Experimental and Numerical Evaluation of the behaviour of laterally-loaded non slender piles
Authors
Borobia Moreno, Alejandro ; Troya Díaz , Jose Luis ; Mikalauskas, Linas
Term
4. term
Education
Publication year
2011
Submitted on
2011-06-17
Pages
265
Abstract
Havvindmøller er en konkurrencedygtig kilde til vedvarende energi. Udbygningen går hurtigt, og der er behov for klare regler og metoder til at designe vindmøllefundamenter. Der findes flere fundamenttyper (gravitation, monopæl, spand, tripod m.fl.). For vanddybder på 15-30 m anvendes ofte monopæle, og dette arbejde fokuserer derfor på monopælfundamenter. Formålet er at forstå og undersøge usikkerheder i den nuværende designmetode for monopæle i sand. Der er udført småskalaforsøg i et trykkar for at begrænse skalaforsøgseffekter. Monopælen var udstyret med tøjningsmålere, der måler deformationer på elleve niveauer langs den indlejrede længde. Siden 2009 er der gennemført 22 forsøg med varierende slankhedsforhold og trykniveauer, hvoraf 10 er udført af Borobia, Mikalauskas og Troya i år. De samme forsøg er modelleret numerisk i 3D med FLAC3D og PLAXIS 3D 2010, inklusive en fuldskalamodel. En dimensionsløs analyse anvendes til at undersøge en jordeksponent og udlede en generel ligning, der knytter lateral last til lateral forskydning af pælen. Den anvendte p-y-kurvemetode (som forbinder jordmodstand p med sideforskydning y) blev udviklet til slanke, lateralt belastede pæle i offshore olieindustrien, ofte i bløde lerer og senere i sand. Vindmøllemonopæle har derimod større diameter og kortere indlejret længde, med slankhedsforhold under 10, så metoden passer ikke altid godt til dem. En yderligere usikkerhed er, at forskellige diametre og den initiale stivhed ikke medtages, fordi design typisk fokuserer på brudgrænsetilstanden og ser bort fra indledende jorddeformationer. I anvendelsesgrænsetilstanden er den initiale stivhed derimod vigtig, da for stor rotation over API-grænser reducerer møllernes effektivitet. Studiet sammenligner forsøg og numeriske modeller for at identificere usikkerheder på tværs af metoder og understøtter en dimensionsløs beskrivelse af forholdet mellem lateral last og forskydning i design af monopæle.
Offshore wind turbines are a competitive source of renewable energy. Development is moving fast, creating a need for clear rules and practical methods to design turbine foundations. Several foundation types exist (gravity, monopile, bucket, tripod, etc.). For water depths of 15-30 m, monopiles are common, so this thesis focuses on monopile foundations. The goal is to understand and investigate uncertainties in the current design method for monopiles in sand. Small-scale tests were carried out in a pressure tank to limit scale effects. The monopile was instrumented with strain gauges that measure deformation at eleven levels along its embedded length. Since 2009, 22 tests with different slenderness ratios and pressure levels have been conducted, 10 of them by Borobia, Mikalauskas and Troya this year. The same tests were modeled numerically in 3D using FLAC3D and PLAXIS 3D 2010, including a full-scale model. A dimensionless analysis is used to study a soil exponent and to derive a general equation that relates the lateral load to the lateral displacement of the pile. The design method used is the p-y curve method (which links soil resistance p to lateral displacement y), originally developed for slender, laterally loaded piles in the offshore oil industry, often in soft clays and later extended to sands. Wind turbine monopiles differ: they have larger diameters and shorter embedded lengths, with slenderness ratios below 10, so the method does not always fit well. Another uncertainty is that pile diameter effects and initial stiffness are not included because designs tend to focus on the ultimate limit state and ignore initial soil deformations. In the serviceability limit state, however, initial stiffness is important, as excessive rotation beyond API limits reduces turbine efficiency. This study compares experiments and numerical models to identify uncertainties across methods and supports a dimensionless description of the load-displacement relationship for monopile design.
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