One day last August, Paul Sharp woke in his room on the Borgsten Dolphin oil rig. He had been on the night shift, then slept for five or six hours, so it was getting on for midday. He showered, dressed, then, after lunch in the canteen, packed his bags and took them up to heli admin to be weighed. Sharp has been a scaffolder on the rigs for nearly 23 years, working two-week blocks of 12-hour shifts, often in brutal weather: horizontal snow, screaming wind, icicles as tall as a room and waves that lapped at 75ft-high platforms. He commutes from East Hull, where he was born and now lives with his wife and daughter in a neat, low house facing a church and a main road.

The flight was running late, so Sharp went back downstairs to watch TV. When the call finally came, he returned to heli admin, a small room for a large and diverse bunch: the crew, plus 18 passengers, 17 men and one woman, working as safety officers, electricians, caterers, welders; old hands like Sharp, and those for whom the whole thing was still new, such as James Nugent, who had also just come off a night shift and was on only his second trip offshore. Nugent is a tall, well-built South African, weathered and emotionally open. After a career in the film industry and running a B&B, he became a rope access instructor, specialising in work at heights or in confined spaces. We talk in a restaurant perched on a point of land just outside Newquay in Cornwall, where he lives.

Everyone climbed into neoprene immersion suits. The five days of mandatory safety training for anyone who goes offshore are conducted in a swimming pool kept at 26C; the North Sea is about 17C in the summer, 6 or 7C in the winter. Survival times at the latter temperature are one to three hours, hence the suits, which fasten with a heavy-duty zip and include feet, spray hood, strobe light, beacons and a rebreather, designed to buy precious minutes in a submersion. There is something comforting about the suit, but it also produces a distinctly unpleasant feeling of entrapment: rubber cuffs, designed to stop any water from entering the suit, fit tightly around your wrists and, harder to get used to, your neck.

When the safety video began and the passengers saw that they would be in a Super Puma L2, more than one heart sank. Everybody in the room knew that since 2009 there had been four serious incidents involving Super Pumas in British waters: three ditchings and one crash – or, to be specific, as helicopter pilots would dearly like people to be, two controlled ditchings, one controlled flight into terrain and one crash, in April 2009, in which an AS 332-L2 fell out of clear skies off Crimond, between Fraserburgh and Peterhead, killing all 16 people on board. The last ditching, in 2012, led to all Super Puma EC225s being withdrawn from service over the North Sea. In fact, they had been cleared to fly again only the previous month; not all were yet back in service.

The victims: Sarah Darnley and Duncan Munro. Photograph: AFP/Police Scotland

Nugent looked around the small room. There was George Allison, safety supervisor, "pretty much the first guy I met". Allison had shoved a newsletter into his hand and said, "'I can't believe this is happening.' And I said, 'I don't know what you're talking about.' And he said, 'Read that.'" It was an account of a briefing on the Borgsten Dolphin a few days previously, on 8 August, in which Will Hanekom, deputy chief pilot of helicopter service company CHC, and managers from rig operators Total tried to prepare nervous workers for the return of the grounded EC225s. One worker had asked what would happen if any of them refused to board. "If you can't live with the risk, then you can't work offshore," came the blunt reply. "How else are we going to get there?" added a CHC pilot. "It is what we do. At some point we have to put our big boy pants on."

"It was absolutely disgusting," says Sharp, who got to the platform two days later. "A lot of people felt kicked in the teeth."

Nugent had come to know Allison relatively well in the two weeks he had been on the Borgsten. "Every night I told him what we were going to do [on that shift]. He signed my risk assessment." Though Allison was big and burly, Nugent found he had a soft side and a sense of fun. "[At quiet times] we sat in the locker room next to George's office and he'd play music, 60s and 70s psychedelic rock. There was a lot of banter. He'd put together a newsletter and five or six questions – general knowledge, like Trivial Pursuit. We'd try to work them out and he would have a look and laugh."

The victims: George Allison and Gary McCrossan. Photograph: Press shots

Then there was Sarah Darnley, the only woman on the flight (just 3% of the British offshore workforce is female). Nugent knew her quite well, too: they had started their shifts on the same day and quickly discovered they had a country in common. She was from Elgin, but had travelled in South Africa and liked to talk to him about it. She was in catering and dealt with laundry, and he met her when he brought in his bag. Or she'd bring him coffee and biscuits, and they would chat.

