Located in Victoria’s offshore Gippsland Basin, the Longtom Gas Field Development involves the production of gas through horizontal subsea wells, tied back to Santos’ recently upgraded onshore Orbost Gas Plant.

The 12 inch diameter, 17 km Longtom Pipeline has been constructed as part of the gas field development. The pipeline links two subsea gas production wells in the gas field to the existing Patricia Baleen Pipeline operated by Santos.

Developing the field

The Longtom Gas Field was first discovered by BHP Billiton in 1995. Since the VIC/L29 licence covering the field was awarded to Nexus in 2003, two wells have been drilled in the field with gas encountered in each.

Nexus Drilling and Operations Manager John Ah-Cann said “The Longtom-3 well, drilled in July 2006, confirmed the commercial potential of the Longtom field when an estimated flow rate of over 75 million cubic feet per day of gas was recorded during the second production test over several reservoir sections.”

Nexus signed a gas sales agreement for Longtom with Santos in December 2005. Santos agreed to process up to 450 petajoules (PJ) of raw gas from the Longtom field through its existing Patricia Baleen facilities near Orbost.

Nexus made a final commitment to the development of Longtom in April 2007 following an independent reserves certification and commercial arrangements for financing. A production licence was granted in September 2007.

In September 2008, the second horizontal development well, Longtom-4, was drilled and successfully flow tested.

Tying back to Patricia Baleen

The three-week pipelay operation commenced in April 2009. Pipe joints of 12.2 metre length were welded together using semi-automatic welding processes and non-destructive testing was carried out using automatic ultrasonic testing technology. The maximum lay rate achieved from the pipelay barge Aussie 1 was in excess of 2.3 km or 190 joints per day, and at its peak the Aussie 1 had over 175 personnel on board.

The project experienced some completion difficulties when the original offshore installation contractor was unable to complete the project with the original dive support vessel. Nexus resolved the problem by contracting the TS Marine Rem Etive to complete the project. This vessel completed installation of the pipeline tie-in spools and other subsea hardware such as the High Integrity Pressure Protection System (HIPPS) and Subsea Control Unit (SCU).

Divers from the vessel were also used to connect up and assist with leak and function testing the electro-hydraulic control system, and pre-commissioning works.

Gas production from the Longtom project commenced on 22 October 2009.

Protecting the pipe

The subsea HIPPS package was installed to protect the pipeline downstream of the HIPPS from over pressure.

“Being a ‘step out’ from the existing Patricia Baleen facilities, the design had to take into consideration specific constraints and plant and operation interfaces issues,” said a Nexus spokesperson.

“The design of the subsea HIPPS brought about some engineering challenges. Two very difficult sinusoidal horizontal wells had to be drilled, and the success of the Longtom project depends on the HIPPS package.”

Staying safe

Nexus has said that the management of safety during the construction phase has been the highest priority.

The company had dedicated safety supervisors present on the offshore vessels throughout the Longtom Pipeline installation.

Health, Safety, Environment and Community Manager Michelle Zaunbrecher said “A focused safety meeting was held prior to every task and activity.

“The strong focus on safety paid off with very good safety performance where the project had no serious safety incidents or any Lost Time Cases.”

Community and environment

During the installation operations for the project, Nexus utilised local infrastructure including ports, industrial areas, accommodation and supplies. A local vessel from Lakes Entrance was used to ferry out fresh food and supplies.

“Nexus also employed up to 160 people on the vessels – ten in Eden, and a further 20 in Melbourne and Perth during the offshore campaign,” said a company spokesperson.

In addition, ongoing employment for a number of local people is being provided at Santos’ Orbost Gas Plant.

Ms Zaunbrecher said “As the Longtom project is located a significant distance offshore, the main stakeholders with an interest in the project are commercial fishermen.

“Nexus undertook regular consultation with the fishing community prior to and during the Longtom installation activities.”

All activities associated with the project, for both the installation and ongoing production operations, underwent environmental impact assessment processes that were approved by regulatory authorities.

“There is very limited offshore infrastructure involved in the Longtom Pipeline project and hence it has a very small environmental footprint, with no significant environmental impacts,” said Ms Zaunbrecher.