importance de la prise en compte de l'état des contraintes initiales après la mise en place d'un pieu pour la détermination de sa capacité portante

Inclusion inversée : une innovation pédagogique au bénéfice de l’inclusion scolaire

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Axial behaviour of jacket piles for offshore wind turbines

International audienceTo improve our understanding and to quantify the behaviour of a jacket foundation for offshore wind turbines, several tests have been conducted using a geotechnical centrifuge at 100g. The monotonic tension and compression tests carried out on single small-scale model piles (prototype dimensions: 1·8 m in diameter and an embedment depth of 40 m in dense sand) jacked at 1g and 100g show the influence of the set-up method on the axial ultimate capacity and on the tip and shaft resistances. The pile axial capacity in compression is improved, whereas the normalised displacement of the pile is bigger in tension when piles are jacked at 1g. Vertical cyclic loads on single piles jacked at 1g (pure tension tests or two-way tests) and horizontal cyclic loads on a four-pile jacket structure are applied to examine the behaviour under more realistic loading paths. The piles of the jacket structure are considered and studied as singles piles. Finally, the stability diagrams for the single and the jacket piles are discussed to visualise the significant difference in performance of a single pile against the comparable pile in the jacket

Projet CHARGEOL : Etude expérimentale et numérique des pieux de fondation des éoliennes offshore

International audienceLes structures offshore sont conçues pour résister à des importantes sollicitations dues au vent, la houle, la rotation des pales ou encore d'origine sismique. Ces charges ont des composantes cycliques de basses et de hautes fréquences et le nombre de cycles peut varier de quelques dizaines (pour un séisme) à plusieurs milliers de cycles (lors d'une tempête). Plus spécifiquement, les pieux des éoliennes offshore se caractérisent par leur important diamètre et grande profondeur (longueur). Leur dimensionnement suit les normes internationales en vigueur [1] et [2] et des méthodes proposées dans le domaine de l'industrie pétrolière, pour lequel les pieux sont de taille inférieure par rapport au cas des éoliennes offshore. Des méthodes alternatives à partir des essais CPT (ICP-05, UWA-05, Fugro-05 ou NGI-05) ont été également développées permettant de déterminer la résistance du pieu à la traction à partir des résultats des essais CPT. L'ensemble de ces méthodes est résumé dans [3]

Multi-scale characterisation of chars mineral species for tar cracking

International audienceSyngas from thermochemical conversion of waste or biomass is a renewable energy carrier that may contain pollutants – such as tar – that should be removed before further syngas utilisation. Chars have proved to be promising catalysts for tar cracking, but the influence of the physico-chemical properties on their reactivity is still unclear. This work aimed to better understand the structure and the composition of the mineral species of pyrolysis char, as well as their catalytic role in tar cracking. For this purpose, a characterisation of the minerals has been performed at bulk, surface (studied at micro and nano-scale) and crystallite scale. Pyrolysis chars were produced from wastes generated on cruise ships – namely used wood pallets (UWP), food waste (FW) and coagulation flocculation sludge (CFS) – having different mineral amount and content. Ethylbenzene was used as surrogate of light aromatic hydrocarbons in a tar cracking process. The results showed that ethylbenzene was converted into lighter gases meaning that the chars were efficient for this. Ethylbenzene conversion at 650 °C was found to be significantly higher with the char from a mixture of sludge and food waste (c.FW/CFS) compared to that of wood-based char (c.UWP): 71 wt.% against 45 wt.%, respectively. The combination of multi-scale and complementary techniques has highlighted that the higher catalytic activity of this char was mainly attributed to the mineral content. Well dispersed mineral particles with various morphologies and natures were observed on the surface of c.FW/CFS using Scanning and Transmission Electron Microscopy (SEM and TEM). Especially, Ca, Al and P were the main mineral species identified using XRFS and SEM. These mineral species in form of oxides and hydroxyapatite were considered to be the main active mineral components for tar cracking. Oxides were identified using EDX-analysis. XRD analysis highlighted the presence of crystalised particles of hydroxyapatite (Ca5(PO4)3(OH)), while Raman spectroscopy revealed that these particles were embedded in the carbon matrix

Does hydrological connectivity control functional characteristics of artificial wetland communities? Evidence from the Rhône River

