Modelling Of Lateral Behaviour Of Large-diameter Monopiles Supporting Offshore Wind Turbines In Soft Clay

Vigorously developing offshore wind power is a crucial measure to achieve China's“double carbon” goal.For offshore wind turbines,the foundation embedded in the seabed is directly related to the safety and stability of the whole turbine structure.At present,large-diameter monopiles are the most popular foundation solution for offshore wind turbines within the water depth of 40 m.For the design of monopiles,it is necessary to ensure that its ultimate bearing capacity can bear the most extreme external load;Secondly,during the 25 year service period of a offshore wind turbine,the cumulative deformation of the monopile at the mudline under the cyclic loading induced from wind and wave shall not exceed the specified value;Finally,it should be sure the wind turbine structure will not be fatigue damaged under the action of the external dynamic loads.Such strict design criteria,combined with the soft clay seabed that is widely distributed in the sea area of offshore wind farms in China,bring severe challenges to the design of monopile foundations.In the past decades,many researchers at home and abroad have carried out a lot of research on the static response,cyclic response,and dynamic fatigue analysis of largediameter monopiles supporting offshore wind turbines in soft clay,but there are still some problems worth our concern.To sum up,there is still no simple and practical analysis model for predicting the static behaviour of monopiles,which fully considers the contribution of additional resistance at the pile tip,the soil failure mechanism of the soil around the pile and the site-specific mechanical response of seabed soil,so that it can incorporate “pile diameter effect” and consequently be suitable for the static analysis of monopile with a wide range of length-to-diameter ratios in different seabed soil;In terms of cyclic behaviour of monopiles,according to the author's knowledge,no research has systematically revealed the episodic cycling and reconsolidation effect on the subsequent cyclic pile response(i.e.,cumulative displacement and pile stiffness),and there is still lacking the corresponding analysis model that can capture the cyclic softening and reconsolidation hardening of monopiles to realize the accurate prediction of cyclic response of monopiles in the whole life;In terms of dynamic fatigue analysis of offshore wind turbine,there is no fully integrated analysis model that can finely consider pile-soil dynamic interaction(including pile-soil damping)and couple with wind-wave-turbine control-upper structure.Thus the dynamic fatigue response of monopiles has not been accurately evaluated.Aiming at the key scientific problems of large-diameter monopiles supporting offshore wind turbines in soft clay mentioned above,this paper carried out a systematic research and investigation by means of theoretical analysis,centrifuge model test and numerical simulation.The main contents and innovations are summarized as follows:1.From the failure mechanisms of soil around the pile,a two-spring conceptual model for static analysis of monopiles is proposed,named the “p-y+M-?” model.The model can well capture the contribution of additional soil resistance contribution,i.e.,pile tip shear and tip reaction moment,and thus solve the existing problem of “pile diameter effect” so that can be applied to the analysis of monopiles with a wide range of length-to-diameter ratios using a single set of parameters.The p-y and M-? responses are investigated by integrating three-dimensional finite element analysis,limit upper bound analysis,and elastic analysis.Based on this,an approach is proposed that allows for the construction of site-specific p-y and M-? curves by directly scaling the soil stress-strain response measured in the laboratory.It is worth noting that the proposed model only needs the stress-strain relationship of the seabed soil and soil strength profile as the input,which fully takes into account the mechanical characteristics of the seabed soil at different sites and locations.This greatly enhances the applicability of the model.The satisfactory predictive capability of the proposed model has been demonstrated by back analyses of the 6 field tests and 7 centrifuge model tests covering flexible,semi-rigid,and rigid piles.2.Large-diameter monopiles supporting offshore wind turbines in the offshore area of China are frequently subjected to intermittent episodes of cycling and reconsolidation induced by typhoons during its' lifetime.The lateral pile-soil stiffness is likely to degrade during the cycling but tends to recover during the subsequent reconsolidation.The former effect has been widely acknowledged in monopile design,while the latter is less commonly recognized.This paper carried out a series of centrifuge tests on two typical pile diameter monopiles(D = 4 m and 6 m)embedded in soft clay subjected to multi-stage episodic cycling and reconsolidation.Based on the centrifuge test results,the episodic cycling and reconsolidation effects on pile loaddisplacement response,cumulative displacement,reloading and unloading stiffness evolution,peak and locked-in moment distribution,and cyclic p-y curve responses are systematically quantified.The episodic cycling and reconsolidation effects on the natural vibration frequency and structural resonance safety of the typical offshore wind turbine are also evaluated.3.Continuation of the proposed “p-y+M-?” framework,combined with the threedimensional stress-strain contour diagram of soil element,a cyclic “p-y+M-?” model based on the cyclic stress-strain relationship of soil element is proposed to consider the cyclic softening of monopiles.On the basis of the proposed cyclic model,a pore pressure degree of freedom is introduced into the soil springs.The value of the accumulated pore pressure after cycling of each soil spring is determined by the soil element pore pressure contour diagram.By doing this,the reconsolidation effect is well quantified through the critical state soil mechanics.Consequently,an analysis model that can capture the combined effect of cycling and reconsolidation is developed.Preliminary model validation against the centrifuge testing results shows that the model can reasonably capture the effect of episodes of cycling and reconsolidation on the monopile response.4.A nonlinear pile-soil damping model is established by an extensive threedimensional finite element analysis.Combined with the soil reaction curves proposed in this paper,a simple-yet-powerful dynamic “p-y+M-?” model considering nonlinear pile-soil damping is proposed.The proposed model is then implemented into a nonlinear aero-hydro-servo-elastic simulation tool FAST to finely consider the dynamic pile-soil interaction.Through the implementation,a fully coupled and integrated analysis model for overall offshore wind turbine structure with aerodynamic and hydrodynamic solution,structural dynamics solution,turbine servo control,and pilesoil dynamic interaction solution is constructed.Based on the integrated model,extensive calculation and analysis are performed.The load characteristics and structure dynamic responses of the offshore wind turbines under different design load cases and different types of foundation modelling are revealed.The effects of foundation modelling and pile-soil on fatigue of offshore wind turbines has also been quantified.