Trenchless Engineering Services (TESPL) based in New Delhi, India was awarded a prestigious project 36 in (914 mm) diameter pipeline project in Taiwan consists of installing two crossings including:

• A 36 in (914 mm) and 12 in (323 mm) and 6 in (150 mm) (22.4 mm wall thickness) gas pipe over a length of 1,800 m pipe bundle running parallel to the gas pipeline over a crossing distance of 1,962 m.
• A 36 in (914 mm) diameter (22.4 mm wall thickness) gas pipe over a length of 1,570 m beneath the Da-Jia River using the Intersection Method.

The project was unique because of several complications with respect to normal HDD Projects.

• The crossing was across sea channel with very limited space for coiling.
• The drill profile had a 36o curve.
• The pipe side was limited and hence three strings had to be prepared which would be welded during pulling.
• 45 m harbour sheet piling both at entry and exit were to be avoided, hence the guidance had to be accurate.

TESPL had further evaluated the design engineering requirements and selected the right equipment for execution of the project. TESPL mobilised its 400 ton HK Rig along with all the necessary auxiliary and supplementary equipment. The company also established contingency plans in case of any problems or unexpected mishaps which included a 500 ton Pipeline Thruster and Pipeline Pneumatic Hammer for retrieval of the pipeline.

The Pipe Thruster and Air Hammer, whilst included in the contingency plans for retrieving pipe if necessary, were also to be utilised for adding thrust to the pipe during the final pipeline pullback operation.

As per the design, the prevailing ground had a back-fill to 3 m deep with a mixture of cobbles, gravel and sand. To mitigate this TESPL designed a cofferdam of 20 m horizontal length from the drilling entry point which was 4 m wide excavated between sheet piles. The coffer dam sloped at an angle of 12o. This resulted in the dam reaching the 3 m depth to the sea bed sand formation ensure the route was clear from debris. As TESPL did not intend to have to handle any cobbles within the drilling fluid returns the site team installed a 12 in (300 mm) diameter casing over a length of 120 m. This also allowed for reinforcing of the drill string preventing buckling during drilling and maintaining uninterrupted mud returns back from the bore to the entry pit.

A DC Beacon was set on the drill path to double check the drilling accuracy. The DC Beacon consists of two solenoids A and B which are energised with 24 V DC power supply. This helped the steering team to establish the right signals from downhole probe as it helps in places where a surface coil could not be laid or to double check coordinates in high interference areas when surrounded by lot of steel on the drill path. A Paratrack 2 surface coil was also laid with cable protection on the cable where road crossing areas were encountered.

A slurry pump was installed by the solid controls recycle pit to pump the soil cuttings from the drill site to an adjacent yard for further disposal. With the pilot bore almost to the reception side of the sea channel, the assembly entered the Paratrack 2 surface coil area of influence and as well as that of the Large DC Beacon placed at the edge of the bank. No surface coil could be laid over the water covered area. The Large DC beacon did however provide a range of 200 m of signal area which helped the drilling team locate the drill head some 200 m prior to reaching the shoreline. The pilot bore successfully punched out on line inside the designated exit area side Cofferdam. Subsequently reaming stages were executed for 24 in (610 mm), 30 in (760 mm), 36 in (914 mm), 42 in (1,067 mm), 48 in (1,220 mm) and finally 44 in (1,118 mm) diameter to give a clean pass.

As the ground conditions has been coarse sand all the way, the drilling team used barrel reamers sized under the diameter of the previously reamed hole diameter, followed by a fly cutter designed to cut to the next reaming size. This way the barrel reamer at the front acts as a hole stabiliser for the fly cutter enabling it to cut a uniform hole instead of sinking downwards during its cutting process. The barrel reamer in the front of the fly cutter also swabs the previous cut by fly cutter. This tool combination cleans the previous cut hole and stabilises it along the new cut.
Meanwhile, when the drilling operation was going on, on the reception side of the bore and simultaneously to the drilling works the pipeline was laid out and welded to length. As the job site had space constraints, it was not possible able to lay the required 1,800 m of pipeline in one continuous length. The design was to lay it in three multiple strings. One string was 640 m long with two more at 540 m each which meant the project was carried out with two golden joints.
However, this job was designed with the Buoyancy control method because the 36 in (914 mm) diameter pipeline was buoyant. So the design was to install an 18 in (458 mm) diameter pipe in the 36 in pipe along with a 6 in (150 mm) diameter steel pipeline to act as a water feed pipe.
To install the 18 in diameter pipe inside the 36 in diameter product pipe, the design was to weld roller saddles at uniform intervals along the length and slide the smaller pipe in thereby not damaging the inner coating of the product pipe and as well making for easier installation. The 6 in (150 mm) diameter water feed pipe was pre-installed in the 18 in (460 mm) pipe, before (914 mm diameter pipeline. placing this inside the product pipe.
On reaching the final stage of reaming and clean at 44 in diameter, as per the contingency plan the Pipe Thruster was positioned and anchored in case by any chance the buoyancy control over the pipe failed. This was meant to reduce any upward lift and reduce the pulling load on the pipe. Also if the pulling load happens to exceed the rig pulling capacity then the Pipe thruster would help to thrust the pipe in from the reception side.
The design called for an exit angle for gas pipe of 6 o , so TESPL had to dig down to bring the Thruster down to match this height and match the incline alignment with the exit angle. A concrete dead man was cast under the thruster to anchor it.
With the cleaning pass ready and the pipe string ready, buoyancy control water tanks were set at the end of the strings to pump water into the 18 in (460 mm) pipe via the 6 in (150 mm) diameter steel pipe inside it.
A water flow meter with a Non Return Valve was installed on the delivery side of the water pump to keep a record of the amount of water being pumped to maintain uniformity as the pipe was being pulled into the hole.
A Pull back Assembly with a 34 in (860 mm) Barrel reamer and swivel with a capacity of 500 tons and a Knuckle joint were connected to product pipe puller head.
The 500 ton Pipe Thruster had the capacity to retrieve the pipe from the ground as well as to install a pipe in the ground. The Thruster played an important role in this project by enabling the moment of inertia to be initiated once pulling in started after the 8 hour long standby period required for the completion of each of the golden joints.
The unit also helped in trying to match and fit the golden joints precisely over the two sections of the pipeline as it enabled the installed pipe to be moved backwards if necessary. As pulling in of the first pipe string advanced, the next string was moved into place on the same alignment rollers from where the previous string had been pulled.