While the basic steps of a shore approach remain the same as a conventional HDD crossing (i.e. pilot, ream, pullback), there are additional challenges in working within a marine environment.
- Pilot Hole Drilling
The drilling tools and rig equipment selected for each job are largely determined based on the results of the geotechnical investigation and the size of the crossing (length and diameter). During the pilot hole drilling, a directional guidance system is used to navigate the pilot hole along its pre-designed profile. In some cases, the pilot hole is drilled from the primary rig at the entry point onshore to the secondary rig located at the exit point offshore following a previously designed profile and alignment.
- Hole Enlargement
In small diameter crossings the pipeline may be directly installed in the pilot hole. However in most cases pilot hole enlarging, known as “pre-reaming,” will be necessary. Pre-reaming is required to provide a bore diameter large enough so that the pipeline can be installed in the drilled crossing. Based on the final desired diameter and soil conditions, this process may include one or more stages.
Once the drilled hole has been enlarged to the required diameter and cleaned adequately, the product pipe, which will have been preassembled offshore (or assembled onshore and then towed offshore), is readied for installation. The pipeline string is aligned behind the underwater borehole exit point and then pulled into the hole towards the onshore drill rig until installation is complete. Variations to this basic scenario may be needed to fit actual conditions.