Σεισμική φόρτιση πασσάλων αγκύρωσης πλωτών κατασκευών υπό καθεστώς ρευστοποίησης

Βασιλείου, Λουκάς; Βασιλόπουλος, Νέστωρ; Vasilopoulos, Nestor; Vasileiou, Loukas

dc.contributor.author	Βασιλείου, Λουκάς	el
dc.contributor.author	Βασιλόπουλος, Νέστωρ	el
dc.contributor.author	Vasilopoulos, Nestor	en
dc.contributor.author	Vasileiou, Loukas	en
dc.date.accessioned	2020-11-09T21:27:27Z
dc.date.available	2020-11-09T21:27:27Z
dc.identifier.uri	https://dspace.lib.ntua.gr/xmlui/handle/123456789/51849
dc.identifier.uri	http://dx.doi.org/10.26240/heal.ntua.19547
dc.rights	Default License
dc.subject	Πλωτή ανεμογεννήτρια	el
dc.subject	Πάσσαλος θεμελίωσης	el
dc.subject	Ρευστοποίηση	el
dc.subject	Εξόλκευση πασσάλου	el
dc.subject	Απώλεια προέντασης	el
dc.subject	TLP	en
dc.subject	Pile foundation	en
dc.subject	Liquefaction	en
dc.subject	Pile pullout	en
dc.subject	Pretension loss	en
dc.title	Σεισμική φόρτιση πασσάλων αγκύρωσης πλωτών κατασκευών υπό καθεστώς ρευστοποίησης	el
dc.title	Seismic loading on anchor piles of offshore floating structures under liquefiable conditions	en
heal.type	bachelorThesis
heal.classification	Γεωτεχνική μηχανική	el
heal.classification	Geotechnical engineering	en
heal.language	el
heal.access	free
heal.recordProvider	ntua	el
heal.publicationDate	2020-03-09
heal.abstract	Extended Summary I. Thesis Outline The scope of the present Diploma Thesis is to examine the behavior of a single foundation pile used to anchor offshore floating structures under the combined effect of pretension and seismic loading that induces liquefaction. The research project was based on the previous work by Bouckovalas et al. (2015) called ARISTEIA-POSEIDON/2041 which deals with the foundation pile of an offshore wind turbine anchored to the seabed as a Tension Leg Platform (TLP). The pile in question has a length of L = 55 m and a diameter of D = 2.5 m. Initially a baseline analysis was performed in order to investigate the static response of the foundation pile, as well as, the response of the pile-soil system under seismic loading with emphasis given to the influence of liquefaction. Whereupon, the impact of the seismic load (Frequency, number of cycles, max acceleration) and the soil conditions (Relative Density, Permeability) on the pullout displacement of the pile head were evaluated. To that purpose, a series of parametric numerical analyses was performed, using the baseline analysis as a reference point. Furthermore, an attempt was made to extend the results of the parametric analyses to actual excitations through “complex” seismic parameters (e.g. Cumulative Absolute Velocity, Arias Intensity). The objective of this parametric investigation was to better understand the “governing mechanisms” for the vertical movement (pull out) of the pile due to the combined action of the cyclic seismic load and the pretension of the steel tendon. Finally, the effect of the pullout displacement on the pretension of the steel tendon and the buoyancy stability of the floater were evaluated. For this purpose, an analytical solution was developed that takes into account the interaction between the pullout displacement of the pile head, the reduction of the pretension load of the tendon and the buoyancy applied to the floater. II. Conclusions Baseline Analysis (a) The tensile capacity of the foundation pile under static load, as it is calculated through the numerical analysis is in good agreement with the expected value from analytical calculations. (b) Soon after the application of seismic shaking, the soil along most of the pile is fully liquefied. As a result, the soil presents reduced shear strength and the pile presents reduced tensile capacity. Consequently, due to the steady pretension force applied at its head, the pile shifts vertically upwards. At the end of shaking (i.e. after about 10 sec) the computed pullout of the pile head was approximately 28 cm and continued to increase (more than doubled) during the subsequent excess pore pressure dissipation. (c) The seismic excitation that is applied at the base of the model results in complete liquefaction of the subsoil, with the excess pore pressure ratio (ru) becoming approximately equal to 1.0. The only exception to this, are two regions located around the outer edge of the pile where ru < 0.7. The first region is located at the pile head, and reaches a depth of around 12m (or 4.8 pile diameters), and the second one is at the base of the pile. It is speculated that the reduction of the excess pore pressure ratio in these regions is the combined result of soil dilation due to soil-pile interaction and incoming water flow from the far-field towards the pile perimeter. (d) In areas where liquefaction is extensive, the ground motion accelerations are considerably reduced, eventually reaching values close to zero. On the other hand, in areas where the soil maintains relatively low excess pore pressure ratio (ru = 0 – 0.6) the ground motion acceleration are amplified. Parametric Investigation of Pile Pullout (e) As expected, the pile pullout increases as: • The maximum acceleration (amax), the number of cycles (N) or the period (Texc) of the excitation increase • The value of various “combined” seismic intensity parameters (e.g. CAV, AI) increase. • The Relative Density (Dr) of the soil decreases. • The Static Safety Factor (FS) for pullout failure of the pile decreases. (f) The effect of soil permeability (k) does not appear to be univocal. Initially, the pile pullout increases as the soil permeability (k) increases, until it reaches a critical value. After that, pullout displacements decrease rapidly with any further increase of soil permeability. For the pile in question, the critical value of the permeability coefficient corresponds to coarse sands and fine gravels. (g) The impact of the intensity of the seismic excitation can be univocally expressed through “combined” seismic intensity parameters that take into consideration the applied acceleration/velocity, as well as, the duration and the frequency of the seismic excitation. (h) Among the “complex” parameters that were taken into consideration, the most promising for the unification of the various excitation characteristics and the