Floating roofs prevent the evaporation of volatile chemicals in tanks. This is achieved by providing a seal between product fluid and the environment. The seal prevents vapors from escaping the tank. Escaped vapors are troublesome for two reasons: financial loss and the potential for fire. As a result, maintaining the integrity of floating roofs is a vital risk concern for terminals, and protecting them must be considered on a regular basis.
Hurricane Harvey ravaged Houston and surrounding communities. Refineries were impacted by the natural disaster, and an estimated 30-50 floating roofs sank. Harvey presented the most difficult conditions a floating roof must endure, yet many sinking events could have been prevented had best practices been applied. Floating roofs are unlikely to fail if constructed properly, inspected regularly and maintained accordingly.
Most floating roofs are either annular pontoon floating roofs (APFR) or double- deck floating roofs (DDFR). APFRs are the more common type of floating roof, and DDFRs are more expensive and sturdy.
Floating roof tanks are sometimes fitted with a fixed roof above to exclude precipitation and wind. Floating roof tanks with a fixed roof above are called internal floating roof tanks (IFRTs), and tanks without one are external floating roof tanks (EFRTs).
API 650 states the design and maintenance criteria for storage tanks in the U.S. EFRTs require steel construction and a more strict design criteria than IFRTs. EFRTs are required to drain a minimum of 10 inches of water off the deck per 24 hours. This design criteria may prove insufficient for tropical storms and hurricanes, as witnessed recently. Similarly, API 650 allows for two adjacent pontoons to be punctured and still continue operation.
The stability of a floating roof depends on the buoyant forces lifting the roof and the weight pulling it down. Buoyancy is the force providing lift to floating roofs, and the buoyant lift is created by pontoons on the bottom of the roof. Pontoons rest inside the liquid, displacing volume and providing buoyancy. If a pontoon is penetrated, then product fluid is free to fill the cavity. Punctured pontoons do not provide any buoyancy to the structure, and volatile liquids will evaporate within the open cavity. The vapor within the pontoon may ignite from sources such as lightning. The explosion of a pontoon is likely to result in total roof failure or surface fire.
Additionally, the deck of a floating roof is constantly exposed to corrosive product fluid, temperature change and precipitation. These factors deteriorate the roof and can allow product to seep into the buoyant compartments of the floating roof. The excess weight of product on the roof may cause the roof to tilt and sink.
Floating roofs have many failure modes beyond development of corrosion holes, such as mechanical impact, rim sticking, fabrication errors, seismic events, failure to drain and many more. Because of this, floating roofs are one of the largest sources of risk for terminals.
Inspection of pontoons and roof compartments requires personnel to climb in access hatches and evaluate visually. Internal inspections require a confined-entry permit through OSHA. This causes inspections to be ignored, overlooked or irregular.
New technology has emerged to combat most of the failure modes for floating roofs. One such technology is the application of instrumentation applied to monitor the roof tilt at several locations to ensure the proper operation of floating roofs. By combining pre-planning of inspection activities with floating roof monitoring approaches, many of the floating roof incidents witnessed during the Harvey storm could have been delayed. Moreover, appropriate risk management and monitoring of floating roofs protect terminals from catastrophic events and property losses.
For more information, call (925) 302-6707 or email phil@pemyconsulting.com.
