- Accident date
- Accident area
- Timor Sea
- Spill area
- Cause of spill
- Oil eruption
- Nature of pollutant
- crude oil
- Quantity spilled
- 4, 800 tonnes
- Ship / structure type
- Oil rig (exploration platform)
- Seadrill Ltd
On the 21st August 2009 an uncontrolled discharge of oil and gas occurred from the Montara wellhead platform, where the West Atlas mobile drilling unit was operating. The incident occurred approximately 230 km off the north-west coast of Australia in the Montara offshore oil field in the Timor Sea.
The 69 engineers and technicians on the rig were immediately evacuated unharmed to Darwin.
The released hydrocarbons were composed of:
- light crude oil (but with a wax content of up to 11% and a pour point of up to 27°C) rapidly forming slicks on the water surface
- a mixture of condensates and gas released into the atmosphere, posing an explosion risk in the vicinity of the rig.
PTTEP Australasia, the platform operator, estimated that 64 tonnes of crude oil was being lost per day. At this rate, an estimated total of 4,800 tonnes was released into the sea, the leak having continued up until the 3rd November 2009. The slick created stretched up to 40 km wide by 136 km long.
Response at sea
AMSA (Australian Maritime Safety Authority) was alerted some hours after the incident and activated the National Plan to Combat Pollution of the Sea by Oil and Other Noxious and Hazardous Substances (National Plan)
The Australian Marine Oil Spill Centre (AMOSC), a response cooperative integrated into the National Plan, was also activated. Around 17 vessels and 9 aircraft were mobilised by AMSA and the oil industry. In total, approximately 300 responders from the public and private sectors were involved in the operations.
Monitoring the slicks and guidance
Aerial reconnaissance surveys were conducted from the outset to monitor the drift of the slicks and to guide the nautical response in the area (recovery vessels, aircraft for spraying dispersant etc.).
The drift of the slick was also monitored using marker buoys released from an AMSA Dornier aircraft.
Over the entire incident period 130 surveillance flights were carried out in a 35 km perimeter around the rig. Confirmation (by sampling from a helicopter) was sometimes necessary to differentiate oil slicks from the natural phenomenon of algal bloom.
From the 2nd day after the spill the oil slicks were treated by spraying dispersants (from aircraft and vessels); 50 tonnes were transported from the AMOSC depot in Geelong. The spraying was initially carried out from a Hercules C130 transport aircraft then from smaller monoplanes, which are easier to manoeuvre over small slicks. These aircraft were used until the 1st November 2009.
The large wax percentage of the crude oil, which tends to solidify the slicks depending on temperature variations, had an impact on the window of opportunity for applying dispersant (more efficient in the afternoon). Furthermore, the distance from the coast also posed operational limits linked to the time necessary for transporting personnel and equipment to the area.
Chemical dispersion proved efficient for this spill.
Containment and recovery
The weather and sea conditions (little agitation) were not favourable for the natural dispersion of the oil but facilitated containment and recovery operations.
These operations were carried out between the 5th September and the 30th November mainly by two pairs of vessels working together. This method enabled the deployment of a 300-metre boom to form a containment pocket (in "U" or "J" formation) within which weir skimmers were used to recover the crude oil.
As with the dispersion operations, the high wax content of the crude oil created certain constraints. As the wax solidified during the night it was sometimes necessary to fit screens to the weir skimmers.
Being far from the coast, the presence of a FPSO (Floating Production, Storage and Offloading) vessel from PTTEP was a major element in recovery operations, offering a rapidly available storage means for the recovered oil.
No recoverable oil was detected after the 15th November, 12 days after the leak was plugged. In total, 844 m3 of emulsion was recovered, 58% of which (493 m3) was crude oil according to AMSA estimates.
Plugging the well
A group of experts was mobilised by PTTEP Australasia to elaborate a strategy for plugging the leak.
They decided to drill a relief well to intercept the main well, 2,600 m below the seabed.
The West Triton mobile drill rig arrived from Singapore on the 21st September to implement the relief well plan and to kill the leaking well by pumping heavy mud into it.
On the 3rd November the main well was intercepted, a real technical challenge, and 540 m3 of heavy mud was pumped in to stop the leak.
The well was definitively capped in late November by pumping cement over 1,500 m down the relief well.
The National Plan was deactivated and the response at sea demobilised on the 3rd December 2009 following the definitive capping of the well.
The spill response was successful, no pollutant arrivals were observed on the shoreline, which included environmentally sensitive areas.
From September 2009, the Australian Department of the Environment (DEWHA) financed two studies to monitor the environmental impact of the spill. One dealt with the impact on birds and mammals and the other with coastal pollution.
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Sea&Shore Technical Newsletter, 2016, n° 44