The Government of the Hong Kong Special Administrative Region through the Drainage Services Department (DSD) awarded a US$6.5M ground investigation contract for HATS Stage 2A on 14 November 2006. This is the first of two major ground investigation contracts planned for the scheme, which will intercept sewage from the north and southwest of Hong Kong Island and convey it to Stonecutters Island for treatment.

Under the contract some 40 vertical and directional boreholes will be drilled to investigate rocks at the tunnel location. Norwegian firm Devico, specialists in directional core drilling technology and borehole surveying, will design the inclined and directional boreholes at the project’s proposed shaft locations. These will be performed using the DeviDrill system, which consists of wireline steerable core barrel, measure while drilling system, and PC software package for borehole design, planning, and survey calculation and reporting. Continuous rock sampling along the tunnel alignment at strategic locations is also being performed using directional coring techniques. Over 5000m of directional core drilling has been undertaken so far.

Borehole design

The aim is to obtain as much information as possible from the directional boreholes for use during further design and tunnel construction. The borehole paths follow the tunnel alignment and are restricted to the safety envelope. Core samples OD 31.5mm are collected, with the paths initially determined based on the specific tunnelling requirements at the location under consideration.

The actual borehole profiles are designed using Devico’s software design package DeviSoft, and various scenarios are discussed with the project Engineer, Metcalf & Eddy/Maunsell JV. The optimal borehole path is then selected and the information passed to the responsible drilling contractor for execution.

Experience gained on other ground investigation projects in Hong Kong, such as the Eagle’s Nest twin tunnels, Tsuen Wan Drainage Tunnel Project and Landslide Preventive Works at Po Shan Road, where granitic rocks dominate, has shown that the optimal length of the directional borehole is in the range up to 1200m -1300m. When drilling longer horizontal boreholes special care is required for orientation of the steerable core barrel and drill string tripping operation.

Drilling operations

Following drill site and drill rig setup, commencement of the drilling operations begins. The long horizontal drill paths mean much higher frictions on the drill string, when compared to vertical boreholes of the same length. It is therefore important that the drill rigs used have the necessary power in terms of torque, WOB (weight on the bit), and pull and push forces to run the drill string in and out of the borehole.

The drilling process includes several steps. First, conventional drilling is employed to drill the straight sections of the planned borehole. Here a wireline NQ core barrel system is used. When the borehole reaches the section where steering is required, the DeviDrill system is adopted.

The drill string is pulled out of the hole and, following hydraulic testing and adjustment of the dogleg settings (the eccentric sub that is responsible for the deviation of the borehole in the direction set), the DeviDrill is employed. Orientation is performed and drilling is initiated. Control borehole surveys are carried out in order to determine whether the borehole is following the designed path. If needed corrections are performed by changing the dogleg setting of the DeviDrill.

For the initial section of the boreholes, where unconsolidated, weathered granites are present within the soil matrix, a casing string is used to maintain borehole integrity. The casing string is not cemented and after completing the borehole it is pulled out.

Drilling fluid additives such as polymers and lubricants are used to reduce the friction between borehole walls and drill string. During the directional core drilling strict and precise control of the drilling parameters – WOB, rotation speed, mudflow rate, etc. is required. These parameters directly affect daily production rates.

Maintaining the mud pump pressure to guarantee a stable tool face is one of the main factors in controlling the borehole path after orientation.

In addition to the geotechnical conditions at the location, deviation response during borehole steering in the directional borehole sections has shown a dependency on the dogleg setting of the DeviDrill system. Experience from the HATS project shows that a build-up rate in the range of 10-12°/30m is the limit where drilling constrains do not influence the overall drilling process.

During drilling, borehole instabilities have been faced in some sections. In these cases cement plugs and grouting are used to stabilise the rock. Then the drilling operations are resumed.

Further applications

Directional core drilling applications provide indisputable advantages compared with conventional wireline drilling. The ability to closely follow borehole trajectory tolerances using this technology helps obtain a better quality of geological information, as well as reducing drilling and project time. Savings in the range of 50% can be both feasible and realistic.


Drill site and drill rig set up Devico project manager Grigor Topev explains the DeviDrill system principals to Drainage Services Department engineers DeviDrill hydraulic test before running in hole DeviDrill orientation Dogleg eccentric housing