The Otway Gas Project covers the Geographe and Thylacine gas fields, which are located between 55 and 70 km offshore Port Campbell, Victoria in 85 and 100 m of water respectively. The project includes the installation of a 20 inch gas export pipeline and a 4 inch service line between the offshore fields and a new onshore gas treatment plant.

The shore crossing was a key component of the project, with Horizontal Directional Drilling technology used to install casing pipes to house the two pipelines.

This shore crossing project was unique in many ways and some of the key aspects included: the five month detailed design phase, which was undertaken as an integrated team with Woodside in Perth; the innovative management strategies to deal with the environmental commitments made by Woodside for the Rifle Range site; the use of a retractable microtunnelling technique to pre-install 210 m of 48 inch conductor casing and the installation of threaded casing (10 and 29 inch) over the drill lengths of 820 m based on the pipe being neutrally buoyant.

The scope of work included up front detailed design and construction engineering, site preparations works, installation of both conductor casings (pre-installed) and associated foundations works, mobilisation of drilling equipment, supply of 29 inch casing (10 inch casing FI by Woodside), drilling of 2 holes and installation of casing pipes and finally installation of bellmouths complete with covers and site clean-up and rig demobilisation from site.

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Installation of conductor casings

During the detailed design phase it was mutually decided that both the conductor casings would be pre-installed before the drilling was scheduled to commence. The 20 inch conductor casing was installed using a small laser guided boring machine.

A major component of the project was the award of a $A2 million dollar contract to DJ & MB MacCormick for the installation of a 48 inch casing at a 17º angle over approximately 210 m. Specialist microtunnelling equipment was purpose designed and built for this application. Typically microtunnelling is performed from pit to pit to facilitate tunnelling machine removal but in this case the casing had to be installed and the tunnelling machine retracted. The tunnelling went very well and took approximately 15 days to complete. See Page 58 for more info.

Drilling and casing installation

Drilling operations commenced late in January and the usual small problems were encountered. These problems were quickly overcome to allow the installation of the 29 inch casing into the 39 inch hole by mid March. The smaller hole was completed on March 28, essentially bringing an end to the drilling/onshore operations, aside from some miscellaneous grouting works, site clean-up and demobilisation.

The installation of threaded casing based on a neutrally buoyant situation downhole was a relatively new concept, as typically long welded pipestrings are used. Issues included design and selection of casing and threads, casings handling and clamping of casing to facilitate screwing together and undoing if necessary.

Marine and diving works

The final activity was the removal of the bullnoses on the casings pipes which had been necessary for installation and the fitting of bellmouths and covers to facilitate the future pipeline installations. Divers were required to carry out these works and in total, Perth-based Fraser Diving took four days to finish the job.

The completion of this shore crossing brings over four years of planning, design and execution to fruition. The end result was a successful project completed on time and under budget, providing a model for how HDD shore crossings should be undertaken on all future projects.