Two wells from the Persephone gas field will be tied back to the North Rankin Complex (NRC). Credit: Woodside Energy.
The Persephone field will supply approximately 140 million barrels of oil equivalent (MMboe) to the Karratha Gas Plant (KGP) onshore. Credit: Woodside Energy.
Subsea 7 will employ its Seven Eagle vessel to perform its contract activities for the Persephone gas field.
The Persephone-1 discovery well was drilled by the Transocean semi-submersible drilling unit Sedco 703. Credit: Karoon Gas.

The Persephone gas field is located within Permit Area WA-1-L (Program Area) offshore north-west coast of Australia, approximately 135km north-west of Karratha, at a water depth of approximately 126m.

The field is being developed as part of the larger North West Shelf (NWS) Project, Australia’s biggest operating oil and gas project. The Persephone project along with the Greater Western Flank II project will enable the NWS joint venture to export an additional 86 million tonnes of LNG.

Front-end engineering and design (FEED) studies for the project started in the third quarter of 2013, the final investment decision was made in November 2014 and first production is expected in early 2018.

The development partners in the project are Woodside Energy (operator, 16.67%), BHP Billiton Petroleum (North West Shelf) (16.67%), BP Developments Australia (16.67%), Chevron Australia (16.67%), Japan Australia LNG (16.67%) and Shell Development (Australia) (16.67%).

The overall investment for the development of the field is estimated to be A$1.2bn (approximately $1bn). The gas field will supply approximately 140 million barrels of oil equivalent (MMboe) to the Karratha Gas Plant (KGP) onshore.

Discovery and geology

The Persephone field was discovered in December 2006 by drilling the Persephone-1 well. The vertical exploration well was drilled by the Transocean semi-submersible drilling rig Sedco 703 to a total depth of 3,366m. The well encountered 151m true vertical depth (TVD) gross gas column in Bathonian age sands of the Legendre Formation.

Persephone gas field development details

The project will involve the development of two large-bore deviated production wells namely PSA-B (PSA01) well and PSA-G (PSA02) well, which will be tied back to the existing North Rankin Complex (NRC).

The drilling of the well’s topholes is scheduled to start in late 2015 whereas the reservoirs will be drilled in mid-2016. The two wells will be drilled from a common drill centre using the Atwood Eagle mobile offshore drilling unit (MODU).

Subsea production and well equipment will include a valve manifold, a 6.9km-long and 12in-diameter flexible flowline, rigid diverless spools between the wells and the manifold, and a 7.2km electro-hydraulic umbilical (EHU). A primary installation vessel, including remotely operated underwater vehicles (ROV), is expected to be used for the installation.

The project also involves piping modifications on the NRC, installation of new equipment to control the subsea infrastructure, and new units to manage hydrates and water.

An existing decommissioned flexible riser earlier used for supplying gas from the Wanaea and Cossack fields to North Rankin A is also being removed as part of the project. This will avoid the need to install a new J-tube pipe on the NRC and enable the Persephone project to use the existing conductor to house a new rigid riser.

A 500m radius exclusion zone will be in place around the rig during the drilling phase. Two supply vessels to support the drill rig and an optional third vessel for assistance during specific operational periods will be required.

Details of the North Rankin Complex (NRC)

The NRC comprises the North Rankin A (NRA) and North Rankin B (NRB) platforms, which operate as a single integrated facility connected by two 100m bridges. It has a gas production capacity of 60,000t a day.

Featuring drilling, production, utilities and accommodation facilities, it currently supplies gas and condensate from the North Rankin and Perseus fields via two trunkline systems to the Karratha Gas Plant onshore for processing.

Contractors involved with the Australian gas field

Wood Group is the engineering, procurement, construction and management contractor for the project. Flexible riser removal works for the project are being performed by Fugro-TSM.

Subsea 7 is providing fabrication, transportation, installation, diving services and pre-commissioning works for the project, employing its Seven Eagle vessel.