Υδροδυναμικά φορτία σε υποθαλάσσιους αγωγούς μεγάλου βάθους

Μυλωνάς, Αντώνιος; Mylonas, Antonios

dc.contributor.author	Μυλωνάς, Αντώνιος	el
dc.contributor.author	Mylonas, Antonios	en
dc.date.accessioned	2024-03-28T10:21:40Z
dc.date.available	2024-03-28T10:21:40Z
dc.identifier.uri	https://dspace.lib.ntua.gr/xmlui/handle/123456789/59065
dc.identifier.uri	http://dx.doi.org/10.26240/heal.ntua.26761
dc.description	Εθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Επιστήμη και Τεχνολογία Υδατικών Πόρων”	el
dc.rights	Αναφορά Δημιουργού-Μη Εμπορική Χρήση-Όχι Παράγωγα Έργα 3.0 Ελλάδα	*
dc.rights.uri	http://creativecommons.org/licenses/by-nc-nd/3.0/gr/	*
dc.subject	Υδροδυναμικά φορτία	el
dc.subject	Υποθαλάσσιοι αγωγοί	el
dc.subject	Χρονοσειρές δεδομένων	el
dc.subject	Ανάλυση ακραίων τιμών	el
dc.subject	Έλεγχοι ευστάθειας	el
dc.subject	Hydrodynamic forces	en
dc.subject	Subsea pipes	en
dc.subject	Data series	en
dc.subject	Extreme value analysis	en
dc.subject	Stability checks	en
dc.title	Υδροδυναμικά φορτία σε υποθαλάσσιους αγωγούς μεγάλου βάθους	el
heal.type	masterThesis
heal.classification	Θαλάσσια υδροδυναμική	el
heal.language	el
heal.access	free
heal.recordProvider	ntua	el
heal.publicationDate	2023-10-31
heal.abstract	Η ανάγκη μεταφοράς καυσίμων, ασφαλέστερα και με μικρότερο μακροπρόθεσμα κόστος από την μεταφορά τους με πλοία, οδήγησε στην ανάπτυξη και εφαρμογή των υποθαλάσσιων αγωγών. Ο σχεδιασμός των υποθαλάσσιων αγωγών αποτελεί ένα απαιτητικό και μακροσκελές αντικείμενο, το οποίο περιλαμβάνει ένα σύνολο ελέγχων καθοριστικής σημασίας, όπως οι έλεγχοι ευστάθειας των αγωγών έναντι άνωσης και ολίσθησης. Η ορθότητα των ελέγχων των υποθαλάσσιων αγωγών προϋποθέτει την ύπαρξη ή την παραγωγή αντιπροσωπευτικών δεδομένων, τα οποία περιγράφουν τα χαρακτηριστικά ροής του νερού στην θέση εγκατάστασης και σχετίζονται με το κυματικό κλίμα της περιοχής και την γεωστροφική κυκλοφορία. Αντικείμενο της παρούσας διπλωματικής αποτελεί η αξιοποίηση της σύγχρονης βάσης δεδομένων του προγράμματος Copernicus, με στόχο τον αντιπροσωπευτικό υπολογισμό των υδροδυναμικών φορτίσεων που ασκούνται σε μια τυπική διατομή υποθαλάσσιου αγωγού μεταφοράς ενέργειας λόγω της συνδυασμένης δράσης των κυματισμών και των ρευμάτων, τον έλεγχο ευστάθειας της διατομής του και την αξιολόγηση της επιρροής των ρευμάτων στα φορτία της κατασκευής, σε σχέση με το βάθος εγκατάστασης και τις τοπικές επικρατούσες συνθήκες. Με σκοπό την αξιολόγηση της επιρροής των ρευμάτων στα ασκούμενα στην κατασκευή φορτία, ο υπολογισμός των φορτίων πραγματοποιείται για ένα κάναβο 48 σημείων με κυμαινόμενα βάθη στην περιοχή της Ανατολικής Μεσογείου, λόγω της στρατηγικής και γεωπολιτικής της σημασίας. Οι παράμετροι που υπεισέρχονται στον υπολογισμό των φορτίων και περιγράφουν τα χαρακτηριστικά της θαλάσσιας κατάστασης αφορούν τις ταχύτητες των ρευμάτων λόγω της γεωστροφικής κυκλοφορίας, των ανέμων και των μακρών κυματισμών, τις ταχύτητες και τις επιταχύνσεις των μορίων του νερού λόγω της δράσης των κυματισμών στη στάθμη σχεδιασμού και την κλιματική αλλαγή. Για τον υπολογισμό των υδροδυναμικών φορτίσεων που αναπτύσσονται στην εξεταζόμενη διάταξη λόγω των προαναφερθέντων παραμέτρων, χρησιμοποιείται στο πλαίσιο της παρούσας μεταπτυχιακής εργασίας, η εξίσωση του Morison σύμφωνα με την οποία, οι εν λόγω φορτίσεις περιλαμβάνουν την δύναμη ώθησης, την οριζόντια και κατακόρυφη δύναμη αδράνειας και την δύναμη υδροδυναμικής άνωσης. Η εξεταζόμενη διάταξη αποτελείται από έναν οριζόντιο αγωγό κυκλικής διατομής, ο οποίος εδράζεται ελεύθερα επάνω στο θαλάσσιο πυθμένα και προορίζεται για την μεταφορά και προστασία καλωδιώσεων ηλεκτροδότησης. Ο υπολογισμός των φορτίων στην εξεταζόμενη διάταξη πραγματοποιείται για δύο περιπτώσεις, την περίπτωση του κενού και την περίπτωση του έμφορτου αγωγού, με σκοπό τον έλεγχο ευστάθειας της διατομής, κατά τη φάση κατασκευής και κατά τη φάση λειτουργίας της. Παράλληλα, έτσι ώστε να είναι δυνατή η αξιολόγηση της επιρροής των ρευμάτων στα φορτία και τους ελέγχους ευστάθειας της διάταξης, οι φορτίσεις υπολογίζονται για δύο υδροδυναμικές καταστάσεις, που διακρίνονται ανάλογα με τον συνυπολογισμό ή όχι του υποθαλάσσιου ρεύματος Σύμφωνα με τα αποτελέσματα των υπολογισμών, η εξέλιξη των υδροδυναμικών φορτίσεων που ασκούνται στην εξεταζόμενη διάταξη ακολουθεί μια πτωτική τάση σε σχέση με την αύξηση του βάθους, η οποία με τη σειρά της διαμορφώνει και την εξέλιξη των ελέγχων ευστάθειας του αγωγού, οι οποίοι προκύπτουν ευνοϊκότεροι, όσο το βάθος εγκατάστασης του αγωγού αυξάνεται. Παράλληλα, μεταβάλλεται και η σχέση κυριαρχίας μεταξύ των κυματισμών και των ρευμάτων επί των φορτίων που αναπτύσσονται στη διάταξη, με την επιρροή των ρευμάτων να γίνεται ολοένα και σημαντικότερη όσο το βάθος αυξάνεται. Η διαμόρφωση των ταχυτήτων των ρευμάτων σε αυξημένα επίπεδα στην στάθμη σχεδιασμού του αγωγού, ακόμα και σε μεγαλύτερα από τα αναμενόμενα βάθη, επηρεάζει σημαντικά την έκβαση των ελέγχων ευστάθειας του αγωγού, οι οποίοι σε ορισμένες περιπτώσεις δεν ικανοποιούνται στο σενάριο συνυπολογισμού των ρευμάτων. Σε κάθε περίπτωση, η σπουδαιότητα της επιρροής των ρευμάτων στις φορτίσεις και τους ελέγχους ευστάθειας του αγωγού εξαρτάται από τις τοπικές συνθήκες, σχετικά με τα χαρακτηριστικά των κυμάτων και των ρευμάτων που επιδρούν στην κατασκευή και ορίζουν την σχέση κυριαρχίας τους επί της ροής.	el
heal.abstract	The need of fuel transportation, safer and with less long-term cost than their transportation through ships, has led to the development and application of subsea pipes, which are used today for a number of applications such as the pumping and disposal of water, the pumping, transport and disposal of fuel and the interconnection of offshore regions with the hinterland by means of cables. The design of a subsea pipeline is a demanding and challenging subject, which includes a set of crucial checks, regarding the stability of the structure. The correctness of the subsea pipeline checks, presupposes the availability of representative data, which describe the water flow characteristics at the installation site and are related to the area’s wave climate and geostrophic circulation. Until the early 2000s, due to the lack of publicly available representative data on the wave climate of the world’s seas, the determination of design values for coastal and marine structures was mainly carried out through the renowned at the time forecast models, such as the JONSWAP energy spectrum of the SMB model, which are often based on incomplete wind data, while their application is bound to specific conditions and describe an idealized sea state. Accordingly, regarding the case of currents, the majority of the scientific community tended to consider that the intensity of currents near the sea bed, especially in the case of deep waters, is negligible, when referring to their influence on an underwater structure. Today, thanks to the development of satellite and terrestrial observation systems, as well as water circulation models, it has become possible to create representative databases on a global scale, such as the European project, Copernicus. The study object of this thesis is the utilization of the modern database of the Copernicus program, in order to representatively determine the hydrodynamic loads exerted on a typical cross-section of a subsea energy transmission pipeline, due to the combined action of waves and currents, the stability control of its cross-section and the evaluation of the influence of currents on the loads of the structure, in relation to the installation depth and local prevailing conditions. In order to evaluate the influence of currents on the loads exerted to the structure, the calculation of the loads is carried out for a set of sites (48-point canvas) with varying depths in the Eastern Mediterranean region, due to its strategic and geopolitical importance. The examined structure consists of a cylindrical pipe, freely set on the surface of the sea bed, constructed by special material, which is used for the housing and protection of electricity cables. In order to check the self-supporting stability of the pipeline, vertically and horizontally, a specific anchoring system is not considered. The parameters that are taken into account, regarding the determination of the loads on the structure and describe the characteristics of the sea state, concern the velocities of currents due to the geostrophic circulation, winds, and long waves, and the velocities and accelerations of water molecules due to the action of waves at the design level and climate change. Design values of the parameters in question are obtained through the utilization of the wind data of the Tymbaki meteorological station of the NMS and the Nicosia meteorological station of the Cyprus Meteorological Department and the data from the products of the Copernicus program, MEDSEA_MULTIYEAR_ WAV_006_012 and MEDSEA_MULTIYEAR_PHY_006_004, which include the data for wave and current characteristics for the periods 1993–2021 and 1987–2021, respectively. The processing of the wave data is carried out corresponding to the philosophy of the