Timeline of the Piper Alpha disaster
Created by confiteor on Jun 30, 2008
Last updated: 03/11/10 at 02:27 PM
The entire platform has gone. Module (A) is all that remains of Piper Alpha.
The generation and utilities Module (D), which includes the fireproofed accommodation block, slips into the sea. The largest part of the platform follows it.
The pipeline connecting Piper Alpha to the Claymore Platform bursts and the disaster claims its final victims.
The second gas line ruptures, spilling millions of litres of gas into the conflagration. Huge flames shoot over three hundred feet in the air. The Tharos is driven off due to the fearsome heat, which begins to melt the surrounding machinery and steelwork. It was after this second explosion that the Claymore stopped pumping oil. Personnel still left alive are either desperately sheltering in the scorched, smoke-filled accommodation block or leaping from the deck some 200ft into the rough, undulating North Sea.
The Tharos, a large fire fighting and rescue platform, draws alongside Piper Alpha. Attempts are made to extend its rescue walkway the 30 metres to the deck. A woeful design flaw in Tharos becomes apparent as the walkway extends too slowly to be able to reach the platform before 22:50.
Tartan's gas line (pressured to 120 Atmospheres) melts and bursts. From this moment on, the platform's destruction is assured. 15 - 30 tonnes of gas are released instantaneously and immediately ignite. Gas bursts out at ½ a tonne per second, equivalent to the entire domestic consumption of gas in the UK. A massive fireball of 150 metres in diameter engulfs Piper Alpha.
The control room is abandoned. Piper Alpha's design made no allowances for the destruction of the control room and the platform's organisation disintegrates. 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 are never seen again. The fire would have burnt out were it not being fed new oil from both Tartan and the Claymore platforms, the resulting backpressure 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 received this directive from his superior. The reason for this procedure was the exorbitant cost of such a shut down. It takes several days to restart production after a stop, with substantial financial consequences. Gas lines of 140 to 146 cm in diameter ran close to Piper Alpha. Two years earlier Occidental management ordered a study, which warned of the dangers of these gas lines. Due to their length and diameter it would take several hours to reduce their pressure, so that it would not be possible to fight a fire fueled 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.
Condensate (LPG) Pump A is switched on. Gas flowed into the pump, and due to the missing safety valve produced an overpressure which the loosely fitted metal disc did not withstand. Gas audibly leaks out at high pressure, drawing the attention of several men and triggering 6 gas alarms including the high level gas alarm, but before anyone can act, the gas ignites and explodes, 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 presses the emergency stop button; closing huge valves in the sea lines and ceasing all oil and gas production. Theoretically, the platform would now have been isolated from the flow of oil and gas and the fire relatively contained. However, because the platform was originally built for oil, the firewalls were designed to resist fire rather than withstand explosions. The first explosion breaks up the firewall and dislodges panels around Module (B). One of the flying panels ruptures a small Condensate pipe, creating another fire.
The permit for the overhaul is 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 were 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 compressor A. The missing valve was not noticed by anyone, particularly since the metal disc replacing the safety valve was located several metres above ground level and obscured by machinery
Condensate (LPG) Pump B stops suddenly and cannot be restarted. 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 (LPG) Pump A could be started.
Like many other offshore platforms, Piper Alpha had an automatic fire-fighting system, driven by both diesel and electric pumps (the latter of which were disabled by the initial explosions). The diesel pumps sucked in large amounts of sea water in order to extinguish any fires. These pumps should automatically switch themselves on in case of fire. However, when divers were working in the water below the platform (approximately 12 hours per day during summer), the pumps were switched to manual and could be started only from one place. Fire pumps on other platforms were switched to manual only if the divers were close to the inlet, to prevent them being sucked in with the sea water. However, Piper Alpha procedures dictated that the pumps be manual whenever divers were in the water, regardless of their location. This meant that the fire-fighting system was on manual on the evening of July 6.
The day shift ends and the night shift starts with 62 men running Piper Alpha. As he found the on-duty custodian busy, the engineer neglects to inform him of the condition of Pump A. Instead he places the permit in the control center and leaves. 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.
Two Condensate pumps on the platform, designated A and B, compressed the gas 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 now open Condensate pipe was temporarily sealed with a flat metal disc. Because the work could not be completed by 6:00 p.m., the metal disc 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.