CASE STUDIES
OAHU RESEARCH STATION
Tying Island Into Mainland Utilities
PROJECT SUMMARY
Coconut Island is a small island off the coast of Oahu, Hawaii, which houses a university research station. All sewer, fiber, telephone and cable utilities were originally installed along the seabed floor. Over time, the conduit for the utilities began to corrode and integrity of the utilities became compromised. This time, horizontal drilling was chosen to tie the island into mainland utilities. This HDD solution involved drilling a 457 mm (18 in) subsurface borehole running under Kaneohe Bay, which would contain a bundle of two HDPE lines: a 102 mm (4 inch) telecommunications conduit and a 152 mm (6 in) force main sewer line. This project was successfully completed within a 64-day timeframe.
KEY CHALLENGES
The island is extremely small with little space for construction operations. As the exit point for the drill, it offered limited area to contain drilling fluid. The HDD Company proposed an innovative solution. The pilot hole was drilled 50 m (164 ft) shy of exit. Push ream methodology was used on the plugged pilot hole. By tripping the reamer back to entry side multiple times, debris was mechanically removed, eliminating the risk of covering the island with drill spoils and fluid. Because there was limited space for the drag section on Coconut Island, crews were required to string the product lines around a walking path prior to installation.
Ground conditions were mostly basalt, with very soft beach sand on the exit. As a result, the geometry of the bore path required a horizontal bend at the approach to exit, which demonstrated our navigational precision.
