Santos Ltd (Santos) proposes a revision to its in-force Bayu-Undan to Darwin Gas Export Pipeline
Environment Plan, to enable the continued operation of the Bayu-Undan to Darwin Gas Export Pipeline (BU GEP) and to allow the BU GEP to enter a preservation phase at the end of the Bayu-Undan field life. The BU GEP is situated within permit areas; BU-1-PL, WA-8-PL, NT/PL1, NTC/PL1 and NTC/PL20. The permit areas are within Timor-Leste, Australian Commonwealth and Northern Territory Coastal waters, in depths ranging from 55 to 120 metres.
The BU GEP is a dry natural gas export pipeline transporting gas from the Bayu-Undan field located in Timor-Leste waters to the Darwin Liquefied Natural Gas (DLNG) plant. The BU GEP has been in operation since 2005 and the Bayu-Undan field is approaching the end of its commercially productive life.
At the end of the Bayu-Undan field life, the BU GEP will transition from the transport of natural gas to DLNG to linepacking of the pipeline with hydrocarbon gas to the BU GEP’s maximum operating pressure. This gas will then be back fed to the Bayu-Undan central processing facility for power generation purposes.
Once gas is no longer available to support power generation at the Bayu-Undan central processing facility, the pipeline will remain filled with hydrocarbon gas in a preservation phase until a decision is made to either repurpose the BU GEP for carbon capture and storage (CCS), or to decommission all or part of the BU GEP. It is envisaged that the total duration of the preservation phase will be up to 36 months. A decision on whether the pipeline will be used for CCS is expected to be made during 2022.
Ongoing integrity management of the BU GEP is as per a Risk Based Inspection (RBI) regime to ensure that the BU GEP can be removed from title. Typical subsea inspection activities carried out may include:

• Internal inspection of the BU GEP using an in-line inspection tool (intelligent pig);
• Surveys using sidescan sonar or a multibeam echo sounder;
• External inspections of the Pipeline, typically using an ROV or divers; and
• Trailing wire cathodic protection surveys.

Typically, inspection vessels will be within the operational area for approximately 5–60 days per year depending on the type of inspection. Events such as cyclones may also trigger inspections. Anomalies identified from planned inspections and condition monitoring will be reviewed, risk assessed, and managed.
Vessels used for inspection activities are expected to range between 30 m and 130 m in length. The vessel type and specifications will depend on availability and specific activity requirements. A 117 m Class DNV with ROV is typical of the type of vessel used for these activities. It will be supported by up to two vessels of the same or lesser class – including a supply/support vessel and/or a hyperbaric rescue vessel. Not all vessels will be active at the same time.