Nugent caught Allison's eye. "I looked at George and he shook his head, and just went, 'I'm going home.' And I thought, he's 25 years down the line. If he's flying helicopters, I'm going home as well."

Finally they heard the Super Puma coming into land. Suddenly heli admin was even more full of people, hustle and bustle, as passengers from Aberdeen, or from the nearby North Alwyn platform, disembarked. There was a great swapping of life jackets, and then they were walking out on to the helideck and into a sunny August afternoon.

Business in the North Sea is booming. Although production is dropping steadily, it still provides 67% of the UK's oil and 53% of its gas, according to a report published by the Department of Energy & Climate Change last month. There are vast new ventures – in the West Shetlands, for instance, where BP is spending £4.5bn extending the Clair field – but the character of the main basin is also changing fast. Ageing rigs such as the Borgsten – a semi-submersible, built in 1975 and registered (because it is movable and thus counts as a ship) in Singapore – require expensive maintenance or complex programmes of skilled decommissioning. As the huge multinationals remove themselves from fields they have largely mined out, smaller companies are sprucing up worn-out installations and using them to extract every last drop – or pioneering new technologies to chase oil previously impossible to reach.

Estimates of what is left in the North Sea vary between 12bn and – if you are Alex Salmond, desperate to persuade Scottish voters to choose oil-subsidised independence – 24bn barrels of oil equivalent. Ten years ago, there were 78 companies operating here; now there are 131. The British oil industry employs 440,000 people; in 2012, 25,760 of them spent more than 100 nights offshore. And they all have to get to work by helicopter, flying over one of the most hostile terrains in the world.

The four variants of Super Pumas, along with Sikorsky S61s and S92s, are the "heavies" of the helicopter world: big and sturdy, they are used for search-and-rescue missions and in the army, as well as at oil industry locations all over the world. The pilots I speak to affirm their faith in them, especially the EC225s – the latest generation of Super Pumas. "It's like driving a Porsche after getting into an old Mini," says Mike Buckley, a spokesman for the British Airline Pilots Association (Balpa). Buckley started flying helicopters in the army before putting in more than 26 years, and counting, over the North Sea. Oil & Gas UK describes the EC225 as "arguably the most examined helicopter in modern history".

"Logically I know they're a pretty safe monster," says oil worker Jonathan Garcia, who was in a CHC-operated Super Puma EC225 when it executed a controlled landing between Shetland and Orkney in October 2012. "Illogically, I hate anything to do with Eurocopter [now Airbus Helicopters] – 95% of the time I'm convinced I'm going to die."

The helicopter Garcia was in dropped 3,000ft in less than a minute, but "I don't think the pilots would have put a ripple in bed linen," he says, speaking on the phone from Middlesbrough, with his lawyer listening in. He was impressed by their sang-froid. "The water came up to our ankles, our knees, our waists. One of the pilots pops off the starboard door and comes round to have a chat. 'Anybody got any questions?' As if he was on a Sunday walk with his dog, not bobbing around in the North Atlantic. One of the guys put his hand up. 'Any chance we can get out?'" Landing on water is not the crisis it's been made out to be, argues CHC pilot Will Hanekom. "For us, landing on the water is a safe solution."

Sitting in the cockpit of an EC225 in CHC's hangar, I begin to see why their pilots wax so lyrical about them: pilot and copilot sit, essentially, in a bowl of glass – glass over them, glass to the side, glass below. Flying, it must feel (bar the extreme noise) as though they are sitting in the sky. "I still get a buzz when I get into the cockpit," Buckley says. "The smell of the aviation fuel when you fire up the engines. The feeling when you take off into low cloud, then push through and hit clear air, or watch the sun rise, the oilfields laid out below – it's a majestic sight."

Rig scaffolder Paul Sharp is unmoved: "They're just taxis to us, really. A flying taxi."

The passengers do have a rather different point of view. The Super Pumas are cramped. It isn't possible to stand upright, and the seats are strikingly narrow. They are also configured so that passengers face each other; it is not uncommon for their legs to be interlocked. When these L-variants first went into service, Buckley notes, "people got on, lit a cigarette, leaned back and read a paper". There were no bulky survival suits or mandatory life jackets, which alone add 3.17kg to a person's weight. And people are bigger now.

