Storage Tanks – Maintenance & Reliability of Floating Roofs
The safe operation, maintenance and reliability of roofs in large tanks, does require special considerations in their design, construction and installation practices. Over the past 60 plus years, thousands of tanks with Fixed Roofs with internal Covers, and Floating Roofs, have been used to store Crude Oil and Middle Distillates. Refiners derived significant advantage through the Floating roofs which are used to store very large volumes from super-tanker shipments, and meet the escalating demand for hydrocarbons in industry.
This blog is Part 1 of two blogs and covers the maintenance and reliability of floating roofs amongst storage tanks. Part 2 deals with internal floating covers on these storage tanks.
Aging storage facilities, which can be 50 to 60 years old, have heightened the challenges to ensure safe operation and integrity in a cost effective manner. Therefore, these facilities must be given similar priorities to on-sites units where operational and maintenance risks have been identified.
The principal API Standards that are used for storage design, construction, operation and maintenance are as follows:
- API Standard 650 for the Design & Construction of New Tanks;
- API Standard 653 for the Inspection, Maintenance, and Repair of Existing Tanks; and
- API Standards 601 and 602 specify Recommended Practices for Cathodic Protection and Glass Reinforcement Polyester Linings (GRP) for New and Existing Tanks.
The requirements for floating roof design and maintenance are specified in the above Standards. The Standards identify areas of inspection, maintenance and recommended criteria for assessing repairs. In addition to these, API Standards also provide guidance in evaluating leak tightness of floating roof tank seals, and measurement (calculation) of vapor losses
Roof Designs Should Be Based on Storage Application & Life Cycle Costs
The recommended methodology to assess the pros and cons for selecting storage tank roof designs type should be risk based and consider the failure modes mentioned in the following sections. Risk assessments should consider the Safety, Health and Environmental Consequences, and the Financial Impact, of having an unplanned tank outage due to roof failure.
The Probability and Consequence Analysis should be done by a Cross-Functional Team with sufficient knowledge and experience in Tank Operation, 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 principal recommended tools for reaching an optimum decision on the selection of an optimum roof design.
Types of Floating Roofs in Storage Tanks
The principal types of floating roof designs which are installed in tanks which store Crude and Middle Distillate products, are summarized below. These are categorized as External Floating Roofs which are designed to meet requirements of API Standards 650 Appendix C. These designs evolved in response to the advent of large diameter tanks in the 1960’s. Industry experience with these roof designs has shown that when roof damage, instability and sinking occurs, the consequences can be catastrophic – causing fires and extended outage times for repairs.
The roofs consist of the following types:
- Single deck roofs which have distributed buoyancy across the entire roof area through the provision of localized ‘buoyancy chambers” which are welded to the top side of the roof center deck.
- Double deck pontoon roofs which are designed with concentric annular pontoons with radial bulkheads in each pontoon section.
- The sketches shown illustrate the above single deck and double deck floating roof, with the critical components for these structures. Roof designs shown have been used in tanks as large as 345 ft. diameter, to store Crude Oil.
Enhanced Design Requirements Improve Reliability for Floating Roofs
API 650 Standard has design requirements for the provision of reserve buoyancy and structural integrity of floating roof designs. However, these requirements must be treated as minimum requirements, and may not be adequate to satisfy tank operating loads under specific site conditions.
Therefore, it is customary for Owner Specifications to supplement these requirements to adequately reflect loading conditions at the tank site. Some of the major considerations in ensuring tank roof integrity and reliable operation throughout its service life are as follows:
- The design of Single-Deck Pontoon Roofs should ensure adequate Buckling Stability to cope with a punctured center deck, or a rainfall condition, that is forecast by the meteorological conditions at 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 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 which cause rippling of the lap welds
- Roof drainage designs, and capacity, should cater for the most severe rainfall loading with the roof in its lowest operating position. The provision of emergency roof drains, for products which can tolerate water ingress, should be considered.
- Traditionally, tanks equal to or greater than 300 ft diameter, with double deck roofs, have demonstrated good integrity through their service life. Recent trends in severe wind loading locations have justified the use of these designs for tanks in the 275 ft diameter.
- The choice between a single deck design and a double deck design requires an assessment of site wind loading conditions, rainfall intensity and frequency and history of prior wind damage to floating roofs.
Poor Maintenance of Floating Roofs Can Have Severe Operational Impact
Large diameter floating roofs are efficient structures but require safe operating practices that are supported by effective inspection and maintenance programs. The history of floating roof operations has been generally good. However, unlike other types of fixed equipment, floating roof incidents, when they occur, 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 which are designed to permit the roof to “breathe” and ensure stable flotation.
Failure Modes of Single Deck Floating Roofs which have contributed to incidents are summarized below. These are listed to emphasize the importance of having a comprehensive set of tank floating roof inspection and maintenance checklists which are implemented by Operations, Inspection, Maintenance personnel who are responsible for these large structures.
Pontoon buckling, causing loss of roof buoyancy and sinking, leads to fires and major damage to the roof structure. These failures, on average, have contributed to repair costs of up to $1.25 million and loss of storage capacity for 4 to 5 months. Types of failures include:
- Corrosion of the roof center deck leading to oil accumulation and pooling of oil, 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.
- Corrosion of rolling ladder wheel bearings causing the ladder to “pin down” the roof in its travel, causing tilting and submergence.
- Uncontrolled roof landings causing severe impact loading on the roof support leg sleeve attachments and damage to the roof membrane.
- Wind induced rippling of the center deck causing fatigue failure of the lap welds on the center deck, 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 and roof structure and the roof sinking.
- Poor roof drainage practices, and inadequate clearance of rain water from the center deck resulting in roof sinking. This is also known to occur with the roof in a landed position causing catastrophic damage to the roof.
- Poor tank turnaround practices, with inadequate checking of roof supports. There was a fatality on a large diameter floating roof tank when a Contractor died under the weight of the roof plates which collapsed on him.
- Failures of floating roof primary and secondary seals (foam filled or liquid filled) have been a significant concern. The Mean Time between failure of these designs has generally been around 10-12 years, requiring expensive replacements.
- Failure of Steel Shoe Seals (Pantographs) due to excessive erosion and wear.
- Failure of Floating Roof drains due to corrosion of the swivel joints has caused expensive environmental consequences and unplanned tank outage.
Becht Engineering Technical Support
Becht Engineering has proprietary software tools and a team of Multi-Disciplinary Senior Tank Specialists, and TA Planning / Execution resource. These Specialists can assist clients in the development and implementation of Storage Tank Maintenance Programs, including Risk Assessments of Existing Storage Tank Operations where needed.
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Contact Becht Reliability Services
or Call Arcot (“Radha”) Radhakrishnan at