Pneumatic hammering may be used to assist with pipe installation and/or removal of stuck pipe during horizontal directional drilling and straight bore crossing operations. Pipe stresses induced during hammering are often poorly understood or undocumented.
To gain a better understanding of the induced stresses, elastic wave analyses were performed to simulate stress waves which travel from the struck end of the pipe and back via reflection from the other end. Due to the nature of the reflections, even compressive hammering (e.g., for pullback assist) results in tensile stress waves (and potential fatigue damage), which sometimes goes unrecognized.
To predict the damage accumulation at planar circumferential flaws (e.g., girth weld defects) from repeated hammering, flaw growth analyses were performed via Paris Law fracture mechanics formulations. An initial flaw based on NDE detection limits was grown to the maximum permitted by API 1104 weld quality requirements. This assumes that the hammered pipe will be used within the pipeline construction assembly and must therefore meet initial inspection requirements for screening of in-situ welds.
For this study, different combinations of pipe diameter, wall thickness, pneumatic hammer model, input energy and soil type were investigated to discover trends and determine limits on hammering force and hammering hours. Allowable hours for a particular set of parameters are provided, along with recommendations concerning monitoring of hammering operations, constructability considerations, and future work to refine the methodology presented herein.