Three-dimensional Stress Analysis Of Tool Joints Under Complex Loads

Since the exploration of oil and gas resources moves further to greater depth andinto harsher environment, the structural and sealing integrity of the drill string isfacing enormous challenges. As the weakest part of the drill string, the tool joints haveto bear extremely high combined loads including tension/compression, bending,torsion, internal/external pressure etc. The failure of the tool joints often occurs duringthe drilling, which results in a larger economic loss. In order to guarantee the safety ofdrilling operations, it is crucial to optimize the tool joints and select bearing capacitybased operation parameters.The stress analysis of the tool joints involves material nonlinearity, geometricnonlinearity and contact nonlinearity. It is difficult to obtain an analytical solution ofthe tool joints under the complex loads. The full-scale experiment technique canevaluate the performance of the tool joints intuitively and reliably. However, itgenerates higher cost and consumes more time. Meanwhile, the complex loads aredifficult to be imposed due to the limitation of the equipment.The finite element method has been proved to be a powerful numerical tool forsolving such problem. Conventionally, a two-dimensional axisymmetric analysis isperformed, in which the helix angle is neglected. It is thus hard to precisely analyzethe three-dimensional stress of the tool joints under the complex loads. In this thesis, athree-dimensional elastic-plastic finite element method is discussed in detail for thetoll joints with fully consideration of geometric nonlinearity, material nonlinearity andcontact nonlinearity. By using this method, the three-dimensional stress of the tooljoints is analyzed. Several key parameters are investigated by performing a seriesnumerical experiment. The stress character of API (American Petroleum Institute)RSCs (Rotary Shoulder Connections) and the double shoulder tool joints is thenidentified. A premium tool joint with higher performance is then proposed. A diagramof the ultimate working torque for various tool joints is obtained, which is useful forthe drilling operation. The main contributions are summarized as follows:First, based on the nonlinear finite element theory, the key factors that influencethe numerical accuracy and the computation efficiency have been discussed.Moreover, the three-dimensional mechanical analysis of the tool joints under thecomplex loads, i.e., tension/compression, bending and torsion, is performed. Theresults show that the helix angle of the thread profile influences significantly thestress character of tool joints. It is thus necessary to perform the three-dimensionalanalysis.Second, the stress of API RSCs and double shoulder tool joints under thecomplicated loads is analyzed. It shows that the double shoulder tool joints have moreuniform load distribution, higher torsional performance. It is thus more adaptable tothe harsh drilling conditions.Third, the distribution of the contact pressure of double shoulder tool joints inseveral typical load cases is investigated. It is found that the threads have high risks to galling in the case of the ultra-deep drilling. Meanwhile, the secondary shoulder hashigh risks to galling in the case of the extended reach drilling. These results areconsistent with the field observation.Fourth, based on the sealing performance the ultimate lifting load of drill stringin ultra-deep drilling is proposed to reduce the stress corrosion of tool joints. If theaxial tensile load exceeds the ultimate value, the tool joints should be broken out andcleaned carefully.Fifth, according to the complex load feature of the tool joints, several keyparameters are analyzed. These parameters are threads angle, counter bore depth andsecondary shoulder clearance. A higher performance double shoulder tool joint SH isachieved by the parameter optimization.Finally, although the ultimate working torque for API RSCs can predicted by APIstandard, the mechanics character of the tool joints under the complex loads,especially in the ultra-deep drilling cannot be reflected. In this thesis, based on thethree-dimensional finite element analysis a method to determine the ultimate workingtorque of tool joints is developed. The operation load limits predicted by this methodare validated by field applications. The diagrams of three widely used tool joints arethen obtained to determine the operation load limits.