What are some of the key steps to take to ensure no damage is caused during an HDD installation?

15.08.16

There are several factors to consider. The five most important things to think about are:

Pilot Hole Profile; the as drilled profile of the bore will have an effect on the pull force and abrasion the pipe is exposed to during pullback. This maybe in areas where doglegs (rapid change in direction) have been created, which often occur at formation changes from soft to hard or hard to soft, or where radii of the pipe have not been maintained. The driller’s log and the steering engineers log and survey data should be examined on completion of the pilot hole to identify any potential areas that could be out of specification or cause a potential problem during reaming and insertion.

Hole Reaming; the speed of the reaming pass should be calculated to ensure the correct pump volume is been used for the given penetration rate. For example the cut volume of a 24” ream following a 12” pilot hole is approximately 2m³/ range 2 drill pipe. If the solids been removed are measured at 20% of mud volume and pump rate is 1000 lpm then the ream should take 10 minutes (1000 x 0.2 x 10 = 2,000 L (2m³). The drillers log should indicate the time per joint and the mud logs / test report should indicate the percentage of solids in the mud returns. Also as a rough guide a volumetric check of the cuttings stockpiled on site can be equated against the complete hole volume.

Mud Properties; On completion of the pilot hole and once the bore is open at both ends the fluid must be configured to suspended and remove the cuttings indicated in the geotechnical investigations. Cuttings suspension and transportation should be observed at the entry pit, and often cuttings will be deposited directly after exiting the bore. This implies the fluid velocity along with the viscosity (gel strengths) is important in cutting transport but as soon as the velocity decreases after exiting the bore the cuttings fall out of suspension. Mud logs and test records should be examined to understand the fluid properties that were employed for each reaming stage and their ability to suspend and remove coarse grained cuttings such as sand and gravel.

Cleaning pass; it is good practice to conduct a cleaning pass with a under gauge barrel reamer after completing the reaming pass. For example the barrel I would recommend for a 24” hole would be 20-22”. A smaller barrel would not correctly identify problem areas and potentially skip over or under any cuttings beds/ restrictions/ instability. This pass should then be used to gauge the condition of the bore and its readiness for pipe insertion. Sometimes it will be observed that sections are exposed to higher torque, which would indicate cuttings, collapse or hole shrinkage. The driller should then swab back through the section to ensure stability before completing the pass. If any concerns remain an additional cleaning pass can be rerun.

Pullback; The pipe must be correctly aligned with the borehole and enter the HDD central to the hole at the correct angle. A lifting plan should be developed to confirm the position and height of the supports. For large diameter steel pipelines and HDPE or FPVC pipes the buoyancy of the pipe should be consider as it displaces the drilling fluid from the bore. It may be necessary to fill or partially fill the pipe to create neutral buoyancy to reduce drag and therefore the insertion force and potential coating abrasion.