Numerical Study For The Buckling Loads Of Offshore Large Diameter Steel Pipe Piles Under Combined Loadings

Offshore large diameter steel pipe piles are usually used as the foundation of offshore wind turbines and offshore platforms. Comparing with the pile foundations on land, offshore pile foundations have characteristic of free pile segment with long length in sea water and above the level of sea water, low radio of diameter to thickness of piles, withstanding combined loads resulting from offshore environment, and being surrounded by soils with complex strength profile, global buckling or local buckling easily occurs to them, which lead to damage to offshore structures or shorter lifespan of offshore structures. Therefore, researching on the buckling capacity of offshore large diameter steel pipe piles under combined loads need to carry out, which is significant to the improving of the design of offshore steel pipe piles and the promoting of corresponding safety of offshore structures.This paper introduces the buckling theory of piles firstly. Then based on the Winkler’s elastic foundation beam theory, a two-dimensional beam(pile)/soil spring model for finite element analysis was established to investigate the buckling loads of offshore steel pipe piles. The influences of constraint conditions at pile head, combined loads applied to piles, deformation behavior of material of pile, distribution of the coefficient of subgrade reaction of soils, and the length of the free segment and embedded part of pile on buckling capacity were examined via eigenvalue buckling analysis and Riks analysis. After that, a three-dimensional finite model of pile/soil of was used to study the influences of elasto-plastic behaviors of soils and piles, the contact behavior of the interface between piles and soils on the buckling loads of piles. Finally, based on the three-dimensional finite element model of thin-wall cylindrical shell, the global buckling and local buckling modes were investigated.(1) The buckling theory of piles was described, including the basic concepts of structural buckling, deformation behavior of piles under different loads, buckling analysis of piles and the influences that affecting the stability of piles. Finally, two kinds of finite element analysis methods of buckling built in software ABAQUS were related, which provide the theoretical basement for numerical study of buckling capacity of large diameter steel pipe pile/soil system.(2) Based on the concept of two-dimensional Winkler elastic foundation beam, finite element numerical model of two dimensional beam(pile)/soil spring was established. The different distribution forms of the coefficient of subgrade reaction of surrounding soils were described by means of the secondary development of software ABAQUS. Its influence, as well as the influence of the ratio of embedded length of piles, on bearing capacity of eigenvalue buckling of offshore large diameter steel pipe piles was studied.(3) Based on the extending of theoretical solution of buckling capacity of axial-compressive column under the complex constrained conditions in pile head, we provided a method to calculate the buckling loads of offshore large diameter steel pipe pile based on a series of finite element buckling analyses. The influence on the buckling capacity of piles with different constraints conditions was analyzed. Then we discussed the influence of elasto-plastic behavior of piles on the buckling capacity.(4) Three-dimensional finite element model was used to analyze the buckling of large diameter steel pile piles. Based on the numerical results, the influence of elastic and plastic behaviors of piles and soils, and the interaction of pile and soil on buckling stability of piles were examined.(5) The numerical and theoretical solutions of buckling capacity for large diameter steel pipe piles, which were viewed as thin-wall cylindrical shell structures, were compared. On this basis, the buckling modes were studied via the numerical eigenvalue buckling analysis and Riks analysis. Then the influence of pile wall thickness, pile length, and deformation behavior of material of piles on the buckling modes was discussed.