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Proceedings of the ASME 2017 Pressure Vessels and Piping Conference
PVP2017
July 16-20, 2017, Waikoloa, Hawaii, USA
PVP2017-65161
WELD REPAIR OF C, CR-MO COKEDRUMS
(& PRESSURE VESSELS) WITHOUT PWHT
Alwyn Kaye, M.Eng P.Eng
Canadian Natural Resources Limited
Alberta, Canada
Patrick Lester, BSWE CWI
AZZ WSI
Norcross, GA, USA
Darren Barborak, Ph.D
AZZ WSI
Norcross, GA, USA
ABSTRACT
Many of the Cr{1-1/4 to 2-1/4}-Mo{1/2 to 1} pressure vessels
in the refining and petrochemical industries such as process
reactors, distillation columns, separators, pressurized storage
vessels, and heat exchangers are typically vertical columns,
most often supported by a circular skirt. Typically, design
considerations for these vessels and support skirts are for
operating under continuous “steady-state” conditions, where
temporary stresses due to short-term “transient” events such as
start-up and shutdown are often ignored. Consequences of
dynamic and cyclic loading play a very significant role in their
life and performance. For Coke drums, survey data from API
shows that the skirt-to-drum attachment weld and adjoining area
appears to be the most problematic, frequently experiencing
low-cycle fatigue cracking due to concentrated stresses.
existing skirt and the attachment weld (knuckle) in segments, 2)
Inspect the cone for remaining flaws, 3) Excavate and repair
flaws in cone using temper bead technique, 4) Rebuild knuckle
area for skirt to cone attachment with an increased radius using
temper bead welding techniques, 5) Install new skirt sections
using controlled deposition welding technique. Temper Bead
and Controlled Deposition repair welding techniques were
utilized to avoid conventional post-weld heat treatment
requirements, significantly improving the turn-around time in
the field.
1 NOMENCLATURE
ABSA
Alberta Boiler Safety Association (Canadian
Government Provincial Regulator)
ASME
American Society of Mechanical Engineers
BPVC
Boiler & Pressure Vessel Code
CDW
Controlled Deposition Welding
D-LPT
Dye- Liquid Penetrant Testing
GMAW
Gas Metal Arc Welding
GTAW
Gas Tungsten Arc Welding
GTAW-HP Hot Pulse Gas Tungsten Arc Welding
HAZ
Heat Affected Zone
MPT
Magnetic Particle Testing
NDT
Non Destructive Testing
PAUT
Phased Array Ultrasonic Testing
PQR
Procedure Qualification Record
PWHT
Post-Weld Heat Treatment
RCA
Root Cause Analysis
SAW
Submerged Arc Welding
TBW
Temper Bead Welding
A methodology for repairing the skirt attachment weld of CrMo pressure vessels is provided. When designing a repair
approach, consideration should include material and aged
condition, extent and location of defects, welding process and
consumables, and codes, standards, and regulatory guidelines.
When repair by weld metal buildup to rebuild a skirt-attachment
weld configuration is considered, weld procedure qualification
and adequate mock-ups should be performed in order to ensure
a sound repair. Further, when invoking a code compliant repair
without post-weld heat treatment by controlled deposition
welding or temper bead techniques, proper training of welder
operators should be conducted to ensure the techniques are
implemented properly.
A case study is provided for a Coke drum, where the original
design and fabrication of the skirt attachment included an initial
SAW weld metal buildup on the 2.25Cr (P5A) cone followed by
an SMAW/GTAW attachment weld to the 1.25Cr skirt (P4).
During a plant shutdown, a surface breaking crack was detected
in the skirt to shell attachment weld by Dye Liquid Penetrant
Testing (D-LPT) and confirmed with Magnetic Particle Testing
(MPT). Subsequent examination by Phased Array Ultrasonic
Testing (PAUT) discovered a large number of volumetric
indications, oriented towards the knuckle section internally.
