Risk management of storage tank farms
Atmospheric storage tanks (ASTs) in the oil, chemical and transportation industry are used to store flammable and toxic liquids. Their content might be kept under atmospheric temperature and pressure but can also be refrigerated. AST leaks can cause serious environmental problems if they reach surface or underground waters. Floor leaks may go undetected over long periods and can cause serious contamination of soil or sub-surface water. Rapid floor failure or catastrophic shell failure are rare events but they do occur and can have extremely serious consequences. Clean-up of the ground, groundwater and surface water are very costly operations.
The main objective of the risk based inspection (RBI) methodology is to provide the basis for managing inspection intervals and inspection methods and coverage through risk assessment.
API 580 is a recommended practice developed by the American Petroleum Institute (API) that outlines the basic elements for developing, implementing and maintaining an RBI program. API RP 581 describes a specific quantitative RBI methodology with full details: data tables, algorithms, equations and models. The essential idea of RBI is to design an optimized inspection program that can reduce the risk of equipment failures and possibly reduce inspection and maintenance costs without compromising safety.
It is known that a large percentage of the total unit risk will be concentrated in a relatively small percent of the equipment items. From all equipment items that are competing for attention, the inspection plan will focus on those components with the highest risk. As the process plant changes with ongoing operations, inspection priorities and frequencies will be guided by the RBI process.
Three major goals are identified in API 580/581:
- Define, quantify and rank the risk of process equipment failure to target the most important elements in a process plant
- Give the ability to review safety, environmental, and business-interruption risks in an integrated, cost-effective manner
- Systematically reduce the likelihood by allocating inspection resources to high risk equipment
Storage tank inspection software
DNV GL’s Synergi Plant – RBI AST software is being used by tank storage farms to optimize inspection strategies for storage tanks. Storage tank farms spend between USD 2 and 3 million on each storage tank refurbishment, which can decommission a tank for 12 to 18 months during repairs and repainting. Storage tank inspections and refurbishment should be done regularly, but as they can have environmental consequences (toxic substances are released when the tank is opened) and can also represent a danger to workers, it is important that refurbishment is done when needed, not before. Some DNV GL customers have used the risk-based inspection methodology of Synergi Plant to improve maintenance strategy to an optimum level that reduces both costs and risk levels.
With Synergi Plant – RBI AST, users can run different scenarios on the computer, quantifying influences of inspections as well as operation and design parameters. Synergi Plant - RBI software includes methods for semi-quantitative and quantitative RBI. The software helps to decide how and when to do inspections and subsequent repairs. It offers detailed calculation of consequence of failure, probability of failure, financial risk, and inspection optimization with quantitative modules.
Synergi Plant has standard RBI modules supporting API 581 and EEMUA 159 methodologies.
The software enables users to evaluate and justify inspection budgets and set inspection targets using standard and user-definable risk matrices.