It thus becomes rather important where, exactly, you sit. "I was quite fortunate," says Sam Bull, 24, a field analyst based in London. He was coming from the North Alwyn and, along with three others, was already on the chopper when the passengers from the Borgsten Dolphin started filing on. "If it's your first time offshore or on to a platform, you get a green armband and they don't let you sit next to the emergency exits. But they forgot to give me a green armband, so I sat right by it."

Nugent was last on. He turned right, shuffling all the way to the back: "Then one of the baggage handlers said, 'No, don't sit there.'" He worked his way back to the middle and took a seat on the aisle, next to Allison. "And the large chap that George was, I had only half an arse cheek on the seat. I remember thinking, 'Well, this is going to be an uncomfortable two hours.'" He did his belt up extra tight, so it was holding him in the chair.

The doors slammed shut and everyone reached for their ear defenders. The pilots announced a 15-minute refuelling stop at Sumburgh airport on Shetland. "Ideal," Sharp thought. "Enough time to stretch my legs and have a quick cup of tea." The rotors roared and finally the Super Puma lifted itself up. It dipped slightly at the front, as though bowing at the waist to those left behind, then rose up and out, over the sea.

Nugent watched the orange bulk of the Borgsten receding below them, then sat back and looked around. Up on the right-hand side of the interior, he noticed a screw loose and a panel beginning to come away. He took off his ear defenders, just to check what he already sensed. It was really very loud, much louder than any previous choppers he'd been on. He ran his eye over the windows, "because you just do. You go, 'There's my exit. If that exit's blocked, where do I go? OK, I've got George next to me, so I've got to fight for my second exit.'" He noticed there was a tab on the window. "I never had that in my survival training." He tried to doze off, but couldn't, so he scanned the cabin, noticed someone reading Nuts and indicated – nobody could hear anyone else talk – that he wanted a look. As he began to flick through the pages, he was overtaken by tiredness. Soon he was asleep. Sharp and Bull fell asleep, too.

RNLI crew members attend the crash. Photograph: EPA/RNLI

When Nugent woke, the sun had gone and they were flying through cloud. He tried to figure out where they were, but wasn't sure until the pilot announced they were 10 minutes from Sumburgh and to prepare for landing. This woke Sharp. He sat up and tightened his belt. Bull woke, too. "It looked as though we were at 30,000ft rather than 2,500." Nugent shut his eyes, wanting to sleep for that extra 10 minutes; 10 stretched to 15, to 20, but they were still above cloud level. Nugent woke again. "Jesus," he thought, "I must have had more than 10 minutes." They all looked at each other.

And then Sharp heard "a whoosh, and a crack like someone cracking a bone. And the chopper turned on its side, in the air." Nugent heard the bang, too: "An almighty bang, metal on metal, above our heads. It was loud. And I was just, 'What the…?' And then there was this twisting of the fuselage, left to right, very rapid, very violent, very sudden, and I thought, oh no, there's something wrong. I looked at the others and there was shock and horror on their faces. I thought, no, we can't be crashing. You don't want to believe it."

Bull says he didn't hear anything break, but he felt a "shudder and banking from side to side. A shudder in the airframe itself, and the engine straining. It happened so quickly – it seemed as if [the helicopter] was working, but just falling. There was no lift. I'd be lying if I said I wasn't scared, but also I didn't believe it. It was incredibly surreal."

Nugent has no idea why, but he started to count: one one thousand, two one thousand. "And when I got to four, I could see this gap of mist or cloud and there was the ocean, and I remember thinking, we're so close to the ocean – why are we so close to the ocean?" The light was dim below the clouds, Bull remembers, and it was hard to see faces. "It was quite misty," Sharp says. "The swell was breaking white, and there were white horses."

They hit the water. The rotor blades shattered, Bull says, and the windows at the back popped out. He remembers water streaming through them like fountains. It was, according to the Air Accidents Investigation Branch (AAIB), 18.17 local time.

"The helicopter filled with water, instantly," Sharp says, then it inverted. "The door buckled on the left-hand side and none of us had a chance to pull our rebreathers out, get our hoods on, nothing like that. And as the water came up to here" – he indicates his chin – "and I took my last breath, I could see people floating around. As soon as my head was covered with water, I looked down and pulled the tab on the window and it just came to bits in my hand. So I hit it with my elbow a couple of times. Nothing." His entire body relives the motions as he describes them. "And then I punched it – I think I punched it three times – and all of a sudden it went pop and away it went."

