Horizontal Vibration Characteristics And Safety Analysis Of Offshore Pipe Pile Considering Soil Plug Effect

With the deep propulsion of ‘Maritime power’ strategy and ‘The Belt and Road’initiative,the major marine facilities such as cross sea bridges,coastal terminals,offshore turbines and drilling platforms are constantly emerging.Due to the advantages of good bearing capacity,convenient construction and stratum adaptability,pipe piles have become the most important foundation form of offshore structures.During the service period,offshore pipe piles bear the horizontal dynamic loads caused by wind,wave,mechanical vibration,ship(vehicle)impact and earthquake disaster for a long time.Therefore,how to ensure the dynamic stability of pile foundation under horizontal dynamic loads and avoid resonance disaster or excessive deformation damage is of great significance for the safe operation and maintenance of offshore structures.The existing pile foundation dynamics mainly focuses on the dynamic interaction between pile and pile surrounding soil,but rarely involves the dynamic interaction between pipe pile and its internal soil plug.Owing to the construction difficulty of offshore pile foundation,large-diameter pipe piles are widely used in offshore structures to reduce the foundation investment cost and reduce the number of piles as much as possible.For large-diameter pipe piles,the diameter of soil plug usually reaches 2～5m.Ignoring the influence of soil plug effect on the horizontal vibration characteristics of pipe piles would bring great potential safety hazards to offshore structures.The existing research on the horizontal vibration characteristics of offshore pipe pile still has the following two deficiencies:(1)in terms of dynamic foundation design,the focus is on the natural frequency and complex impedance of pile foundation.However,due to the failure to establish a reasonable interaction model between pipe pile and soil plug,the horizontal complex impedance of pile top can not be accurately estimated,and the pile stiffness design is often wasteful.This phenomenon not only great increases the investment cost,but also makes the natural frequency of pile foundation close to the frequency range of external load for vibration sensitive structures such as offshore wind turbines or sea crossing bridges,which greatly shorten the service period of offshore structures and induce serious resonance disasters.(2)in terms of anti-seismic design of pile foundation,the focus is on the resistance capacity of pile foundation to dynamic deformation and energy dissipation.The theoretical model of horizontal vibration of offshore pipe piles considering the strict coupling interaction among pipe pile,fluid,pile surrounding soil and soil plug has not been established,and the influence mechanism of soil plug on the horizontal vibration characteristics of pipe pile under seismic load is still not clear.It is still unknown to what extent the existing seismic design principles will change when considering the hydrodynamic pressure and soil inertia effect.How to choose an economic and reasonable optimal design scheme of pile-soil system when ensuring the seismic safety of offshore structures has become an urgent technical problem in engineering.In view of the shortcomings of the existing research,this thesis studies the horizontal vibration characteristics and safety analysis of offshore pipe pile considering soil plug effect.The research results can provide a theoretical basis for the dynamic foundation design and anti-seismic design of offshore pipe piles,and effectively improve the operation and maintenance safety and service period of offshore structures.The main research work and results of this thesis are summarized as follows:(1)A new dynamic interaction model of pipe pile and soil plug,namely continuous additional mass model,is established.This model can consider both the resistance effect and inertial fluctuation effect of soil plug at the same time,and effectively improve the calculation accuracy of natural frequency and complex impedance of pile foundation.The theoretical modeling idea is as follows: the continuous additional mass model is used to simulate the dynamic interaction between pipe pile and soil plug,the Biot porous medium theory based on Novak plane strain model is used to simulate the dynamic interaction between pipe pile saturated pile surrounding soil,and the Euler beam model is used to simulate the horizontal vibration of pipe pile.Then,the theoretical solution of horizontal dynamic response of pipe pile considering soil plug effect under inertial force(external load)is established by virtue of Laplace transform,operator decomposition theory,potential function decomposition and matrix transfer method.The main conclusions are summarized as follows: ignoring the inertia effect of soil would overestimate the dynamic stiffness and underestimate the dynamic damping of pipe pile,and the predicted resonance frequency and resonance amplitude of pipe pile are higher than the actual working condition.The worse the soil properties and the larger the diameter of the pipe pile is,the greater the calculation error of the Biot porous medium model is.At this time,there are great unknown safety risks when Biot porous media model is applied to the dynamic foundation design of large-diameter pipe pile in soft soil area.With the increase of soil plug height,the dynamic stiffness of pile top decreases and the dynamic damping increases.