The repair approach consisted of 1) Completely remove the
WPS
Welding Procedure Specification
2 INTRODUCTION
Many of the pressure vessels in the refining and petrochemical
industries such as process reactors, distillation columns,
separators, pressurized storage vessels, and heat exchangers are
typically vertical columns, most often supported by a circular
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skirt (Figure 1). The design code for the majority of these
vessels and support skirts are based on the rules of ASME
BPVC Section VIII Division I1, where considerations for
dynamic and cyclic loading are ambiguous and left up to the
discretion of the designer.
due to concentrated stresses. An earlier survey indicated that
failures typically occur within 3000 to 5000 operating cycles7.
For vessels operating under continuous “steady-state”
conditions, temporary stresses due to short-term “transient”
events such as start-up and shutdown are often ignored.
However there is increasing evidence2,3,4,5,6,7 that cyclic,
transient events, residual stresses, fatigue, design, operational
decisions, and other previously overlooked factors play a very
significant role in life and performance of all forms of
equipment including static equipment and pressure vessels.
Typical Pressure Vessel
Figure 2. Typical “coking” Cycle2
The most recent survey conducted in 2013 indicated that 80%
of the respondents accounting for approximately 131 coke
drums experienced cracks originating in the skirt, with 12 years
being the average age before cracks appeared2. It should be
noted that all respondents indicated that the cracks did not
propagate into the drum or cone i.e. the pressure boundary.
The detail of the skirt to shell connection has a significant
consequence on increased stress levels and cracking.
Influencing factors include the Joint Design, the Knuckle or
Crotch Radius, the Eccentricity or lack of vertical alignment of
the skirt and shell, and the Taper or Transition from the drum
bottom head as it connects to the skirt ring (see Figure 3).
Support Skirt to repair
Figure 1. Typical Pressure Vessel showing Skirt region.
In early coke drums, the skirt attachment consisted of a fillet
weld below the lower tangent line of the drum (Figure 3a). This
configuration produces a very large stress concentration point
on the backside of the fillet weld (inside crotch) that quickly
leads to crack initiation and propagation. Early attempts to
overcome the limitations of this design simply moved the skirt
attachment fillet weld above the lower tangent line (Figure 3b).
While this configuration moves the skirt attachment to an area
with lower stresses, its effectiveness is only realized if the gap
between the skirt and the shell are kept to a minimum which is
difficult to accomplish during fabrication as the shell and skirt
components are rarely concentric. Another modified skirt
attachment design implements a smooth internal weld crotch
radius (Figure 3c) which minimizes the stress concentration and
provides improved life. A variation of this utilizes a forging
(Figure 3d) with an integrated internal crotch radius.
Of all of the batch type operations, the delayed coking process
subjects coke drums and their support skirts to some of the most
severe cyclic thermo-mechanical loading. A coking cycle
typically ranges from 18 to 36 hours, with shorter 9 to 12 hour
cycles becoming more common.2 The cycle starts by preheating the coke drum from ambient temperature to around 250
to 350oC (482 to 662oF) by injecting steam. Next residual oil
feed at a temperature of 400 to 500oC (752 to 932oF) at 300 to
350kPa (43.5 to 50.8 psi) is introduced into the drum, following
by steam injection to facilitate the thermal coke cracking
process. Finally, water is injected into the drum to expedite the
cooling and solidification of the coke. A typical cycle is shown
in Figure 2.
Recent operating trends have tended towards shorter cycle times
resulting in more thermal cycles experienced by a drum within a
year, increasing the thermal stresses due to the accelerated
heating and quench cycles. Additionally, the density of the
residual feed has tended to increase, resulting in a denser coke
bed, which contributes to channeling of cool quench water to
the hot coke drum wall, resulting in thermal shock.2 Thermal
stresses are manifested by shell course deformation such as
bulging and bending (often called banana effect), as well as
cracking at the shell course circumferential seam welds and
skirt attachment welds.
