The following is an incident investigation report from the UNITED STATES DEPARTMENT OF THE INTERIOR BUREAU OF OCEAN ENERGY MANAGEMENT, REGULATION AND ENFORCEMENT. The accident was a result of having a flammable liquid tank within the 35′ fire safe zone required for safe hotwork (e.g. cutting, welding, brazing, grinding, etc.) zone.
On 21 May 2010, two 500 bbl oil storage tanks exploded/ruptured on a platform as a result of the ignition of hydrocarbon vapors in the headspace of the tanks. The explosion caused both tanks (wet and dry oil storage tanks) to rupture and immediately discharge the water and crude oil contents of the tanks. The explosion occurred while a contract construction crew was conducting operations to relocate the platform’s LLOG production separator from
the southeast portion of the platform’s main deck. The BOEMRE investigation of the incident revealed that, at the time of the explosion, the work crew was using an oxyacetylene cutting torch to remove (for repositioning) an air tugger that had been welded to a main deck I-beam approximately 25 feet from the explosion site. Slag/sparks from the cutting/washing operation is believed to be the source of the ignition in the tanks.
According to the company’s reconciliation report of the contents of the oil tanks, the 500 bbl dry oil tank contained 34 bbls of crude oil before the explosion, and the adjacent 500 bbl wet oil tank contained from 8 – 25 bbls of crude oil before the explosion, depending on the assumption of the water content in the wet oil tank. Additionally, the report indicated that after the explosion, a total of 18.5 – 29.4 bbls of crude oil were not recovered in platform containment vessels, and this oil was assumed to be spilled overboard. According to McMoran, the dry oil tank had 3 feet of fluid and the wet oil tank had 6 feet of fluid before the explosion.
BOEMRE inspection staff collected the following information regarding the subject incident and of during an interview conducted on 24 May 2010. On 21 May 2010, the construction crew started at 0600 hours with a safety meeting, pre-job planner (discussion of how job is to be conducted) and Job Safety Analysis (JSA). After the morning meetings, the construction crew started work on the top deck to remove piping. After piping was removed, the construction crew blinded open ended piping located above and below the top deck that led to the production separator on the top deck so that the piping would not act as a conduit for ignition of hydrocarbon vapors. The construction crew then removed an additional line from under the main deck using a band saw. A hole was cut in the grating on the main deck where the tugger was welded to an I-beam for stability during the skidding operation. The tugger was welded to the beam in a position that is on the deck above and about 25 feet horizontally from the wet oil storage tank. The company’s study of the explosion indicated “The last “washing” activity was located above and approximately 12′ laterally south/southwest of the southern portion of the wet oil storage tank”. The tanks were neither rendered inert nor shielded with metal or fire resistant guards or curtains as required by 30 CFR 250.113(a). The crew then proceeded to wash the separator skid (which was on solid decking) welds with a cutting torch so that the separator skid could be relocated. When the construction crew completed washing the welds on the skid, they attempted to move the separator skid closer to the crane with the tugger so that the lift of the separator could be made within the specifications indicated on the crane’s load chart. After this attempt failed, they picked up the front end of the separator skid with the crane in order to help the tugger slide the separator skid. Using this method, they successfully slid the separator skid approximately 35 feet from its original location. After further evaluation, it was determined that the tugger would need to be moved again in order to position the separator skid to the exact location needed to make the separator skid lift with the crane.
Work was stopped to hold a pre-job meeting on deck to discuss the relocation of the tugger and separator skid. At the conclusion of the meeting, went downstairs to monitor for any signs of gas using a portable gas detector, and to wet down the area with a water hose. At this point, personnel started cutting/washing the tugger welds using an oxy-acetylene cutting torch. A worker was on the cellar deck and equipped with a dry chemical extinguisher and a potable water hose in order to perform the fire watch duty. Worker said that he heard a small rumbling noise, and then started walking around the backside of the production separators on the cellar deck to evaluate the situation. Worker then heard a louder popping noise and became covered in oil. Worker stated there was no sign of fire, and the smoke cleared fairly fast from the tank area after spraying down the area with water. After all was clear, a production operator called the field foreman and then shutdown the platform generator.
The Oil Storage Tanks Explosion Analysis performed by the company’s consultant, Engineering Systems, Inc, found that several deficiencies existed in the tank system including:
1) the rubber flange gasket below the thief hatch was deteriorated and could have allowed gas leakage,
2) the flame arrestor was occluded with corrosion which resulted in a diminished flow to the extent that an application of a pressurized air stream didn’t result in the detection of an air flow through the arrestor
3) the 8-inch, Enardo 600-lb thief hatch was not adequately sized for this application and
4) the storage tanks’ bottom-to-shell connection was not welded according to applicable API standards.
Although the study was not able to reach a conclusion as to the precise cause of the explosion, it did not discount the adjacent cutting operation as the source of ignition of the explosion and stated, “Sparks/spray normally associated with oxygen/acetylene torch cutting activities could have a casual relationship with the ignition of ignitable liquid vapors and/or flammable gas exterior to and in the vicinity of the oil storage tanks on the platform’s cellar deck”.
The probable cause of the incident was the propagating combustion reaction of the volatile vapors and air mixture within the headspaces of the tanks and the torch cutting operation, as a result of the vapors not being properly vented through the flame arrestor. Cursory evaluation of the flame arrestor indicated the presence of corrosion deposits and occlusion of the arrestor, resulting in insufficient venting of the volatile vapors and air mixture. The most likely scenario for the volatile vapor/air mixture resulted from the “breathing” of the tanks through the thief hatch. This breathing allowed gas to escape during hot daytime hours and allowed air to enter the tanks during the cooler nighttime hours.