HDD | TRENCHLESS
Left: Figure 4: SBU
Right: Figure 5: NPS 72 rotating head Bottom: Figure 6:
Drill pipe string break #1
Prime Contractor with The Crossing Co as HDD contractor, and The Tunneling Co (TTC) responsible for installing the entry and exit casings. CCI performed onsite inspection team to ensure compliance and quality.
Casing Installation At the entry location, 42m of casing was installed against 32m planned, while the exit location utilised a telescoping casing method, consisting of 72m of NPS 72 casing, was installed within 41m of NPS 84 casing. The project parties chose to embed the casing deeper but standard installation procedures had changeover at the bedrock interface to a forward boring unit. Similarly, at the exit location, reaching the target depth
was critical due to a thick gravel layer above bedrock. Hard ground conditions, including large cobbles, presented difficulties for augering and a Small Boring Unit (SBU) was ultimately used. Hollow stem augers were used to distribute bentonite slurry to the face of the cutting wheel where clay/ silt might be met.
Water Infiltration Plan A comprehensive water mitigation plan was developed to address potential groundwater ingress into the HDD bore. At the upper slope, at the entry point a rotating head was used, capable of slow fluid flushes. Also, supplementary containment infrastructure was constructed and the access road could take heavy traffic under soft ground conditions. During the wet spring season, the entry pad experienced significant flooding in June 2023 and delayed the start of HDD operations by three weeks.
Pilot Hole/Reaming Prior to initiating pilot hole operations, a 16” centraliser within the casing helped to ensure proper alignment. Drilling progressed smoothly but achieving the desired build rate beyond the bore bottom presented challenges. Adjustments were made to the mud motor bend angle, increasing the sub angle to improve steering performance. On the exit side, operations were impacted by fluid losses and reduced circulation. The 30” reaming pass with a TCI reamer took 15 days; the 42” reaming pass also proceeded without major complications with a TCI reamer and took 21 days.
Drill Pipe Breaks 1 & 2 In the 54” reaming phase, a significant volume of cuttings was observed and vacuum trucks helped to manage
removal. A parasite string increased fluid flow and improve cuttings transport. But a drill pipe failure occurred at a measured depth (MD) of 910m from the entry rig side, within a section of the bore not supported by drilling fluid. The break was located in joint 1, directly connected to the reamer. Three attempts failed to reach and fish out the broken pipe; video inspection helped; a fourth attempt— extracting the assembly with the reamer—got lodged in the bore; a secondary string was deployed to clean and remove cuttings. After a total of nine days the reamer was retrieved. After reaming resumed, resistance at 942m MD resulted
in significant tool wear; a collapse was suspected. Reaming operations switched to the exit side, first with a 42” pass to stabilise and enlarge the caved-in zone; and then the 54” reamer was run and then the 60” reaming commenced from the entry side, only for high torque readings and a switch over to the exit rig to continue boring. But the pipe then failed directly behind the reamer, at 930m MD, in a portion of the bore not supported by drilling fluid; only 75m of 54” bore remained to be reamed out to 60”. Fishing and video attempts were made. A mechanical disconnect left three pipe joints attached to the reamer within the borehole.
Left: Figure 7: Drill pipe break #1
October 2026 | 31
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