Investigation of the Behaviour of Laterally Loaded Monopiles in Cohesionless Soil
Authors
Wolf, Torben Kirk ; Rasmussen, Kristian Lange ; Hansen, Mette
Term
4. term
Education
Publication year
2012
Submitted on
2012-06-11
Pages
64
Abstract
Efterspørgslen på vedvarende energi har ført til flere og større offshore vindmøller, der ofte placeres længere fra kysten i dybere vand (typisk 15–30 m). Den mest udbredte fundering er monopæle med indlejrede længder omkring 20–30 m og diametre på 4–6 m. Nuværende designvejledninger (DNV 2010, API 2007) for sidebelastede pæle bygger på p-y kurver, som stammer fra få forsøg på slanke, fleksible pæle fra 1970’erne. Disse kurver anvender kun få jord- og pæleparametre og antager en slank pæleadfærd. Dagens monopæle har derimod et slankhedsforhold under 10 og reagerer mere stift, hvilket ikke afspejles i vejledningerne. Samtidig behandles langtidscyklisk sidebelastning kun begrænset, selv om den kan ændre stivheden i jord-pæl-systemet og give hældning af turbinen. I specialet udføres en 3D finit-element-analyse (FEA) for at genvurdere p-y kurver for lateralt belastede monopæle. Analysen viser, at valget af materialemodel har stor indflydelse på den beregnede stivhed. P-y kurver udledt ved henholdsvis pålagt forskydning og pålagt last stemmer ikke tydeligt overens. Sammenlignet med de konventionelle formuleringer giver FEA-baserede p-y kurver en blødere respons, mens standardkurverne er betydeligt stivere. For at belyse effekten af cyklisk sidebelastning gennemføres et lille laboratorieforsøg med en 100 mm bred pæl (slankhedsforhold 6) i mættet sand. Den cykliske last sættes til 35 % af den laterale bæreevne, defineret ved en rotation på 3. Kraft og forskydning måles for at bestemme rotationen. Forsøget viser aftagende forskydningsspring med stigende antal lastcyklusser, men der opnås ikke stabilisering. Et litteraturstudie gennemgår teorier om stivhedsdegradering (Long & Vanneste, 1994; Lin & Liao, 1999) og nyere forsøg (Peng m.fl., 2006; Peralta & Achmus, 2010; LeBlanc m.fl., 2010; Roesen m.fl., 2011). De målte resultater stemmer med nyere forsøg i, at rotationen fortsætter med at vokse med antallet af cyklusser; i modsætning hertil rapporterer Roesen m.fl. (2011) stabilisering efter 15.000 cyklusser. Samlet set peger resultaterne på, at gældende p-y designkurver kan overvurdere stivheden for moderne monopæle, og at langtidscyklisk sidebelastning kan medføre vedvarende rotation, som bør håndteres i designet.
The push for renewable energy has led to more and larger offshore wind turbines, often placed farther from shore in deeper waters (typically 15–30 m). The most common foundations are monopiles with embedded lengths of about 20–30 m and diameters of 4–6 m. Current design guidelines (DNV 2010, API 2007) for laterally loaded piles rely on p-y curves derived from a small number of tests on slender, flexible piles from the 1970s. These curves use only a few soil and pile parameters and assume slender-pile behavior. Modern monopiles, however, have a slenderness ratio below 10 and respond more rigidly, which is not captured by the guidelines. The guidelines also give limited attention to long-term cyclic lateral loading, even though it can change the stiffness of the soil–pile system and cause turbine tilt. This thesis uses 3D finite element analysis (FEA) to reassess p-y curves for laterally loaded monopiles. The results show that the chosen soil material model strongly influences the calculated stiffness. P-y curves obtained under displacement-controlled and load-controlled evaluations do not clearly agree. Compared with conventional formulations, the FEA-based p-y curves are softer, while the standard curves predict a much stiffer response. To study cyclic effects, a small-scale laboratory test was performed with a 100 mm wide pile (slenderness ratio 6) in saturated sand. The cyclic load was set to 35% of the lateral bearing capacity defined at a rotation of 3. Force and displacement were measured to evaluate rotation. The test shows decreasing displacement increments as the number of load cycles increases, but no stabilization was reached. A literature review covers theories of stiffness degradation (Long & Vanneste, 1994; Lin & Liao, 1999) and recent experiments (Peng et al., 2006; Peralta & Achmus, 2010; LeBlanc et al., 2010; Roesen et al., 2011). The measured results agree with recent experiments in that pile rotation keeps increasing with more cycles; in contrast, Roesen et al. (2011) reported stabilization after 15,000 cycles. Overall, the findings indicate that current p-y design curves may overestimate stiffness for modern monopiles and that long-term cyclic lateral loading can lead to ongoing rotation that should be addressed in design.
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