By: Sean Norburn, Senior Consultant - Structural Integrity
The inspection of assets typically found in power and process plants is not only costly; it requires careful planning and preparation work. This article illustrates how Quest Integrity Group uses fitness-for-service methods such as API 579/ASME FFS-1  and BS 7910:2005  to optimise inspection programmes and minimise the amount of inspection required, therefore reducing down-time and the associated loss in revenue.
Upon finding defects, these methods can also assist in decision making regarding the most appropriate course of action to take. This could be repair, replace or re-rate. In some instances, the repair may be unavoidable, but with the knowledge of remaining life or safety factors it may be possible to delay the repair until appropriate procedures are developed.
Advanced techniques such as finite element analysis are used to determine where plant equipment has been subjected to the highest levels of stress, strain or damage. This technique requires knowledge of the equipment geometry, which can either be obtained from available engineering drawings or an on-site dimensioning survey. The materials of fabrication and the operational loading of the equipment are also required. Once this information is available, a finite element model can be developed and the stresses, strains, displacements and temperatures can be determined using finite element analysis.
This technique has been successfully applied on many occasions for Quest Integrity clients operating high energy steam pipework, where there are potentially many welds to inspect. Use of finite element analysis combined with advanced creep damage algorithms helps identify the welds most at-risk from exhausting their available creep life due to operation at elevated temperatures and pressures. Using a prioritised risk ranking of each weld in the pipework significantly reduces the number of welds to inspect, and therefore reduces the associated down-time and loss in revenue for the client (see Figure 1).
Figure 1. Stresses calculated by finite element analysis in a high energy steam pipework assessment
Quest Integrity also performs fitness-for-service assessments prior to inspection for clients that own large atmospheric chemical storage tanks (see Figure 2). These assessments are based on finite element analysis to determine locations of high stress, which in turn are used to determine the associated critical flaw sizes (the size of defect or flaw at which tank rupture could occur).
Fitness-for-service codes offer guidelines on how to compute such critical flaw sizes, however Quest Integrity has developed specialised software, Signal™ Fitness-For-Service (FFS) , which automates the implementation of fitness-for-service standards API 579/ASME FFS-1 and BS 7910.
In addition, if cracks are found in atmospheric storage tanks that require removal by grinding, local thinning analysis can determine how much metal loss can be potentially tolerated at the tank wall or floor without compromising safety. Performing assessments such as this prior to a weld inspection programme has the advantage of reducing the lead time associated with defect repairs significantly and thus corresponds to a significant reduction in down-time for the plant.
Figure 2. Fitness-for-service assessment of atmospheric chemical storage tank
Upon finding defects, assessment methods based on advanced techniques such as finite element analysis enable asset owners to understand whether or not they can run the equipment safely, or whether they need to lower the operational loading (re-rate), or alternatively repair.
In the instance that a repair is required (perhaps the assessment concluded that the defect was too large relative to the calculated critical flaw size), a fitness-for-service assessment approach is used to investigate the proposed repair scenario and ensure that the repaired equipment is fit-for-service. If the equipment is subjected to fatigue loading (cyclic, repeated loading over time), the number of repeated load cycles to grow the defect, sized by a Non Destructive Inspection (NDI) method, to the calculated critical flaw size can be readily calculated using Signal FFS. In other words, these advanced methods are used to calculate the remnant life of the equipment based on the rate of crack growth associated with the future operation of the equipment.
Furthermore, asset life extension philosophies such as safety-by-inspection or retirement-for-cause enable the life of assets to be extended in a safe and efficient manner. With these approaches, there is always a conservative assumption that flaws exists at all welds and are growing under service loading. The remnant life associated with the time for these flaws to grow from the minimum detectable flaw size (associated with some NDI technique) to the critical flaw size is calculated using fracture mechanics. The inspection interval is determined by dividing the calculated remnant life by an appropriate safety factor. Then repeated application of the associated NDI technique at the critical weld locations is performed during the life of the asset (see Figure 3). If no defects are found with subsequent inspections, then the assumed flaw size is re-set to the minimum detectable flaw size of the NDI technique. Thus a decision on repairing or retiring is only required when a flaw is actually detected after many years of service beyond the original manufacturers’ stated design life.
Figure 3. Illustration of life extension through life assessment and inspection
The assessment of equipment subjected to environmental cracking can also be performed through fitness-for-service methods. Figure 4 summarises stress corrosion cracking found in a reducer cone weld that is associated with a particular piece of high-temperature plant exposed to a corrosive environment. The assessment sized the repair weld that was required to guarantee 18 months of continued safe operation. This 18 month target was set by the asset owner to reach the next planned major outage when extensive repairs on the plant could be adequately planned for. This assessment was also based on finite element analysis to ascertain the levels of stress at the weld and critical crack size calculations, as well as a leak-before-break assessment as outlined in BS 7910:2005.
Figure 4. Summary of weld repair sizing and life assessment
Fitness-for-service methods involving advanced analysis techniques such as finite element analysis optimise large weld inspection programmes by locating and prioritising the most at-risk welds, which consequently reduces down-time and the associated loss in revenue for the asset owner. Asset management programmes have also benefited from using fitness-for-service methodology to support life extension programmes and assessment of suitable repair methods.