Explosion in the North Sea is the 10th episode of Seconds from Disaster and dives to unreveal the truth about the offshore disaster.
A series of explosions and fires on the Piper Alpha, an oil platform 110 miles off the coast of Scotland that had been converted to liquified petroleum gas (LPG) production, results in the deaths of 167 people and the collapse of the platform.
CauseEditA new gas pipeline was built in the weeks before the 6 July explosion, and while this work disrupted the normal
12:00 p.m. Two condensate pumps, designated A and B, displaced the platform's condensate for transport to the coast. On the morning of July 6, Pump A's pressure safety valve (PSV #504) was removed for routine maintenance. The pump's fortnightly overhaul was planned but had not started. The open condensate pipe was temporarily sealed with a blind flange (flat metal disc). Because the work could not be completed by 6:00 p.m., the blind flange remained in place. The on-duty engineer filled out a permit which stated that Pump A was not ready and must not be switched on under any circumstances.
6:00 p.m. The day shift ended, and the night shift started with 62 men running Piper Alpha. As he found the on-duty custodian busy, the engineer neglected to inform him of the condition of Pump A. Instead he placed the permit in the control centre and left. This permit disappeared and was not found. Coincidentally there was another permit issued for the general overhaul of Pump A that had not yet begun.
7:00 p.m. Like many other offshore platforms, Piper Alpha had an automatic fire-fighting system, driven by both diesel and electric pumps (the latter were disabled by the initial explosions). The diesel pumps were designed to suck in large amounts of sea water for fire fighting; the pumps had an automatic control to start them in case of fire. However, the fire-fighting system was under manual control on the evening of 6 July: Piper Alpha procedures required manual control of the pumps whenever divers were in the water (as they were for approximately 12 hours a day during summer) regardless of their location, to prevent divers from being sucked in with the sea water (fire pumps on other platforms were switched to manual control only if the divers were close to the inlet).
9:45 p.m. Condensate (natural gas liquids NGL) Pump B stopped suddenly and could not be restarted. As the entire power supply of the offshore construction work depended on this pump, the manager had only a few minutes to bring the pump back online, otherwise the power supply would fail completely. A search was made through the documents to determine whether Condensate Pump A could be started.
9:52 p.m. The permit for the overhaul was found, but not the other permit stating that the pump must not be started under any circumstances due to the missing safety valve. The valve was in a different location from the pump and therefore the permits were stored in different boxes, as they were sorted by location. None of those present was aware that a vital part of the machine had been removed. The manager assumed from the existing documents that it would be safe to start Pump A. The missing valve was not noticed by anyone, particularly as the metal disc replacing the safety valve was several metres above ground level and obscured by machinery.
9:55 p.m. Condensate Pump A was switched on. Gas flowed into the pump, and because of the missing safety valve, produced an overpressure which the loosely fitted metal disc did not withstand.
Gas audibly leaked out at high pressure, drawing the attention of several men and triggering six gas alarms including the high level gas alarm, but before anyone could act, the gas ignited and exploded, blowing through the firewall made up of 2.5 x 1.5 metre panels bolted together, which were not designed to withstand explosions. The custodian pressed the emergency stop button, closing huge valves in the sea lines and ceasing all oil and gas production.
Theoretically, the platform would then have been isolated from the flow of oil and gas and the fire contained. However, because the platform was originally built for oil, the firewalls were designed to resist fire rather than withstand explosions. The first explosion broke the firewall and dislodged panels around Module (B). One of the flying panels ruptured a small condensate pipe, creating another fire.
10:04 p.m. The control room was abandoned. Piper Alpha's design made no allowances for the destruction of the control room, and the platform's organisation disintegrated. No attempt was made to use loudspeakers or to order an evacuation.
Emergency procedures instructed personnel to make their way to lifeboat stations, but the fire prevented them from doing so. Instead the men moved to the fireproofed accommodation block beneath the helicopter deck to await further instructions. Wind, fire and smoke prevented helicopter landings and no further instructions were given, with smoke beginning to penetrate the personnel block.
As the crisis mounted, two men donned protective gear in an attempt to reach the diesel pumping machinery below decks and activate the firefighting system. They were never seen again.
The fire would have burnt out were it not being fed with oil from both Tartan and the Claymore platforms, the resulting back pressure forcing fresh fuel out of ruptured pipework on Piper, directly into the heart of the fire. The Claymore continued pumping until the second explosion because the manager had no permission from the Occidental control centre to shut down. Also, the connecting pipeline to Tartan continued to pump, as its manager had been directed by his superior. The reason for this procedure was the exorbitant cost of such a shut down. It would have taken several days to restart production after a stop, with substantial financial consequences.
Gas lines of 140 to 146 cm in diameter ran to Piper Alpha. Two years earlier Occidental management ordered a study, the results of which warned of the dangers of these gas lines. Due to their length and diameter it would have taken several hours to reduce their pressure, so that it would not have been possible to fight a fire fuelled by them. Although the management admitted how devastating a gas explosion would be, Claymore and Tartan were not switched off with the first emergency call.
10:20 p.m. Tartan's gas line (pressurised to 120 Atmospheres) melted and burst, releasing 15-30 tonnes of gas every second, which immediately ignited. A massive fireball 150 metres in diameter engulfed Piper Alpha, killing two crewmen on a fast rescue boat launched from the standby vessel Sandhaven and the six Piper Alpha crewmen they had rescued from the water. From that moment on, the platform's destruction was assured.
10:30 p.m. The Tharos, a large semi-submersible fire fighting, rescue and accommodation vessel, drew alongside Piper Alpha. The Tharos used its water cannons where it could, but it was restricted, because the cannons were so powerful they would injure or kill anyone hit by the water.
10:50 p.m. The second gas line ruptured, spilling millions of litres of gas into the conflagration. Huge flames shot over 300 ft (90 m) in the air. The Tharos was driven off by the fearsome heat, which began to melt the surrounding machinery and steelwork. It was only after this second explosion that the Claymore stopped pumping oil. Personnel still left alive were either desperately sheltering in the scorched, smoke-filled accommodation block or leaping from the deck some 200 ft (60 m) into the North Sea.
11:20 p.m. The pipeline connecting Piper Alpha to the Claymore Platform burst.
11:50 p.m. The generation and utilities Module (D), which included the fireproofed accommodation block, slipped into the sea. The largest part of the platform followed it.
12:45 a.m., 7 July The entire platform had gone. Module (A) was all that remained of Piper Alpha.
At the time of the disaster 224 people were on the platform; 167 died and 59 survived. Two men from the Standby Vessel Sandhaven were also killed.