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An Australian-first for HDD intersect technology

In mid-2011 Coe Drilling was awarded the horizontal directional drilling crossing contract on the Curtis Island Water and Trade Waste Infrastructure Project, which involved the first use of horizontal directional drilling intersect technology in Australia. In September 2013, Coe Drilling was recognised for its work on the project at the Australasian Society of Trenchless Technology International No-Dig Down Under 2013 event, where it won the Project of the Year 2013 Award and the New Technology Award.

The scope of Coe Drilling’s contract included the design, development, procurement and construction of two, 2,150 m, DN250 and DN500 potable water pipelines and a 2,150 m, DN250 sewer pressure main beneath the entrance to Queensland’s Gladstone harbour and the Narrows to facilitate the provision of utility services to meet demand growth associated with LNG facilities in the area.

The project heralded a number of innovative technical milestones for trenchless technology, with the first horizontal directional drilling (HDD) intersect technology in Australia used on all three harbour crossings. It also encompassed the first use of gyroscopic steering tools on hard rock crossings involving HDD intersects and also the deepest recorded high density polyethylene (HDPE) marine crossings installed at a depth of over 75 m.

Design and engineering

The use of HDD was the logical choice for crossing the Gladstone harbour, however, detailed project engineering and design was required to confirm that the proposed methodology could achieve a successful installation of the pipelines.

The initial review revealed that a conventional HDD crossing would create excessive downhole pressure on the formation leading to a potential frac-out beneath the harbour. A conventional HDD would also prove difficult to guide and maintain directional control. Given the above risks and with the sensitivity of the works within Gladstone harbour, the HDD intersect using the gyroscopic steering tool procedure was developed with Mears Inc., Coe’s parent company, which had already completed several HDD intersect projects in the United States and had previous experience with the Gyro steering tools.

The hydrofracture analysis of the formation was undertaken to understand and put in place drilling parameters to ensure that drilling fluid pressures were monitored and modelled against the design. The use of a Centrifuge as part of the drilling fluids solids separation equipment proved vital in the management of the drilling fluid downhole weight.

Product pipe selection and installation at depths in excess of 75 m had to be designed and engineered.

The detailed engineering analysis proposed to install steel enveloper pipelines within the boreholes first and then install the PN25 HDPE product pipelines within the steel enveloper pipelines. The final design for the project included a 610 mm steel enveloper for the DN500 water pipeline and a 324 mm steel enveloper for the DN250 water and sewerage pipelines.

HDD drilling operations

Drilling operations commenced in early November 2011, using the American Augers DD 1080 drilling spread. With the presence of loose overburden gravels and cobbles material, steel conductor casings were required to be installed for up to a 120 m borehole depth. In total, 508 mm and 914 mm steel casings were installed using a washover technique on the smaller 508 mm casings and applying a Grundoram Tauras pneumatic hammer, which has a dynamic loading in excess of 2,000 tonnes (t), on the larger 914 mm casing.

The pilot holes were drilled using 311 mm tungsten carbide insert drill bits driven by 203 mm mud motors using both conventional magnetic guidance systems to set the entry steel conductor casings and followed by the gyroscopic steering tools to provide guidance beneath the Gladstone harbour. The specialist gyroscopic steering tools and operators were mobilised from Europe. Simultaneous drilling operations took place from the mainland and Curtis Island followed by the use of rotating magnets to complete the first intersect on 20 December 2011. Hole opening was achieved using split bit hole openers to a maximum of 813 mm diameter on the largest of the boreholes.

As part of the procedure, the steering tools were equipped with pressure while drilling modules, which allowed the pilot hole annulas and internal pipe pressure to be continuously monitored to limit any potential drilling fluid releases and environmental damage.

Pipeline installations

On completion of the hole opening operations, the first of the prepared steel enveloper pipelines were installed. The steel casings were then flooded with water prior to installation of the pipelines, to minimise stresses in the pipeline during installation.

Commissioning of the pipelines included gauge pigging, hydrostatic testing and chlorination of the pipelines prior to tie-in to the existing works on both the island and mainland. At one stage in the operations a third 100 t HDD rig was mobilised and put to use to install drill string within the steel casing and install the DN250 pipeline while the HDD drilling and intersect works took place in the adjacent borehole less than 8 m offset.

Environmental factors

The project was situated close to the Great Barrier Reef Marine Park border, which meant that the construction methodology had to meet the stringent conditions of environmental protection and considerations set by the client and that of the Government Co-ordinator General conditions.

Such environmental significance was integral to the choice of the HDD intersect drilling methodology. The selection of a gyro steering tool ensured that accuracy was achieved without the need to lay tracking cables within the harbour in order to avoid impact on marine life.

Safety

Prestarts, toolboxes and safety training were tools used by senior management and the HSEQ Solutions managers onsite to ensure safety was always foremost in the mind. Specific programs were developed to adhere to the strong safety and health policies of the client and Coe Drilling.

One such innovation was the implementation of myosh® on the project. myosh® is a remote and real time occupational, safety and health system computer compliance management system. Utilising the capacity of this system meant that, risks could be identified early and mitigation measures implemented in real-time to limit exposure of potential risks and hazards to the workforce.

In September 2013, Coe Drilling was recognised for its work on the project at the Australasian Society of Trenchless Technology International No-Dig Down Under 2013 event, where it won the Project of the Year 2013 Award and the New Technology Award.

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