resulting extension to real excitations seems to be the Cumulative Absolute Velocity (CAV). (i) The intensity of the seismic excitation needs to exceed a lower boundary (trigger level) in order to induce extensive soil liquefaction around the pile and thus lead to pullout. (j) The maximum pullout value that was measured during the parametric analyses was δ = 2.25m [for the critical permeability coefficient discussed in (f) above]. Effect of the Pile Pullout on Floater Stability (k) In simple terms, the pile-steel tendon-floater system can be simulated by two linear springs connected in series: (i) one for the steel tendon with stiffness kst and (ii) another that expresses the resistance to submersion of the floater due to buoyancy, with a stiffness kπλ. As a result, the equivalent stiffness of the system is calculated as: For the platform in question, the equivalent stiffness is keq = 973.6 kN/m. (l) The equivalent stiffness is always smaller than each of the corresponding stiffnesses of the steel tendon (kst) and the floater (kπλ). Therefore, a crude estimation of the expected reduction of the pretension force can be conservatively achieved by assuming that the pullout translates directly to upwards movement of the floater. In that case, the reduction of the pretension force is calculated as: with a relatively small error margin for common platforms. (m) The expected pullout value, as it was calculated in the baseline analysis (δ = 28 – 56 cm), does not pose a threat to the stability of this particular platform, keeping in mind that, in order for the Stability Safety Factor to be reduced to a value of FSlim=1.5 a pullout of more than 6 meters is required. In any case, a corrective reloading of about ΔF = 270 - 545 kN should be made in order to restore the initial operating conditions. (n) However, the impact of the pullout on the pretension force can increase significantly since the parametric analyses showed that, for specific soil conditions, the vertical movement of the pile can reach a value of up to δ = 2.25 – 2.68 meters. In that case, the reduction of the pretension force is calculated at ΔF = 2190 – 2610 kN and the corresponding Stability Safety Factor is reduced from a value of FSo = 2 to a value of FS = 1.9. (o) The numerical analysis using the software of Finite Differences FLAC3D v5. as it was performed in this particular Thesis, results in an adequate estimation of the pullout and the resulting loss of pretension force when the value of the pullout is relatively small. On the other hand, when the pullout value is relatively high (e.g. δ > 2 m) taking into consideration the reduction of the pretension force as the phenomenon progresses becomes crucial and the iterative procedure that is described in Chapter 6 should be used. For the pile in question and for the worst-case scenario the iterative procedure led to a reduction of the expected pullout value of about 44% (≈1.2 m). III. Proposals for Future Research In the process of compiling the present Thesis certain questions arose that require further consideration. The aforementioned issues are presented below and constitute proposals for future research. (a) As a primary research proposal, it is suggested that the post-shaking tensile capacity of the pile is confirmed. Due to the dynamic nature of the seismic load and the resulting pullout of the pile, as well as due to the redistribution of the geostatic stresses in the soil surrounding the pile, it is expected that the pile will lose a portion of its tensile capacity. Although the post-shaking tensile capacity can be calculated through analytical equations, it is vital that it should be also verified through experiments or numerical analyses since it is directly connected to the stability and regular operation of the structure. (i) Further investigation is required to evaluate the impact of the soil profile and the pile geometry on liquefaction-induced pullout. Specifically, relative to the soil profile, it is expected that any stratigraphy other than a uniform one will have considerable influence on the behavior of the soil itself as well as its interaction with the foundation. At the same time, it is obvious that any change of the pile geometry will result in different behavior for the foundation. It is worth mentioning that, the pile diameter is of utmost importance when it comes to flow issues, like the ones investigated in this particular Thesis, and caution should be exercised when selecting it. (ii) Additional research is required to accurately assess the long-term pullout value of the pile following the time required for excess pore pressures to fully dissipate. Especially, in the case of lower permeability values (fine sands), where the time required for the pore pressures to stabilize is greater, the additional movement of the pile can result in a total pullout value that is more than double the value that was calculated at the end of shaking. In contrast, for higher permeability values (gravel, coarse sands) the drainage is over relatively quickly and the additional pile movement is a small percentage of the value calculated at the end of shaking. (iii) Finally, it has been shown (in Chapter 6) that the pile-steel tendon-floater system can be simulated with two springs in series (one for the tendon and the other for the floater, applied at the pile head. Such an arrangement can be relatively easily simulated in various numerical codes, including FLAC3D. The construction of such a model will eliminate the need for iterative computation of the pullout displacements, thereby allowing the direct estimation of the final pile pullout and the corresponding reduction in the pretension of the tendon and the buoyancy applied to the floater.	en
heal.advisorName	Μπουκοβάλας, Γεώργιος	el
heal.advisorName	Bouckovalas, George	en
heal.committeeMemberName	Παπαδημητρίου, Αχιλλέας	el
heal.committeeMemberName	Γερόλυμος, Νικόλαος	el
heal.committeeMemberName	Papadimitriou, Achilleas	en
heal.committeeMemberName	Gerolimos, Nikos	en
heal.academicPublisher	Εθνικό Μετσόβιο Πολυτεχνείο. Σχολή Πολιτικών Μηχανικών. Τομέας Γεωτεχνικής	el
heal.academicPublisherID	ntua
heal.numberOfPages	171 σ.	el
heal.fullTextAvailability	false