Fitness For Service Assessments Research Articles

Circular Economy Opportunity Utilizing Fitness for Service Methodology for Aged HIC Defected Reboiler

The phenomenon of Hydrogen-Induced Cracking (HIC) is a cracking mechanism of carbon steel material as in ASME SA- 516 pressure containing equipment operating in a wet sour H2S environment. Such phenomenon is mainly driven by the diffusion of hydrogen atoms into the carbon steel due to corrosion reaction. The Structural integrity of an in-service equipment that contain a flaw or damage such as HIC could be scrutinised using Fitness For Service (FFS) assessments as quantitative engineering evaluations. The assessments are conducted using methodologies specifically prepared for pressurized equipment. These assessments provide standard procedure that can be used to determine if pressurized equipment containing flaws could continue to operate safely for certain period of time. FFS assessments are recognized and referenced by the API Codes and Standards such as API- 510, 570, & 653, and by National Board-23. Those assessments are considered suitable methodologies for evaluating the structural integrity of pressurized equipment as heat exchangers, pressure vessels, piping systems and storage tanks where inspection has revealed degradation and flaws in the equipment. The objective of this paper isto illustrate API-579 case study of HIC of fifty (50) years operating pressurized equipment, namely reboiler.

Railroad bridge fatigue life estimation using the probabilistic method and new fatigue resistance for riveted details

Existing fatigue life evaluation procedures are often over-simplified and do not account accurately for the uncertainty in applied load and fatigue resistance of steel railway bridges. Simplified calculations and conservative assumptions often lead to significant reductions in estimated life that indicate older structures ceased to be functional decades ago. Fitness for service assessments that utilise probabilistic methods and that are informed by and consistent with detailed physical inspections of the structures provide a more accurate assessment of the expected life of bridges and can be used to establish improved bridge inspection programmes. This paper focuses on development of probabilistic, risk-based methods and models for assessing steel bridge Fitness for Service (FFS) and remaining life. The fatigue resistance models developed in this project are calibrated, validated, and verified against all available and relevant fatigue test results. For some bridge types, these models provide remaining fatigue life estimates that are double and sometimes even triple those obtained by using existing simplified methods. In this paper, a probabilistic method is demonstrated on two, riveted deck plate girder spans that exceed 100 years of service and are currently located at the Facility for Accelerated Service Testing (FAST) near Pueblo, CO.

Fitness for Service Assessment on Local Metal Loss Near a Discontinuity Using Elastic-Plastic Stress Analysis

Abstract In fitness for service (FFS) assessments, one issue that people often encounter is a corroded area near a structural discontinuity. In this case, the formula-based sections of the FFS standard are incapable of evaluating the component without resorting to finite element analysis (FEA). In this paper, an FEA-based technical approach for evaluating FFS assessments using an elastic-plastic material model and reformed criteria is proposed.

Assessment of Corrosion Damage in a Finger-Type Slug Catcher

Corrosion pits of up to 6 mm depth, associated with areas of high solid deposition were discovered in the finger-type slug catcher and glycol sump during routine inspection (first in 2005 and then in 2013) following lengthy periods of below-threshold CI injection and partial implementation of the corrosion management plan. Corrosion rates were predicted to have increased from 0.46 mm/year in 2005 to 0.71 mm/year in 2007 and then decreased to 0.41 mm/year in 2008. Subsequent integrity assessments concluded that the slug catcher was not fit for continued operation at the current rate of pitting and was therefore shut down to prevent potential loss of containment. Results of API 579 fitness for service assessments of the corroded areas justified the subsequent decision to de-rate the system and that no further corrosion can be tolerated at the original design pressure, at least until the rates of corrosion and its mechanisms were fully understood. Based on the damage morphology, chemical and monitoring data and theoretical concepts, a synergy of mechanisms (under deposit corrosion, CO2 corrosion, preferential weld corrosion and microbial-induced corrosion) is identified as being responsible for the observed damage. The short-term mitigation plan is to routinely inspect the damaged locations (by UT wall thickness checks) while simultaneously implementing a corrosion management and control plan which consists of: continuous inhibitor dosing, pH stabilisation (with MEA/DEA) to above pH 6, batch biociding, sampling of residuals, chlorides, iron, SRB/GHB count, water cut, water content and pH of lean glycol, glycol content of pipeline fluid and online monitoring. The inspection, sampling and monitoring results should then be reviewed initially monthly, subsequently quarterly by the corrosion and integrity team in order to assess the rate of pitting. If the target (<0.125 mm/year) is being breached frequently, then alternative but more costly mitigation should be considered such as repair or outright replacement of the finger-type by vessel-type slug catchers which have built-in sand jetting systems designed to deal with high deposition rates and are therefore considered less prone to associated corrosion damage.

The Behavior of Shallow Cracks in a Pipeline Steel Operating in a Sour Environment

Setting conditions for the avoidance of in-service crack growth in aggressive corroding environments has long been a major challenge due to the number of variables that have a significant effect on material behavior. One area where both experimental data and a validated assessment methodology are lacking is the behavior of shallow cracks. This paper describes the early results of an ongoing research program aimed at addressing the shortfall in experimental data to characterize material behavior in the shallow-crack regime, with the long-term aim of improving the understanding and assessment of the early stages of environment assisted cracking. There is an industry need for a better understanding of material behavior under these conditions and for the development of a more robust assessment methodology. API 5L X65 pipeline steel parent material was tested in a sour environment with initial flaw sizes in the range 1–2 mm. Fatigue crack growth rate tests have been performed to investigate the influence of crack depth on crack growth rate (da/dN). Initial results suggest that crack growth rates for deep flaws can increase by a factor of 5–100 compared with air depending on the applied stress intensity factor range (ΔK). Shallow cracks have been shown to grow up to 130 times faster in a sour environment than in air and up to an order of magnitude faster than deep cracks in a sour environment at the same value of ΔK. Constant load tests have also been performed to investigate the influence of crack depth on the threshold stress intensity factor for stress corrosion cracking (KISCC). Preliminary results suggest that in this case there is no crack depth dependence in the range of flaw sizes tested. While further experimental work is required, the results obtained to date highlight the potential nonconservatism associated with extrapolating deep-crack data. Guidance is therefore provided on how to generate appropriate experimental data to ensure that subsequent fitness for service assessments are conservative.

Determining Effective Thickness of Cylinders in Pressure Equipment With Significant Thickness Variations for Fitness for Service Assessments

In many in-service issues involving pressure equipment, it is often the case that the thickness of a cylinder reveals significant variations in thickness around its cross section. This variation could represent loss of thickness due to corrosion or erosion, or could involve additions of thickness from repair activities. A common issue deals with determining the overall affect of such variations on the integrity of the vessel. This paper describes a spreadsheet tool employing common geometric properties such that these thickness variations may be simply reduced to an effective thickness of the cylinder cross section. This effective thickness may then be used for purposes of conducting fitness for service assessments or to assist in the planning of repairs. The use of this tool is illustrated with two case studies.

Background to the derivation of partial safety factors for BS 7910 and API 579

The background to the derivation of partial safety factors (PSFs) given in two standards, BS 7910 and API 579, is described. In both cases, PSFs are provided to achieve selected target reliability levels against the failure modes of fracture and plastic collapse in structural components. The recommended PSFs in the two standards are compared in order to investigate differences. Example fitness for service assessments are also conducted in order to explore the potential impact of differences on the output of an assessment. Work being carried out to review and update the current partial safety factors for the next revision of BS 7910 is discussed.