Singapore's southern coastline turned black after a major oil spill caused by a dredging boat hitting a stationary cargo tanker at Pasir Panjang Terminal on June 14.

Oil slicks quickly spread to the waters near Labrador Nature Reserve, Sentosa, East Coast Park, the Southern Islands and Changi, despite the authorities swinging into action within 11 minutes of being alerted to the incident.

After a massive cleanup over days, the removal of bulk oil from the sea and beaches is nearly complete, the authorities said in an update on June 24.

Based on satellite and drone images, there has been no oil slick observed at East Coast and Changi since June 18. Most of the public beaches affected have had the bulk of oil-soaked sand cleared, except Sentosa’s Tanjong and Palawan beaches.

The authorities said they are moving to the next phase of the cleanup operation, which will focus on oil remnants trapped in areas such as coastal features, waterside infrastructure and rock bunds that are not easily accessible.

This phase is expected to take about three months as it is more complex and requires trained personnel and specialised equipment.

Here’s a comparison of the body of water between HarbourFront Centre and Sentosa, before and after the oil spill:

The spill occurred after Netherlands-flagged dredger Vox Maxima, suffering a sudden loss of engine and steering control, hit stationary bunker vessel Marine Honour at Pasir Panjang Terminal. The incident caused half – or 400 tonnes – of the low-sulphur fuel oil in one of Marine Honour’s tanks to flow into the sea.

In Singapore’s largest oil spill in 1997, about 28,000 tonnes of oil spilt into the sea after two tankers collided 5km south of Singapore.

According to ship-tracking website MarineTraffic, Vox Maxima was built in 2010 and is 198m long and 31m wide. Singapore-flagged Marine Honour, built in 2007, is 103.3m long and 18.6m wide.

Dr Tkalich, who specialises in coastal hydrodynamics, noted that tide currents can be predicted, while wind speed and wind direction vary according to local weather conditions.

He added that the oil slick could have been moving at an average speed of 2 per cent to 4 per cent of the wind speed then, from the west to the east.

Concurrently, the tide currents were responsible for dispersing the oil slick – from its initial single, compact shape – into multiple smaller oil slicks across the various beachfronts and islands, he said.

The cleanup operation has three stages: an emergency phase that focuses on containing the spill; a project phase that targets areas not easily accessible, such as rock bunds; and a final polishing phase to remove remnants on land.

Areas where cleanup processes are taking place

This phase involves the removal of bulk oil – or major black oil slicks – from the sea and beaches, together with the removal of oil-soaked sand from the affected beaches.

Cleanup of oil slicks from sea and beaches following the incident

This stage focuses on the more difficult cleanup of oil remnants trapped in areas such as coastal features, waterside infrastructure and rock bunds.

This is more challenging as these areas are not as easily accessible, and oil could be trapped in crevices and below beach surfaces. The process has to be done carefully, to minimise the oil on rock bunds re-polluting the waters and other areas.

The Singapore Civil Defence Force has deployed a rapid response fire vessel, used for firefighting operations, to support the cleaning operation of the rock bunds at Siloso Beach from June 21.

This phase primarily involves picking up tar balls, or small blobs of coagulated oil that are the size of a fingernail, and monitoring water quality. The authorities are looking into whether volunteers can be safely involved in this final phase of cleanup.

Containment booms are floating devices that create a barrier to trap oil. Once the oil is contained, oil skimmer machines help collect it from the water surface. More than 3,400m of booms have been laid so far.

The Current Buster system, towed by two vessels, goes to affected areas to collect contaminated seawater and sieve oil out through a pump for skimmers to collect. Up to 50 cubic metres of oil can be collected at a time.

Drones that can travel up to about 60m above sea level, with a battery life of 25 minutes each, are used to direct the Current Buster systems to affected areas. The drones can capture clearer aerial views of the water surface. They do a better job as there is a lot of reflection on the water surface when observed from a vessel.

An action plan to protect and restore coastal habitats from the effects of the oil spill is being worked out by government agencies and the community, Minister for National Development Desmond Lee said on June 24.

Suggestions for what this plan could look like include strengthening wildlife rescue and recovery processes, and conducting scientific research to understand the longer-term impact of oil spills on biodiversity.

The Straits Times had earlier reported that four collared kingfishers affected by the oil had been rescued, although two died later. No other oil-coated animal has been retrieved by the authorities to date, although volunteers, nature groups and NParks are continuing to keep an eye on the situation.

Mr Lee cautioned that there could be a time lag between the incident of the oil spill and its effects on biodiversity and habitats. For example, the impact on sensitive species such as corals, sea grasses and sea stars may only be apparent weeks or months later, such as during spawning periods.

After Singapore’s last major oil spill in 2017, which affected areas like the biodiversity-rich Chek Jawa Wetlands on Pulau Ubin, recovery took a few months. Pulau Ubin is not affected by the recent oil spill.

Mr Lester Tan, chair of the Nature Society Singapore’s marine conservation group, said that while the oil spill involved a low-sulphur fuel that is less viscous than crude oil, it can still affect coastal and marine life negatively.