APPLICATION OF THE HORIZONTAL DIRECTIONAL DRILLING METHOD IN THE DESIGN AND CONSTRUCTION OF A WATER

PIPELINE RIVER CROSSING

Ir. Abd Isham Abd Karim, P. Eng., MIEM, Director,HLA Associates Sdn Bhd, Kuala Lumpur Malaysia.

ABSTRACT

Normally in Malaysia, the design and construction of water pipeline river crossings are by aboveground bridged crossings. Use of trenchless crossings is uncommon. This paper highlights the recent trenchless construction technique of Horizontal Directional Drilling, traditionally used by the oil and gas industry. This technique is now being applied to an ongoing water supply project in Terengganu, Malaysia. Details of this construction method are described along with the planning and design considerations required for application to two pipeline river crossings in the project. Construction risks involved in the application of this technique are also mentioned.

KEYWORDS

Construction risks; horizontal directional drilling; planning and design; river crossings; trenchless; water pipeline.

1.0 Background

In a water supply scheme, apart from the sourceworks (intake and treatment plant), the pipeline transmission system is a vital link in the water supply distribution system. In the Besut Water Supply Scheme Stage 2, a new water transmission pipeline is required to connect two new reservoirs being constructed.

This pipeline will mainly traverse along existing road shoulders and right-of-way, occasionally crossing public roads, crossing numerous streams and two main rivers in the Besut District. These two rivers are the Besut River and Keluang River, the former being the larger watercourse. The mild steel pipeline gravitates from the new Bukit Puteri reservoir with a nominal pipe size of 600mm and crosses the Besut River. It later tapers to a nominal pipe size of 500mm before the Keluang River crossing, before feeding into the new Bukit Bubus reservoir.

Locations of the above crossings are shown in Figure 1 below.

It has been decided that the above two river crossings will be executed by a trenchless construction method to minimise impacts on the environment and to existing river traffic at these two major rivers in Besut District. The following will highlight the salient features of the Horizontal Directional Drilling (HDD) method that has been adopted for the water pipeline river crossings.

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2.0 Development of Horizontal Directional Drilling (HDD)

The HDD method was developed in the 1970’s by combining techniques used by conventional road boring and the horizontal directional drilling of oil and gas wells. It is a trenchless installation technique developed to install pipelines under natural and man-made obstacles, especially waterways and highways. Today it is also used for parallel installations along the right-of-way as an alternative to digging trenches.

In Malaysia, the HDD technique was initially adopted by Petronas for laying their gas pipelines beneath rivers, in the early 1990’s. Today its use has been widespread to the other utility sectors including water supply.

3.0 Basic HDD Process and Its Advantages

The HDD process in brief, begins with the boring of a small, horizontal (pilot hole) under the crossing obstacle (i.e. the river) with a continuous string of steel drill rod. When the drill head and rod emerge on the opposite side of the crossing, a special cutter, called a back reamer, is attached and pulled back through the pilot hole. The reamer bores out the pilot hole so that the pipe can be pulled through. Usually it is pulled through from the side of the crossing opposite the drill rig.

In the Besut project, a steel casing pipe is first pulled through the river crossing before the main carrier pipe of a size smaller than the casing is inserted. This main water pipe inserted is then pulled through in a similar way as the casing pipe. After installation, the annular space between the casing pipe and carrier pipe will be filled with pumped cement grout.

The above directional crossings have the significant advantage of having the least environmental impact of any alternative method of river crossing. It also offers

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optimum depth of cover under the obstacle thereby providing maximum protection to the pipeline and minimizing maintenance costs. As most of the work is confined to relatively small areas on either of the riverbanks, river traffic is not interrupted. These directional crossings can also have a predictable and short construction schedule. Safety concerns associated with trenching or open cutting in rivers are also eliminated.

4.0 Planning and Design

In the planning of the river crossing, various factors need to be considered. Firstly, there must be adequate and proper access at both sides of the river for working space and set-up of the drilling rig and associated equipment. In the Besut project, a rectangular area approximately 45 metres long (L) and 30 metres wide (D) need to be prepared at the entrance or rig side. The longer side being parallel to the proposed drilling direction and the mid-point of the shorter side facing the drilling direction at the entrance to the proposed drill hole. Operations will be facilitated if the area is level, hardstanding and clear of overhead obstructions. Since a large volume of water is required for the mixing of the drilling slurry during the drilling operation, a nearby source of water is necessary.