(2)Novak plane strain model has been widely used due to its relatively simple mathematical modeling.However,since it ignores the influence of soil longitudinal continuity,there are some design problems,and the design theories based on this model have some potential safety hazards.In view of this,the continuous medium model is further used to simulate the strict coupling interaction between pile and soil,and the theoretical solution of horizontal vibration characteristics of pipe pile considering the strict coupling interaction between pile and soil and axial load is derived.Based on the present solution,the applicability of Novak plane strain model,the effective pile length considering soil plug effect and the influence of axial load are systematically studied.The main conclusions are summarized as follows: ignoring the longitudinal continuity of soil would lead to the underestimation of dynamic stiffness and overestimation of dynamic damping of pile top.The smaller the length diameter ratio of pile body,the larger the young’s modulus of soil and the better the permeability coefficient is,the greater the calculation error of plane strain model is.The smaller the pile diameter is,the shorter the effective pile length is.For the same stratum conditions,the ratio of effective pile length to pile diameter remains basically unchanged.The plane strain model will underestimate the effective pile length range and the safety factor is low,which is easy to cause large unknown safety hazards.With the increase of axial load,the dynamic stiffness,dynamic damping and resonance frequency of pile top gradually decrease,the amplitude of resonance peak gradually increases,and the dynamic stability of pile body decreases,which is easy to induce safte problems.(3)The radiation wave theory is used to simulate the dynamic interaction between pipe pile and fluid,the saturated continuum model is used to simulate the dynamic interaction between pipe pile and pile surrounding soil,and the additional mass model is used to simulate the dynamic interaction between pipe pile and soil plug.The theoretical solution of horizontal vibration characteristics of offshore pipe pile considering the strict coupling interaction of fluid-pipe pile-soil under inertial force is established.Based on the present solution,the influence mechanism of hydrodynamic pressure on the horizontal vibration characteristics of pipe pile is studied.The main conclusions are as follows: ignoring the influence of hydrodynamic pressure would overestimate the horizontal dynamic stiffness of pile top,underestimate its dynamic damping,and significantly overestimate the resonance frequency and resonance peak amplitude of pile body.The greater the water depth and pile diameter,and the weaker the soil strength is,the more obvious the influence of hydrodynamic pressure is.For large-diameter offshore pipe piles on soft soil foundation,ignoring hydrodynamic pressure in dynamic foundation design would produce major potential safety hazards.(4)The above three studies mainly serve the dynamic foundation design of offshore pipe piles,that is,the horizontal vibration characteristics of pipe piles under external loads such as wave,wind load and mechanical vibration,in which the power source is the external load and the soil provides resistance.For the anti-seismic design,the seismic wave is transmitted from the soil to the pile foundation,and the soil is the power source.At this time,the pile body is taken as the resistance term.In view of this,according to the anti-seismic design requirements of pipe pile,the theoretical solution of horizontal vibration characteristics of pipe pile considering the strict coupling interaction between pile and soil under SH simple harmonic seismic wave is established.The idea of theoretical modeling is as follows: firstly,based on the one-dimensional wave theory,the horizontal displacement of soil free field under the vertical incidence of SH simple harmonic seismic wave is obtained.Based on the three-dimensional axisymmetric elastic dynamics theory,the dynamic interaction model between pipe pile and pile surrounding soil is established.The scattering wave equation of soil caused by pile under earthquake is deduced by using potential function decomposition and variable separation method.The real horizontal displacement field equation of soil considering the existence of pile under earthquake is obtained by superposition of soil free displacement field and scattered displacement field.Finally,the analytical expressions of the horizontal displacement fields of pipe pile,soil and scattered wave are obtained by using the boundary conditions and initial conditions of pile-soil system.Based on the present solutions,the effects of soil inertia effect,pile diameter,wall thickness,buried depth and soil properties on the amplification factor of horizontal displacement at the top of pile and the curvature distribution of pile body under SH simple harmonic seismic wave are systematically studied.(5)Soil and fluid are the external force sources of pile foundation vibration under seismic load,but the fluctuation of fluid is more rapid and intense.For the offshore pipe pile undergoing SH simple harmonic seismic wave,the theoretical solution of horizontal vibration characteristics of offshore pipe pile considering the strict coupling interaction among fluid,pipe pile and soil is established by further using the radiation wave theory.Based on the present solution,the influence of hydrodynamic pressure on the horizontal vibration characteristics and safety analysis of offshore pipe pile under seismic load is systematically studied.The main conclusions are as follows: the existence of hydrodynamic pressure would significantly increase the amplitude of displacement amplification factor at the top of pile,and the shear deformation of pile body would increase sharply.The existence of fluid would significantly reduce the dynamic stability of pipe pile under seismic load.The decrease of pile diameter would weaken the strength of pile body,but it would also reduce the adverse effect of hydrodynamic pressure.In the anti-seismic design of offshore pile foundation,it is not that the larger the pile diameter is,the safer it is,but there is an optimal pile diameter range.The existence of soil plug in the pipe pile is conducive to accelerate the absorption and attenuation of seismic wave energy,which significantly reduce the shear deformation of the pile body,and play a positive role in the anti-seismic design of pile foundation.