The Shetland Oil Disaster: Questions over ship maintenance team: Susan Watts considers what may have happened to make both sets of engines stop on the Braer

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Only the scantest of information has emerged so far to explain the chain of events that led to engine failure on the Braer.

Marine engineers agreed yesterday that something catastrophic must have happened for both sets of engines to stop.

The most likely scenario is that the tanker's small diesel engines failed first - probably through water contamination of their fuel, although other problems with fuel supply lines cannot be ruled out.

Questions remain over why a four-strong Polish maintenance team was on board the ship, working on 'machinery' in the Braer's engine room as she travelled. The tanker's small auxiliary engines are vital. They run generators that provide the ship's electrical power and supply power for cooling and lubrication systems for the main engine, and the pumps that feed this engine with its fuel oil.

The main engine, often the size of a building, drives the ship's propellor, and an alternator to supply lights when the vessel is at sea. If the small engines stop, so will the main engine.

Chris Jenman, chairman of a London-based marine engineering consultancy, Global Maritime, said the water purifiers for the auxiliary engines could have been set incorrectly, allowing water to seep into them. He said the crew should have checked the ship's purifiers and sludge valves every four hours.

A spokesman for the ship's owners, B & H Ship Management, dismissed reports that the tanker's auxiliary fuel tanks were empty during its voyage. He said the tanker docked in New York on 21 December, when it loaded up with fuel 'to full capacity' for the round trip to Norway and back to North America.

When the Braer arrived at New York it was already carrying some fuel, but topped this up with 2,084 tonnes of 'intermediate', or 'bunker', fuel oil for its main engines, and 124 tonnes of diesel fuel for its auxiliary engines. When the vessel left New York it was carrying a total of 2,634 tonnes of intermediate fuel and 223 tonnes of diesel. It took no extra fuel at the Mongstad terminal while in dock there between New Year's Eve and 3 January.

The B & H spokesman said: 'We know there is a technical problem on board the ship, but we don't know what that is.' He said it is now up to the various inquiries to find out exactly what went wrong.

A spokeswoman for Statoil, the company that controls the Mongstad terminal, yesterday denied having played any role in assessing the seaworthiness of the Braer. 'We were delivering oil at the quayside. . .that was our only role.'

She said Statoil had no responsibility to inspect repairs to faulty pipework connected to the tanker's steam boilers. There is a slim possibility that this fault allowed steam to contaminate the fuel tanks for the auxiliary engines, although Statoil insisted yesterday that the Braer's captain assured it the repair was successful.

The ship's boilers burn fuel oil to produce steam to heat the vessel. Exhaust from the boilers is rich in nitrogen, an inert gas pumped into the cargo hold to help minimise the risk of explosion as oil is loaded and unloaded from the tanker.

A spokeswoman from Statoil said she understood that the faulty pipework meant the boiler system was not producing enough inert gas, but the problem was sorted out promptly and loading continued.

'We were told by the captain that it (the boiler) produced steam to power lifting cranes and loading pumps and also produced an inert gas. He said he had a minor problem.'

She said Statoil had no information on the repair, which was carried out by the ship's crew and was the responsibility of the ship's owner and captain. 'We had no authority, or qualification, to go on board and inspect that.'

She stressed that if Statoil had had any suspicions that the Braer should not set to sea it would not have let the ship go. Statoil would have passed its concerns on to the Norwegian authorities.

Norsk Hydro, the Norwegian oil company, yesterday confirmed that in August of last year it inspected the Braer at its Sture terminal, checking that it was of a sufficiently high standard to carry its oil cargoes. Statoil carried out no further inspection once the Braer docked at Mongstad, choosing to rely on Norsk Hydro's report. Norsk Hydro said it had identified only minor problems.

Yesterday, Det Norske Veritas, the Norwegian equivalent of Lloyd's List, said it had no further information on the cause of the Braer's engine failure. A spokesman said it remained a surprise since the vessel had passed the company's annual inspection in May 1992 and several strict port authority checks.

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