Peak Over Threshold (POT) method, where the set of the top twenty, temporally independent, wave phenomena, in relation to the wave height, is selected, in order to form the sample of the extreme data to be used to determine the characteristics of the design wave affecting the construction, at each point of the canvas. The sample of extreme wave data is then fitted to the Weibull distribution, using the least square fitting method, and taking into account the return period chosen for the structure which is set to 60 years, the characteristics of the design wave and by extension the velocities and the accelerations of the water particles due to waves at the reference level of the array are calculated, utilizing the equations of Airy's linear theory. Then, the velocities of the water molecules due to currents are calculated, utilizing the data related to the wind climate and the characteristics of the geostrophic current per set point, according to the physical laws that govern the behavior of the geostrophic circulation, of wind currents and currents developed due to long waves. The final design values of these parameters are formed, taking into account the influence of climate change. In order to determine the hydrodynamic loads that are exerted on the considered pipeline due to the aforementioned parameters, Morison's equation is used in the context of this master's thesis. Morison's equation is an analytical equation for calculating the hydrodynamic loads on cylindrical elements, due to the combined action of waves and currents, according to which, the loads in question include the drag force, the horizontal and vertical inertia force and the hydrodynamic buoyancy force. Due to the sinusoidal shape of the wave, the design loads are determined based on the phases, for which the total horizontal and the total vertical force exerted on the pipeline are maximized, in order to perform the stability checks of the assembly for the worst-case scenario. The pipeline stability checks include the vertical and horizontal stability checks, which are carried out taking into account the weight of the pipeline and the hydrostatic buoyancy force. The considered assembly consists of a horizontal pipeline of circular cross-section, which rests freely on the seabed and is intended for the housing and protection of electricity cables. The calculation of the loads exerted on the structure, is carried out for two scenarios, the scenario of the empty and the scenario of the loaded pipeline, in order to check the stability of the cross-section, during its construction and its operational phase. At the same time, in order to evaluate the influence of the currents on the loads and the stability checks of the structure, the loads are calculated for two hydrodynamic states, distinguished by the inclusion or not of the subsea current. For the purposes of this application, the outer and inner diameters of the pipeline, S and S_0, are set to 400 mm and 350 mm, respectively, while the pipe is chosen to be constructed, for stability reasons, from a special material, resulting to a weight of 150 kg/m for the typical cross-section of the structure. Regarding the proposed cables, their weight, corresponding to a diameter of 350 mm, is estimated at 50 kg/m. As part of the research of this master's thesis, certain assumptions are also made regarding the position of the pipeline and the morphology of the sea bed, so that the calculation of the loads for the worst-case scenario can be carried out. Regarding the position of the pipeline, it is considered to be placed directly on the seabed, without excavations and without any specific anchoring system, so that it is possible to check the self-supporting balance of the pipeline. In addition, the pipeline is assumed to be in constant contact with the seabed for the entire length for which the loads are determined, at every possible location. Finally, regarding the morphology of the bottom, the assumption of a rocky and flat sea bed is made, so that the horizontal stability of the pipeline is not affected by the existence of protrusions or depressions in the surface of the bottom. Due to the number of points of the research canvas of the present master's thesis and the strong diversity that characterizes them, in relation to the maximum depth of the sea bed, the set points are grouped into depth zones, which represent the general tendency of the shaping loads at the points they include. These depth zones include the shallow-water zone with a depth of 0 to 20m, the