At the HDD exit area, a similar sized area is required at the exit point itself, including a receiving pit excavated to collect any bentonite / slurry discharged from the drill hole during backreaming and pullback. For the Besut project, the receiving pit sizes are 5m (L) x 2m (W) x 1.5m depth (D). Here also a sufficient length of workspace should be provided to fabricate the casing pipe, and later the main water pipeline, into one string.

Secondly, prior to any design work, subsurface utility investigations need to be carried out. Use of geophysical investigating methods such as electromagnetic pipe detectors to determine the horizontal position of subsurface facilities is usually necessary. Locating or trial pits should be carried out to expose the underground facility at critical points along its path to determine the horizontal and vertical position of the facility. The located facility information is surveyed and mapped. These data provides precise plan and profile information for the subsequent crossing design work.

Design considerations for the river crossings that the HDD designer should consider and design for include the following:

Minimum horizontal and vertical clearance requirements to existing surface features, expected future river bottom considering scour or planned dredging, and existing under ground utilities and structures. (For the Besut project, a minimum depth of cover of 5.5 metres is required under the lowest section of the river crossing.)

Casing or carrier pipes and reamer diameter requirement. Normally the over- sizing of the reamer diameter ranges from 120% to 150% of the casing or carrier pipe diameter. (For the Besut project, the casing pipe diameter used is 914 mm.)

The bore geometry for the given ground profile including bore lengths and depth requirements, bending radii for the final casing or carrier pipe (for the

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Besut project, a minimum radius of 1000 metres is typically specified), and entry and exit pit locations with the required plan size and depth.

Drilling equipment requirements for the given geotechnical conditions, geometry, and final pipeline diameter including thrust and pullback ratings, mud motors, jetting heads, wireline or walkover tracking systems.

Equipment and material handling requirements including drilling fluid and its containment, drill operation and final pipeline staging.

Pipeline material strengths, capacities and jointing methods.

Stress analysis for the pipeline during installation such as spanning stresses while the pipeline is on roller conveyor, the curvature, tensile and hoop stresses occurring during the pullback operation and after installation.

The plan and profile of the Besut River and Keluang River HDD Crossings are as shown below in Figure 2 and Figure 3 respectively.

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5.0 Geotechnical Investigation

The geotechnical investigation is normally the first step in a HDD project. Upon completion of this investigation, a determination of whether HDD is feasible can be made. The design of both the casing and carrier pipe and the installation can then begin.

The number of exploration holes is dependent on the crossing length and the complexity of the underlying soil strata. For a crossing of about 300 metres, a borehole made on each side of the crossing may suffice. However, for the Keluang River crossing, an additional borehole was made in the mid-river section. All boreholes were terminated at 21.45 metres below existing ground level.

Generally, all boreholes should be made to a minimum depth of 12 metres (40 feet) below the lowest point in the river crossing or 6 metres (20 feet) below the proposed depth of the crossing, whichever is greater. Where possible the boreholes should be conducted at least 7.5 metres (25 feet) off the proposed centreline. These boreholes should be grouted upon completion to help prevent the loss of drilling slurry during the crossing installation.

6.0 Construction Execution

6.1 Pilot Drill

The drilling operation begins by drilling a pilot hole using the hard soil drilling unit (140mm Jaw-shape drill bit), the steering tool and the high-pressure injection of bentonite slurry. Drilling is carried out continuously for an interval of 4.57 metres (approx. 15 feet), equivalent to one length of drill pipe.

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Guidance of the drill bit is monitored using two sets of a Downhole Survey System namely Digitrack and Short Steering Tools. The former is a type of walkover downhole survey system, where the sensing unit (sonde or transmitter) is fixed to the rear of the drill bit. This sensing unit transmits the location of the drill bit back to the receiver, held by a survey engineer above ground, by producing an electromagnetic field that is received and interpreted by the receiver. From this information, the location of the drill bit can be determined at all times.

Meanwhile the latter is a type of wireline tracking system where the transmitter transmits drill head information (pitch, roll, inclination, and azimuth) to the receiver via wireline installed along the inner diameter of the drill rods. This transmitter is fixed behind the Digitrack’s transmitter. The Short Steering Tool will only be used along with the Digitrack to track the drill head when it is under the river.

The accuracy of the drilling is then verified by comparing readings of the actual drill bit location and the planned drill profile at any particular point. The engineer can then be able to either adjust the drilling profile or pull back a certain distance and re-drill to the correct profile. The pilot drilling operation is continued until the drill head exit at an accepted point. The location of the drill head for every three metres is recorded in a log sheet. Drilling parameters are also recorded every 30 minutes or more frequent as necessary.

