Pipelines are widely used to transport of fluids, such as water, oil and natural gas, both onshore and offshore. Offshore pipelines are simply laid on the bottom of the sea, whereas onshore pipes are installed at the bottom of a trench and in order to limit costs fully covered with the previously removed soil. If pipelines are embedded into granular liquefiable soils, they may develop unusual movements. Indeed, if fully saturated granular materials are subjected to cyclic loads such as earthquakes or ocean tides, they could find themselves in a condition for which they are uncapable of dissipating excess pore water pressure and consequently effective stresses decrease. This leads to the nullification of their shear resistance: as a consequence, they start behaving as viscous fluids. Usually, the unit weight of pipes is less than the unit weight of the soil, which means that the pipeline lifts from the bottom of the trench and floats until the ground surface is reached: this phenomenon may determine non negligible structural damages, not to mention all its possible catastrophic consequences. In order to evaluate the interaction between the pipeline and the soil, experimental tests have been performed with the aim of determining the main factors influencing the mechanical response of the structure. Experiments have been performed by both changing the diameter of the tube and the hydraulic boundary conditions imposed to the experimental apparatus. This is made by a box filled with sand, inside of which the pipeline was installed. Liquefaction has been triggered into the system, by imposing a difference in the hydraulic gradient between the ground surface and the bottom of the sandy layer in the box, so that a vertical flow was generated. As a consequence, a seepage force develops, progressively reducing the effective stresses due to the self-weight of the soil, until their nullification. Furthermore, a possible mitigation strategy has been taken into consideration, in order to limit the uplift of the tube, so as to reduce the risk associated to their damage. At the same time, numerical finite elements analyses have been performed, by using the Material Point Method. To this aim, the software Anura 3D has been used to simulate the uplift of the pipeline and to gain information about the displacements and the pressures developing in the system during the process. Numerical simulations have been performed by employing two different constitutive models for the soil: at first the sandy layer has been considered fully liquefied, therefore the soil has been assimilated to a Newtonian fluid; subsequently a condition of partial development of liquefaction has been taken into account, and the soil has been represented through the constitutive model of Binghamian fluid, in order to partially reintroduce the contribution of the force chains developing at the contacts among grains. On the basis of the results obtained from the numerical simulations, an analytical model has been introduced, in order to predict the temporal evolution of the pipeline displacements, on the basis of the dynamic equilibrium of the forces acting on the system. Each contribution to the equation has been calibrated according to the numerical results. Indeed, throughout numerical simulations it has been possible to obtain information about the development of resisting forces and to introduce, under precise hypotheses, simplified analytical expressions to evaluate them. Finally, the analytical model has been compared with the experimental evidence, in order to check its limits and to verify its validity.
Le condotte sono ampiamente utilizzate, sia in ambiente onshore che offshore, per il trasporto di fluidi sia allo stato liquido (acqua e petrolio) che gassoso (gas naturale). Mentre nel caso offshore le tubature vengono semplicemente posate sul fondale marino, nel caso onshore il processo di installazione si effettua realizzando una trincea all’interno del terreno, sul cui fondo si posa la condotta; successivamente, per limitare i costi, lo scavo viene ricoperto dello stesso materiale precedentemente rimosso. Qualora tali condotte siano immerse in terreni granulari, questi ultimi possono essere suscettibili al fenomeno della liquefazione. Infatti, sotto l’effetto di carichi ciclici, quali maree, terremoti, correnti ecc, i terreni sabbiosi in condizioni di totale saturazione, nel caso in cui non riescano a dissipare la pressione in eccesso, perdono la resistenza al taglio che li caratterizza e iniziano a comportarsi come fluidi viscosi. Dal momento che, in generale, il peso specifico della condotta è inferiore rispetto al peso saturo del terreno, il tubo, si solleva dal fondo della trincea, fino a risalire progressivamente verso la superficie: tale fenomeno risulta essere drammatico, visti i danni che potrebbe provocare alla condotta dal punto di vista strutturale e tutte le possibili problematiche ad essi associati. Al fine di valutare l’interazione tra il tubo e il terreno, sono state eseguite delle prove sperimentali, con l’obiettivo di determinare i principali fattori che influenzano la risposta meccanica della condotta. Le prove sono state effettuate variando sia il diametro del tubo che le condizioni idrauliche imposte al sistema, costituito da una scatola contenente terreno sabbioso, all’interno del quale viene interrata una tubazione. La liquefazione è stata indotta applicando una differenza di carico idraulico tra il piano campagna e la superficie inferiore dello strato sabbioso, in modo da generare un moto di filtrazione verticale dal basso verso l’alto. La diretta conseguenza è lo sviluppo di una forza di filtrazione che