intermediate-water zone with a depth of 20 to 100m, the deep-water zone with a depth of 100 to 300m and the very deep-water zone with a depth of more than 300m. The calculation of the hydrodynamic loads exerted on the considered pipeline, utilizing the data of the Copernicus project database, leads to useful and important conclusions, regarding the real picture of the marine loading environment that affects the structure and the formation of the resulting loads as a function of the pipelines installation depth. Both the formation of the loads, as well as the influence of the currents on the forces exerted on the pipeline, are evaluated in relation to the depth zones for which the determination of the loads and the stability checks of the pipeline were carried out. In the shallow-water zone, the combination of the increased velocities of water particles due to waves and currents at the design level of the pipeline, contribute to the development of significant hydrodynamic loads, resulting in the general instability of the structure, both in the scenarios of the empty and the loaded pipeline. According to the local wave conditions, and due to the relatively shallow water depth, the velocities of the molecules due to waves are significantly higher, in relation to the corresponding velocities due to currents, thus shaping the dominance relationship between these actions. In the case of intermediate waters, the hydrodynamic forces exerted on the structure are reduced, resulting in more favorable stability checks for the considered pipeline. The diminishing trend of the loads is to some extent expected, since the velocities of the water molecules due to waves decrease, as the distance from the free surface of the sea grows larger. However, the increased velocities of the currents on the reference level of the pipe, in this depth zone, could not be considered expected. This fact, combined with the reduction of the velocities of the water molecules due to waves, changes the dominance relationship of these actions, highlighting the currents as the dominant load source. Consequently, during the construction phase of the pipeline, the stability checks of the assembly are not satisfied, resulting in the need of external anchoring. Conversely, during the pipeline operational phase, the beneficial effect of the overall weight increase of the assembly allows it to stabilize vertically, but not horizontally. In the deep-water zone, the set of hydrodynamic loads on the structure form at milder levels, compared to the intermediate waters, a fact strongly related to the further increase in depth. The low order of magnitude of the calculated wave velocities of the water particles makes the currents, the main source of hydrodynamic loads exerted on the pipeline in the zone in question. The formation of current velocities at higher than expected levels, significantly affects the stability of the pipeline, which is not achieved horizontally, during the construction phase of the pipeline. On the contrary, and due to the combined selection of the assembly material and the housed load, the vertical balance of the arrangement is achieved, regardless of the loading phases and scenarios. Finally, in the very deep-water zone, the downward trend of the formation of hydrodynamic loads leads to the development of even milder loads on the considered structure, resulting in the achievement of its self-supporting stability for each loading scenario. Due to the particularly low velocities of water molecules due to waves, which are formed in this zone, the currents emerge as practically the only source of hydrodynamic forces of the pipeline, reflecting their importance. Despite this fact, their contribution is not enough to affect the outcome of the pipeline's stability checks, which are satisfied, both during the construction and the operational phase of the pipe. In any case, the importance of the influence of currents on the hydrodynamic loads exerted on the pipeline and its stability checks, depends on local conditions regarding the characteristics of waves and currents acting on the structure and defining their dominance relationship over the flow.	en
heal.advisorName	Μέμος, Κωνσταντίνος	el
heal.committeeMemberName	Μέμος, Κωνσταντίνος	el
heal.committeeMemberName	Τσουκαλά, Βασιλική	el
heal.committeeMemberName	Μαλαμής, Συμεών	el
heal.academicPublisher	Εθνικό Μετσόβιο Πολυτεχνείο. Σχολή Πολιτικών Μηχανικών	el
heal.academicPublisherID	ntua
heal.numberOfPages	246 σ.	el
heal.fullTextAvailability	false