International audienceThe importance of artificial wetlands for the biodiversity of the Rhône River floodplain was assessed at twelve stations along two sections of the river. The relative roles of the local characteristics of the wetlands (e.g. available nutrients, dissolved oxygen, sediment quality) and of their connectivity with the main channel were considered for three different groups of organisms (benthic micr obes, benthic invertebrates and phytoplankton communities). The functional characteristics of these organisms were determined through their enzymatic activities (for microbes) and biological traits (for invertebrates and phytoplankton). The predicted major role of connections between the wetlands and the river channel was confirmed for environmental characteristics (water and sediment) and benthic invertebrates. Connectivity appeared to be less of a driver of benthic microbial communities and phytoplankton, which were partly or mainly controlled by wetland geometry (water depth, light penetration, and resulting organic matter production). We conclude that connectivity with the river channel needs to be considered jointly with the geometry of wetlands to provide functional solutions when defining management and restoration plans

Reactivity and deactivation mechanisms of pyrolysis chars from bio-waste during catalytic cracking of tar

International audienceThe catalytic activity of pyrolysis chars from bio-waste was investigated for the cracking of model tar compounds (ethylbenzene and benzene). Two pyrolysis chars were produced at 700 °C from (1) used wood pallets (UWP), and (2) a 50/50 dry% mixture of food waste (FW) and coagulation-flocculation sludge (CFS). Steam activation at 850 °C was used to study the influence of the porous structure. While coke deposition is known to be responsible for the deactivation of carbonaceous chars and metal catalysts during tar cracking reactions, the deactivation of complex materials such as bio-waste chars has scarcely been studied. For this reason, special attention was paid on the relationships between the physicochemical properties of the chars, the operating conditions, and the deactivation mechanisms. To this aim, the cracking tests were performed over a wide temperature range: 400–650 °C for the ethylbenzene cracking, and 850–950 °C for benzene cracking. After the ethylbenzene cracking tests at 650 °C, the characterisations performed with SEM, BET, FTIR and Raman revealed that coke deposition was responsible for the char’s deactivation. The high specific surface area of activated chars explained their higher catalytic activity, and mesoporous catalysts were proved to be more resistant to coke deactivation than microporous catalysts. For these reasons, the higher ethylbenzene conversion (85.8%) was reached with the activated char from food waste and sludge (ac.FW/CFS). For benzene cracking at higher temperature (850 and 950 °C), the chars from food waste and sludge (FW/CFS) were the most active catalysts, despite their deactivation by the melting, diffusion and sintering of the inorganic species. This original deactivation mechanism, reported for the first time, led to the formation of an inorganic layer composed of P and Ca species at the char surface, with some areas rich in KCl and NaCl. Non-activated char from food waste and sludge (c.FW/CFS) was surprisingly proved to be more resistant to deactivation by inorganic species than the activated char (ac.FW/CFS) during the benzene cracking tests at 950 °C. This extended catalytic activity was explained by the activation of the non-activated char (c.FW/CFS) with the CO2 contained in the syngas which simultaneously developed the porosity and created new available active sites. This study marks a step forward in the understanding of the relationships between the deactivation mechanisms, the physicochemical properties of the chars, and the cracking temperature. Finally, a proposal for process integration is presented to consider the possibility to valorise the chars as catalysts to decompose the tar generated in the same pyro-gasification process

Advanced characterization unravels the structure and reactivity of wood-based chars

International audienceThis study aims at understanding the structural changes occurring in the carbonaceous matrix of wood-based chars during their thermal conversion. Although chars are routinely characterized by porosity measurements or scanning electron microscopy, the composition and structure of the carbonaceous matrix is often not investigated. Here, advanced characterization using X-ray synchrotron microtomography, transmission electron microscopy, Raman spectroscopy and X-ray diffraction provided a precise description of the char properties, allowing for an accurate discussion of their catalytic properties. Two chars were produced by slow pyrolysis of wood waste (400 and 700 °C) and a third one was fabricated by activation under steam at 850 °C of the char obtained at 700 °C. The results show that the pyrolysis temperature and the activation performed did not affect the macrostructure of the chars and that the pores were interconnected at the macroscopic scale. However, at 700 °C, the micro- and nanostructures were modified: short-range organized graphene fringes were observed. The activated char showed a homogeneous microstructure similar to that of its precursor. Besides, the ratio of graphene-like structures, the local organization of graphene sheets, and the imperfections in graphene-like sheets were clearly improved by the post-treatment. To our knowledge, this is the first time that such an approach, combining various tools, is applied for the study of pyrolysis chars