Manawanui sinking: An expert explains why a speedy cleanup will be crucial – and the main challenges ahead

Analysis - Speed will be of the essence as salvage crews attempt to stop fuel leaking from the sunken New Zealand naval ship off the coast of Samoa.

The HMNZS Manawanui ran aground last weekend on a reef about one nautical mile off the south coast of Upolu, Samoa's most populated island. The specialist dive and hydrographic vessel was on its third deployment, conducting a reef survey, when it caught fire and sank.

The ship has come to rest at a depth of up to 150 metres, which means it may be relatively undisturbed even during storms. Any hull cracks from the impact should not be exacerbated.

But depth makes the salvage operation challenging. Crew may need decompression chambers, and there's only a narrow window of time to seal any fuel leaks - and, ideally, pipe out more than 900 tonnes of marine diesel the ship carries.

Fuel leaks the first priority

The Manawanui's sinking is a marine disaster. But it arguably poses a lesser risk than the oil spill caused by the container ship MV Rena, which ran aground near the Astrolabe Reef off Tauranga in 2011.

The Rena was loaded with 1368 containers, some of which contained hazardous materials, as well as 1700 tonnes of heavy fuel oil. It also carried thousands of tonnes of dairy products, which effectively fertilised the ocean and caused massive algal blooms, visible from space.

The fuel oil on board the Manawanui is lighter. Its most toxic short-chain hydrocarbons will likely evaporate with wave action. But if the remaining slick washes up on beaches, it will be harder to remove. During low tides, it will be running onto the reef, likely killing off corals and fish in a swath moving inshore or driven by wind and currents.

The naval chart for Safata Harbour, Samoa, and MarineTraffic referencing the position HMNZS Manawanui went aground and sunkhttps://t.co/SoBMux80Fi pic.twitter.com/MTwSGeo6RL

— Michael Field (@MichaelFieldNZ) October 6, 2024

The salvage crew's first focus will probably be on mitigating fuel leaks. But they will also need to clean up any crushed coral and contaminated sediment around the reef and wreck as quickly as possible, as it may have been exposed to the ship's anti-fouling paint. In calm weather this would be possible as it's shallow on the reef crest.

Ships in the past have been painted with the anti-fouling paint Tributylin. It has now been banned because of its toxicity, but many ships simply painted over it with modern paints. Any damage to the hull could expose old layers. Without a thorough cleanup, this could preclude coral recovery.

My experience, and that of colleague's both in New Zealand and in tropical Australia, shows a speedy cleanup can make all the difference for the environmental recovery after ships ground on reefs and sometimes sink.

When the Malaysian-flagged container ship Bunga Teratai Satu ran into the Sudbury Reef in the Great Barrier Reef in 2001, the vessel was refloated without losing any cargo or fuel. But it had scraped against the reef, spreading tributyltin-coated fragments. The salvage operation cleaned up the toxic material and the coral was on a recovery trajectory within four years.

In contrast, the sinking of the Shen Neng 1 in 2010 flattened 8000 square metres of reef east of Great Keppel Island on Queensland's Capricorn coast. While the ship was also refloated and removed, there was no cleanup and no signs of coral recovery a decade after the disaster.

Should oil dispersants be used?

All 75 crew and passengers have been taken off the Manawanui by life rafts and other boats that came to the rescue. A Court of Inquiry is under way to establish exactly what caused the sinking.

The focus is now on mitigating environmental impacts.

At this point, there are no signs of oil on the beaches where the vessel sank, but locals are reporting an oil-like substance in the water around the wreck.

Should fuel oil spill ashore, locals will have to find ways of harnessing the public for the beach cleanup. When the oil slick from the Rena contaminated local beaches, thousands of volunteers helped with the recovery operation.

Locals will likely also face decisions about using oil dispersants, which break up oil into smaller droplets into the water column.

At the time of the Rena operation, there was a public outcry against the use of dispersants because they spread the pollution further into the marine environment, and the chemical combination of oil and dispersant can be more toxic than either alone.

The use of dispersants makes sense however, if an oil spill threatens turtle nesting areas for example, as it did when the cargo ship Pacific Adventurer was caught in a cyclone off Queensland in 2009 and 270 tonnes of oil created a 5.5 kilometre long slick.

An oil spill originating from the sunken naval vessel HMNZS Manawanui has already made an impact on the surrounding environment. Within its natural habitat, a sea turtle is observed struggling amongst spilled fuel and debris from the sunken ship.

Story: https://t.co/hEsbZD3U5h pic.twitter.com/xd2qnQfWrr

— Samoa Observer (@samoaobserver) October 7, 2024

The reef where the Manawanui struck is well known for its large population of sea turtles, which come to feed in the area. They are likely to sense the pollution and eventually stay away, as will pelagic fish.

Given the area is the local villagers' food basket and a tourist destination, any deleterious effects on the coastal environment and coral reefs will be keenly felt. As in Aotearoa, the intimate bond between people and the sea is profound. Drawing on past experiences will empower speedy action and hasten ecological restoration.

* Christopher Battershill is Professor in Coastal Science at University of Otago.

** Disclosure statement: Christopher Battershill received funding from the Ministry for the Environment to examine the environmental effects of the MV Rena ship wreck. He is affiliated with the Oil Pollution Advisory Committee and has previously worked with the Australian Institute of Marine Science.