Is Titanium Drillpipe Applicable to Offshore Drilling? A Question from a Corrosion Fatigue Perspective

Summary In offshore horizontal well drilling, one of the major challenges is the large dogleg severity in the buildup section due to the shallow depth of reservoirs. In such a case, the drillstring has to bend itself to fit the borehole trajectory and suffers greatly from the high alternating stress while rotary drilling. This could lead to fatigue fracture of the drillpipe within a short period. The corrosion from drilling mud may exacerbate the failure risk. Titanium alloy, as a new drillpipe material, has the characteristics of excellent corrosion resistance and low elastic modulus, which can theoretically extend the fatigue life. To study the performance of titanium alloy drillpipe quantitatively, titanium alloy material, and G105 steel of the same grade are compared to obtain the microscopic characteristics and macroscopic properties through experimental tests. Moreover, the mechanism of passive film formation of the two materials is analyzed in the corrosion fatigue (CF) environment. Then, the experimental results are extended to modeling the actual drillstring assembly and simulating its service life in the field practice of offshore drilling. Our numerical results indicated that the titanium alloy drillpipe has much better corrosion resistance but only half the stress level of G105 steel under the same dogleg severity and axial force, which makes its fatigue life over 23 times longer. Eventually an economic appraisal is given by considering the full-service life of the drillpipe. The research results of this paper can provide a detailed theoretical basis and reference for field application and can popularize the use of titanium alloy drillpipe.