Fitness for Services (FFS) Assessment and re-rating
Fitness for Services (FFS)
The Oil& gas and process pressure equipment’s are safely designed to complete it design life/Cycle without any injuries to personnel and environmental damages. But while in service, often they degrading itself by different corrosion mechanism, operating conditions, service up & down, temperatures, construction anomalies & aging that may risk to continues operation of equipment until its design purpose and equipment cycle. By definition FSS is “Quantitative engineering evaluations that are performed to demonstrate the structural integrity of an in-service component that may contain a flaw or damage”. In other words, the pressure retaining such as pressure vessels, piping, and storage tanks contain a flaw that can be evaluated by means of Inspection and engineering calculation if they can continue to operate safely until their design life and avoid injuries to personnel and the environmental and unexpected equipment downtime.
API RP 579-1/ASME FFS-1, Fitness-For-Service is a kind of methodology that is being currently adopted by most of the Oil and gas and petrochemical industries experts. The FFS assessment divided into three levels such as level 1, Level 2& level 3 assessments. Each successive Level requires increasing amounts of data, specific calculations, efforts to have most accurate output results and possible to extend equipment service life. For Level 1 and level 2 calculation are made by formulae. The level 3 assessment is made by numerical analysis i.e. Finite Element Analysis (FEA). This is non conservative approach that gives accurate outputs
Rerate:
Due to demand, the manufacturer may require increase the capacity of output, for this they need to increase equipment operating efficiencies without changing original construction condition. The pressure equipment design condition such as design temperature, Maximum allowable working pressure (MAWP), design pressure, higher allowable stresses or/and Minimum Design metal temperature (MDMT) to coincide the propose increasing design condition. This process of re-evaluating (analysis and inspections) the equipment for more severe design conditions is usually referred to as re-rating. It is important to re-evaluate if the equipment has sufficient corrosion allowance until the next inspection and also the original manufacturer’s design calculations, codes used, design drawings, manufacturer’s data reports (MDR), and equipment inspection reports are major consideration. Additional NDE inspections check may be required for general corrosion, localized metal loss, high temperature corrosion, environmental cracking and Metallurgical Failure Mechanisms.