Survey data from API2,4,6,7shows that the skirt-to-drum
attachment weld and adjoining area appears to be the most
problematic, frequently experiencing low-cycle fatigue cracking
3a: Fillet Weld
2
3b: Elevated Fillet Weld
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3 DISCUSSION
3c: Modified Crotch
During a plant shutdown in 2015, one surface breaking crack
was detected in the skirt to shell attachment weld of a coke
drum by Dye Liquid Penetrant Testing (D-LPT) and confirmed
with Magnetic Particle Testing (MPT).
A subsequent
examination by Phased Array Ultrasonic Testing (PAUT)
discovered a large number of volumetric cracks. The majority
of the cracks appeared to be oriented towards the knuckle
section internally. This is a known area of susceptibility in
Coke drums but also C, Cr-Mo pressure vessels in general. A
follow-up Root Cause Analysis (RCA) with metallurgical
testing from boat samples identified the plausible cause as
Reheat Cracking. This mechanism is a known issue in pressure
vessels of these compositions with welds produced by the
Submerged Arc Welding (SAW) process.
3d: Forged Fitting
Figure 3. Typical Skirt Attachment Weld Configurations2
Many researchers have investigated the design and failure of
coke drum skirt attachment configurations.8,9,10,11 The general
consensus is that cracking at this location can be attributed to
the severe thermal gradients that exist between the coke drum
shell and skirt. The skirt acts as a heat sink that enhances the
thermal gradient that exists at the shell to skirt junction of a
drum during a typical operating cycle. During the drum heating
cycle, the relatively cooler skirt tends to restrain the shell/head
expansion while during the quench cycle the relatively hotter
skirt tends to restrain the shell/head from shrinking back to its
cool position. In each case, significant bending stresses of
opposite signs (between heat up and cool down) occur around
the skirt to shell joint from the thermal cycling. The more
severe the temperature gradient is between the shell/head and
the skirt the more severe the bending stresses generated in the
skirt attachment weld are. Figure 4 demonstrates the reversal of
the tensile and compressive stresses from the fill cycle to the
quench cycle via finite element analysis simulation.
The original design and fabrication of the skirt attachment
(Figure 5) included an initial SAW weld metal buildup on the
2.25Cr (P5A) cone followed by an SMAW/GTAW attachment
weld to the 1.25Cr skirt (P4), all using Cr.-Mo. filler metal
(ASME SFA-5.23) followed by a PWHT. The materials of
construction are listed in Table 1
Because there are a large number of coke drums in operation
throughout the world, many sites with multiple coke drums,
there is a need for sound design and repair methodologies. This
paper explores the challenges of performing a skirt attachment
weld repair and provides a case study for skirt attachment weld
replacement.
Figure 5. Original Skirt to Cone Weld Detail
Table 1. Materials of Construction
Material Spec.
Min. Thick.
Bottom Shell
SA-387 Gr.11 Cl.1 (P4 G1)
59 mm.
SA-240 Tp.410S (P7 G1)
3 mm
Bottom Head
SA-387 Gr.11 Cl.1 (P4 G1)
59-62 mm
SA-240 Tp.410S (P7 G1)
3 mm
Cone
SA-387 Gr.22 Cl.1 (P5A G1)
41mm
SA-240 Tp.410S (P7 G1)
3 mm
Top of Skirt
SA-387 Gr.11 Cl.1 (P4 G1)
37 mm
Bottom of Skirt SA-516 Gr.70N (P1 G2)
37 mm
Repair Planning & Evaluation
Fill Cycle
Plans for repair were initiated and the following considerations
were taken into account:
 Evaluate this repair protocol for code compliance and
the repair requirements of the Alberta Safety Authority
(ABSA), the government pressure vessel regulator.
 Develop a sound repair method to ensure safe long
term operations.
Quench Cycle
Figure 4. Resultant Stresses in the Skirt Attachment during
Fill and Quench Cycles
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



Evaluate capabilities of potential service providers
who have competencies in using this technique.
Develop critical design inputs for further design and
development of detail design.
Reduce overall duration of the coke drum repair outage
by adopting temper bead welding technique or
controlled deposition welding technique or a
combination of these techniques.
