Horizontal Directional Drilling (HDD) is often a preferred trenchless pipe installation approach. It is completed in multiple stages including pullback, where a large reamer enlarges a previously drilled hole. The pullback stage is often overlooked in HDD design, yet it is a critical component of successful HDD installation. This article covers an overview of pullback planning, timing to have the planning completed, critical components in a good pullback planning, importance of pullback planning, and possible risks of an oversight in a pullback design.
Pullback Planning Overview
Pullback planning includes design and review of proposed pipe specification and the calculation of forces and resistance involved. It should consider scenarios for both installation/temporary conditions and long term/permanent conditions. The designer should look for aspects experienced during installation that may have permanent implications. The pullback should be designed to ensure no permanent deformation on the proposed pipe.
When it should be done: the planning is typically completed as soon as practical in the design stage, especially prior to the bidding/ contract of the project. The pullback planning will be best commenced after background information is obtained such as geotechnical investigation results, required pipe size (hydraulic requirement) and site constraints.
Download: The Importance of Pullback in HDD
Designing a Pullback Plan
During pullback motion, the pipe is subjected to axial tensile forces caused by:
- The frictional drag between the pipe and the borehole or slurry.
- The frictional drag on the ground surface.
- The capstan effect around drill-path bends.
- Hydrokinetic drag.
In addition, the pipe may be subjected to external hoop pressures due to net external fluid head and bending stresses.
The pipe’s collapse resistance to external pressure is reduced as it is in tension exerted by the axial pulling force. Furthermore, the drill path curvature may be limited by the pipe’s bending radius. Quite often, considerable judgment is also required to predict the pullback force due to a complex interaction between pipe and soil.
In a pullback design, the borehole is depicted as an ideal borehole that behaves like a rigid tunnel with gradual curvature, smooth profile, no hole collapses, complete cuttings removal, and good slurry circulation. There are several forces and pressures that the pipe would experience. (Read: How HDD Drilling Rigs Work.)
Large HDD rigs can exert up to 500,000 lbs. of pull force. Axial tensile stress increases over the length of the pull. Duration of the pull stress is longest at the pull-nose. The tail end of the pipe segment has zero applied tensile stress for zero time. The incremental time duration of stress intensity along the length of the pipeline from nose to tail causes a varying degree of recoverable elastic strain and viscoelastic stretch per foot of length along the pipe. Thicker walled pipe generally reduces stress.
Frictional Drag Resistance
Pipe resistance to pullback in the borehole depends primarily on the frictional force created between the pipe and the borehole or the pipe and the ground surface in the entry area, the frictional drag between pipe and drilling slurry. The frictional force is also dictated by the weight of the pipe.
For curves in the borehole, the force can be factored into horizontal and vertical components. The capstan effect increases frictional resistance when pulling along a curved path. In a mild curve, capstan force appears to be minimal. In tight bends, capstan force increases; therefore it’s prudent to consider the number and degree of turns or bends in the bore.
During pulling, pipe movement is resisted by the drag force of the drilling fluid. This hydrokinetic force is difficult to estimate and depends on the drilling slurry, slurry flow rate pipe pullback rate, and borehole and pipe sizes.
Tensile Stress During Pullback
The maximum outer fiber tensile stress should not exceed an allowable stress. The maximum outer fiber tensile stress is obtained by adding the tensile stresses in the pipe due to the pullback force, the hydrokinetic pulling force, and the tensile bending stress due to pipe curvature. During pullback, it is advisable to monitor the pulling force.
The Importance of Pullback Planning
Determining the amount of additional temporary workspace that may be required for the preparation of the pipe section is an important consideration. A full support design would include evaluation of horizontal curves (or “roping”) along the pullback section to confirm if the pipe section can fit within the constraints of existing right of way (ROW) and temporary workspace inflections.
This evaluation at the design stage can allow for additional temporary workspace to be obtained that would allow for the line pull to be completed in one continuous section and which would otherwise not be obtained prior to construction. If ROW and workspace cannot achieve layout in a single section due to other impediments such as elevation differences, roads or rails, alternatives such as culverts or temporary supports above roadways can be evaluated at the design stage to assess construction challenges and impacts to the public. (Read also: Easements and Contracts: The Legalities of Trenchless Projects.)
Risks of Not Prioritizing Pullback Planning
Construction is arguably the most difficult stage in the project. Therefore it’s critical to prioritize and complete the pullback planning properly. Risks that the project would be exposed where a planning was incomplete are as follows:
- Encountering unforeseen conditions during construction such as delay in project schedule, cost overrun, impact to public (for example, encroachment to neighbouring property or public right-of-way).
- No clear understanding of the pipe support requirements at the design stage, and therefore confusion in the bidding stage which may lead to project cost overrun.
- Inadequate machine capacity. Considering that machine pullback pressures increase as more of the product pipe enters the borehole during the pullback process. Also, the pullback pressures are greater for a larger diameter pipe. Additionally, the slope of the pullback pressure is greater for a larger diameter pipe. These factors should be reviewed and considered in advance prior to the site work.
- Insufficient pipe capacity. Loading on the pipe during the pullback phase increases as more of the product pipe enters the borehole. The magnitude of the loading is responsive to the installed pipe diameter, as the load is greater for a larger diameter pipe. As such, the pipe may be damaged/ experience permanent deflection that reduces its’ long term capacity.
- Jeopardizing structability of the installed pipe. Permanent structural damage on pipe may occur as strain increases during pullback.