Nugent grabbed desperately for his rebreather, but he was clawing at the wrong side. "I remember the water washing over my face and I told myself, 'Take your last breath' and then the impact of the chopper I think knocked me out. There's a definite gap there. All that came over me was, don't worry, you're going to see Indi [his two-year-old] again. And I thought, well, OK, if I'm going to see Indi again, do your worst. And I kind of relaxed into it – but I was aware we were turning upside down and I was hung in my harness. Then, maybe 45 seconds, a minute, later, I heard that same voice again: 'James, it's time to get out now.' And I was like, oh, OK. I don't remember undoing my buckle, but I felt myself floating and I reached over, grabbed the window and pulled myself through it."

"As I hit the surface, the first thing I did was push myself away from the aircraft," Sharp says. "Then I went for the tab on my jacket and pulled it, and nothing happened. I tore the Velcro away, grabbed hold of the straw, started blowing it up and it went bouff! by itself, which was a bit of a shock." Then he looked around: "I saw this guy beside me. I grabbed him to get him away from the chopper. His eyes were massive, his pupils were black and he had sick coming out of his mouth – I just knew he was dead."

He heard someone shouting, "Me back! Me back! I think I broke me back!" Sharp grabbed hold of him: "You're alive," he said. "Let's get away!" They were surrounded by aviation fuel, and Sharp kept being sick. His movement was also oddly constricted – a piece of rotor blade had torn a hole in his suit. When he pushed the blade away, his suit began to fill with freezing water.

When Nugent broke the surface, he saw land not far away. His first thought was, that's fine! And then, close upon it, 30 seconds more and we'd have hit it. And he began to shiver in earnest.

He climbed up on to the fuselage and looked around him. There was gear everywhere, rope, pieces of helicopter, "guys in the water, and I was screaming at them, 'Raft up! Raft up!' There were other guys holding on to the fuselage and I was saying, 'Climb on to the fuselage!'" – where Bull joined him – but then he realised that wasn't a good idea at all, because the aluminium underside of the helicopter had ripped jaggedly open. So he yelled, "Stay in the water! Hold on!"

They tried to deploy the life raft, but only about 10% of it emerged and "we had to fight and push and tug it out of its casing. Obviously it's not been designed to be deployed when the chopper's upside down. I looked at Gary [McCrossan], who was as white as a sheet – that white-grey colour – and I recognised he was going into a heart attack. I pulled him into the life raft. Then I grabbed one of the younger chaps and said, 'Listen, sort him out.'" He helped five or six more men climb in.

When he finally looked up, Nugent saw there were still people in the water, drifting away, so he made for the second lifeboat, where he found the copilot. "But [the lifeboat] is just a big inner tube, there's no way you can steer it. And I was like, 'This is useless. What do we do now? Did you mayday?'" The copilot said no, there was no chance; but they were so close to landing, they would have fallen off the radar and triggered a rescue.

It was a long half-hour before they heard the familiar thud-thud-thud of rotor blades and another Super Puma appeared. It lowered a winch man and secured McCrossan first. But as it began to lift him into the air, one end of a cord from the life raft caught in the winch and the other end round the foot of a man in the raft, throwing him out. Quickly he was dragged back in, but the raft was now attached, precariously, to the helicopter. Bull scrambled to cut them free. "I doubt I was the only survivor to fear another Super Puma falling from the sky that day," he wrote later. "Finally, one by one, all the survivors were winched to safety.

Meanwhile, at 18.33, Jim Nicholson, a retired English teacher who has volunteered with the RNLI at Aith on Shetland since 1970, got a call from the coastguard. He alerted his crew; by 18.40 they had launched. They began by searching north-west of the crash site but, finding nothing, returned to the wreck, now drifting towards the rocks. Eventually the fuselage broke open and two bodies emerged. It was after 8pm and getting dark. The crew knew there should be one more body in the helicopter, so they drew it out into the bay and held it there all night, rocking quietly, until another lifeboat relieved them in the morning. Sarah Darnley's body was retrieved by divers the next day.