Develop testing protocols for the above
Recently, Temper Bead Welding (TBW) and Controlled
Deposition Welding (CDW) techniques have found widespread
use in hydrocarbon industry for repairs and modifications.
These code approved repair techniques can be applied to steels
that are prone to suffer high hardness, or loss of fracture
toughness in the HAZ as a result of welding. Often TBW or
CDW techniques are used where PWHT is either impossible or
impractical (e.g., large vessel repair, safety concerns due to
buckling, schedule constraints).
Figure 6. Nomenclature for Temper Bead Welding12.
It has been shown that mechanical properties in the HAZ can be
improved with the TBW and CDW techniques, similar to the
mechanical properties achieved with conventional post weld
heat treatment. The TBW technique has been specifically
developed to refine the coarse grained HAZ in the parent metal
by sequential passes of new weld beads on top of previous weld
bead passes, thereby “tempering” the passes below. The
placement of a second weld bead will affect the properties of
the first weld bead and its associated HAZ and so on. Similarly,
the placement of a second weld layer will affect the properties
of the first weld layer and its associated HAZ. The tempering
effect from the subsequent weld beads and layers simulates the
tempering that occurs during a conventional PWHT. The goal
is to improve fracture toughness and reduce the peak hardness
within the HAZ.
The National Board Inspection Code (NBIC) directly references
the qualification rules of ASME Section IX, QW-290 for their
Alternative Welding Method 2 and Alternative Welding Method
4 governing repairs of P4 and P5A materials with or without
Charpy Impact requirements, respectively.
NBIC §Cl.2.5.3 Alternative Welding Methods without
PWHT
§2.5.3.2 Welding Method 2
d) The Welding Procedures Specifications shall be
qualified in accordance with the temper bead procedure
qualification requirements in QW-290 of ASME Section
IX.
§2.5.3.4 Welding Method 4
g) The welding procedures (WPS) shall be qualified in
accordance with the temper bead procedure qualification
requirements in QW-290 of ASME Section IX.
The rules for qualification of a temper bead welding procedure
require strict control of essential variables such as weld bead
placement, overlap, and heat input. A diagram of a partial
penetration temper bead weld deposit is shown in Figure 6.
Depending on the technique utilized, the heat input may be
consistent or vary from layer to layer. For Ferritic weld
deposits, removal of the final 1.5 layers may be desirable in
order to ensure the weld deposit also has desirable mechanical
properties. Placement of the weld beads at the outer edge of the
weld deposit is critical to ensure that the outer regions of the
HAZ have been properly tempered. If the bead placement is
incorrect, a region with elevated hardness may form at the weld
toe.
API 510 Pressure Vessel Inspection Code also provides repair
processes alternative to PWHT. API 510 refers to their repairs
as Preheat or Controlled Deposition Welding (CDW) Methods
for repairs with or without Charpy Impact requirements,
respectively. API 510 defines CDW as:
Any welding technique used to obtain controlled grain
refinement and tempering of the underlying heat-affected
zone in the base metal. Various controlled-deposition
techniques, such as temper bead (tempering of the layer
below the current bead being deposited) and half bead
(requiring removal of one-half of the first layer), are
included.13
The ASME Boiler and Pressure Vessel Code defines Temper
Bead Welding as follows: ASME Section IX QG-109:
A weld bead placed at a specific location in or at the
surface of a weld for the purpose of affecting the
metallurgical properties of the heat affected zone or
previously deposited weld metal. The bead may be above,
flush with, or below the surrounding base metal surface. If
above the base metal surface, the beads may cover all or
only part of the weld deposit and may or may not be
removed following welding.12
API 510, however, does not directly reference ASME Section
IX, QW-290, instead the supplementary essentials variables of
QW-250 are invoked.
From a metallurgical perspective TBW and CDW can be
beneficial to materials that are susceptible to re-heat cracking
since the heat input, cooling rate and thus resulting
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
microstructures can with be controlled and manipulated with
careful attention.
Install new skirt sections using controlled deposition
welding technique.
