Preventing Catastrophe: Best Practices for Maintaining External Floating Roofs (EFRs) on Storage Tanks

The safe operation, maintenance, and reliability of floating roofs on storage tanks requires special considerations in their design, construction, and inspection practices. Over the past 100+ years, thousands of tanks with External Floating Roofs (EFRs), fixed roofs with Internal Floating Roofs (IFRs), and EFRs covered with domes have been used to store crude oil, refined fuels, volatile chemicals, and other liquids. Refiners, terminals, pipeline operators, and other markets can benefit from optimizing their floating roofs to meet the escalating demand for hydrocarbons in many industries.

Aging storage facilities, which can be 70 or more years old, have made it more difficult to ensure safe operation and integrity in a cost-effective manner. These facilities must be given additional attention where operational and maintenance risks have been identified.

This blog is the first of a two-part series and covers the maintenance and reliability of EFRs on storage tanks. Part 2 will explore IFRs.

Industry Standards

The requirements for floating roof design and maintenance are specified by a number of API Standards and Recommended Practices. The standards identify areas of inspection, maintenance, and recommended criteria for assessing repairs. Both the standards and practices also provide guidance in evaluating the condition and leak tightness of floating roof tank seals and the measurement (calculation) of vapor losses.

Relevant API Standards include:

• API Standard 650: Design and Construction of New Welded Atmospheric Storage Tanks (ASTs);
• API-653: Inspection, Repair, Alteration, and Reconstruction of Existing Tanks
• API-651 and API-652: Recommended Practices for Cathodic Protection and Linings for New and Existing Tanks

Additionally, there are a few recommended Practices for inspection of ASTs:

• API-575: Inspection Practices of Atmospheric and Low-Pressure ASTs
• API-576: Inspection of Pressure-relieving Devices

Roof Designs Should Be Based on Storage Application and Life Cycle Costs

The methodology for assessing the pros and cons of storage tank fixed and floating roof design types should be risk-based and consider the failure modes mentioned in the following sections. Risk assessments should consider the safety, health, and environmental consequences, as well as the financial impact, of an unplanned tank outage due to roof failures.

The Probability and Consequence Analysis should be managed by a cross-functional team with sufficient knowledge and experience in tank operations, maintenance, and turnarounds. An environmental specialist should provide input on the consequences of an unplanned tank roof event.

Risk-Based Benefit Cost Analysis (RBBCA) and Risk-Based Life Cycle Cost Analysis (RBLCCA) are two of the recommended tools for reaching an optimized decision on the selection of a roof design.

Types of External Floating Roofs in Storage Tanks

External Floating Roofs (EFRs) comprise one category of floating roof design appropriate for installation in tanks that store Volatile Organic Compounds (VOCs) and other products with a vapor pressure of less than 11.1 psia (79 kPa). EFRs are designed to meet the requirements of API-650 Annex C. These designs originated in the 1920s and evolved through the 1960s, with many remaining in service today. Industry experience with these roof designs has shown that when roof damage, instability, or sinking occurs, the consequences can be catastrophic – causing fires and extended outage times for repairs.

EFRs are made exclusively of welded steel sheets and are most commonly found in one of two designs. Single-deck roofs have distributed buoyancy across the entire roof area through the provision of localized “buoyancy chambers,” or pontoons, which are welded to the top side of the roof deck. Generally, these pontoons are arranged in an annular configuration with liquid-tight compartments around the perimeter of the roof. Some designs may integrate additional pontoons in the center or around the intermediate portion for additional floatation and stability.

Double-deck roofs are designed with compartments across the entire roof. These compartments are typically located in concentric circles around the deck, and the top deck is sloped to a center roof drain or to a ring of drains located in the middle of the deck.

Enhanced Design Requirements Improve Reliability for Floating Roofs

The API 650 Standard has design requirements for reserve buoyancy and structural integrity of floating roof designs. However, these must be treated as minimum requirements and may not be adequate to satisfy tank operating loads under specific site conditions.

