Australian Water Management Review Vol 2 2013 | Page 55

In order to create a continuous borehole that has been drilled from each side of the crossing and also control and monitor the drilling fluids pressures in the Pilot Hole, a number of critical HDD tools need to be used to ensure horizontal and vertical accuracy . They also measure actual Pilot Hole Drilling fluid pressures both inside and outside of the drill pipe. The external drill pipe pressure is known as the Annular Pressure. Conventional HDD Steering Tool systems used in Australia are known as Magnetic Steering Tools

and provide directional control based on sensing Magnetic North and require a surface cable system to provide a secondary verification of the Drilling Bits location when sensitive to magnetic interference. The geographic location of the Gladstone Harbour Crossing Project provided challenges and risks such as magnetic interference and the lack of access to place tracking cables on the shore and on the Harbour bed, given the volume of marine construction craft, such as work barges and crew transfer vessels operating on a continual basis between the mainland and Curtis Island. In addition any potential disturbance to marine megafauna whilst installing tracking cables would require lengthy and complex approvals to be sought and may have delays associated with the timing of the works. Coe Drilling proposed the use of Gyroscopic Steering Tools to undertake the Pilot Hole Drilling Operations. The use of such Gyroscopic Steering Tools in HDD applications dismissed the need for any surface tracking cables and ensured constant accurate positioning of the Drill Bit at all times, even in areas of high interference from metallic sources nearby. The Gyroscopic steering tool uses Optical Gyroscopes which makes it completely independent of the need to sense magnetic North and its insensitive to any and all magnetic disturbance. The Gyroscopic steering tools were equipped with additional pressure monitoring modules which allowed the drilling fluid pressure to be accurately measured both inside and outside the drill pipe. This New HDD Tooling approach allowed Coe Drilling to engage a robust drilling plan which would ultimately lead to the success of the HDD Intersect process on all three of the Gladstone Harbour Crossings. A further advantages of the Gyroscopic Steering Tool as opposed to the conventional Magnetic Steering Tools previously used in Australia is that Non Magnetic Drill Collars are not required and this means the distance between the Steering Tool Sensors and the Drill Bit is reduced by 6 to 8 meters which allows for a more accurate drilled tracectory. The published accuracy of the Gyroscopic Steering Tool is 0.04? of Azimuth ( Horizontal Directional) and 0.02?of Inclination ( Vertical Position) as opposed to the conventional Magnetic Steering Tool accuracy of 0.1? Azimuth ( Horizontal Directional) and 0.1?of Inclination ( Vertical Position). Coe completed the project in April 2013. It was the deepest recorded HDPE marine crossings installed at depth of over 75m. For a crossing of this length and depth a PN25 pipe was developed to withstand the external forces during service and also the tensile forces required to allow the installation. Following on from the recent success of the first HDD Intersects in Australia and the use of the Gyro Steering Tools, Coe has received several enquiries on the benefits and is confident that the Technology breakthrough will lead to further advances for the HDD Industry in Australia
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