TBW technology was easier to apply in sections or segments of
the skirt replacement and this segmented skirt replacement
approach is also more practical and easier to execute.
Additionally, the segmented approach allowed for static support
of the structure on the remaining segments without the need for
a crane to stabilize the structure. Figure 7 shows a general
overview of the repair sequence.
ASME Section VIII Div. 1 Part UCS-56 of the ASME Boiler &
Pressure Vessel Code permits weld repairs to be made utilizing
the TBW technique. UCS-56 stipulates that TBW may be
completed “specifically” using the half bead weld repair and
weld temper bead reinforcement technique.
Referring to Section VIII UCS-56 4(c)
“(4) In addition to the requirements of Section IX for
qualification of Welding Procedure Specifications for
groove welds, the following requirements shall apply: …
(c) ... the repair weld method shall be limited to the
half bead weld repair and weld temper bead reinforcement
technique ... Approximately one-half the thickness of this
layer shall be removed by grinding before depositing
subsequent layers ...”
“A final temper bead weld shall be applied to a level above
the surface being repaired without contacting the base
material but close enough to the edge of the underlying
weld bead to assure tempering of the base material heat
affected zone ...
The final temper bead reinforcement layer shall be removed
substantially flush with the surface of the base material.”
Greater caution and assessment is required where TBW and
CDW is used as a substitute for PWHT in circumstances where
operating conditions require a significant reduction in residual
stress levels. This particularly applies to cyclic applications
although there is some evidence that Thermal Cycling (due to
in-service operations) can facilitate stress relaxation14.
After a thorough review the evidence indicated that performing
the repair by Temper Bead Welding (TBW) or similar was a
better option than a conventional repair with PWHT. The
Temper Bead option was chosen for the following advantages:
1. Potential for better quality weld.
2. Controlled heat input.
3. Better ability to control heating and cooling rates.
4. Better control on metallurgy and microstructure outcome
via 2) & 3) above.
5. Avoiding SAW with the known propensity to Reheat
Cracking in these material compositions.
6. Temper Bead provided the option of saving one PWHT
Cycle in the life of the drum.
7. Better fatigue resistance properties
Figure 7. Repair Sequence
Welding Procedure Development
The material combination of the skirt and cone presented some
particular challenges with respect to repair codes and welding
requirements. The cone is constructed of SA-387 Gr. 22 Cl 1
(ASME P5A, Group 1) and the skirt is constructed of SA-387
Gr. 11 Cl 1 (ASME P4, Group 1). Due to the materials
involved, it wasn’t possible to address the entire repair within
one repair code methodology. The alternative welding methods
within the NBIC are often the clearest path for these types of
repairs, however thru-wall repairs of P4 and P5A materials are
prohibited within the NBIC alternative welding methods. The
CDW methods in API-510 would allow a thru-wall repair to be
made, but API-510 does not address P5A materials.
Jurisdictional requirements add another level of complexity as
NBIC alternative welding methods are commonly used and
accepted by ABSA, but API-510 repairs are far less common
and require additional scrutiny for approval. With these factors
in mind, the code path chosen was to use NBIC alternative
welding methods wherever possible and limit the use of API510 CDW to only the thru wall weld attaching the new skirt
windows to the new knuckle attachment weld.
Repair Approach
Accounting for the combined considerations the following
approach was devised:
 Completely remove the existing skirt and the
attachment weld (knuckle) in segments.
 Inspect the cone for remaining flaws via MPT and
PAUT.
 Excavate and repair flaws in cone using temper bead
technique.
 Rebuild knuckle area for skirt to cone attachment with
an increased radius using temper bead welding
techniques and ceramic backing material.
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AZZ|WSI Hot PulseTM technology (GTAW-HP) was to be
utilized for this portion of the repair. Hot Pulse technology uses
a combination of hot-wire current and pulsating (reciprocating)
wire feed for enhanced characteristics including; increased
deposition rates, improved weld quality with decreased weld
spatter.
GTAW-HP has both Semi-Auto and Machine
capabilities. An example of the increased deposition rates for
hot-wire GTAW versus cold-wire GTAW is shown in Figure 8.
