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:
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
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.
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:
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:
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:
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 (949) 208-6882
Radha's career has spanned over 45 years of worldwide involvement in the petroleum, chemicals and oil producing industries, including service with Shell Chemicals, Exxon Corporation and with ExxonMobil Corporation. At Exxon his R&D contributions included development of equipment technology for synthetic fuels (DOE) and the development/implementation of storage tank design and construction practices for atmospheric and low temperature services. His career has included a diverse range of technology assignments relating to capital projects, commissioning of new facilities, R&D, trouble-shooting, and incident investigation.
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