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Otway Gas Project – Microtunnelling retraction project

On behalf of the Otway Gas Project Venture, Woodside is developing gas condensate fields located off the south west coast of Victoria in the Otway Basin. The project involves installing a 20 inch gas export pipeline and a 4 inch service line between the offshore fields and a new onshore gas treatment plant.

Woodside awarded the HDD Shore Crossing to Drilltec Australia who engaged DJ & MB MacCormick Civil Engineering to undertake the microtunnelling retraction.

A key element of the project was the shore crossing where a microtunnelling retraction system installed a 1200 mm outside diameter steel casing in 6 m lengths, welded flush joints and wall thickness of pipe 20 mm at an incline angle of 17 degrees with an end location approximately 120 m below ground level. Once reaching this target location the machine and equipment was to be retracted back to surface.

The next stage involved HDD operations, using the casing to ensure no break out of drilling fluids/environmental impacts in these softer ground strata. Upon reaching the end of the casing, works continued approximately 800 m out to sea in more stable, harder ground and finished at sea bed level. From here, divers were to disconnect the equipment and attach the gas pipe for pullback operations.

The planning, design and procurement for the microtunnelling procedure took over ten months. This stage involved thrust shaft design, custom designing of weld joints and pipe, contingency planning, project specific quality, safety and environmental plans, testing the various ground stratas to ensure the lubrication mix was adequate, safety workshops in identifying potential hazards and risks and the development of action plans.

The process was undertaken by MacCormick in conjunction with Woodside, Drilltec, Eric Jas, Herrenknecht and Sinclair, Knight Merz. MacCormick awarded the supply and design of the specialised tunnelling equipment for the project to Herrenknecht and the design of the thrust shaft to Sinclair, Knight Merz.

The tunnelling equipment from Herrenknecht was tested in their workshops prior to shipping. The quantity of tunnelling equipment required for the project was considerable, with three 40 foot and two 20 foot containers required. Following the arrival of the equipment in Melbourne port, four hours of road transport to get to the project site was required. The shipment was then unpacked ready for works and retesting prior to use.

Within five days all containers had been unloaded, the concrete pad for the jacking rails constructed, the thrust block, entrance ring and gasket were placed and all cables, power, separation systems and tunnelling equipment were installed in the thrust shaft.

The project commenced on site from 6 December 2004, with shipping containers arriving on site from December 9. On December 14 the machine was launched and driven through the entrance ring.

The Project Manager for the tunnelling operations was Donald MacCormick. In risk analysis it was decided that works would be required 24 hours a day using two crews to ensure no risk exposure to high jacking tonnage which would have occurred if the machine had been stopped for any considerable time, increasing the friction of the pipes. A further consideration was that bentonite ports could not be placed along the pipeline, given the HDD operations required the internal casing to be flush smooth to ensure equipment did not catch on any obstacles during operations.

The works progressed at a rate of two to three pipes per shift, including the welding of pipe joints for each 6 m length of pipe.

On December 29 the final completion point of the microtunnelling operations was achieved and on December 30 the pullback system methodology was discussed with the work team and developed by JHA. At this point the jacking frame was changed to entrance wall for commencement of pullback. On commencing the pullback system the initial critical point was the shearing of the bolts, reached at 66 tonnes. All pullback tonnage after this was around 26 tonnes.

The equipment was removed at each joint location as the retraction works were completed during day shifts only. On January 3 at 10 am the tunnelling machine had returned to the thrust shaft. In total the operation was completed in 20 days.

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