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Specialist subsea drilling for Woodside North Rankin A

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USING a specialist subsea caisson drilling machine developed by Furmanite , eight caissons of varying diameters have been successfully fitted with arrestor bars on Woodside Energy’s North Rankin A platform, on Western Australia’s North West Shelf, situated 135 kilometres offshore north-west of Dampier.

The bars were fitted to prevent the eight tonne caisson pumps from falling to the seabed, some 150 metres below, in the event of a failure of the pump or its pipework.

To fit the bars, sixteen 190 mm holes had to be drilled in the caissons at depths of eight metres below sea level. These caissons were of three different outside diameters: 750 mm, 900 mm and 1012 mm.

Furmanite’s brief was to develop diver-operated equipment and maintain it throughout the drilling process, and also to provide a technician to instruct the divers during the operation.

The company purpose-designed a hydraulic subsea drilling machine measuring 1.25 metres wide and 3.25 metres long, and weighing some 620 kilograms. Adjustable legs enable the machine to be externally mounted on the three different caisson diameters.

The machine was designed in two halves, with a removable hinge pin, enabling it to be split for simplified installation on each caisson.

Special heavy duty cutters were manufactured to drill through the caissons. Some of these were over 80 mm thick (comprising a 16 mm outer steel skin, 50 mm grout and a further 15 mm inner steel skin).

Furmanite also supplied a hydraulic pump arrangement, which was slung below the rig platform during operations, and 120 metres of hydraulic hose. This connected the hydraulic pump to the subsea caisson drilling machine.

Due to the weight of the pump, precise positioning of the arrestor bars in the caissons below the pumps was required, notes Furmanite Australia general manager Colin Bickerstaff.

The distance had to be “short enough so that if the pump fell it would not shear off the arrestor bar”, explained Bickerstaff.

“The positions of the holes were marked on the caissons but, because accuracy was paramount, they had to be checked before the final holes were drilled,” he said.

To ensure that the distance between each hole and the underside of the pump was accurate, Furmanite’s drilling machine was used to cut initial 80 mm inspection holes.

“By initially drilling a smaller inspection hole than the 200 mm hole required for installation of the bars, this allowed for a degree of adjustment to take place to ensure the precise positioning of the final hole,” Bickerstaff pointed out.

Before going offshore, trials were undertaken successfully off Fremantle to fit and function-test the arrestor bars and the specialist installation equipment required.

These included an in-air fit of the drilling and measurement equipment on a dummy caisson test piece, and in-water diving trials to function-test the equipment on the dummy caisson piece by simulating the installation procedure of an arrestor bar and hole plug.

Offshore, the project was completed in just 10 days, with arrestor bars successfully installed on all eight caissons.

Commenting on the achievement, Woodside Energy’s completions and subsea systems project engineer said, “I was impressed with the double drilling machine. It was well suited to the application and performed well.”

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