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DCU Coke Drum Skirt Failure

Delayed Coker Unit equipment was introduced in the 1930's but by the late 1950's, several problems became sufficiently pressing that the first of several API surveys was undertaken to understand the damage mechanism and prescribe remedies. The analytical tools were limited but correlations were introduced; some were quite insightful. The Unit Quench Factor or UQF was one such measure that has actual foundation in the physics of coke drum damage and reliability. Unfortunately, conclusions were misinterpreted and motivated designers to use increasingly higher strength materials such as SA 387 22 (2¼ Cr - 1Mo) and even SA 387 Grade 21 (3Cr - 1Mo).

Equally perplexing is the misunderstanding by vessel designers of basic engineering principles impacting drum component reliability and, specifically for this discussion, skirt cracking.

Classic Coke Drum Skirt Cracking Failure

Photo 1 provides illustration of the typical OD cracking location observed in coke drums. In "recent times", "improvements" were implemented for the purpose of increasing reliability but, instead, have led to premature failure. Drum designs previously demonstrating 50 years of trouble-free skirt weld integrity currently appear to experience cone-side cracking in less than 10 years. Also, surprisingly, cracking is initiating at the toe of the skirt cone side ID joint weld; these are newer drums with purported enhanced skirt-to-drum dimensional and weld details.

Recent Coke Drum Skirt / Cone Cracking Failure

In Figure 1, an FEA was completed on a drum which had experienced "pre-mature" cracking; this cracking was extensive and entered into the cone from the toe of the 2" radius skirt ID weld. The generous contour and more robust skirt dimensions had been intended to reduce discontinuity stresses but inadvertently caused substantially increased stress & strain concentrations by virtue of the disproportional material distribution. In effect, the susceptible crack initiation relocated to the cone. The strain at the toe of the crotch is approximately 7,600 µstrain; the consequential predicted fatigue life is some 1,550 operational cycles or only some 5 years of operation. This nominally matched actual operation; the joint, in essence, was inadvertently designed to fail.

Photo 1 Classic OD Crack at Skirt to Shell Joint in Repair
drum skirt crack

Figure 1 Relocated Crack Initiation Point

Proposed Remedy for Damaged Skirt-to-Drum Joint

Consequently, short term and long term reliability and safety concerns were raised. The decision was made to replace the drums for a design incorporating a completely new drum-to-skirt detail. As an interim mitigation, a decision was made to add multiple longitudinal brackets to the upper portion of the skirt to provide an independent support path should the cone or skirt weld crack through prior to a turnaround scheduled 4 years out.

An FEA of the proposed "fix" using these brackets spanning the skirt attachment point indicates that the high strain location on the cone incurs further strain concentration since the brackets further stiffen the joint. This is a "rookie" design mistake when designing for displacement controlled loads. In this specific case, the strain increases to approximately 12,800 µstrain; and the fatigue life is reduced to some 1¼ years. Activation of specific fracture mechanics processes, easily determined, may accelerate crack propagation. Hence, the proposed "fix" exacerbates the situation to the extent an uplanned shutdown is likely well prior to the planned turnaround. This uncertainty prompts a risk assessment explicitly addressing personnel safety for the contingent event of uncontrolled release of vessel contents.

As a point of interest, modelling of the "classic" detail indicates an apparent crack initiation life at some 20 years by pushing the location of high strain into wrought skirt material. Avoiding weld residual stresses and taking advantage of fracture mechanics mechanisms contribute to substantially reducing the velocity of crack propagation . The major advantage, as demonstrated by successful operation of older units, is that cracking associated with joint discontinuity is precluded from occurring in the cone or progressing from skirt into the shell. Astute efforts during the design stage can avoid selecting "fatigue incompatible" coker skirt designs. Designers, thus, need to also be alert to specific deficient design guidance in some industry standard practice documents such as API TR 934 G.

A followup screening analysis of several industry recommended drum-to-skirt joint designs suggests that there are significant differences in performance. In extreme situations, owners have mistakenly replaced drums in the belief that prematurely failed skirt joints were an indicator of balance-of-drum integrity.

The details for implementing design and design check methodologies for DCU coke drums are covered in detail in our course Delayed Coke Drum FFS & Damage Assessment to ASME FFS - 1 / API 579 - 1

John Aumuller, P. Eng., Ph. D.