Lessons from Offshore Accidents

Transcription

1 Lessons from Offshore Accidents Tekna Prosessikkerhet, Bergen Jon Erik Pettersvold/DNV Energy With extracts from Petrobras Presentation June 2001 Source:

2 Lessons from Offshore Accidents Each Accident is Unique! It could never happen to us! Slide 2

3 Accidents to Offshore Installations March 1980: Alexander Kielland accommodation semi on the Ekofisk field of North Sea broke up with fatigue fracture and capsized, killing 123 people. October 1981: U.S. oil rig Ocean Ranger keeled over in the North Atlantic, killing 84 people. January 1985: Two men were killed and two injured in a pump room explosion on drilling unit Glomar Arctic II rig in the North Sea. July 1988: In the world's worst oil rig disaster, 167 people were killed when Occidental Petroleum's Piper Alpha oil rig in the North Sea exploded after a gas leak. September 1988: One person was killed and 66 people rescued uninjured after Ocean Odyssey drilling rig burst into flames in the North Sea. March 2001: Eleven people were killed after explosions/capsizing of the P-36 offshore floating platform belonging to Petrobras. Slide 3

4 P-36 Roncador Field, Brazil March dead $500M Asset Lost Lost Production Slide 4

5 10 bar Starboard Tank Leak Scenario Valve for isolation Initial Condition Port & Stbd 50% water content Stbd Tk isolated 26th February Vent blinded March 9th At 50% filling, tank to be emptied Slide 5

6 The P-36 Accident development First event occurred at 00:22 on March 15 as a thud in Starboard Aft Column - no advance warning, no prior upsets Alarms sound almost immediately: low firewater pressure, flammable gas, ESD-3 (Fire-Mode) P-36 lists 2 o within 5 minutes Emergency Brigade, EB, responds to location, expecting to find a fire No apparent damage, slight smoke or mist, EB descends into Column Operators and Ballast staff try to diagnose situation - complex pattern of alarms Violent explosion at 00:39 kills 10 immediately, 1 dies later of burns, ESD-4 Rescue operations commenced for some time - access difficult, no firewater Evacuation commenced at 01:45 and completed at 03:30 (by crane) Loss of all power and control at 04:30, final abandonment 06:00 (by helicopter) Progressive sinking, recovery attempts fail, sinks March 20 Slide 6

7 Details of the aft starboard column 3rd level Ventilation duct existing Waste oil tank Seawater supply line Drain storage tank Drain storage pump 4th level Waste oil pump Slide 7

8 10 bar Starboard Tank Leak Scenario Valve for isolation Initial Condition Port & Stbd 50% water content Stbd Tk isolated 26th February Vent blinded March 9th At 50% filling, tank to be emptied Slide 8

9 10 bar Starboard Tank burst scenario Valve leaks Pressure builds Decision made to pump out water from port EDT Production header lined up with port/stbd. EDT Pump start delayed 1 hour Inlet valve to stbd. EDT leaking app. 70 t of crude Pressure build up in stbd. EDT Starboard Emergency Drain Tank Slide 9

10 Starboard Tank burst scenario 19 bar Valve leaks Hi Press Oil Water Pumping from port EDT starts at 23:15 Cont. pressurisation of port EDT Port EDT burst at 00:22 at P 10 bar 450 mm seawater line breaks Starboard Emergency Drain Tank Slide 10

11 Basis for conclusion on immediate cause Interviews of witnesses Documentation/material retrieved from P-36 ESD and Fire & Gas alarm records (approx alarms in 17 min.) Platform project documentation Photographic evidence Timeline analysis HAZOP reviews Stability analysis Dispersion and explosion analysis Visit to sister unit Jack Bates Slide 11

12 Most Likely Accident Scenario Burst tank ruptures 450mm seawater line Also ruptures nearby buoyancy tank vent lines Firewater system alarms Low Pressure Crude oil outgasses flammable vapors Gas alarms on Main Deck + elsewhere P-36 enters ESD-3, flaring & alarms Water level builds up quickly to 2m height Water flows down ventilation pipes Pontoon spaces start to flood P-36 lists 2 degrees Slide 12

13 Most Likely Accident Scenario, contd. Emergency brigade enters Starboard Aft Column Flammable gas ignites, major explosion inside P-36 Local Emergency Brigade caught in explosion Flooding continues until downflooding points are reached Unit capsizes and sinks Slide 13

14 Sinking of P-36 Slide 14

15 Combination of Errors Fault Description of fault 1 Leak condition of inlet valve of Starboard EDT Outcome if Fault did not occur (No explosion means also no fatalities) No accident 2 Starboard EDT vent isolated with blind No accident, but overflow into vent header, not serious 3 No blind for inlet valve of Starboard EDT No accident 4 Select Production Header for outlet rather than Caisson Elastic deformation only of EDT, no explosion, no sinking 5 Delay in start of pump for Port EDT Plastic deformation and maybe rupture, but no explosion and no sinking 6 Failure of compartment watertight dampers Seawater pipe rupture and explosion, but no sinking 7 Opening of Tanks 26S and 61S for natural ventilation Seawater pipe rupture and explosion, borderline sinking - but more time available to diagnose problem 8 Two seawater pumps out-of-service Seawater pipe rupture and explosion, but no sinking as immediately switch away from pump D Slide 15

16 Incorrect conditions: Leak condition of inlet valve of Starboard EDT: NO ACCIDENT Starboard EDT vent isolated with blind: NO ACCIDENT 19 bar Valve leaks Hi Press Oil Water No blind for inlet valve of Starboard EDT: NO ACCIDENT Select Production Header for outlet rather than Caisson: ELASTIC DEFORMATION Delay in start of pump for Port EDT: PLASTIC DEFORMATION Slide 16

17 Lessons learned from the P-36 accident Operators of similar units have performed reviews together with DNV (typically SWIFT) in order to identify potential for similar faults - WP system to address all critical situations - Focus on combined operations - MoC to be given focus - No HC tanks un lower hull - Training of operators - Limit use of WT dampers - Separate alarm VDU for critical marine alarms - Etc. Applicable DNV rules address some relevant issues - Safety assessment to be carried out if HC tanks are located in lower hull of a unit - The number of alarms during abnormal situations shall be assessed and reduced as far as practicable - Location of safety systems to give maximum availability during DALs Slide 17

18 This could never have happened to you...?????? Slide 18

19 Slide 19