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Horizontal directional drilling is one of the most important process in Drilling Technology.Drill pipes are the most important tools for horizontal directional drilling, which can fail easily.Galling of rotary-shouldered thread connections is the usual failure style of drill pipe, which threatens drilling process safety seriously and leads to great economic losses.In past several years, lots of galling failure cases of drill pipes' rotary shouldered thread connections for horizontal directional drilling happened, which caused great economic loses and bad society influence.However, research of galling failure reason is rare at present, which leads to short of prevention for this failure style.In order to control quantity of galling failure cases, we analyzed various factors affecting galling failure process of rotary shouldered thread connections based on typical failure cases,including material chemical composition, mechanical properties, thread parameter size, thread compound, well trajectory, field operation and so on.Through the analysis, it' s found that thread parameters, compound performance, drill string helical bucking and make-up torque have great effect on galling failure process of rotary shouldered thread connections.In order to control galling failure accidents, it is necessary to control compound quality and standard field operation.