Finite Element Analysis of Suction Bucket Foundations in Sand Subjected to Cyclic Loading
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Finite Element Analysis of Suction Bucket Foundations in Sand Subjected to Cyclic Loading Ingerid Elisabeth Rolstad Jahren Civil and Environmental Engineering Submission date: June 2018 Supervisor: Hans Petter Jostad, IBM Norwegian University of Science and Technology Department of Civil and Environmental Engineering i Preface This is a master thesis written in the spring of 2018 as the final part of my M.Sc. degree in Civil and Environmental Engineering at the Norwegian University of Science and Technology (NTNU) in Trondheim. The thesis is a part of the master’s programme in Geotechnical Engi- neering at the department of Civil and Environmental Engineering. The thesis has been carried out in a cooperation with the Norwegian Geotechnical Institute (NGI), which also proposed the thesis. Trondheim, 2018-06-10 Ingerid Rolstad Jahren iii Acknowledgement I would like to express my gratitude to my academic supervisor Adjunct Prof. Hans Petter Jostad, NGI, who provided great insight and expertise throughout the thesis work. His interest and enthusiasm for the topic in addition to great knowledge is truly inspiring. I also thank members of the staff at the Geotechnical Division at NTNU for kindly sharing their wisdom during my years as a student at NTNU. Finally, thank you to all my fellow students for valuable discussions and support. I.R.J. v Abstract A suction bucket or suction caisson is a foundation concept for supporting offshore installa- tions. The practical experience related to this concept is mainly based on applications in the oil and gas industry. Observations show significant difference in response for wind turbines compared to more traditional installations offshore. The lightweight structure of a wind turbine makes the caisson susceptible to cyclic loading due to metocean conditions. As this foundation concept is being increasingly considered for offshore wind turbines, a proper understanding of its behaviour is important for a functional and effective design of suction caissons for this purpose. A series of centrifuge experiments have been performed at the University of Western Australia to investigate the response of a suction caisson in dense Baskarp sand. The experiments aim to represent the load conditions of a caisson supporting an offshore wind turbine in the North Sea. Raw data from the centrifuge experiments have in this thesis been analysed focusing on the cyclic response, and the work demonstrates a significant difference in response for compression and tension loading. The permeability of the soil is also found to be decisive for the caisson response. A model of the centrifuge experiment in prototype scale is in this study recreated in the finite element software PLAXIS. Model parameters are based on tests on similar sand available at NGI, due to limitations of sand specific cyclic laboratory tests related to the centrifuge exper- iments. Arbitrary sets of parameters, representing sands of different densities, are also consid- ered. Loads identical to those of the centrifuge experiments is applied to the simulations. Results from the simulations differ from the centrifuge experiments. Displacements are in gen- eral underestimated and total stress and pore pressure are overestimated. Several reasons to the differences are discussed in the thesis, and these are assumed to be related to calibration of the permeability, loosening of the sand due to installation, lack of fully contact between caisson lid and soil in addition to air bubbles in the pore fluid. The differences emphasise the challenge of capturing all features of the complex caisson response in a simplified model. vii Sammendrag Bøttefundament eller sugeanker er et fundamenteringskonsept for offshore-installasjoner. Den praktiske erfaringen tilknyttet dette konseptet er i hovedsak basert på anvendelser i olje- og gassektoren. Undersøkelser viser en betydelig forskjell i oppførsel for vindturbiner sammen- lignet med mer tradisjonelle installasjoner offshore. Den lette vindmøllekonstruksjonen gjør bøttefundamentet sårbart for syklisk last fra vind, bølger og havstrømmer. Dette fundamenter- ingskonseptet blir i økende grad vurdert for offshore vindmøller, og en god forståelse av dets oppførsel er viktig for en funksjonell og effektiv utforming av bøttefundament til dette formålet. En rekke sentrifugeforsøk er utført ved University of Western Australia for å analysere oppførse- len til et bøttefundamet plassert i tettpakket Baskarp-sand. Forsøkene streber etter å gjenskape lastforholdene for en offshore vindmølle plassert på toppen av et fundament i Nordsjøen. Rå- data fra forsøkene er i denne avhandling analysert med fokus på syklisk respons, og arbei- det viser en