In total, five (5) new welding procedures were successfully
qualified and registered with ABSA for the execution of this
repair.
To repair the cone, a Semi-Automatic GMAW Temper Bead
welding procedure was developed in accordance with NBIC
Alternative Welding Method 2. Semi-Automatic GMAW was
chosen for flexibility in repair scenarios in the instance that
asymmetric flaws required excavation and repair. Since many
of the known indications were oriented vertically, the PQR was
performed 3G-Uphill to allow the WPS to be used in all
positions. The base material used for the qualification of this
WPS was SA-387 Gr. 22 Cl 1 cone material previously
removed from the coke drum, with the NBIC mandated
minimum preheat of 400°F. ER80S-B3L filler metal was
utilized for this portion of the repair to provide more favorable
hardness levels over the typical ER90S-B3 while still matching
the alloy composition of the cone.
To start rebuilding the knuckle on the cone a Machine GMAW
Temper Bead welding procedure was developed in accordance
with NBIC Alternative Welding Method 2. Machine GMAW
was chosen for high deposition rates and consistent tempering.
The base material used for the qualification of this WPS was
SA-387 Gr. 22 Cl 1 cone material previously removed from the
coke drum. The first few layers that would be applied to the
cone were considered a “buildup” of the existing pressure
boundary. This was the basis for using a P5A (2-1/4Cr) base
material with a P4 (1-1/4 Cr) filler material. ER70S-B2L filler
metal would be utilized for this portion of the repair to provide
more favorable hardness levels over the typical ER80S-B2.
After these first few layers are deposited, the buildup will be
considered a deposited P4 material for all remaining welding.
Figure 8. HOT wire vs. Cold wire GTAW Deposition Rates15
The root and hot pass were installed using Semi-Auto GTAWHP. Then two temper bead “like” layers were installed using
Machine GTAW-HP. After the two layers were completed, the
weld joint was filled out with Machine GTAW-HP. The SemiAuto GTAW-HP would allow for the root and hot pass to be
installed with greater heat input control and productivity during
implementation than Manual GTAW while allowing for greater
tolerance for mismatch in the fit-up of the new skirt windows.
Mismatch of skirt segments or in the vertical alignment of skirt
to drum diameters can make fit-up more difficult, requiring
additional welding. The Machine GTAW-HP layers, fill and cap
would allow for the attachment of the skirt to be completed with
a high integrity process for the critical attachment while still
maintaining higher than typical deposition rates. This WPS was
qualified in the 3G-Uphill position to cover welding the
horizontal seams between the new skirt window and the
knuckle and vertical seams between adjacent skirt windows.
For rebuilding the rest of the knuckle attachment weld both
Semi-Auto and Machine GMAW Temper Bead welding
procedures were developed in accordance with NBIC
Alternative Welding Method 2. The focus and intent would be
to use Machine GMAW for the greatest extent possible but in
certain areas, especially while forming the radius of the
knuckle, the possibility of using Semi-Auto GMAW Temper
Bead would exist. SA-387 Gr. 11 Cl 1 & 2 base material was
used along with ER70S-B2L filler material for both
qualifications. The NBIC mandated minimum preheat of 300°F
was used for development of these WPS’s.
The installation of the new skirt windows to the new knuckle
attachment weld buildup took a somewhat different approach.
The geometry, but more importantly the access, of the joint
would not allow access to the back side of the joint in between
the skirt and the cone, so all welding would need to be carried
out entirely from the outside of the skirt. In lieu of NBIC
requirements, API-510 requirements for controlled deposition
welding were followed for attaching the new skirt windows to
the knuckle attachment weld on the cone. This welding
procedure was developed using SA-387 Gr. 11 Cl 1 & 2
material and ER70S-B2L filler material. API 510 does not
dictate preheat and interpass temperatures, but sets the
minimum preheat and maximum interpass used for qualification
as the min and max required for the WPS. For consistency with
the GMAW temper bead WPS’s and based off of previous
success, a minimum preheat of 300°F was utilized. GTAW with
Mock-up
A mock-up is a prudent measure to allow verification of the
proposed method of repair. A full mock-up using existing
pieces removed from one of the drums was employed for
procedure development and testing.
