Storage tank external floating roof: the importance of inspection and maintenance to retain buoyancy of the roof structure

Health and Safety Executive - Safety alert

Introduction:

This safety notice describes the potential hazards relating to the specification, inspection and maintenance of external floating roofs for storage tanks built to BS 2654¹ , BS EN 14015² and API 650³ . There has been a recent incident, along with findings from further investigation, that suggest the examination and maintenance of floating roofs is not given sufficiently priority. This safety notice stresses the importance of maintaining the integrity of the roof, including taking precautions to maintain the buoyancy of the structure. Failure of the roof can lead to a loss of containment, and has the potential to lead to a major accident (as defined in the Control of Major Accident Hazards Regulations), such as by evaporation or ignition of hydrocarbons, or by pollution to ground via open roof drains.

Background:

Design

Large above ground hydrocarbons storage tanks, typically in excess of 20m diameter, are often fitted with an external floating roof. They float on the surface of the stored fluid, and remain buoyant by means of pontoons around their perimeter or as double-deck roofs, with integral chambers to provide buoyancy. The roof structure has a central open deck, which needs periodical draining, usually by one or more central drains. The outer edge of the roof is fitted with at least one seal to retain the hydrocarbon vapour beneath. A cross-section of a popular design with an outer ring of pontoons is shown right.

Construction standards require that floating roofs to remain buoyant in certain arduous situations. BS EN 14015 for example, calls for the roof to remain afloat with 250mm of water on the centre deck or two adjacent pontoons flooded. Operators should note that while these requirements are not cumulative, they should be aware that a roof which has one or more compromised pontoons is more vulnerable to sinking from rainfall. The roof may well have listed or even deformed in such a situation, limiting the ability to remove rainwater using the centre drain. In such circumstances, continued rainfall could well result in the roof sinking. In addition, the extreme conditions of flooded pontoons or accumulated rainfall could also lead to permanent damage to the roof, as localised stresses could well be in excess of design limits.

In order to demonstrate that the floating roof will remain intact and function properly, operators should be able to demonstrate that for all operating conditions, the roof will remain buoyant and perform in accordance with the original design intent. In this case, operators should have a design calculation which demonstrates that for the fluid stored, the roof will continue to function even in certain fault conditions (these are defined in the design codes - for a single deck pontoon roof for example, BS EN 14015 states either two adjacent pontoons flooded or 250mm of water on the centre deck should be accommodated). Operators should ensure the roof buoyancy calculation uses a sufficiently low specific gravity - API650 for example, states either a specific gravity of 0.7 is used, or the specific gravity of the stored product, whichever is the lower (consideration should be given to all foreseeable variations in operating conditions, for example, warm material being charged to the tank. Note that a fluid with a specific gravity below 0.7 is believed to have been a contributory factor in a recent failure). Operators are also reminded that for tanks storing relatively light hydrocarbons, excessive vapour pressures can also cause upset conditions. Finally, as part of a managed change, the calculation should be re-visited to ensure it remains valid if the tank contents are changed and the SG reduced.

Single 3" or 4" diameter roof-drains may be fitted to tanks designed to API 650³. At low roof levels, the relatively small head leads to a slow discharge rate through the drain, so it is vital that the roof does not accumulate excessive water during periods of heavy rain, and that the roof remains sufficiently buoyant at all times. Operators should be aware that other fittings in the system, such as non-return valves and filters, will further slow the discharge rates unless properly managed, and also that line restrictions, such as reducers, should be avoided.

In service examination

As well as the checks on other aspects of tank integrity detailed within industry accepted guidance ^4,5,6), in-service examinations that should be in place that guard against floating roof failure, namely, check:

• deck integrity, including checks for excessive corrosion;
• for pools of standing water that may indicate distortion of the roof;
• roof level, ensuring the deck is not listing by checking cardinal points around the roof perimeter;
• roof seal integrity along with associated fittings, such as foam dams and vents;
• pontoon integrity, including visual inspection and checking for flammable vapour;
• Tank roundness and distortion, and
• Access ways and ladders.

Out-of-service examination

A suitable internal and external examination of pontoons for on-going integrity should be made during out-of-service inspections. It is important to ensure that pontoons will remain intact for the duration of the following period in service, so appropriate checks should be made (using suitable inspection and testing techniques) during the out of service examination, along with an assessment of roof integrity by a competent person. Guidance for tank inspection calls for checks of the pontoons to ensure that they remain either 'liquid tight' or 'vapour tight', so checks should include internal bulkheads and division plates as well as the primary boundary of the roof and pontoons.