Owner/operator specifications can supplement these requirements to better reflect loading conditions at the tank site. Major considerations in ensuring tank roof integrity and reliable operation throughout its service life include:

• The design of single-deck pontoon roofs should ensure adequate buckling stability to cope with a punctured center deck or a rainfall condition forecasted by the meteorological conditions at the site.
• The design of the roof support legs of single-deck pontoon roofs should ensure that roof deflection in the lowest operating position (with rainfall accumulation on the center deck, due to inadequate drainage) will not result in “uncontrolled landings” during operation.
• The roofs should be designed with adequate resistance to wind-induced ripping and fatigue cracking of the welds in the center deck. The wind design loads should reflect site meteorological conditions. If these wind loads are more severe than the API 650 Standard, they should govern. Experience with wind damage in roofs subjected to severe loads indicates that designers must focus on damping provisions for the center deck and minimize “excitation” frequencies that cause rippling of the lap welds.
• Roof drainage designs and capacity should cater to the most severe rainfall loading, with the roof in its lowest operating position. Emergency roof drains should be considered for products that can tolerate water ingress.

The choice between a single-deck or double-deck design requires an assessment of site wind loading conditions, rainfall intensity and frequency, and any history of prior wind damage to floating roofs.

Large-diameter tanks with double-deck roofs have demonstrated good integrity throughout their service life. Recent trends in severe wind loading locations have justified the use of these designs for large-diameter tanks.

Poor Maintenance of Floating Roofs Can Have Severe Operational Impact

External floating roofs are efficient structures but require safe operating practices supported by effective inspection and maintenance programs. The history of floating roof operations has generally been good. However, unlike incidents with other types of fixed equipment, floating roof incidents have resulted in expensive and hazardous consequences.

The roof structures are membranes, which are subjected to hydraulic loads, wind damage due to displacements of the tank shell, and potential roof tilting during filling and emptying cycles of tanks. The roof design provides a system of roof and deck vents designed to allow the roof to “breathe” and ensure stable flotation.

Various failure modes have contributed to incidents leading to millions of dollars in repair costs and prolonged loss of storage capacity. As such, it is vital to develop a comprehensive set of tank floating roof inspection and maintenance checklists to be implemented by operations, inspection, and maintenance personnel responsible for large structures. Types of failures include:

• Corrosion of the roof center deck, leading to oil accumulation and pooling of oil and exposing the surface to potential ignition
• Excessive shell settlement (differential and planar tilt), resulting in tank out-of-roundness and roof seal abrasion and binding
• Uncontrolled inadvertent roof landings or frequent intentional service landings, which can impart impact loading on the roof support leg sleeve attachments and fatigue damage to the roof membrane
• Wind-induced rippling of the center deck, causing fatigue failure of the lap welds on the center deck and leading to oil accumulation and exposure to potential fires
• Corrosion failure of the guide pole to shell attachment welds, resulting in a loss of restraint (anti-rotation) and significant damage to the shell, roof structure, and in many cases the roof sinking
• Poor roof drainage practices, freezing conditions, or accumulation of debris, preventing rainwater draining from the center deck and resulting in roof sinking; this can occur with the roof in a landed position, causing catastrophic damage to the roof and often to the tank bottom as well
• Poor inspection practices with inadequate checking of roof supports. There have been fatalities on floating roof tanks where roof plates collapsed
• Failures of floating roof primary and secondary seals; mean time between failure of these designs has generally been around 10-12 years, requiring expensive replacements
• Failure of mechanical shoe seals due to excessive erosion and wear
• Failure of floating Roof drain sumps and piping, leading to expensive environmental consequences and unplanned tank outages

Technical Support for Higher Storage Tank Reliability

At Becht, our expertise in storage tank management is backed by cutting-edge proprietary software and a team of multi-disciplinary senior tank specialists. With extensive experience in Fitness-for-Service (FFS), Risk-Based Inspection (RBI), and turnaround planning/execution, we are uniquely positioned to help you safeguard your assets and prevent costly unplanned incidents. Trust Becht to enhance the reliability and longevity of your External Floating Roofs and overall storage tank operations.

Have a question or need expert advice? Contact our team of specialists today. [Link to becht.com/contact]