Explosions and a fire on the Piper Alpha platform July 6, 1988, killed 167 offshore workers and shook the U.K. oil and gas industry's concepts about operational safety in the North Sea.
The full effects of that blast on offshore safety legislation are only now being seen as fields developed after the explosion begin production.
Key among the new installations is the replacement Piper Bravo platform, which started production from U.K. North Sea Block 15/17 (OGJ, Feb. 8, p. 31).
Piper Bravo features an accommodation module that doubles as a safe refuge, shielded escape routes leading to freefall lifeboats, and duplicated oil and gas isolation valves.
They are a direct response to the major revision in legislation that followed the inquiry into the Piper Alpha disaster undertaken by Lord Cullen. Results were reported in October 1990.
THE EXPLOSION
The Cullen report said the Piper Alpha accident stemmed from a breakdown in communications at shift handover between maintenance crews. Failure of the permit to work system meant the new crew was unaware that a pressure safety valve had been removed by the previous crew.
The valve belonged to a condensate injection pump, which the new crew tried to restart when a similar pump tripped. This caused an explosion in the oil separation module and a subsequent fire.
The fire, fed by oil from the platform and a leak from the main oil line to shore, eventually ruptured the gas riser from the nearby Tartan platform. This caused a second major explosion and massive intensification of the fire.
Main power supplies and the control room were knocked out of action by the first explosion. Emergency shutdown systems were activated, but many failed immediately or shortly after the first blast.
Firewater systems were rendered useless by damage or power loss. Smoke entered the accommodation module, where many workers gathered, but there was no organized attempt to evacuate the accommodation block.
"To remain in the accommodation meant certain death," the Cullen report said. Twenty-eight of the 61 survivors got out of the accommodation using ropes and hoses or by jumping from the platform into the sea.
LESSONS LEARNED
Central to recommendations flowing from the Cullen inquiry was the concept of the safety case. This is a document required of all operators for each U.K. offshore installation. Cullen recommended that the government's regulatory body approve each safety case before a producer was allowed to continue operation of the installation concerned.
New offshore safety regulations, based on Cullen's concept, take effect this year for new and existing installations (OGJ, Dec. 14, 1992, p. 25).
Among 106 recommendations of the Cullen report were:
- Operators must have an approved safety case for each installation showing that the design and operation of each installation are safe and major hazards have been identified and appropriate safety measures provided.
- A temporary safe refuge (TSR) must be provided, along with facilities for safe and full evacuation, escape, and rescue.
- The TSR normally should be the accommodation.
- The permit to work system should form part of the safety case.
PIPER BRAVO
Elf Enterprise Caledonia Ltd. is now operator of Piper field, having taken over redevelopment by buying the North Sea assets of previous operator Occidental Petroleum Corp.
The new Piper platform structure is centered around an accommodation/TSR module that is completely enclosed and pressured. It is designed to provide protection for at least 2 hr.
Piper Bravo is the first platform from which the crew can escape from accommodations to sea without outdoor exposure.
Accommodation is set as far as possible from the wellhead/drilling, process, and compression modules. The nonhazardous utilities module lies between the accommodation and production sections.
Blast walls and decks were tested to ensure they could withstand blast pressures and contain an explosion within one area. Four diesel firewater pumps are located in both wellhead and utilities areas. They drive the deluge system and supply the firewater main line.
The process plant is equipped with a "blowdown" facility, which rapidly removes hydrocarbons from the platform in an emergency.
The platform's six risers are located under the wellbay module, again as far as possible from accommodations. Three main fire and blast walls separate the wellhead/drilling, process, and compression modules from each other and from the utilities and TSR.
Pipelines to and from the platform are fitted with emergency shutdown valves on the seabed and on the topsides. These are held open by hydraulic pressure against a spring actuator and close automatically when pressure falls.
ESCAPE ROUTES
Two routes from anywhere on the platform lead to the main escape method, freefall lifeboats. Kickover life rafts form the first backup, while as a last resort crewmen can descend to sea level by rope.
Piper Bravo was the first U.K. platform equipped with freefall lifeboats. They are launched from a heat-shielded stairway to emerge 150 ft away from the platform.
Each lifeboat can carry 45 passengers. Six lifeboats are in the TSR, while three more are in the wellbay module. They are designed to withstand outside temperatures of 1,000 C. while maintaining a 30 C. average inside temperature.
"Piper B is regarded as the first of a new generation of North Sea platforms with safety as an inherent feature," said Elf Enterprise, when the field began production Feb. 3.
The new platform will produce remaining reserves of 172 million bbl of oil and 14 bcf of gas. Original reserves were 1 billion bbl and 120 bcf. The field originally went on production in December 1976.
PLANS MARK PROGRESS
A group led by Woodside Petroleum Pty. Ltd. has completed feasibility studies for development of Cossack and Wanaea oil fields on the Northwest Shelf off Western Australia.
The project now will proceed to the definition stage, which includes preliminary engineering and preparation of detailed budget estimates.
At the same time, plans call for a comprehensive study of the feasibility of a liquefied petroleum gas export program fed by natural gas supplies from Wanaea/Cossack as well as North Rankin and Goodwyn fields-the two mainstays of the Northwest Shelf liquefied natural gas export project.
PROJECT DETAILS
Preliminary plans for Wanaea/Cossack development call for subsea wellheads tied back to a floating oil production storage and offloading facility installed in Wanaea field.
Woodside group also plans to lay a gas pipeline to the nearby North Rankin platform with a connection to the main trunk line to shore.
LPG export would involve construction of an onshore LPG chilling plant, two 50,000 cu m capacity storage tanks, and a 500 m long loading jetty.
When in full production, Wanaea and Cossack are expected to produce 70,000 b/d of crude oil.
Total project output of LPG is expected to reach 350,000 metric tons/year.
If the separate projects are approved by third quarter 1993, production will begin in 1995.
Total combined development costs are pegged at about $1.1 billion (Australian).
Copyright 1993 Oil & Gas Journal. All Rights Reserved.