When it comes to such disasters, veteran North Sea pilots start making analogies to Swiss cheese. There are so many checks and balances keeping the helicopters flying that normally they "catch things going through", Buckley says. "Obviously in the last five years there have been a series of holes lining up, and we need to know why."

Jim McAuslan, general secretary of Balpa, is even blunter: "We believe something is wrong in the North Sea." The question is, what?

‘The helicopter filled with water, instantly’: survivors (from left) Paul Sharp, Sam Bull and James Nugent. Photograph: Gary Calton; Martin Godwin; Phil Bunt/cornwall-photos.com, all for the Guardian

The passengers I spoke to have doubts about whether pilot error was involved, but mechanical failure was ruled out quite early, and the Helicopter Safety Steering Group cleared the Super Pumas for flight after just one week's suspension. The debris has been taken to Farnborough to be examined by the AAIB. Meanwhile, the helicopter operators and oil companies have a further, serious, problem: confidence in their helicopters is extremely low. A recent poll by Unite found that 53% of its members are not confident about their safety.

Airbus Helicopters president and CEO, Guillaume Faury, asked last September about the previous month's crash, said, "The Super Pumas' overall safety record is excellent. One reason there are so many in the North Sea oil and gas business is their safety." But he also admitted: "We have reputation and image damage in the UK that we must face."

Ask survivors what the problem is and the answer is immediate; they seem surprised I even ask. "Money." Sharp rubs thumb and forefinger together. "Money," says Nugent. Balpa has said it is particularly concerned by "cut-throat" competition between helicopter operators bidding for oil firm contracts. Buckley notes that in the 1990s the oil companies brought in an initiative called Crine. "It stood for Cost Reduction In the New Era and was the basis for oil companies cutting back on routine maintenance, and other cost-saving measures. It doesn't formally exist now, but the ethos is still very prevalent."

Suggest there might be pressures on maintenance, however, and the unions say there cannot be – everything is controlled from Europe; leave a lightbulb in a day too long and you might lose your job. Oil & Gas UK retorts that Balpa has yet to produce evidence, and that in January helicopter operators told the transport select committee there was no commercial pressure – but "there is competition and safety is not jeopardised". Certainly there is an ostentatious fetishisation of safety: in the land-based CHC offices, I must hold the handrail going up steps and even on the level, which feels ridiculous. Sheriff Principal Derek Pyle, who conducted the inquiry into the 2009 crash off Fraserburgh, concluded this month that "everyone in [Bond Offshore Helicopters] well knew that maintenance must be done by the book. On one occasion, that fundamental rule was broken. It resulted in the failure to detect a significant fault in the helicopter's gearbox, which possibly – but only possibly – resulted in the crash."

"Everything affects safety," Buckley says. "If the pilots are frustrated because they're delayed – you've started engines, you're burning fuel, you miss your slot, you're at the back of the queue, then you have to taxi back to get more fuel – these are little holes that are beginning to line up. It's not just one person, it's everybody who's involved. It includes the managers, it includes the culture."

And there is a fear of speaking up. "It's supposed to be a no-blame culture," Sharp says. When you report a safety issue on the rigs, "you're supposed to put your name at the bottom, but a lot of people leave that blank". Of course, safety on the rigs is different from safety on the helicopters, but it is probably instructive to look at aspects of the wider North Sea culture. There used to be, for instance, a culture of blacklisting, or NRB – Not Required Back. "They say you can't NRB anyone any more. That's rubbish," Sharp says, and laughs.

After the incident in August, the oil companies made a show of saying they would not force anyone to fly, but it's difficult to see what choice they have. There are already stories of workers mentioning they are afraid of the helicopters, only to be told, "If you don't think they're safe, son, find yourself another career." Sharp says: "That's why people don't say anything. They're afraid."

"After the accident, a lot of safety reps emailed us with questions," says John Taylor at Unite. "Every one asked to be kept anonymous."

Are there issues with pilot training? Buckley wonders if they do enough hours on automatics. The autopilot systems are radically different on each variant of the Super Puma. Do they need to support copilots better? "Some copilots with low hours don't have a lot of confidence, and if you're flying next to a grizzled captain who doesn't like copilots anyway…" When a Sikorsky S92 crashed off Newfoundland in March 2009, killing all but one on board, Buckley says this was found to be one of the reasons: the copilot suggested, three times, that they ditch, but the captain overruled him.

