COSMO OIL : Overview of the Fire and Explosion at Chiba Refinery, the Cause of the Accident and the Action Plan to Prevent Recurrence
08/02/2011 | 01:40am(1) Many braces supporting the legs holding the LPG tank broke, which caused the legs to bend,
leading to the collapse of the tank.
Damaged LPG Tank No. 364 met all the earthquake-proof structural requirements. However, since at the time of the earthquake, Tank No. 364 was filled with water, instead of lighter LPG, for a regular inspection, the initial earthquake off the coast of the Tohoku region put such a heavy load on the braces that some of them collapsed . This coincided with the second earthquake off the coast of the Ibaraki Prefecture, which led to the tank falling down. It was a temporary measure to fill the tank with water for regular inspection and we recognize that we were insufficiently prepared to deal with an earthquake of this magnitude at that time.
For future regular inspections, we will minimize the period of time for filling up the tank with water . When we build new LPG tanks, we will incorporate safety measures into the tank installation design to withstand even during the period when it is filled with water. We will also assess the durability of the existing LPG tanks and provide enhancements, if necessary. Furthermore, when we fill up a tank with water for inspection in the future, we will protect, isolate, or separate the pipelines and other facilities adjacent to the tank preventing any harms should there be any major damages to the tanks.
It is conceivable that the earthquake jolted the LPG tank and pipes and that the collapse of the tank damaged the pipes, causing the LPG to leak. This led us to infer that LPG might have continued to leak through three cracks in the pipes, and one of these damaged parts was found on the pipe connected to an emergency shut-off valve*5 that was set as open in a fixed position.
Before the earthquake, we had confirmed a tiny amount of air leaking from the pipe to which air was supplied in order to activate the emergency shut-off valve, which was fixed open until we fix the pipe. This was a temporary measure taken to keep the emergency shut-off valve from closing if air pressure was reduced in the pipe. The operation procedure was to manually close the valve at the time of emergency, but on March 11 we could not gain access to close the valve due to LPG leaks.
When we have an opportunity to design a new pipe around the LPG tanks, we will ensure that an appropriate level of flexuosity*6 will be incorporated into the structure of the new pipes.
We will not set the emergency shut-off valve to open in any situation in the future.
Notes:
*5. The emergency shut-off valve refers to the valve installed in the pipe to ensure that it will
be closed, and safely and quickly shut off the LPG flow, if LPG should leak from the pipe.
*6. A structure with flexuosity refers to a structure in which pipes are made flexible and bendable
enough to absorb any displacements caused by deflection, etc.
(3) Source of Fire, Explosion and Spreading
We carefully investigated the ignition cause but was unable to identify this.
Upon the breakout of the fire, we continued to spray water over the LPG tanks around Tank No. LPG364 to cool them down but we judge that the fire, which broke out near Tank No. LPG364, gathered momentum to increase the surface temperatures of the adjacent LPG tank units, which reduced their strengths, rising their internal pressure to an unsustainable amount, leading to explosions, spreading the fire from one tank to another.
