Fitness for Service (FFS) assessments have rapidly become an important evaluation methodology of deteriorating piping and equipment that may no longer meet applicable design code requirements, owing to several potential benefits. These include an extended life for the aging pressure vessels and reduced costs associated with inspection, repair, replacement, and reduced production. FFS analysis also provides stakeholders with a greater understanding of the degradation mechanism at work and the contributing factors. 

To maximize efficiency while maintaining the safe operation of pressurized vessels beyond their design life, FFS procedures can be leveraged to predict the vessels’ end-of-life (EOL), downrate their operating pressure below design specifications, and ultimately reduce the duration and frequency of outages and overhauls. 

NSI team relies heavily on API 579-1/ASME FFS-1 to assess Fitness for Service of various components. A wide range of flaw types and degradation mechanisms are addressed through API 579-1. The most common mechanisms are:

  • General metal loss 
  • Local metal loss
  • Pitting corrosion
  • Crack-like flaws 

A Multi-level assessment strategy is developed based on clients’ requirements. The applicable FFS assessment starts with data collection and will progress through each level until the assessment is complete.  

  • Level-1 assessment is the most conservative and expedient assessment and requires the least amount of data and computational effort 
  • Level-2 assessment reduces the level of conservatism while requiring a more accurate description of the damage and limited computations 
  • Level-3 assessment involves sophisticated finite element analysis and an increasing degree of accuracy in the modeling of material behaviour

Data and Measurement Analysis 

A suite of in-house developed scripts for data processing, interpretation, and organization allows for a seamless transfer of information between the raw data sets acquired in the filed and our analysis, maximizing expediency and efficiency. Automation strategies, routinely utilized to curtail computational effort, involve program-specific scripting for the pre-and post-processing of selected data such as: 

  • Processing of thickness maps acquired from ultrasonic measurements of corroded vessels 
  • Automatic identification of general metal loss and sizing of local thin areas and pits 
  • Formatting of processed thickness data as suitable input to ANSYS and ABAQUS 
  • Implementation of parametric studies 
  • Creation and modification of databases 
  • Access and management of output data

Our automated data processing algorithms include: 

  • Statistical analysis of wall thickness distribution to determine a baseline for general metal loss 
  • Identification of local thin areas via data clustering algorithms: FFS assessment levels 1&2 
  • Filling gaps in the dataset based on pre-established assessment criteria and determination of the origin of shifted frame of reference 
  • Projection of current thickness values to planned end-of-life, based on estimated corrosion rate 
  • Expunging data redundancies and data down-sampling 

Level-1 & 2 Assessment

We have developed our in-house tools for Level1-2 assessment. These tools include: 

  • Matchad dedicated worksheets 
  • MAPLE dedicated worksheets 
  • Matlab dedicated scripts 

Level-3 Assessment

Our experts perform state-of-the-art Finite Element Analysis to account for material behaviour via elastic, limit-load, and elastic-plastic stress analysis, depending on the complexity and degree of conservatism required by the problem. The stress-strain relationship may be characterized as linear, bilinear, and fully non-linear, to capture the tri-axial stress-strain relationship in plastic materials such as fibre reinforced polymers, brittle materials such as reinforced concrete, and ductile materials such as steel and cast iron. 

Acceptance criteria are rigorously applied to provide adequate protection against a variety of collapse mechanisms: 

  • Plastic collapse 
  • Local failure 
  • Buckling 

In addition to the more mondain loading conditions acting on degraded equipment, such as self-weight, internal pressure, and hydrostatic water head, our team of analyst will utilize their expertise with the modelling of upset, emergency, and faulted conditions, encompassing a wide range of phenomena: 

  • Thermal transients 
  • Steady-state, flow-induced vibrations 
  • Transient vibrations induced by equipment operation, e.g. pump start-up and shutdown 
  • Seismic accelerations accounted for either via floor response spectrum methodology or time-history analysis 
  • Reduced material toughness instigated by maintenance operations involving super-cooling procedures and consequent ice accretion in piping and equipment 

The location, size, and reduced thickness associated with one or multiple concurrent flaws may greatly affect the structural stability of a vessel. Flow progression and growth over time are a major concern that is routinely addressed by NSI to estimate the vessel’s degraded state at its forecasted end-of-life: 

  • In volumetric-type flaws, the potential for increased metal loss and expansion of the corroded area is evaluated 
  • In crack-like flaws, the possibility of crack growth by fatigue, corrosion-fatigue, stress corrosion cracking, and creep is evaluated 

Specific issues that are routinely addressed by our analysis team include planar flaws as well as degraded welded connections: 

  • When present, planar flaws parallel to a plate or shell surface in the direction of compressive stress require particular attention in that the buckling strength of the material between the flaw and the vessel’s surface may be significantly diminished, depending on flaw size and location 
  • Flaw occurring in close proximity to a welded connection––joining, say, a stiffener to a shell or plate loaded in compression––may significantly reduce the effective length over which the stiffener is attached to the plate, thereby compromising the strength of the connection