But the most urgent thing, right now, is why did four people die last August? In mid-December, the BBC reported that, according to the AAIB, the crash was "survivable" and that the death certificates for three of the four fatalities recorded drowning as the cause of death (the fourth was Gary McCrossan, who died of a heart attack).

Survivors, and offshore workers polled by Unite, are clear: put fewer people on the helicopters, Sharp says – 16 at most – and change the seat configuration so that more people are closer to the windows. (When, after the crash, Nugent told another worker where he had been sitting, he discovered it was known as "the death seat".) Training should be made more realistic. The window Sharp dragged himself out of was, he estimates, only just over a foot high and 18 inches wide: "In training, the windows are massive. In reality it doesn't work like that." He suggests adding wave machines to give a sense of actual sea, but this is not allowed: health and safety guidelines bar putting trainees under "undue stress", says Matt Scaife at Falck Nutec, which runs many of these courses.

By far the most difficult issue to broach is that of body mass. There is understandable sensitivity about this. Food is a major morale booster in an industry in which there is no alcohol allowed (there is drug and alcohol testing at takeoff, and random bag searches) and there is nowhere to go; it also feels unfair to put pressure on men who've spent years in the industry to lose weight. But as Nugent says: "I've looked at some of those guys and thought, there's no way you're going to fit through that window. Even if you were naked and greased up, you wouldn't fit." Buckley points out that those who did not survive the August crash were the larger people on the flight. "I feel you should put on your survival suit, your life jacket, and try to get out of the smallest window in the smallest helicopter," Garcia says. "If you can't do that, you need to get your weight down."

At the end of February, the Civil Aviation Authority (CAA) published a 293-page safety review of offshore helicopter operations, stipulating that, from 1 April 2015, helicopter operators will be prohibited from carrying anyone "whose body size, including required safety and survival equipment, is incompatible with push-out window emergency exit size". Most dramatically, from June this year, no one is allowed to sit in a seat not adjacent to a window or door, unless they have been provided with Category A breathing systems or the helicopter has side-floats – but the latter are not available and the former not certified (even if they were, the entire workforce would have to be trained to use them). Which means that, unless many new helicopters and pilots are made available in the next three months, the number of people who can be flown out to the rigs will be reduced by 40%, with an obvious impact on the industry.

"We welcome the recommendations in the CAA's report, especially those that encourage standardisation in safety procedures," says Melinda de Boer, director of communications at CHC. "The industry is safer now than ever… It's also an industry that knows one accident, one injury, much less one loss of life, is too many."

"I have nothing but admiration for a chap prepared to work offshore in the environments they do," Buckley says, "but there's a feeling that their wellbeing is not being thought of as perhaps it should be, and it manifests itself in the design of the helicopters." Nugent is blunter: "Those helicopters are not fit for purpose – and what do you do with a piece of kit not fit for purpose? Scrap it. Simple as." It's not as if there isn't the money. "We're talking about the oil and gas industry: these people make profits in the billions." Oil & Gas UK points out that they do not own the helicopters and, more pertinently, that it takes many years to design them.

The hope is that it will not take that long for the five current investigations into the August crash to report their findings, and in the meantime, life must go on. Bull tries not to think about it too hard. Sharp expected to be able to get a 15-minute test flight, then be back at work in January ("It's not the flying that's the problem," he said drily, "it's the crashing"), but there have been endless delays and now: "I'm skint." He says he keeps being asked for paperwork – physiotherapists' and counsellors' reports, back-to-work certificates: "We're the victims and we're doing all the running about." Garcia had such bad post-traumatic stress disorder that he only recently went back to work. Nugent, who fractured his spine, still has severe migraines if he exerts himself. He worked freelance for a company called Axiom; unlike those with permanent jobs, he has not been paid since the crash.

He struggles with the surrealness of it – the sudden rent in normality, which stretches out as usual on either side – and never wants to get in a helicopter again. He has flashbacks during the day and still dreams about the crash: about struggling to get out and being unable to undo his seatbelt, about someone holding him back, about going back to find George, or Sarah, and either can't find them or can't find his way out again, can't work out which way is up and which is down. He is overwhelmed by guilt and wakes sweating, fighting his way out of the duvet, night after night.