How are the pulling forces and pipe stresses calculated during a pullback in HDD?

The Pulling forces and pipe stresses during the pullback phase of HDD These are calculated by estimating the traction required to pull the pipe through the drilled borehole, taking into account the pipe’s length, diameter, submerged weight, buoyancy, friction with the soil or fluid, alignment geometry, bend radii, external pressure, pipe stiffness, and the material’s allowable strength. The objective is to verify that the pipe can be installed without exceeding allowable stresses, deformations, ovalization, minimum radii, or joint and lining limits.

In projects of horizontal directional drilling, infrastructure crossings, river and watercourse crossings or when running pipes under existing utilities, the pullback calculation must be performed before determining the diameter, material, SDR/wall thickness, bend radius, drilling mud, pulling equipment, and installation procedure.

What forces are at work during a pullback?

During the pullback, the pipe is not only subjected to tensile forces. It may also be subjected to bending, external pressure, friction, localized thrusts, and stresses on linings or joints.

Key variables in the calculation

To estimate the pulling force, the following are analyzed:

1. Length of the intersection

The longer the length, the greater the contact area and the greater the accumulation of friction. The pulling force tends to increase as the pullback progresses, especially on long, curved, or uneven sections.

2. Pipe Diameter and Thickness

The diameter affects weight, buoyancy, stiffness, contact area, and external pressure. The wall thickness, or SDR, affects tensile strength, resistance to ovalization, and the ability to withstand external pressure.

3. Pipe Material

The calculation varies depending on whether the material is HDPE, steel, cast iron, FRP, or other materials. Each has different allowable limits for tensile strength, bending, ovalization, deformation, fatigue, temperature, welding, or joints.

4. Submerged Weight and Buoyancy

In HDD, the pipe is inserted into a borehole filled with mud or fluid. That is why the actual weight of the pipe, taking into account its own weight, its internal contents if filled with water, external buoyancy, and the density of the sludge.

The pipe may tend to float, rest on the wellhead, or rest on the lower generatrix, depending on the balance between its weight and buoyancy.

5. Friction

Friction depends on the contact between the pipe and the borehole, the coefficient of friction, geometry, drilling mud, roughness, curvature, effective weight, and the quality of the pre-expansion. An irregular, collapsed, or insufficiently cleaned borehole can significantly increase the actual pulling force.

6. Geometry and Radii of Curvature

The HDD route includes entry and exit angles, a curved section, and a horizontal section. The radii must be compatible with the pipe material, stiffness, diameter, weld or joint, and allowable deformation.

7. Drilling mud

Drilling mud affects borehole stability, lubrication, cuttings transport, borehole cleaning, and friction. Poor borehole cleaning or inadequate rheology can increase pulling forces and the risk of sticking.

How it is calculated conceptually

The calculation is usually broken down by sections of the route:

1. Entrance section
The pull required to initiate the drag, overcome supports and rollers, determine the angle of entry, and establish initial contact with the borehole is verified.

2. Curved section
The combined effect of traction, bending, and friction is calculated. In curves, contact may increase, and with it, the required force.

3. Horizontal or deep section
The cumulative friction along the intersection is estimated, taking into account the effective weight and fluid pressure.

4. Outbound Leg
Final forces, exit angle, radius, alignment control, and the risk of damage to the lining or joints are checked.

The total pull-out force is obtained by summing the resistances of each section and applying allowances for geotechnical uncertainty, actual friction, borehole cleaning, variations in drilling mud, and contingencies.

Stresses that the pipe must withstand

The piping must be inspected for:

Axial tensile stress

The maximum tensile force during a pullback must be less than the material's allowable stress, taking into account temperature, duration of loading, welds, joints, fittings, and the safety factor.

Combined tensile and bending stress

In bends, the pipe is subjected to both tensile and bending forces simultaneously. It must be verified that the combination of these forces does not exceed the allowable capacity.

Ovalization or deformation

For flexible pipes, such as HDPE, it is verified that deformation during installation and operation is compatible with the SDR, external pressure, depth, and soil conditions.

Buckling or collapse due to external pressure

If the pipe is subjected to external pressure from sludge, groundwater, or soil loads, its resistance to collapse is verified, especially in empty pipes or those with low rigidity.

Damage to the coating

In metal or protected pipes, it is verified that tensile, bending, contact, and friction forces do not damage the anticorrosive coating.

Joint and Connection Capacity

If the pipe is not continuously welded, the tensile strength, angular deviation, leak tightness, and joint integrity must be verified during dragging.

Measures to Reduce Pull Forces

To limit the forces during the pullback, the following can be applied:

• Optimize the path and radii of curvature.
• Increase the input or output radius.
• Improve the cleanliness of the borehole.
• Perform the appropriate pre-expansions.
• Monitor the rheology and flow rate of the slurry.
• Use a compatible lubricant.
• Partially fill the pipe to check buoyancy.
• Use appropriate rollers and supports on the surface.
• Minimize obstacles, sudden changes, or irregularities.
• Control the pullback speed.
• Avoid prolonged downtime.
• Check the capacity of the shooting equipment.
• Protect surfaces and joints.
• Continuously monitor actual force.

Coordination with the technical assistance and engineering for trenchless construction sites It helps validate the layout, equipment, piping, and procedures before construction begins.

What Is Monitored During Execution

During the pullback, the following are recorded:

• Instantaneous and maximum pulling force.
• Feed rate.
• Installed length.
• Sludge pressure and flow rate.
• Fluid rheology.
• Return volume.
• Incidents involving jamming or increased friction.
• Stops and restarts.
• Behavior of rollers and supports.
• Condition of the exterior cladding.
• Surface alignment.
• Internal pressure when the pipe fills up.
• Ambient and material temperature, if applicable.
• Final leak or pressure tests.

This data makes it possible to compare theoretical calculations with actual conditions, detect abnormal friction, and take action before the pipe's allowable capacity is exceeded.

Risks of Insufficient Calculation

Underestimating firing forces can lead to:

• Exceeding the allowable stress.
• Excessive elongation.
• Damage to welds or joints.
• Ovalization.
• Collapse due to external pressure.
• Damage to the anti-corrosion coating.
• A jam during the pullback.
• Pipe break.
• Loss of seal.
• Need to abandon the drilling operation.
• Cost overruns and delays.
• Failure in final exams.

At crossings under roads, railways, waterways, urban areas, or environmentally sensitive areas, these risks can affect permits, service continuity, and environmental safety.

Information Needed to Review the Project

To calculate pulling forces and stresses during the pullback phase of HDD, the following information should be provided: crossing length, outside diameter, wall thickness or SDR, pipe material, weight, stiffness, operating pressure, external pressure, internal fluid during installation, plan view and profile, entry angle, exit angle, bend radii, depth, geotechnical conditions, water table, mud type, fluid density, rheology, pilot borehole diameter, pre-reaming diameter, estimated friction coefficient, HDD equipment, pulling capacity, cleaning method, rollers, supports, joint type, anti-corrosion coating, manufacturer’s allowable limits, acceptance criteria, and contingency plan.

Request a Technical review of thrust forces, stresses, and pullback feasibility in HDD before finalizing the route, diameter, pipe material, or RFQ documentation.