Countdown to a tanker disaster: From new information, Nicholas Schoon pieces together the events on board the oil tanker that caused last month's Shetlands disaster

WHAT went wrong on the tanker Braer to leave her heading towards disaster on the fierce rocks of Fitful Head? New information offers a much clearer picture of how an ageing, but far from primitive, Japanese- built tanker came to be drifting without power on a foul night off the Shetland Islands.

This information comes not from the Ministry of Transport inquiry - that, for the usual British reasons, remains a state secret for now - but from the tanker's operators, B & H Ship Management.

True, the eight-page document issued from its Connecticut headquarters raises almost as many questions as it answers. But the company was willing to clarify at least some points.

The facts provided are credible to marine engineers and square with the first version of events given by the crew soon after they had been airlifted off the pitching tanker. 'This account seems to hang together,' Chris Jenman, chairman of the London-based consultants Global Maritime, says.

On 4 January, the day before the tanker was wrecked, four large steel pipes, inadequately secured on the main deck, were seen to come loose as huge waves washed across her. The 16ft (4.8m) pipes weighed as much as three-quarters of a ton each. It seems probable that they smashed into air vents on deck that allow gas to escape from the ship's diesel fuel tanks.

To prevent seawater getting in, these vents have one-way valves and openings that face downwards. But if the vents were cracked or broken at the base, water could trickle down into the fuel. That, it now seems, was the main cause of the Shetland oil spill.

The ship's huge main engine, which is the size of a small building and capable of generating 20,300hp, normally runs on heavy fuel oil. This is a cheap fuel, but is so viscous that it has to be heated by steam from a boiler to make it liquid enough to inject into the engine. The boiler burns this heavy fuel oil as well.

The electrical generators on the Braer were powered by two much smaller engines, running on lighter, more expensive diesel fuel. The Braer carried 125 tons of this, and 14 times as much heavy fuel oil. Diesel can be used as a reserve fuel for the main engine and the boiler. It is also used to restart the boiler after a shutdown.

On the night of 4 January, the engineer on watch shut down the boiler to adjust the water level inside. He then went through the sequence of operations to relight it using diesel fuel, but the boiler failed to re-ignite. It was 10.30pm.

The engineers repeatedly tried to relight the boiler and find out what was wrong. Meanwhile, the heavy fuel oil was cooling and becoming more and more sticky. As a result, shortly after midnight, the main engine was switched over to run on diesel fuel.

Diesel is stored in two large tanks at the bottom of the ship. It first flows in to a settling tank, where any water should settle to the bottom, and then into a service tank from where it is pumped directly to the engines. Between the settling and service tanks the fuel should be purified by a centrifuge, which removes any water or particles - contaminated fuel could damage or stop the engines. The boiler, however, is less fussy about the purity of its fuel, and is served direct from the diesel settling tank.

As the engineers went through the troubleshooting sequence, they opened a tap at the bottom of the settling tank and a large slug of water came out.

They continued trying to relight the boiler and to drain the water in the bottom of the settling tank, but it kept reappearing. The chief engineer was summoned from his bunk at 2.30am. Large quantities of water were entering the settling tank, and the wild rolling and pitching of the ship made it impossible to settle this out. The water may also have been emulsifying - mixing into the diesel in tiny droplets - making it impossible to relight the boiler.

The chief engineer asked the ship's master, Alexandros Gelis, to find a safe anchorage where the diesel oil and water could be separated and the leaks repaired.

But just after the tanker altered course southwards for the Moray Firth the main engine stopped and, a few minutes later the diesel generators failed. This was almost certainly because seawater had penetrated the final service tank and then the engines. It may be that the centrifugal purifiers had been inadequately maintained, or that they were overwhelmed by the volume of water. Inside the crew quarters, there was now only dim emergency lighting and the sound of the ship being battered by the elements. The combination of waves, storm winds and powerful tidal currents pushed the Braer north, straight towards the southern tip of Shetland, 10 miles away.

The tanker was now in desperate need of electrical power to restart the main engine, but all efforts to revive the generators failed. The feeble light and the tanker's violent pitching must have made the engineers' task of trying to purge severely contaminated diesel from the tanks, fuel lines and engine a nightmare.

The only hope was that the Braer would avoid land for long enough to allow a powerful tug to reach her. The Department of Transport's Marine Accident Investigation Branch will consider whether the captain should have told the coastguard sooner of his predicament. There are also questions over whether the tug Star Sirius could have left Lerwick earlier, and whether the tanker's entire crew should have been taken off by helicopter, leaving nobody on board to take a line from the tug.

The Braer hit the rocks with her anchors up. The waves washing across her main deck prevented anyone going forward to the bow to attempt to lower them.

Even the most modern ships have their anchors firmly 'secured for sea' once they have left port - anchors cannot be released by pushing a lever on the bridge. One question for the inquiry is whether a fail-safe system can be devised to allow a vessel's anchors to be lowered from the bridge in an emergency.

Our series on modern pesticides will continue next week.

(Photograph and graphic omitted)