6.2 Pre-reaming or Hole Enlargement

The pilot drill assembly (sonde housing and drill bit) is then dismantled once it exits at the other end of the drill hole. A 354mm diameter Fly-cutter reamer is attached to the drill rod at the exit pit and then pulled and rotated back into the pilot hole. A swivel is attached to the back of the hole-opener to allow the drill rods to remain free from rotation while the rig is rotating the drill string. Drill rods are attached from the exit pit as the hole-opener is being pulled.

During this hole enlargement process, bentonite drilling mud is pumped under high pressure through the drill string to the reamer. Bentonite fluid stabilizes the tunnel and transports the cut materials out from the created tunnel. This process is repeated for successively increasing diameters of 457mm, 660mm, 914mm and finally 1220mm. To enhance the hole opening process a centralizer is attached at the front of the reamer.

The required final size of the enlarged tunnel is at least 30% bigger than the size of the casing pipe to reduce friction during the pullback operation. The pre-reaming parameters are recorded every 30 minutes or more frequent as necessary.

6.3 Preparation of Steel Pipes and Pullback Operation

During the hole enlargement process, the 914mm diameter American Petroleum Institute (API) 5L Grade B casing pipe is strung along the drill alignment at the exit. These pipes are then jointed by qualified welders for a length of 440 metres for Keluang River and 262 metres for Besut River. The casing pipe string is then placed on rollers prior to the pullback activity.

The API pipe string is then connected to the drill string via a swivel and connected to a 1067mm diameter swab reamer. The swab reamer is to clean and condition the

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enlarged tunnel during pulling in the pipes. During pullback activity, pulling pressure, rotation torque and other parameters are monitored and recorded into the log sheets to ensure the API pipes are not over stressed. The pulling back operation on the API pipe string towards the entrance area by the drill rig will stop when the pull-head reaches the tie-in pit location. The installed casing pipe will be left in a proper end-capped condition for the subsequent insertion of the mild steel water pipe and tie-in works.

Exhibit A below summarises the above construction process.

Exhibit A

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7.0 Construction Records and As-Built Documentation

The HDD Contractor needs to keep detailed and accurate records of all activities associated with the HDD process. Upon completion of HDD installations, the Contractor shall provide the Client with As-Built plans and any supporting documents for records. The HDD construction records and As-Built plans shall include the following:

Daily operator logs and HDD tracking data. These field records and operator notes shall specify the type of tracking equipment used, the length and depth of the HDD installation, and additional information such as steering adjustments and other equipment performance parameters.

As-Built plans shall be derived from the tracking data and operator logs. The drawings shall indicate at a minimum, horizontal and vertical HDD alignment (dimensioned), and existing utility horizontal locations and depths (where exposed or at trial pit locations).

8.0 Management of Risks during HDD Operation

No construction operations are without any risks. Similarly in HDD operation, it is prudent for the HDD Contractor to plan for contingencies to minimise or manage the risks involved. Some of the risks identified in the application of HDD to the pipeline river crossings are:

Sticking of drill pipe

Recovery of drilling tools in case of obstruction or jammed cutting head

The pipeline sticking or stuck during pullback

Equipment breakdown

Pilot hole deviates out of the predetermined parameters

Bentonite fluid flow into the river

Workmen safety while crossing the river during pilot drill

9.0 Conclusions

From the foregoing, the HDD technique has potential in the design and construction of water pipeline crossings in the country. Once completed the above river crossings will provide much information, and knowledge will be gained in the application of this technique in the water industry. Problems that will be encountered undoubtedly can provide useful lessons for the planning and implementation of future trenchless water pipeline crossings.

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Acknowledgments

The author wishes to thank Mersing Construction and Engineering Sdn. Bhd. for providing the information used in this paper and many thanks to Cawangan Bekalan Air, Ibu Pejabat Jabatan Kerja Raya Malaysia for consent in using the project information.

References

Guidelines for a Successful Directional Crossing Bid Package (1995). Guidelines Committee of Directional Crossing Contractors Association, USA.

Guidelines for Successful Mid-Sized Directional Drilling Projects (1997). Mid-sized Rig Task Group of the Directional Crossing Contractors Association, USA.

Procedure for Installation of Proposed 500mm MS Water Main Pipeline across Sg. Keluang and 600mm MS Water Pipe across Sg. Besut in Terengganu through Horizontal Directional Drilling Method Using Slurry (2003). Mersing Construction Sdn. Bhd., Kuala Lumpur, Malaysia.

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