The utility industry is regularly required to call on an enormous and varied range of specialists; from mapping, to drilling, to wastewater treatment, to asset management, to pipe relining, to pipeline integrity, to land access, to risk management, and the list goes on. To make the process a little easier, Utility magazine is bringing together experts from various fields to answer your questions.
There are several factors to consider. The five most important things to think about are:
1. 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, 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.
2. Hole reaming
The speed of the reaming pass should be calculated to ensure the correct pump volume has 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 two drill pipe. If the solids being removed are measured at 20 per cent of mud volume and pump rate is 1,000Lpm, then the ream should take ten minutes (1,000 x 0.2 x 10 = 2,000 L (2m3).
The driller’s 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.
3. 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 the cuttings indicated in the geotechnical investigations must be removed.
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 strength), 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.
4. 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.
5. 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 considered, 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.
About Charles Stockton
UK-born Charles Stockton has been a part of the HDD sector in Australasia since 2003. He is the Managing Director of Stockton Drilling Services, a leading engineering consultancy specialising in HDD and other trenchless pipeline installation methods.
