The 12 inch diameter, 17 km Longtom Pipeline has been constructed as part of Nexus Energy’s Longtom Gas Field Development, located in Victoria’s offshore Gippsland Basin, which involves the production of gas through horizontal subsea wells, tied back to Santos’ recently upgraded onshore Orbost Gas Plant.

The Longtom Pipeline, which is made of DNV HFW450 ISUD ERW pipe, 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, three 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 MMcf/d was recorded during the second production test over several reservoir sections.”

A gas sales agreement for Longtom was signed with Santos in December 2005. Santos agreed to process up to 450 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.

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

Construction process

The pipelay operation commenced with the Aussie 1 pipelay barge in April 2009 and was completed over three weeks. Pipe joints, 12.2 m in 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 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.

In addition to the pipeline, a 21 km electro-hydraulic umbilical control system, consisting of seven thermoplastic hydraulic hoses and two 16 mm electrical cables. were installed using a dynamically positioned diving and construction support vessel.

However, the project experienced 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 HIPPS package has a safety integrity level of three and was built in Melbourne by Cameron in collaboration with designers and manufacturers from Leeds, United Kingdom; Celle, Germany; Houston, United States; and Melbourne, Australia.

“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.”

Accessing a pipelay vessel

Several challenges were posed due to the location of the project. “While Australia is not considered a remote location, there is limited choice of contractors for some roles in major projects,” a Nexus spokesperson said.

“For the Longtom project, Nexus was reliant on the construction of a pipelay barge out of Singapore – delivery of which took longer than expected. Once the barge did arrive, however, it proved highly efficient in the actual pipelay program.

Another challenge arose when the original dive support vessel (DSV) left without completing the work, and Nexus had to step in and contract an alternate DSV to complete the project.

“There were no vessels available in the area, and it took three months to locate, contract and mobilise a vessel from South East Asia to the area,” said a Nexus spokesperson.

An eye on the local community

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.

At the end of the installation activities, Nexus donated some materials to the local community of Eden.

“Nexus also employed up to 160 people on the vessels – 10 in Eden, and a further 20 in Melbourne and Perth during the offshore campaign,” said a Nexus spokesperson.

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

Ensuring minimal impact to community and environment

During the offshore installation process, Nexus said that there were no particular stakeholder issues identified.

“As the Longtom project is located a significant distance offshore, the main stakeholders with an interest in the project are commercial fishermen,” said Nexus Health, Safety, Environment and Community Manager Michelle Zaunbrecher.

“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 a very small environmental footprint, with no significant environmental impacts,” said Ms Zaunbrecher.

“In addition, there is no facility above the ocean surface and there are no areas of high environmental sensitivity within the immediate project region. The hydrocarbons produced from the Longtom field are predominantly gas with only a small volume of condensate so there is a very low risk of oil spill.”