Wonnich, Carnarvon Basin, Australia

Wonnich lies in Licence TL/8 off the Western Australian coast, approximately 25km north-west of Varanus Island. The field contains up to 214 petajoules of gas at a 50% probability level and 4.1 million barrels (bbl) of condensate.

The Wonnich 1 discovery well was drilled in August 1995. In order to delineate the reservoir, Apache carried out back-to-back drilling of the Wonnich-2 and -3 appraisal wells, in August 1997.

PLATFORM

Wonnich is developed by an unmanned platform, tied back to production facilities at Varanus Island, 31km away, from which the platform is remotely operated. The field’s well-head pressure is sufficiently high to permit a flow of the wellstream to the island, without the need for compression or pumping.

The three-leg lattice platform stands in 30m of water. The design includes a helideck, facilities for boat access and a platform crane. It incorporates chemical-injection facilities for corrosion inhibition and a utility gas system. There is a vent-collection system with a cold vent, an open-drain system and a closed-drain system, with pump disposal to export pipelines. The platform-control system consists of automation for remote control and there is a solar and TEG direct-current power system.

The platform consists of two decks. Inside, it has the necessary facilities for one producing well and the additional capacity for three spare slots for the potential tie-in of future wells. There are two production manifolds, as well as provision for a third riser.

The top of the structure stands 55m above the seabed. The topside is 115t, while the lower and upper substructure are 335t. The main piles and insert piles weigh 195t, giving a total weight of 645t.

The platform and pipeline for Wonnich are designed to produce up to 120 terajoules per day (tj/d) and the field is currently producing at its designed operating level of around AUS$0.80/day.

The jacket was built in two parts - an upper parallel-sided structure and a lower substructure including the pile guides. A parallel tri-leg lattice structure was preferred to the more common single tubular design. It was based upon the 'gas-gatherer' concept, developed by Brown & Root, for the North Sea.

Using a two-part substructure afforded the operators a significant increase of flexibility for installation, with either a conventional crane barge of a jack-up drill rig.

FABRICATION

The conceptual engineering studies for the field development were carried out by Brown & Root. The fabrication contract was awarded to BARBOS, a joint venture between Brown & Root and BOS Australia. It was carried out at BOS Australia’s Kwinana yard. The fabrication was completed at the Jervoise Bay offshore projects facility in February 1999 and it was loaded out in March that year.

The platform components were transported to Jervoise Bay for final assembly, then loaded on-board the Jaya transport barge. On arrival at the TL/8 site, the transport barge was manoeuvred under the Ensco 56 jack-up drill rig cantilever deck and secured by a four-point mooring system. The structure was unloaded using power from the jack up rig’s draw works.

This project set an Australian water depth record for installation with a jack-up drilling rig.

The Ensco 56 was also used for the drilling and setting of the piled foundations. These were drilled on a batter, with a subsea deflector frame to minimise movements of the jack-up drilling rig cantilever deck.

PIPELINE

The export system consists of two 31km-long, 8in-diameter pipelines, connecting the platform to Varanus Island. The well-stream flows to the island without any treatment other than the injection of corrosion inhibitor, which is required due to the moderately high carbon dioxide content of the gas.

Click here for printable version