Conséquences de l'effet de fond sur l'instabilité des conduites soumises à une pression interne

Cet article a pour objet d'éclairer, par quelques exemples concrets et simples, les problèmes inhérents à la mécanique statique des tubes élancés soumis à une pression interne. En effet, la production pétrolière en mer, par grande profondeur d'eau, amène de plus en plus souvent à des solutions originales d'assemblage des conduites ou à envisager l'emploi de nouveaux matériaux anisotropes. Cela peut conduire à des instabilités mécaniques (flambement) des tubes, ou bien entraîner des surcontraintes de traction ou de flexion importantes qu'il est nécessaire d'analyser avec précaution. Cette analyse est au demeurant indispensable pour déterminer avec plus de précision les spécifications et les coefficients de sécurité applicables aux nouveaux produits tubulaires envisageables pour ces objectifs

Flambage vertical des conduites en souillées Vertical Buckling of Buried Pipes

Si l'apparition d'un flambage vertical des conduites rigides enfouies dans une tranchée est un phénomène assez rare, il peut ne pas en être de même pour des conduites flexibles dont les propriétés mécaniques sont de nature différente. Une étude théorique et expérimentale, ayant pour but de proposer une méthode analytique de calcul de l'apparition du flambage et de son évolution sous l'effet de la pression interne, a été réalisée. II apparaît que les conduites flexibles actuelles sont très sensibles à ce phénomène et qu'il serait nécessaire, pour l'éliminer à coup sûr, de réexaminer la structure des flexibles ou d'imaginer des artifices dans la procédure d'ensouillage <br> Whereas the appearance of vertical buckling in rigid pipes buried in a trench is a relatively rare phenomenon, the same cannot be said for flexible pipes which have mechanical properties of a different nature. A theoretical and experimental study has been made with the aim of proposing an analytical method for computing the appearance of buckling and its evolution under the effect of outside pressure. Current flexible pipes appear to be very sensitive to this phenomenon, and to be certain of eliminating it the structure of flexible pipes should be reexamined or stratagems in the burying procedure should be devised

Tubes légers à haute résistance pour des applications pétrolières marines Light High Strength Tubes for Offshore Petroeleum Applications

Les besoins nouveaux en tubes à haute résistance, créés par le développement actuel des gisements pétroliers marins, nous ont conduits à étudier des tubes composites particuliers, élaborés par enrouiement hélicoïdal de rubans d'acier minces à haute résistance enrobés d'une résine époxy polymérisable. Un mode de recouvrement adapté des rubans d'acier confère au composite une bonne étanchéité et des caractéristiques mécaniques élevées. Les propriétés physiques et chimiques de la matrice de résine permettent également d'obtenir une faible densité et une résistance convenable à la corrosion par les fluides pétroliers, l'eau de mer ou les boues de forage. Cette étude préliminaire comporte une analyse théorique simplifiée de ce type de structure composite, puis une partie expérimentale destinée a sélectionner les caractéristiques optimales de l'acier et de la résine et à vérifier les propriétés mécaniques et physico-chimiques obtenues sur ces tubes. Enfin, des exemples d'application en mer profonde destinés à comparer ces tubes composites aux tubes en acier sont présentés. Les premières conclusions de cette étude semblent confirmer l'intérêt de ce type de matériau pour ces applications. <br> The new requirements for high-strength tubes created by thé current development of offshore oilfields led thé authors ta study thé development ot spécial composite tubes formed by thé spiral winding of high-strength thin steel strips coated with a polymerïzable epoxy resin. A particular method of overlapping thé steel strips makes the composite leakproof and leads to good mechanical properties: The physical and chemical properties of thé resin matrix also make for a low specific gravity and suitable résistance to corrosion coused by petroleum fluids, seawater or drilling mud. This preliminary article presents a simplified theoretical analysis of this type of composite structure, followed by an expérimental study of thé choice of thé optimum steel and resin properties and thé controls of thé mechanical and physico-chemical properties of these tubes. Finally, deep offshore applications are described in an effort to compare thé composite tubes with steel tubes. The initial conclusions of this study appear to comfirm the interest of this type of material for such applications

Alteration of a cement matrix subjected to biolixiviation test

International audienceThis paper reports on the assessment of durability and long-term performance of a cement matrix subjected to heterotrophic microbial mediated degradation. In near surface disposal facilities for hazardous and radioactive wastes, microbial activities may likely, in a long-term perspective threaten the integrity of cement-solidified wastes. To investigate the detrimental impact of heterotrophic microorganisms on cement matrices, Aspergillus niger reputed as versatile and prevalent fungus in soil flora was selected as candidate. It was shown that this fungus has the potential of severely degrading ordinary Portland cement pastes through organic acids production. Cement pastes experienced chemical alterations such as substantial leaching of calcium, and mechanical degradation was evident as highlighted by the drastic decline in Young’s modulus. Their poor behaviour with respect to heterotrophic biodeterioration and susceptibility to failure were therefore demonstrated. Consequently, biolixiviation scenario should be seriously considered in order to ensure safe long-term disposal for cement-solidified wastes

Quantum modeling, beyond secularity, of the collisional dissipation of molecular alignment using the energy-corrected sudden approximation

International audienceWe propose a Markovian quantum model for the time dependence of the pressure-induced decoherence of rotational wave packets of gas-phase molecules beyond the secular approximation. It is based on a collisional relaxation matrix constructed using the energy-corrected sudden approximation, which improves the previously proposed infinite order sudden one by taking the molecule rotation during collisions into account. The model is tested by comparisons with time-domain measurements of the pressure-induced decays of molecular-axis alignment features (revivals and echoes) for HCl and CO2 gases, pure and diluted in He. For the Markovian systems HCl-He and CO2-He, the comparisons between computed and measured data demonstrate the robustness of our approach, even when the secular approximation largely breaks down. In contrast, significant differences are obtained in the cases of pure HCl and CO2, for which the model underestimates the decay rate of the alignment at short times. This result is attributed to the non-Markovianity of HCl-HCl and CO2-CO2 interactions and the important contribution of those collisions that are ongoing at the time when the system is excited by the aligning laser pulse

Effects of dietary electrolyte balance and calcium supply on mineral and acid−base status of piglets fed a diversified diet

International audienceDietary electrolyte balance (dEB) is known to affect acid−base status and mineral metabolism, but is rarely considered in diet formulation for pigs. Yet, the use of a wide variety of local feedstuffs in Europe contributes to lowering the dEB and increasing the fibre content. Hence, mineral requirements may be modified and skeletal health affected. Therefore, the effects of a lower dEB and a higher dietary Ca level on acid−base balance and mineral status were assessed in young pigs fed a diversified diet. A total of twenty-four weaned pigs were fed a control moderate-dEB diet (C) or a diversified moderate-dEB (D), low-dEB (D-A) or low-dEB supplemented with Ca (D-CA) diet. Growth performance, venous blood gas and chemistry, urine pH, mineral balance and femur characteristics were determined. With an equivalent dEB compared with the C diet, the D diet caused an acidification of the urine and increased the excretion of P as a result of a higher dietary content of S. Low-grade metabolic acidosis occurred in piglets fed the D-A diet with changes at systemic and urine levels. A higher excretion of ammonia and P in urine was observed and some bone characteristics tended to be negatively affected. Ca supplementation partially counteracted the effects of low-grade acidosis. Urine excretion of P and ammonia was alleviated and bone characteristics improved. In conclusion, a higher Ca supply must be considered in more diversified diets to counteract the risk of evolving towards low-grade metabolic acidosis which can negatively affect bone