Study Of Loads Calculation And Strength Checking Method On Deepwater Landing String

In jetting operations of conductor and casing running operations,the exposed landing string in the sea sustains very complicated loads without the protection of the riser.Once failure occurs,there will be serious economic and environmental losses.For dangerous conditions such as wellhead sinking and slip crushing during deepwater drilling operation,this paper conducts researches on loads calculation of the landing string,ultimate tensile load model,slip crushing load model,strength verification and design methods,which provide the theoretical basis for certain design and selection of the landing string and safety operation in the deepwater drilling operation.A quasi-static mechanical model of the landing string in the deepwater drilling without risers is established according to material mechanics and Euler-Bernoulli theory.Marine environmental loads,vessel offset movement and weight of casing strings are considered into the model.It is solved by the weighted residual method to analyze the changing regulation of landing strings' lateral deformation,bending moment,dig angle of the conductor and stress distribution.Ultimate tensile load model is established under the condition of wellhead sinking.According to the soil mechanics and pile foundation theory,focusing on the influence of time effects,the relationship between real-time lateral friction and adsorption time is obtained.By establishing the finite element model of the landing string considering marine environment loads,the axial force increment caused by the bending deformation of the string is calculated,then the quantitative regression relation of axial force increment has been developed based on nonlinear least squares consideration.Based on the analysis of the slip crushing strings,the slip crushing load model is established based on the theory of thick-walled cylinder and the fourth strength theory.The ABAQUS finite element software is used for calculation and analysis to verify the accuracy of the theoretical model.Considering the influence of the bending normal stress caused by the bending deformation of the landing string,a set of strength design and verification method of the landing string is established in the deepwater drilling operation.The research results demonstrate that sea current and wave have great effect on lateral deformation and top bending moment of the landing string respectively.The increase of tensile load in the bottom of strings will lead to the reduction of lateral deformation,bending moment and dig angle of the conductor,while the stress level of the landing string considerably rises.As the vessel offset decreasing along the negative direction and increasing along the positive direction of sea current,bending moment and dig angle of the conductor firstly decrease and then increase.The ultimate tensile load calculated during the wellhead sinking is in order to provide a theoretical basis for strength design of the landing string.Through the finite element analysis of the slip crushing model,the error between the obtained results and the theoretical model is between 6.8 and 9.9% under the influence of marine environmental loads,indicating that the predicted value is conservative and feasible.Through a case study of a deepwater well in the South China Sea,taking into account the dangerous conditions of wellhead sinking and slip crushing,the safety margin,offset range,and critical water depth of the different types of strings are obtained based on the axial composite stress and allowable tensile strength.An optimized landing string selection is proposed,and it is considered that 6-5/8in landing string is more safe.