Ford island waterline replacement

The primary water transmission main serving Ford Island and the Pearl Harbor Naval Complex was over 75 years old, posing a significant risk to ongoing and future operations. A failure of this critical line could result in prolonged water outages, potentially disrupting essential functions at Ford Island and the Pearl Harbor Naval Shipyard. To enhance system reliability and support future development, a replacement waterline was planned and constructed as part of this project. Fukunaga & Associates, Inc. was selected by Naval Facilities Engineering Command Pacific (NAVFAC Pacific) to provide engineering design services for the new transmission main.

While Horizontal Directional Drilling (HDD) is a well-established construction method used globally, this project required an expanded application of conventional HDD to overcome a number of unique and complex site-specific challenges. The most significant technical hurdle was installing a pipeline beneath Pearl Harbor, spanning over 5,000 linear feet from shore to shore - well beyond the feasible range of a single HDD pull using standard thermoplastic piping. 

Several alternatives were explored. One involved installing a steel casing via HDD and then pulling the plastic pipe through the casing, followed by grouting the annular space for support. Another option was a traditional underwater trenching method, which would have required dredging, laying the pipe on the harbor floor, and backfilling - an approach that risked disturbing sediment potentially containing hazardous materials and posed significant environmental concerns. Additionally, the dredging operation could have impacted the foundation of the nearby Admiral Clarey Bridge and compromised Navy harbor operations, given the area must accommodate aircraft carriers and other large vessels. To ensure safe clearance, any pipeline would need to be placed at least 10 feet below the harbor bed. 

FAINC and their HDD/ geotechnical subconsultant, URS, ultimately developed an innovative dual-bore solution. Early in design, they considered an underwater splice using two bore paths to avoid the limitations of a single, extended HDD pull. However, design refinements were necessary to avoid mid-pipeline high points that could trap air and reduce hydraulic efficiency. Bathymetric surveys revealed a sharp drop-off beyond a shallow shelf near Ford Island. This allowed for a refined two-bore profile: Bore No. 2 originated on the Ford Island side and exited at the toe of the drop-off, while Bore No. 1 started from the Halawa shoreline and exited just past the main navigation channel. The resulting sweeping alignment avoided any problematic elevation changes. 

The underwater splice was reinforced with crushed stone backfill and secured using articulated concrete matting to both protect and ballast the pipeline. Proper anchoring and ballast volume were essential to maintain pipeline position during both installation and long-term operation.

The remainder of the Ford Island waterline system — about 19,550 linear feet - was constructed using a combination of traditional HDD and open-trench techniques. Overall, the project was completed for $12.0 million, coming in under the original $12.6 million construction budget. 

This project showcases how established technologies can be adapted and expanded to address complex engineering challenges. By employing a dual-bore horizontal directional drilling (HDD) technique, the team successfully exceeded the typical limitations of single-entry HDD installations. This innovative two-bore approach enabled the placement of a pipeline nearly twice the length typically achievable with conventional HDD methods. While underwater dive operations were necessary to complete the mid-channel splice, the extent of this work was significantly reduced compared to what would have been required using traditional underwater pipe laying techniques.  

This project's modified horizontal directional drilling (HDD) approach involved several intricate design considerations. These included optimizing the vertical alignment to maintain minimum pipe bend radius, managing pulling forces on the high-density polyethylene (HDPE) pipe string, and designing a hydraulic profile that eliminated mid-span high points. Permanent ballast was also incorporated along the harbor bottom to ensure the long-term stability of the installed pipeline. 

To protect harbor waters, special containment measures were implemented at the mid-channel bore exit to control drilling fluid discharge. Detailed analysis was conducted to assess borehole stability, potential heave, and hydro-fracture risks under various drilling fluid conditions. Finite element modeling was used to evaluate soil-structure interactions and to ensure that HDD would not compromise the stability of the nearby Admiral Clarey Bridge. These evaluations confirmed that HDD was the optimal approach, offering a lower-risk alternative to traditional trenching methods. 

Precision during construction was critical. Tru-tracker offshore navigation technology was employed to guide the drill head along the intended bore path, avoiding interference with bridge foundations and ensuring a smooth, consistent pipe alignment. Pullback loads were continuously monitored to prevent overstressing the pipe, preserving its pressure rating and service life. The installation also required complex overwater staging, including barge-mounted support systems and dive operations for the final underwater pipeline splice.