In recent years the understanding of the relationship between drum damage and bulge sharpness has improved significantly. The authors of this paper developed a new parameter called bulge sharpness and have previously shown the relationship between sharpness and observed damage. Further to this study, the authors have exhaustively studied the evolution of stress cracking (elephant skin) on mid-course bulges and have estimated the likelihood of finding a particular type of surface damage based on the observed sharpness levels. This correlation has led to a proposed scale to categorize stress cracking into three levels: minor, intermediate, and significant. In addition, the progression of bulge sharpness over time was analyzed and it was determined through statistical modeling that bulge sharpness can have a range of rates of change or sharpness growth rates: low, medium, and high. These sharpness growth rates were subsequently studied and their relationship with overall cycle times analyzed. The study also shows that individual coke drums can experience different sharpness growth rates and there can be a distribution of these rates. To determine when repairs should be conducted, coke drum operators must consider the expected operational run. While the random nature of coke drum damage can defy such targets, bulge sharpness growth assessments can be used to better define when repairs should be conducted. Understanding current bulge sharpness levels, year-over-year sharpness growth rates and their distribution, can significantly assist in targeting areas of concern for optimized repair strategies and can also be used to avoid unnecessary repairs.

Laser mapping is a well-accepted technique for obtaining surface profiles of coke drum walls to identify bulges. The resulting data is used to track and trend vessel distortions and mapped to illustrate the shape of the vessel from a base radius and or previous inspection. Monitoring the development and evolution of these distortions over time in an accurate and consistent manner has been demonstrated to be an effective tool for predicting bulging induced crack in a coke drum. In this paper, the authors discuss several aspects of the laser mapping technique such as scanner positioning, data noise, laser range accuracy, missing data and the repercussions in the assessment of bulging. The effects on second derivative-based analysis are covered in detail and the use of techniques to reduce the effects of data noise and sensor motion are discussed.

The frequency and extent of vessel bulging and cracking being registered in delayed coke drums throughout the global coking industry has accelerated significantly as refinery operators reduce their cycle times. Several theoretical approaches have been developed to identify how a bulged area may lead to drum damage; however, limited information has been presented to match the theoretical predictions with actual surface damage reported by coke drum operators. The results of hundreds of laser scans spanning the last 25 years have been analyzed to correlate vessel bulging with observed surface damage. Specific categorizations of bulge profiles, and the proximity of these to circumferential weld seams (circs), have been calibrated against hundreds of real-world examples of drum damage and failure, including through wall cracking and stress cracking of the cladding, and further associated with the triggers for repair strategies implemented by industry leading refiners. Strong correlations between specific aspects of bulge profiles and the presence of surface damage were found resulting in an assessment tool that can rank and prioritize coke drum distortions on the likelihood of damage, and can serve as a useful guide for planning future coke drum maintenance.

Weld overlay is a repair method that has been used over the last 10 years to limit the growth of bulges and to extend the remaining life of delayed coking drums. Different refinery operators have used varied approaches, ranging from localized patches on specific regions of concern, to bands along circumferential welds, to large sections of structural repair that completely cover a bulged area. The authors have observed the evolution of regions repaired with internal weld overlay on 18 drums over periods of 5 and 8 years. A comparison of bulge sharpness and bulge depth on a year-over-year basis is presented to measure the effectiveness of metal weld overlay and how the overlay impacts continued distortion of the vessel. Furthermore, factors such as the taper ratio in the transition zones, and the distance between the peak of a bulged area and the edges of the weld overlay, are presented as key parameters that affect the likelihood of cracks developing along the transition zones at the upper and lower edges of the repair.