A Coupled Discrete-Finite Element Method For The Ice-Induced Vibration Of The Offshore Platform Structure

In cold regions,the ice load is a major affecting factor of structural safety.The ice-induced vibrations(IIVs)can be harmful for the offshore structures and the related equipment.IIVs play a key role in structural design,fatigue analysis and alert security.However,due to the complexities of the mechanical properties of sea ice and the interaction between sea ice and offshore platforms,numerical simulations of the IIVs on offshore structures have great challenges.This thesis develops a numerical method to analyze the IIVs of offshore structures.In this study,a coupled algrithem of discrete element method(DEM)and finite element method(FEM)is developed to simulate the ice-structure interaction.The DEM for sea ice is constructed by spheres,which incorporate the bond-breaking parallel bond between spheres to simulate the breakage of ice cover.The offshore structures are modeled by the beam,shell and solid finite elements(FEs)to analyze the responses of structures.A DEM-FEM parameter transmission scheme is proposed in this thesis.To improve the computational efficiency and scale of the coupled DEM-FEM model,two coupled-models are developed,respectively.One is a model based on the domain decomposition method(DDM),the other is a high performance model based on the graphics processing unit(GPU).On the base of the proposed methods,the IIVs characteristics of offshore platform structures are investigated by the coupled DEM-FEM method.The main studies of this thesis includes:(1)A coupled DEM-FEM model for the IIVs of offshore structuresTo model the breakage of the sea ice,the DEM with bond-breaking spherical elements is adopted.Meanwhile,the FEM(with the beam,shell and solid FEs)is applied to model the IIVs of offshore structures.An efficient transmission scheme between the discrete elements(DEs)and the FEs is proposed.The coupled model is verified by two cases,i.e.,the ball impacting and the elastic rod collision.Finally,the m illing between propeller and sea ice are simulated by the coupled model.The damage process of sea ice,ice load and stress distribution of the propeller blade are analyzed.This demonstrates the perfect applicability of the coupled DEM-FEM model in ice-structure interaction.(2)A coupled DEM-FEM model based on the DDMThe small time step in the DEM.as the time step of the coupled model,is time-consuming.This thesis proposes a coupled DEM-FEM model based on the DDM to solve this problem.The computational domain is split into several subdomains based on whether DEs interacts with FEs.Each subdomain can then be solved individually with different time steps.To verify the proposed method,the interaction between the sea ice and the offshore platform are simulated with different ratios of coarse time step to fine time step.The numerical results show that the proposed time-efficient method is reliable and stable for the simulations of ice-platform interactions.Moreover,the computational efficiency is improved greatly by using the proposed method.(3)The coupled DEM-FEM model with GPU-based parallel algorithmTo improve the computational efficiency and scale of the coupled model,a GPU-based parallel algorithm is developed.The coupled model based on the GPU parallel algorithm include four parts:global search between DEs and FEs,FEM stiffness assembly,dynamic equation solving and DEM-FEM parameter transmission.In order to verify the accuracy and efficiency of the improvement parts,three cases,i.e.,ice-conical interaction,lateral vibration of cantilever beam and elastic rod collision,are analyzed.These numerical results are similar to the original DEM-FEM coupling model,and the improved model have a better calculation speedup.(4)The ?Vs characteristics of offshore platform structuresBased on the coupled DEM-FEM method,the ?Vs characteristics of various offshore platform structures are analyzed.Firstly,the ice load on the multi-pile structure is obtained by ice DEM model in various ice drifting directions.The mechanism of shadowing effect of ice load on multi-pile structure is discussed.Afterwards,the interaction between sea ice and multi-pile,mono-pile conical jacket platforms are simulated under various ice conditions(e.g.,velocity and thickness).The simulated ice loads are verified by comparing them with the full-scale data and different ice load functions.The simulated IIV accelerations of the multi-pile and mono-pile conical jacket platforms are consistent with the full-scale data.The numerical results show that the IIV acceleration increases linearly with the ice velocity and the square of the ice thickness.Finally,summary of the coupled DEM-FEM model and IIVs of the offshore structures is concluded,and the major future works are pointed out.