Pullback, or pullback force, in the context of horizontal directional drilling (HDD), can be defined as the tensile load that is applied to the drill string when it is withdrawn to the entry point through the bore hole. Usually, when the drill string is pulled back, the pipe is simultaneously installed. Calculating the pullback force is therefore very important before a project is started. When the pipe is pulled back for installation through the hole, it will experience different stresses and load conditions, which should be estimated to help prevent the failure of a pipeline project.
Pullback is the final and most important stage of the horizontal directional drilling (HDD) process.
Trenchlesspedia Explains Pullback
In the pullback stage of trenchless construction, the pull section of the pipe is made up and supported by some combination of roller stands or pipe-handling equipment at the bore's exit side. When the reamer or bit exits the end of the bore, the pull section is attached to drill string behind the bottom hole or reaming assembly using a swivel. The pull section is then drawn through the bore's exit side toward the rig, using the bottom hole or reaming assembly.
HDD is still relatively new in the field of trenchless construction. Often, engineers and owners rely extensively on the contractor to determine the feasibility of a project. These contractors usually consider factors influencing the drill rig, successful boring, reaming and pulling of product pipe back through the hole if it is within specified tolerance levels. However, the stresses on the pipe itself are often not considered. It is therefore essential for the engineers to give a design to the contractor that takes into consideration the stresses and load that a pipe will experience, and can withstand, rather than considering the forces that the HDD rig will experience. Some of the aspects of a bore that affect pulling loads include bore hole diameter, the stability of surrounding soil, subsurface conditions, drilling fluid properties and buoyancy control measures.