Lateral Behaviour Of Offshore Monopile And Bucket Foundations In Sand

Developing offshore wind energy is an important strategy to solve the environmental problem,energy crisis and upgrade the energy structure of China.For a typical offshore wind farm,the cost of foundation can take up more than 30%in the total project,which plays an important role in the economic feasibility of the project.Currently,the monopile is the most widely used supporting substructure for the offshore wind turbines in water depth less than 40 m.At the same time,due to the limitation of monopile installation technology,the suction bucket foundation is considered as a promising option for its easier installation,reusable and economy in deeper water depth.Unlike traditional oil and gas platforms,self-weight of the offshore wind turbine is much smaller compared with the lateral load from wind,wave and current.Besides,the offshore wind turbine structure is a long slender,dynamic sensitivity structure,foundation design needs to guarantee that the natural frequency is in the range of 0.27?0.35Hz and the accumulated deflection in the design life is less than 0.25°.For the design of laterally loaded monopile,existing design theory are mainly from the experience of small diameter long piles in the oil and gas industry.Its applicability to the large diameter monopile in offshore wind farm has been extensively questioned by both industry engineers and academic researchers.Due to the widely distributed soft soil seabed in China,the monopile foundations are designed to be large diameter long slender pile.However,no research has been performed to investigate the monotonic and cyclic lateral behaviour of the large diameter long slender piles.Furthermore,few existing researches on the influence of "diameter effect",loading eccentricity,soil-pile relative stiffness,"base effect" and soil relative density on the pile-soil interaction were controversial with each other.Systematic studies need to be performed to clarify the influence of these factors on pile-soil interaction under lateral loading.For the lateral response of bucket foundation,existing studies are mainly focused on the ultimate capacity of foundations in clay.Few experimental studies in sand were carried out at 1g small scale condition,which can't correctly reflect the foundation behaviour at prototype scale.No studies were performed to systematically compare the lateral response of monopod and tripod bucket foundations under monotonic and cyclic loading.Given the aforementioned limitations,theoretical analyses,finite element simulations,centrifuge model tests and field tests were carried out to investigate the lateral response of large diameter monopile and suction bucket foundation in sand.The following key issues were addressed in this study:1.The centrifuge tests in medium dense sand were carried out on two different diameter monopiles(4 m and 6 m)with the same embedded length of 60 m to investigate the monotonic and cyclic behaviour of large diameter long slender monopiles.The influence of pile diameter on the pile-soil interaction(i.e.p-y curve)was quantified.The applicability of existing p-y models in API code and literature were evaluated.Comparison between the larger diameter slender pile and small diameter flexible pile and large diameter stiff piles in literature were performed to quantify the influence of pile diameter,soil relative density and soil-pile relative stiffness on the long-term cyclic response.2.The finite element model with the advanced hypoplastic model,which can reflect the state dependency,small strain stiffness,and strain-path dependency behaviour of sand,was built.The model was first validated against the centrifuge tests.Then,the influence of pile diameter,loading eccentricity and soil relative density on the p-y curve of large diameter flexible piles were systematically investigated.A four-parameter conic function-based p-y model was proposed for the large diameter flexible piles under lateral loading.The model was thoroughly validated both by the 3D FEM simulations and centrifuge tests in the existing literature.3.Centrifuge tests on large diameter flexible pile and stiff pile were performed first to provide high-quality validation input data.The finite element model with the advanced hypoplastic model for sand was validated by the centrifuge test results and employed to study the lateral response of large diameter rigid piles.A total of 108 cases were simulated to quantify the influence of pile diameter,length to diameter ratio,loading eccentricity and soil relative density on the p-y curve of large diameter rigid piles.The failure mechanism of rigid piles under lateral loading was revealed where its influence on the distribution of ultimate soil resistance was also investigated.A mechanism-based p-y model and "p-y+MR-?R" analysis model was proposed for the laterally loaded rigid piles.4.Field tests and finite element simulation were carried out to study the influence of pile base condition on the lateral response of rigid piles.By manually removing the soil plug after installation,the influence of pile base shear force and moment produced by the rigid pile under lateral loading on the pile-soil interaction and pile capacity was explicitly quantified.Applicability of the p-y method for the rigid pile was verified.Based on the field tests in this study and those in the literature,a simple CPT based approach for predicting the ultimate lateral capacity of a rigid pile in sand was proposed.Explicit design equations are also presented for soil profiles with a constant qc value and where qc increases linearly with depth.The proposed equations are seen to provide good estimates of capacities for rigid piles with a wide range of diameters.5.A series of centrifuge tests and finite element simulations using an advanced hypoplastic model were carried out in medium dense sand to investigate and compare the lateral monotonic and cyclic behaviour of a monopod and a tripod bucket foundation.The failure mechanism of monopod and tripod bucket foundation under lateral loading was revealed,when the degradation of foundation stiffness with deflection was also quantified.Furthermore,the development of long-term cyclic deflection accumulation,unloading stiffness and damping ratio was studied.A unique feature of "self-healing"in cumulative rotation and dynamic stiffness of the tripod bucket foundation was first identified.