Pontoon compartment explosion on floating roof tanks

Published:  28 March, 2014

The International Association of Oil and Gas Producers (OGP) issues safety alert following an explosion.

An explosion occurred at a location during restoration activities on a crude oil tank which had been cleaned up (Figure 1). Two workers were carrying out hot work on the top of the tank floating roof (cutting rim seal brackets in close proximity to the pontoon). One of the pontoon compartments exploded. Flammable residue was released onto the tank floor causing a small fire.

Figure 1

In this case, workers sustained moderate injuries. However similar accidents (eg Figure 2) involving hot work on floating roofs after tank clean-up have taken place in the past and led to more severe injuries and fatalities.

Figure 2

What went wrong?

Ignition sources generated by the hot work came in contact with the flammable mixture of hydrocarbons inside a nearby pontoon compartment that was contaminated.

• Tank Cleaning activities had been executed by contractors with little/no experience with tank cleaning

• There was no detailed procedure describing the tank cleaning activities

• The tank cleanliness inspection was incomplete as the pontoon compartments were not inspected

• The possible presence of hydrocarbons in the pontoon had not been anticipated during risk assessments (hence gas testing was ineffective because not done inside pontoon).

Corrective actions and recommendations

Have engagement sessions with staff to increase awareness on risks related to tank cleaning and hot work inside, on or near tanks.

NB: Consider very carefully the risks of enclosed volumes, e.g. tank pontoons. Unless clearly proven otherwise, they should always be treated as if they contain volatile hydrocarbons!

• Tank cleaning activities are critical operations that require specialized interventions using detailed procedures.

• Pontoon compartments are enclosed volumes potentially containing hydrocarbons that sometimes go unnoticed.

• Other enclosed volumes exist on floating roofs and present the same risks as with pontoons (e.g. some parts of seal systems, radial stiffeners).

• Risk assessments must systematically address the hazards related to enclosed volumes and include adequate safety measures (eg pontoon cleanliness verification, inside pontoon gas detection).

Safety Alert Number: 255

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