Fierce gusts buffeted the helicopter, and Johanson straightened his headphones. They were all wearing ear-protectors and heavy clothing, and were packed in so tightly that their knees touched. The noise made talk impossible. Lund had closed her eyes.

The helicopter wheeled and proceeded south-west. They were heading for Gullfaks, a group of production platforms belonging to Statoil. Gullfaks C was one of the largest structures in the northern reaches of the North Sea. With 280 workers, it was practically a community in its own right and Johanson shouldn't have been allowed to disembark there. It was years since he'd taken the compulsory safety course for visitors to the platforms. Since then, the regulations had been tightened, but Lund's contacts had cleared the way. In any case, they were only landing in order to board the Thorvaldson, which was anchored off Gullfaks.

A sudden gust caused the helicopter to drop. Johanson clutched his armrests but nobody else stirred: the passengers were used to stronger gales than this. Lund opened her eyes and winked at him.

Kare Sverdrup was a lucky man, thought Johanson, but he'd need more than luck to keep up with Tina Lund.

After a while the helicopter dipped and started to bank. The sea tilted up towards Johanson, then a white building came into view. The pilot prepared to land. For a moment the helicopter's side window showed the whole of Gullfaks C, a colossus supported by four steel-reinforced pillars, weighing 1.5 million tonnes altogether, and with a total height of nearly four hundred metres. Over half of the construction lay under water, its pillars extending from the seabed surrounded by a forest of storage tanks. The white tower block where the workers slept was only a small section of the platform. Bundles of pipes, each a metre or more in diameter, connected the layers of decks, which were flanked by cranes and crowned with the derrick – the cathedral of the oil world. A flame shot over the sea from the tip of an enormous steel boom, burning natural gas that had separated from the oil.

Touch-down was surprisingly gentle. Lund yawned and stretched as far as she could. 'Well, that was pleasant,' she said, and someone laughed.

The hatch opened and they clambered out. Johanson walked to the edge of the helipad and looked down. A hundred and fifty metres below, the waves rose and fell. A biting wind cut through his overalls. 'Is anything capable of knocking this thing over?'

'There's nothing on earth that can't be toppled. Get a move on, will you? We don't have time to hang about.' Lund grabbed him by the arm and pulled him after the other passengers, who were disappearing over the side of the helipad. A small, stocky man with a white moustache was standing at the top of the steel steps, waving at them.

'Tina!' he shouted. 'Have you been missing the oil?'

'That's Lars Jörensen,' said Lund. 'He's responsible for monitoring the helicopter and seagoing traffic on Gullfaks C. He's an excellent chess player too.'

Jörensen was wearing a Statoil T-shirt and reminded Johanson of a petrol-pump attendant. He clasped Lund to his chest, then shook hands with Johanson. 'You've picked an inhospitable day,' he said. 'In good weather you can see the full pride of the Norwegian oil industry from here, every last platform.'

'Are you busy at the moment?' asked Johanson, as they climbed down the spiral steps.

'No more so than usual. Your first time on a platform, is it?'

'It's been a while. How much are you producing these days?'

'Less and less. Production on Gullfaks has been stable for a while now, with two hundred thousand barrels coming from twenty-one wellheads. We should be pleased with that, but we're not' He pointed to a tanker moored to a loading buoy a few hundred metres away. 'We're filling her up. There'll be another along later, and that's it for today. Soon we'll start running out.'

The wellheads weren't directly below the platform but were scattered a fair distance away. The oil was extracted, separated from the natural gas and water, then stored in the tanks on the seabed. From there it was pumped to the loading buoys. A safety zone stretched five hundred metres around the platform and only its maintenance vessels were allowed to cross it.

Johanson peered over the iron railings. 'Hasn't the Thorvaldson arrived?' he asked.

'She's at the other loading buoy, just out of sight.'

'So, you don't even let research vessels come close?'

'The Thorvaldson doesn't belong to Gullfaks and she's too big for our liking. It's enough trouble trying to persuade the fishermen to steer clear.'

'Do you have much trouble with them?'

'Last week we had to chase away a couple of guys after they'd followed a shoal right under the platform, and at Gullfaks A recently a tanker drifted loose – engine problems. We sent a few people to help, but the crew got it sorted just in time.'

Jörensen spoke casually, but he had described the catastrophe that everyone prayed would never happen: a loaded tanker heading straight for a platform. The impact would send shudders through some of the smaller structures, but, worse still, the tanker might explode. Every platform was equipped with sprinklers that would release several tonnes of water at the least sign of fire, but an exploding tanker could tear a platform to pieces. Such accidents were rare, and usually happened in South America where safety regulations weren't as strictly observed.

'You're looking slim,' said Lund, as Jörensen held the door open for her. They went into the accommodation module and walked down a corridor lined with identical doors that led into the living quarters. 'Don't they feed you well enough?'

'Too well,' laughed Jörensen. 'The chefs amazing. You should see our dining room,' he added quickly to Johanson. 'It makes the Ritz look like a roadside cafe. No, the platform boss doesn't like North Sea bellies. He's told us to get rid of any extra kilos, or else he'll ban us from the platform.'

'Seriously?'

'Directive from Statoil. I don't know if they'd really go that far. In any case the threat was effective. No one wants to lose their job.'

They reached a narrow staircase and walked down, passing a group of oil workers whom Jörensen greeted. Their footsteps echoed in the steel stairwell.

'Right, this is the end of the line. You've got a choice. Either we go left, grab a coffee and chat for half an hour, or right, to the boat.'

'Coffee sounds good,' said Johanson.

'We haven't time.' Lund told him.

'The Thorvaldson won't leave without you,' said Jörensen. 'You could easily -'

'I don't want to have to race there. Next time I'll stay longer, I promise. And I'll bring Sigur too. It's about time someone played you into a corner.'

Jörensen laughed, and Lund and Johanson followed him outside. Wind blasted their faces. They were at the bottom edge of the accommodation module, standing on a thick steel grating, through which they caught glimpses of billowing waves. A constant hissing and droning filled the air. Jörensen led them towards another short gangway. An orange launch was suspended from a crane. 'What are you doing on the Thorvaldson?' he asked casually. 'I heard Statoil might be building further out.'

'It's possible,' said Lund.

'A new platform?'

'Not necessarily. Maybe a SWOP.'

Single Well Offshore Production Systems were enormous vessels similar to tankers with their own oil-recovery facility, used in depths of more than three hundred and fifty metres. A flexible flowline kept the vessel in position over the well while the oil was pumped into the hold, which served as a temporary storage tank.

They got into the launch. It was spacious inside, with several rows of benches. Apart from the helmsman they were the only ones on board. The boat jerked as the crane lowered them into the sea. Cracked grey concrete flashed past the side windows, then they were bobbing on the waves. The crane detached itself from the boat and they motored away from the platform.