April 28, 2014 | Mark Paradies


An incident report and lessons learned from the OPG Safety Alert network:

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.

In this case, workers sustained moderate injuries. However similar accidents (e.g. 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.


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).

Figure 1



Figure 2



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 (e.g. pontoon cleanliness verification, inside pontoon gas detection).

 safety alert number: 255 

OGP Safety Alerts http://info.ogp.org.uk/safety/


Whilst every effort has been made to ensure the accuracy of the information contained in this publication, neither the OGP nor any of its members past present or future warrants its accuracy or will, regardless of its or their negligence, assume liability for any foreseeable or unforeseeable use made thereof, which liability is hereby excluded. Consequently, such use is at the recipient’s own risk on the basis that any use by the recipient constitutes agreement to the terms of this disclaimer. The recipient is obliged to inform any subsequent recipient of such terms.

This document may provide guidance supplemental to the requirements of local legislation. Nothing herein, however, is intended to replace, amend, supersede or otherwise depart from such requirements. In the event of any conflict or contradiction between the provisions of this document and local legislation, applicable laws shall prevail.

Show Comments

Leave a Reply

Your email address will not be published. Required fields are marked *