betydelig forskjell i respons avhengig av strekk- og trykkbelastning. Resultater fra forsøkene viser også at permeabiliteten i sanden er avgjørende for oppførselen til bøttefunda- mentet. En modell av sentrifugeforsøkene i prototypeskala er i forbindelse med dette studiet laget i finite element-programmet PLAXIS. Sandparametre er basert på tester av tilsvarende materi- ale tilgjengelig ved NGI, da slike type tester ikke er gjennomført i forbindelse med sentrifuge- forsøkene. Et sett av materialer med ulike densitet er også undersøkt. Lasttilfeller identisk til sentrifugeforsøkene er benyttet i simuleringene. Resultater fra simuleringene viser avvik fra sentrifugetestene. Forskyvningen er generelt under- estimert og total- og poretrykk er overestimert. Oppgaven diskuterer mulige årsaker til avviket, og disse er antatt å være tilknyttet kalibrering av permeabilitet, oppløsning av sand som følge av installasjon, mangel på full kontakt mellom fundamentlokk og jord samt luftbobler i pore- vannet. Avviket understreker utfordringen med å fange opp alle egenskapene i den komplekse fundamentoppførselen i en forenklet modell. Contents Preface................................................i Acknowledgement......................................... iii Abstract................................................v Sammendrag............................................ vii List of Symbols xi 1 Introduction 1 1.1 Background..........................................1 1.2 Objectives...........................................3 1.3 Limitations...........................................3 1.4 Scientific Approach......................................4 1.5 Outline of the Thesis.....................................4 2 Behaviour of Suction Caissons for Offshore Wind Turbines5 2.1 Installation and Applications of Suction Caissons....................5 2.1.1 Installation.......................................5 2.1.2 Applications......................................6 2.2 Limiting Conditions for Offshore Wind Turbines.....................7 3 Cyclic Behaviour of Sand and the HCA Model 11 3.1 Cyclic Behaviour of Sand................................... 11 3.1.1 Monotonic Loading................................. 12 3.1.2 Cyclic Loading..................................... 13 ix x CONTENTS 3.2 High-Cycle Accumulation Model.............................. 14 3.2.1 Explicit Calculation Strategy............................ 14 3.2.2 Parameters in the HCA Model........................... 15 4 Centrifuge Experiments of Suction Caissons in Sand 19 4.1 Measurements and Equipment............................... 19 4.2 Cyclic Loading......................................... 20 4.3 Material Parameters..................................... 22 4.4 Extraction Resistance..................................... 24 5 Recalculation of Centrifuge Experiments in PLAXIS 25 5.1 Structure of the Model.................................... 25 5.1.1 Finite Element Mesh................................. 27 5.1.2 Variations of the Model............................... 29 5.1.3 Calculation Types and Loading........................... 30 5.2 Adaption of Parameters................................... 31 5.2.1 Stiffness Parameters for the Hardening Soil Model............... 31 5.2.2 The Hardening Soil model with Small-Strain Stiffness............. 35 5.2.3 Extraction Resistance................................ 36 5.2.4 Consolidation Parameters.............................. 36 5.2.5 Overview of Input Parameters and Description of Other Materials...... 38 6 Results 39 6.1 Centrifuge Experiments................................... 39 6.1.1 Displacement..................................... 41 6.1.2 Total Stress and Pore Pressure............................ 45 6.1.3 Amplitudes of Cyclic Applied Load, Displacement, Total Stress and Pore Pressure........................................ 45 6.2 Recalculation Using PLAXIS................................. 50 6.3 Miscellaneous Variations for PLAXIS Simulations.................... 59 6.3.1 Replaced Soil Tip................................... 59 6.3.2 Variation in Permeability.............................. 60 CONTENTS xi 6.3.3 Water Pocket Beneath Caisson Lid......................... 61 7 Discussion 63 7.1 Discussion on Cyclic Peak Amplitudes........................... 63 7.1.1 Centrifuge Experiments............................... 63 7.1.2 PLAXIS Simulations................................. 67 7.2 Type of Behaviour and Material............................... 69 7.2.1 Drained and Undrained Behaviour........................ 69 7.2.2 Materials........................................ 70 7.2.3 Time Histories and Phase Changes......................... 72 7.3 Reasons for Differences in Centrifuge Experiments and PLAXIS Simulations.... 73 7.3.1 Loosening of Soil Due to Installation....................... 74 7.3.2 Consolidation and Redistribution