The use of directional drilling techniques for pipeline river crossings has increased sharply over the past few years in Canada and the United States. Improvements in drilling technology and increased experience among a growing number of specialty contractors has helped to reduce the cost of directionally drilled installations and to reduce the risks. The advantages associated with reducing disturbance of the water course by the use of directional drilling are often considered to outweigh the additional costs typically associated with the method.
While the advantages of using directional drilling methods are compelling, the technique is not universally suited to all river valleys due to considerations of valley topography and geological setting. Specifically, there are certain geological and geometrical conditions that make the method completely unsuitable. In other cases, the geology beneath the river channel and the valley geometry may present a challenge to a drilled installation that can be overcome with adjustments to the design and drilling technique if anticipated.
The implications of encountering unfavourable geological conditions during construction can be significant. The implications can range from substantial construction cost overruns up to several times the original bid price, to installations that cannot be safely put into service and must be abandoned. Under certain geological and geometrical conditions, the risk of blowout or fluid leakage to the water course during installation may be significant.
The role of geotechnical and subsurface investigations to identify geological conditions prior to commencing construction is more critical for a drilled installation than for conventional trench techniques, as the consequences of encountering unanticipated conditions can be much more severe with drilled crossings. In addition, a trenched crossing is inherently more flexible than a directional crossing in terms of the ability of the contractor to adapt to different conditions than those anticipated at the start of the work.