Integrity and quality checks should be performed on the roof structure after any repairs or modifications. There are recent instances of poor quality welding of the supports for replacement roof seals, which have led to perforations of the pontoon side wall (above the liquid line but below the vapour seal). These perforations have allowed the formation of an explosive atmosphere within the pontoon. By checking the quality of these changes and the overall pontoon integrity before the roof was put back in to service, these instances would have been found and corrected and the tank not reinstated with holes in the roof pontoons. Regulation 6 of the Dangerous Substances and Explosive Atmosphere Regulations requires that, so far as is reasonably practicable, the risks from fire and explosion should be reduced by avoiding the release of dangerous substances and by preventing the formation of explosive atmospheres.

Routine Monitoring

In addition to in-service examination, regular operator checks will give further assurance of roof integrity, and provide early warning of developing problems. Observations of the roof deck, seal integrity, pontoon hatches in place and secure earthing, amongst other visual checks, will help in assuring on-going roof integrity. It is appropriate to log these observations so that the origin of any problem can be identified, and where repairs are required, that they are notified in the maintenance management system with appropriate priority.

Level Indication & alarm

In some instances, separate independent level indication of both the roof position and liquid level can also indicate problems. Comparison of roof position with liquid level over a period of time can show gradual loss of buoyancy. Where reasonably practicable, duty holders should provide a deviation alarm so that a warning is given if there is excessive differential movement of the roof and liquid.

Action required:

In summary, operators of tanks with external floating roofs are recommended to ensure that:

• Calculations are undertaken to demonstrate that for the fault conditions of a flooded centre deck or two adjacent pontoons flooded, the roof will remain buoyant in the tank fluid. The calculations should be checked for validity when operational changes occur, such as a change to tank contents of reduced specific gravity.
• In-service examinations are in place that guard against floating roof failure.
• Out-of-service examinations are in place that include a suitable internal and external examination of pontoons for on-going integrity.
• Integrity checks are completed following any repairs or modifications.
• Routine operator checks are carried out and findings logged within the maintenance system.
• Build up of standing water on the roof deck is managed - heavy rainfall coincident with low roof levels could lead to slow rainwater discharge rates and increased risk of failure due to water build up on the tank roof.
• Where reasonably practicable, duty holders should provide a deviation alarm so that a warning is given if there is excessive differential movement of the roof and liquid.

Relevant legal documents:

• Health and Safety at Work etc Act 1974
• Provision and use of Work Equipment Regulations 1998
• Control of Major Accident Hazards Regulations 1999
• Dangerous Substances and Explosive Atmospheres Regulations 2002

References:

1. BS 2654:1989 (withdrawn) Specification for Manufacture of vertical steel welded non-refrigerated storage tanks with butt welded shells for the petroleum industry. Back to reference of footnote 1
2. BS EN 14015:2004 Specification for the design and manufacture of site built, vertical, cylindrical, flat bottomed, above ground, welded steel tanks for the storage of liquids at ambient temperature and above. Back to reference of footnote 2
3. American Petroleum Institute, Std 650, Welded tanks for oil storage, Twelfth Edition March 2013. Back to reference of footnote 3
4. Engineering Equipment and Materials Users Association (EEMUA) – Publication No. 159, 'Users Guide to the Inspection, Maintenance and Repair of Aboveground Vertical Cylindrical Steel Storage Tanks.' Back to reference of footnote 4
5. American Petroleum Institute, Std 653, Tank Inspection, Repair, Alteration and Reconstruction, 4th edition 2009. Product Number C65303. Back to reference of footnote 5
6. American Petroleum Institute, RP 575, Inspection of Atmospheric & Low Pressure Storage Tanks, 2nd edition. Product Number C57502. Back to reference of footnote 6

Further information:

Hazardous Installations Directorate
Chemical, Explosive and Microbiological Hazards Division Unit 1
(Mechanical Engineering)
Health & Safety Executive,
BP6301 Alnwick House,
Benton Park View,
Newcastle upon Tyne
NE98 1YX

General note:

Please pass this information to a colleague who may have this Product/ Equipment or operate this type of system/process.