The University of Maine DigitalCommons@UMaine Electronic Theses and Dissertations Fogler Library 8-2012 Physical Modeling of Suction Caissons Loaded in Two Orthogonal Directions for Efficient Mooring of Offshore Wind Platforms Jade Chung Follow this and additional works at: http://digitalcommons.library.umaine.edu/etd Part of the Civil Engineering Commons Recommended Citation Chung, Jade, "Physical Modeling of Suction Caissons Loaded in Two Orthogonal Directions for Efficient Mooring of Offshore Wind Platforms" (2012). Electronic Theses and Dissertations. 1754. http://digitalcommons.library.umaine.edu/etd/1754 This Open-Access Thesis is brought to you for free and open access by DigitalCommons@UMaine. It has been accepted for inclusion in Electronic Theses and Dissertations by an authorized administrator of DigitalCommons@UMaine. PHYSICAL MODELING OF SUCTION CAISSONS LOADED IN TWO ORTHOGONAL DIRECTIONS FOR EFFICIENT MOORING OF OFFSHORE WIND PLATFORMS By Jade Chung B. Eng. Dalhousie University, 2007 A THESIS Submitted in Partial Fulfillment of the Requirements for the Degree of Masters of Science in Civil Engineering The Graduate School The University of Maine August, 2012 Advisory Committee: Melissa Landon Maynard, Assistant Professor of Civil Engineering, Advisor Thomas C Sandford, Associate Professor of Civil Engineering William G. Davids, Professor of Civil Engineering Joseph T. Kelley, Professor of Earth Sciences ll TMSIS ACCO?TANCESTAIEMDNT On behalfofthe CrrafualeCmnittee fc JadeChrmg; I affrm tbatthis nanuscrig is tte. firnl ad acceptedttrcsis. Signatures of all committeemeinbers arc or file with the Ctraduate Schoolet tlrc UniversityofMaine, 42 Stod&r llall, Orono,lvfaine. Melissal*udotr l{aprd, PlrD. Aryust 2012 d)/1// c.. -rfl-atZ otlot /P,> I iii © 2012 Jade Chung All Rights Reserved PHYSICAL MODELING OF SUCTION CAISSONS LOADED IN TWO ORTHOGONAL DIRECTIONS FOR EFFICIENT MOORING OF OFFSHORE WIND PLATFORMS By Jade Chung Thesis Advisor: Dr. Melissa Landon Maynard An Abstract of the Thesis Presented in Partial Fulfillment of theRequirements for the Degree of Masters of Science in Civil Engineering August 2012 Over the past decade a number of Federal and State policies and programs have promoted the development of the wind energy industry, including the establishment of offshore wind. A strategy by the Department of Energy set objectives of reducing cost and reducing time to deployment through specific deliverables such as innovative anchor and mooring design for floating offshore systems and hardware design concepts including turbine array grids. This research program proposes and investigates use of suction caissons as combined anchors to resist line loads from multiple platforms as an efficient solution for anchoring a network of wind turbine platforms. Suction caissons are a ‘mature’ anchor technology in the offshore oil and gas industry, yet there is minimal experience with application for offshore wind platforms. Established design methods, standards and recommended practices from the oil and gas industry, serve as a starting point for further adaptation. Considerations of the differences in conditions (e.g. loads, risk, failure and serviceability tolerances) between the two applications, is important for developing efficient anchor design suited to offshore wind platforms. A physical modelling program was developed to investigate the behaviour of caissons subjected to orthogonal cyclic and post-cyclic monotonic line loads, compared to the behaviour of single line loaded caissons. Modeling was performed in a geotechnical centrifuge in order to simulate in-situ stress profile at model scale, as stresses are critical to soil and foundation behavior. Load tests were performed on a model suction caisson anchor installed by jacking into normally consolidated kaolin clay (in-flight). Baseline tests were performed with single line loading for comparison to the multi-line loading tests. Line loads were applied in orthogonal directions for the multi-line load tests. The effect of varying cyclic mean load and cyclic load amplitude was also investigated. Comparison of test results was based on line displacement, applied line load, caisson rotation and internal pore pressure at the underside of the caisson cap. Centrifuge test results appear to indicate that the line load-displacement response during monotonic loading is similar for the multi-line and the single line loaded suction caisson anchors. The post-cyclic peak monotonic line load resistance provided by the caisson loaded in multiple directions was greater than the resistance provided by the caisson loaded in a single direction (accounting for the total resultant load applied). For all selected load cases, the accumulated permanent displacements during the cyclic loading did not result in a displacement (serviceability) failure of the suction caisson nor contribute significantly to the displacement correlating to the peak line load resistance of the caisson. Test results indicate that the resistance capacity of a given caisson is not reduced by applying line loads in multiple directions, when considering the resistance to the total resultant load. Test observations appear to support conceptualizations of a modified “zone of influence” (active/passive earth pressure wedges) due to the changing load orientation from resolving multiple out of phase line loads. iv DEDICATION To my loving and supportive husband Michael Jean To my father whose work ethic and dedication inspires me See-Wing Chung To the loving memory of my late mother Yew-Teck Chung v ACKNOWLEDGEMENTS I would like to thank the Department of Energy for sponsoring this research through the DeepCwind Consortium program. I would like to express my sincere gratitude to Dr. Melissa Landon Maynard, for accepting me into this research program and for serving as my advisor throughout my graduate studies. Her dedication to research and development endeavours is nothing short of inspirational, and her many hours of guidance and review work has helped me tremendously throughout this program. I am also thankful for the assistance that I received from Dr. William Davids, Dr. Tom Sandford and Dr. Joe Kelley during my research and studies. I would like to thank all of the staff at the Center for Offshore Foundations at the University of Western Australia. I am particularly appreciative of Dr. Christophe Gaudin, for co- ordinating the centrifuge program, without his expert help this research program would not have been possible. Many thanks also to: Don Herley and Manuel Palacios, for the extended hours of work on the centrifuge operations and the endless patience they had with my unfortunate inexperience; Philip Hortin, Shane De Catania and John Breen for all the instrumentation, electronics, digital systems and general support they provided; Dave Jones for the machinery work and being sympathetic to last minute requests. I am so thankful for the many blessings that I have received from the Lord, including this unique research opportunity, and amazing support from my family and colleagues. vi TABLE OF CONTENTS DEDICATION ................................................................................................................................ iv ACKNOWLEDGEMENTS ............................................................................................................. v LIST OF TABLES ........................................................................................................................... x LIST OF FIGURES ........................................................................................................................ xi LIST OF ABBREVIATIONS ....................................................................................................... xiv LIST OF SYMBOLS .................................................................................................................... xvi 1. INTRODUCTION ..................................................................................................................... 1 1.1. Federal Wind Energy Legislation and Federal Programs ........................................... 1 1.2. Maine State Programs ................................................................................................. 3 1.3. Locating Offshore Energy Developments ................................................................... 3 1.4. Wind Energy Developments in Maine ........................................................................ 4 1.5. Wind Energy Economics ............................................................................................ 6 1.6. Selection of Anchor Type ........................................................................................... 7 1.7. Anchorage Design Concept ...................................................................................... 10 1.8. Research Objectives .................................................................................................. 10 1.9. Thesis Structure......................................................................................................... 11 2. BACKGROUND ..................................................................................................................... 13 2.1. Suction Caissons ....................................................................................................... 13 2.1.1. Suction Caisson History .......................................................................... 13 2.1.2. Suction Caisson Anchors for Mooring Systems