The development has recently received ministerial approval under the WA Aboriginal Heritage Act, and the formal Final Investment Decision for the Pluto project is to be considered in mid-2007. Engineering design for the onshore and offshore facilities remains on schedule to be completed in mid-2007.
The Pluto gas project will comprise an onshore processing train with a production capacity of 5 – 6 MMt/a of LNG located on the Burrup Peninsula. The multi-billion project will involve the development of the Pluto and Xena gas fields, located about 190 km from the Burrup Peninsula in the North West Shelf Area, which contains approximately 4.5 Tcf of dry gas and approximately 48 MMbbl of condensate.
Wellstream gas and liquids from the riser platform, including produced water will be transferred via a subsea gas trunkline from the riser platform to shore for treatment in the initial years of operation.
Once received at the onshore gas processing plant, the gas, condensate and other liquids will be separated. The gas will be processed into LNG, and the LNG and condensate will be piped to storage and export facilities.
Offshore development drilling for the Pluto gas field will be staged, commencing initially with three to seven wells and up to twelve wells in total as the field matures. Wellstream products will be delivered to an unmanned offshore riser platform located in a water depth of 80 – 85 metres via two manifolds and flowlines. The riser platform will not support any processing facilities but will be equipped with control systems.
Subsea installation activities will be conducted from specialist surface vessels, which will control Remotely Operated Vehicles (ROVs). The ROVs will be used for activities such as installing the manifolds and hooking up flowlines.
Following construction of the second compression platform, required in around 2017, the wellstream gas and liquids will be transferred from the riser platform to the second platform for partial produced water removal. Produced water removed will be discharged to sea from the second platform. Following removal of produced water, the gas, condensate and residual produced water will be transferred from the second platform via the trunkline to the onshore gas processing plant.
Spanning 180 km, the subsea pipeline provides one of the key challenges for the development. Currently, two sites are being considered for shore crossing. The first reaches landfall at the Karratha Gas Plant while the second reaches at Holden Point.
In both cases, the pipeline will traverse through seabed which is comprised of igneous rock that exceeds the cutting capability of conventional dredging equipment. This challenge will be overcome by pre-treatment with drill and blasting techniques to ensure the construction of a suitably deep trench for trunkline stabilisation and protection.
Due to the deep water and cold temperatures encountered at the Pluto gas field, hydrate formation in the flowlines and trunkline will need to be managed. The formation of hydrates in the flowlines can be prevented with a combination of chemical treatment and thermal insulation of the flowlines. Chemical treatment will involve injection of monoethylene glycol (MEG) which will be recovered onshore and returned to the offshore facility by a separate return pipeline.
Construction of the offshore gas trunkline is anticipated to commence around April 2009. The shore approach and pre-dredge activities will commence approximately six to eight months in advance of pipelaying. It is anticipated that it will take approximately six months to lay the trunkline.
A series of onshore pipelines will be required including a trunkline delivering gas from the Pluto gas field to the gas processing plant. The total onshore section of the gas trunkline will vary according to the route, and is expected to be in the range of 1.5 – 3 km.
All trunkline construction activity will be confined to a 50 m construction corridor. The construction of the onshore gas trunkline is anticipated to take one month to complete.
Other smaller onshore above ground pipelines will be required to transport LNG, condensate and boil off gas water and waste water between the gas processing plant at and the storage and export facilities. Additionally, sub-cooled LNG will be piped from the processing plant to the LNG cryogenic tanks through two insulated pipelines.
The considerable infrastructure associated with the development will require an ongoing maintenance campaign.
Some of the elements include:
* Intelligent Pigging – as a wet trunkline, the trunkline will require inspection pigging to demonstrate that corrosion risks are being effectively managed.
* Corrosion Management – periodic pigging may be required to assist corrosion management by distributing corrosion inhibitors, displacing stagnant water and removing deposited solids, although this is considered unlikely.
* Liquid Management – operational strategies will try to limit accumulation of liquids under all operational conditions. Additional actions may be required to limit liquids under unusual severe turndown scenarios. These include pigging of the trunkline, or periodically gas sweeping the fluids through the flowlines.