Research On Constitutive Model Of Gas Hydrate-Bearing Sediments Based On Unified Hardening Parameters And Its Secondary Development For ABAQUS

Gas hydrate is considered to be a new strategic alternative energy source with great development prospects in the 21 st century.It is a metastable substance,which elevated temperature and reduced pressure can cause hydrate decomposition.Gas Hydrate Bearing Sediments(GHBS)refers to deep-sea sedimentary soils containing natural gas hydrates.The formation and decomposition of hydrates affect the strength,stiffness and dilatancy of the gas hydrate bearing sediments.Therefore,the simulation of the constitutive characteristics of GHBS is essential for safe exploitation of natural gas hydrates.In this research,the effects of hydrate formation on the strength,stiffness and dilatancy of GHBS are studied.The constitutive model establishment of GHBS and its validation of UMAT subroutine are mainly carried out.This study has mainly completed the following works:(1)The research status on mechanical properties of GHBS and constitutive models at home and abroad are reviewed,and the progress on UMAT secondary development of constitutive models is described.The related theories of constitutive model of geomaterials are reviewed,including yielding criteria,loading and unloading criteria,flow laws and hardening laws.The unified hardening parameters proposed by Yao Yangping and the basic formula of the UH model are briefly described in this research.(2)According to the mechanical properties of GHBS,such as strength,dilatancy and softening,the influence mechanism of hydrate saturation and occurrence morphology was analyzed.Two saturation parameters,density parameter and structural parameter,are introduced to reflect the influence of hydrate on strength and deformation.Based on the modified Cam-Clay model,the density parameter and structural parameter are introduced.The unified hardening parameters and associated flow rules are used to establish the constitutive relation of GHBS,and the corresponding elastoplastic matrix is derived.Through the comparison between the simulation results and the existing experimental data,the rationality and effectiveness of the model are verified.(3)Combine the model with the finite element software to provide a model reference for the analysis of deformation and stability problems involved in the hydrate mining process.The study summarizes the related theories of ABAQUS and its UMAT subroutine,briefly introduces the display stress integration algorithm,focuses on the formula of semi-implicit stress integration algorithm,and summarizes the semi-implicit integration algorithm flow,in order to realize the constitutive model.(4)According to the expression of the elastoplastic constitutive model of GHBS and the stress-strain renewal formula,the user material subroutine interface UMAT provided by ABAQUS is used,and the FORTRAN language is adopted.The UMAT subroutine is developed with the semi-implicit backward Euler integration algorithm,complementing the material library of the ABAQUS software.The triaxial test path simulation of a single unit and multiple unit models are carried out to verify the correctness of the secondary development subroutine.