| Recently, drill tool work condition becomes more and more complex as increasing the well depth. There are neither provisions about drill tool’s fatigue performance test nor requirement about fatigue performance of high strength drill tool in the relevant API standards. The150ksi,160ksi drill tools have been widely used in ultra deep well drilling abroad instead of135ksi drill tool which is the highest strength in the present API standards. So, a materials selection technology based on drill tool fatigue performance is important to ultra-deep well drill tool materials development, test and selection.Based on researches of a large number of literatures on drill tool failure, drill tool failure reason and mechanism are analyzed in the paper. Convention mechanics performance of commonly used drill tool domestically and a new drill pipe materials are also tested. The probabilistic fatigue strength of the five kind of drill tool material is test by small sample up-and-down method and the P-S-N curves also have obtained through fatigue life distribution model selection and fatigue stree-life fitting. On this basis, test method of fatigue strength and P-S-N curves of a real drill pipe are discussed.The modified fatigue performance test method provides basis for materials selection technology. Moreover, drill tool fatigue influence factor including mechanics, inclusions, H2S are also studied. It improves the drill tool material performance index in API standars from the point of fatigue performance. Based on the above researches, the following conclusiongs can be got.(1) Compared with shallow well, fatigue failure caused by bottom hole assembly vibration and inversion is more frequent. Drill tool fatigue failure could be divided into three stages:crack initiation, crack metastable growth and crack unstable propagation.(2) The strength and impact performance of the test materials (S135, G105,4145H, X2and X1) meet the demand of relevant API standards. The strength of X1reaches160ksi.(3) Small sample up-and-down method is recommended in fatigue strength experiment and normal distribution model is recommended in faigue strength data process. Three Parameters Weibull distribution model is recommended in fatigue life determination so the S-N curves under different survival probability P can be got. Based on drill tool material P-S-N curves, the real drill tool P-S-N curves could also be determined when stress concentration, scale effect and surface roughness are taken into account.(4) The parameter in Murakami’s fatigue strength-the biggest inclusion size equation has been revised so that more accurate fatigue strength could be calculated.(5) A drill tool material selection and evaluation method has been initially formed. The properties indicators (strength, fatigue strength, inclusion size) of high strength drill tool for ultra-deep well are discussed in the method. It can provide guidance for ultra-deep well drill tool development, test and selection. |
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