riduce progressivamente la componente efficace di sforzo dovuta al peso proprio del terreno, fino al suo completo annullamento. Si è inoltre considerata una possibile strategia di mitigazione per il fenomeno di sollevamento, che consenta di ridurre il potenziale rischio associato al danneggiamento delle condotte. Parallelamente alle prove sperimentali, sono state eseguite delle analisi numeriche agli elementi finiti, adottando il Metodo del Punto Materiale. A tal fine si è utilizzato il software Anura3D, in modo da simulare il processo di risalita del tubo e ottenere informazioni sull’andamento degli spostamenti e delle pressioni che si innescano nel sistema. Le analisi sono state effettuate considerando due possibili comportamenti per il terreno, all’innesco del fenomeno della liquefazione: inizialmente il comportamento dello strato sabbioso, ipotizzato completamente e idealmente liquefatto, è stato assimilato a quello di un Fluido Newtoniano. Successivamente si è considerata una condizione di “liquefazione parziale” in cui le catene di sforzo sono ancora parzialmente presenti e il comportamento del terreno è stato associato a quello di un fluido Binghamiano. A partire dai risultati numerici, si è infine introdotto un modello analitico in grado di prevedere l’evoluzione temporale degli spostamenti del tubo, sulla base dell’equilibrio dinamico di tutte le forze agenti all’interno del sistema. Ciascun contributo è stato valutato in riferimento ai risultati ricavati dalle analisi numeriche. Queste, infatti, hanno consentito di ottenere informazioni sull’andamento delle forze resistenti e di introdurre, sotto determinate ipotesi semplificative, delle espressioni analitiche per il calcolo di queste forze. Il modello è stato poi confrontato con i risultati sperimentali per valutarne i limiti e per verificarne la validità.
Analisi sperimentale, numerica e teorica del comportamento di condotte interrate in sabbia soggetta a liquefazione
LISCO, ARIANNA;LODO, FEDERICA
2020/2021
Abstract
Pipelines are widely used to transport of fluids, such as water, oil and natural gas, both onshore and offshore. Offshore pipelines are simply laid on the bottom of the sea, whereas onshore pipes are installed at the bottom of a trench and in order to limit costs fully covered with the previously removed soil. If pipelines are embedded into granular liquefiable soils, they may develop unusual movements. Indeed, if fully saturated granular materials are subjected to cyclic loads such as earthquakes or ocean tides, they could find themselves in a condition for which they are uncapable of dissipating excess pore water pressure and consequently effective stresses decrease. This leads to the nullification of their shear resistance: as a consequence, they start behaving as viscous fluids. Usually, the unit weight of pipes is less than the unit weight of the soil, which means that the pipeline lifts from the bottom of the trench and floats until the ground surface is reached: this phenomenon may determine non negligible structural damages, not to mention all its possible catastrophic consequences. In order to evaluate the interaction between the pipeline and the soil, experimental tests have been performed with the aim of determining the main factors influencing the mechanical response of the structure. Experiments have been performed by both changing the diameter of the tube and the hydraulic boundary conditions imposed to the experimental apparatus. This is made by a box filled with sand, inside of which the pipeline was installed. Liquefaction has been triggered into the system, by imposing a difference in the hydraulic gradient between the ground surface and the bottom of the sandy layer in the box, so that a vertical flow was generated. As a consequence, a seepage force develops, progressively reducing the effective stresses due to the self-weight of the soil, until their nullification. Furthermore, a possible mitigation strategy has been taken into consideration, in order to limit the uplift of the tube, so as to reduce the risk associated to their damage. At the same time, numerical finite elements analyses have been performed, by using the Material Point Method. To this aim, the software Anura 3D has been used to simulate the uplift of the pipeline and to gain information about the displacements and the pressures developing in the system during the process. Numerical simulations have been performed by employing two different constitutive models for the soil: at first the sandy layer has been considered fully liquefied, therefore the soil has been assimilated to a Newtonian fluid; subsequently a condition of partial development of liquefaction has been taken into account, and the soil has been represented through the constitutive model of Binghamian fluid, in order to partially reintroduce the contribution of the force chains developing at the contacts among grains. On the basis of the results obtained from the numerical simulations, an analytical model has been introduced, in order to predict the temporal evolution of the pipeline displacements, on the basis of the dynamic equilibrium of the forces acting on the system. Each contribution to the equation has been calibrated according to the numerical results. Indeed, throughout numerical simulations it has been possible to obtain information about the development of resisting forces and to introduce, under precise hypotheses, simplified analytical expressions to evaluate them. Finally, the analytical model has been compared with the experimental evidence, in order to check its limits and to verify its validity.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/175230