HDD installation methodology; cleaning the bore - Part 2

This blog is part of our deliver better outcomes series.

Our Technical Sales Manager Clay recently wrote Part 1 of HDD installation methodology; cleaning the bore. He looked at project design, fluid design and the pilot phase. These steps are essential to allow the next stages to be completed successfully.

In Part 2 - Clay looks at the ream and installing the pipe.

Ream

Following the pilot phase; a reamer is attached to the drill rods (most commonly at the far end of the bore), and pulled back towards the rig following the pilot hole. The style of reamer is influenced by the formation type. Hard rock requires a different design to sand or clay. Having good geotechnical data at the start of the tender process, allows the contractor to purchase the correct tooling, which may have manufacturing and freight lead times to contend with. 

When planning the reaming phase, the speed at which the reamer can travel is related to the volumes of mud being pumped and the reamer's rotational speed. The formation needs to be cut to the appropriate size and mixed with the drilling fluid to enable it to flow out of the bore. The ratio of fluid to cuttings is determined again by formation type, with some formations requiring higher ratios to allow the fluid and cuttings to flow out. 

Essentially the correct ratio of reaming speed and pump volume allows the cuttings to be turned into a thin slurry, which will flow easily along the bore to the exit pit.

During reaming, attention should be given to the mud level in the pits. Typically, the mud level will be slightly higher than the crown of the bore to provide hydrostatic pressure and help stabilise the bore. Managing these levels is critical to controlling pressure within the bore and enabling effective cutting transport. 

Pullback speed is usually calculated using proprietary software or based on observing the flow of drilling fluid and cuttings from the bore. If the pullback speed is too fast, you risk not drilling the pump. This is where the fluid pump rate is not high enough to produce the correct cuttings/drilling fluid ratio and a slurry-like cutting flow. The cuttings coming out of the hole will thicken so they will not flow, and borehole pressure will potentially rise, resulting in a frac out.

Depending on the drill rig's final cut size and capability, the ream may be conducted in stages. In this situation, barrel stabilisers are normally used in front of the reamer for each reaming pass to prevent the reamer from sinking into the bore and interfering with the alignment.

Observing the cuttings flow into the pit can provide information on how successful the reaming is. A cuttings flow with a thin flowing consistency, suspended cuttings, and substantial flow shows the correct ratio of fluid to cuttings. This will generally result in a clean bore. A cuttings bed, which is too thick to flow but is rather extruded in a sausage-like fashion, indicates the fluid-to-cuttings ratio is out of balance. The bore risks retaining a cuttings bed, over-pressurised, and potentially damaging adjacent services through hydrofracture and heaving.

The volume of fluid pumped during the reaming phase is higher than during the pilot phase; however, due to the larger hole size, the pressure is lower. 

In a larger diameter bore, many contractors will use fluid recycling systems to reuse water and drilling fluid additives. Recycling fluid itself has economic advantages by enabling the reuse of large volumes of fluid and chemicals. It also has environmental advantages by reducing project volumes. 

Recycling fluids incurs maintenance requirements that need to be respected. Drilled solids pass through mechanical equipment, making them smaller and more difficult to process/remove. Beneficial chemicals are also removed from the system while removing the solids. The benefit of having our qualified drilling fluid engineer on site is around regular fluid testing, observing changes in real-time, and making adjustments to bring the drilling fluid back to specification.

Pipe pull

The pipe can be installed once the contractor is confident the bore is clean. 

Large-diameter pipe can float when it enters the bore, causing friction between the top of the pipe and the bore roof. This can increase resistance to the pressure the drill places on the pipe, damage the pipe, or damage the bore, potentially altering the grade. To prevent this, the pipe can be ballasted in. Our qualified drilling fluid engineers can calculate a ballast solution whereby water is pumped into the pipe as it is installed, intending to sink the pipe into the middle of the bore.

If the bore is clean, the pipe will be installed with very low pullback pressure on the drill. 

If you observe the pit the pipe is moving towards and only drilling fluid with suspending cuttings is displaced from the hole, then you can have confidence the bore path was clean. 

If you observe large amounts of semi solid cuttings being displaced, it is evident there is a cuttings bed which was not cleaned during the reaming phase. 

The resistance this causes may be overcome by the power of the drill. The risks associated with doing things this way could impact the integrity of the bore or the asset being installed, including frac out and tension on the pipe greater than the rated loading, causing stretched, snapped, or stuck pipe.

The preference is to be more effective at cleaning the hole on the pilot and reaming passes so that when it comes to pipe pull, there is only limited material to clean from the hole, and pressures can be maintained at a low level.

From the design to the installation of the pipe, each phase of the drilling process contributes to the success or otherwise of the next phase. There are a myriad of technical details and challenges to solve, as well as opportunities to improve the quality of the installation and provide greater value for money to the asset owner.

We work closely with councils, engineers, and contractors on all stages of the design and installation process. If you would like to talk through any stage of your HDD installation process, our team is here to help. Whether it’s project design, fluid design, the pilot phase, reaming, pipe pull, or the whole process, give us a call. Our Australia team is available on 03 9068 5688, and the New Zealand team on 07 849 2366.