The Longtom Gas Field, which was first discovered by BHP Petroleum in 1995, is located offshore Victoria’s eastern coast in the Gippsland Basin, 31 km from Orbost.

The development project was a result of a gas sales agreement negotiated between Nexus and Santos. Santos agreed to purchase and process 350 PJ of sales gas and 4 MMbbl of condensate from the gas field through its Patricia-Baleen Gas Plant, which was modified to accommodate the Longtom gas.

From here, gas is delivered into the Eastern Gas Pipeline, which connects the Melbourne and Sydney gas markets, and condensate is trucked and sold to the Shell refinery in Geelong. First gas was achieved in October 2009, and gas from the field is capable of supplying 10 percent of Victoria’s annual gas requirements.

The Longtom Gas Development consists of three subsea wells and production trees in water depths of approximately 51–57m, 19 km of offshore pipeline connecting the subsea wells to the Patricia Baleen offshore gas pipeline, a high integrity pressure protection system (HIPPS) and control umbilicals. The development makes Nexus the third producer of gas in Bass Strait in 40 years.

Connecting the pipe

The 300 mm internal diameter Longtom pipeline runs from the pipeline end manifold (PLEM) at the future Longtom-4 subsea tree location through the Longtom HIPPS to the Longtom-5 PLEM at the offshore end of the Patricia Baleen pipeline. It includes a 150 mm diameter tie-in spool from the Longtom-3 subsea tree to the pipeline. Future tie-in spools will be installed between the pipeline and the future Longtom-4 and -5 subsea trees.

Trident Darwin Offshore Logistics Base Joint Venture was contracted to perform all pipe and umbilical installation activities for the project.

The gas pipeline was laid by the newly-built Aussie-1 pipelay barge, owned and operated by Trident Australasia. The barge has a cargo carrying capacity of 11,000 t and a 250 t deck crane, and can operate in water depths ranging between 4 and 120 m. Coated line pipe was already loaded on the vessel when it left Singapore.

A smooth process

Mr Pinzone was the Nexus Senior Environmental Adviser at the time of project construction. “From an environmental perspective, planning for and installation of the pipeline was straightforward, due in large part to the non-sensitive nature of the receiving environment and the small scale of the offshore works,” said Mr Pinzone.

“The fact that the Longtom gas pipeline was to connect with existing offshore infrastructure helped pave the way for a relatively smooth environmental approvals process. This meant that a shore crossing through the sensitive (geomorphologically, ecologically and archaeologically) sand dunes of the Ninety Mile Beach and the wetlands behind them was not necessary, and the construction of another gas processing facility was avoided.”

Mr Pinzone noted that another “bonus” for the project was the fact that the existing Patricia-Baleen gas pipeline was mothballed at the time of the Longtom tie-in, so there were none of the usual issues associated with connection to a live pipeline.

Being compliant

A Referral made under the Environment Protection and Biodiversity Conservation Act 1999 was approved by the then Commonwealth Department of Environment and Heritage in October 2006, while an offshore Environment Plan submitted under the then Petroleum (Submerged Lands) Act 1967 was approved by the Victorian Department of Primary Industries (DPI) in May 2007 (with a subsequent addendum).

Part of Mr Pinzone’s role involved guiding two members of the DPI around the pipelay barge as they conducted their own audit, but Mr Pinzone said there were no non-compliances with the Environment Plan for the pipelay.

“The DPI audit was conducted in addition to another audit on the West Triton jack-up drill rig in July 2008 when it was drilling the Longtom-4 production well,” he added.

Seeing it come together

Being an environmental practitioner more familiar with the onshore installation of pipelines and audits of offshore drill rigs, Mr Pinzone said he found auditing the Aussie-1’s compliance with the Environment Plan an exciting experience.

“Watching the pipe string being welded together in the ‘firing line’ in such a rapid manner and with such tight quality control was awe-inspiring,” he said.

Mr Pinzone also added that the team had spent years planning the campaign, so watching the pipeline resting on the seabed via the remote-operated vehicle vision was reassuring.