This represented a significant advantage by proving the actual
in-service materials and their condition would provide
acceptable mechanical properties after the weld repair is
applied. It is possible for actual in-service material to be
different and even altered versus using new replacement
material which has not seen service. It was fortunate in this
case to have some actual existing material to test with, but this
is rarely the case in most repairs. When existing material is not
available, additional testing and engineering assessment may be
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required to ascertain any differences and their possible
consequences.
The testing and procedure development was as close to in-situ
reality as possible giving some certainty that the testing on the
same as original material, represented reality in the field and in
the plant.
The knuckle attachment weld was created with a combination
of GMAW Temper Bead processes on the service aged cone
section. Then a mechanized torch was used to cut a bevel for
installing a skirt window. The new skirt window was installed
using the Controlled Deposition GTAW process. The final weld
surface was ground smooth and blended to give the skirt
attachment a smooth transition into the cone as shown in Figure
9.
Figure 10 shows the completed Mock-up.
Figure 11. Encoded PAUT of Mock-up Attachment Weld
Site Implementation
The site implementation followed very closely with the mockup with few key differences:

Figure 9. Mock-up Configuration


Crack repair of the cone was not mocked-up but was
none the less an important part of the repair process as
the repairs were numerous around the drum.
An additional difference that was not anticipated was
existing skirt misalignment with the coke drum. This
contributed to significant additional weld buildup in
some areas resulting in an increased radius in the
knuckle in those areas.
The scale of the work was significantly different from
the mock-up. The coke drum was divided up into 6
segments with 3 segments at a time worked in parallel
and the other 3 segments left in place to support the
coke drum. However, each segment was significantly
larger than the mock-up and the geometry was slightly
different given the portion of the cone material that
was available for use for the mock-up.
After removal of the existing skirt section and knuckle weld
were performed, the coke drum cone was examined for
cracking. Flaws were identified, excavated and repaired.
After the cone repairs were completed, the initial two layers of
weld metal buildup were performed on the cone. Figure 12
shows a section of the cone during the knuckle buildup
application.
Figure 10. Completed mock-up
The completed mock-up was inspected using encoded PAUT
and then sectioned for metallurgical evaluation by Canadian
Natural Resources Limited. (see Figure 11 ).
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crotch radius during contouring (Figure 15), and the final weld
prep applied to the knuckle buildup (Figure 16).
Figure 12. Cone Section during P4 (1.25Cr) filler weld
buildup - after (left side) skirt removal
Then 1” diameter segments of ceramic bar were installed at the
base of the buildup. Semi-Auto GMAW temper bead was then
used to attach a 1-1/4 Cr “shelf bar” tangent to the ceramic
backing (Figure 13). After the shelf bar was connected the
previously applied buildup a significant portion of the SemiAuto Temper Bead weld was removed limiting the amount of
Semi-Auto applied GMAW weld in the knuckle. The shelf bar
would be removed entirely later, but was found to be essential
to allow Machine GMAW Temper Bead to rebuild the
remaining portions of the knuckle attachment weld.
Figure 14. Groove Weld Prep to the Knuckle
Knuckle attachment
weld buildup
Figure 15. Crotch Radius During Contouring
1-1/4 Cr Shelf Bar
Figure 13. Connection of “shelf bar” to ceramic backing
When adequate material was deposited to complete the knuckle,
the completed weld was contoured and the ceramic backing was
removed. At this point the radius was cleaned up smooth and a
track mounted torch was utilized to cut a consistent groove prep
on the knuckle attachment weld to facilitate welding the skirt
window in place. The figures below show the track torch
applying the groove weld prep to the knuckle (Figure 14), the
Figure 16. Final Prep Applied to the Knuckle Buildup
8
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With the knuckle prep complete, the new skirt windows were fit
into place to form a 1-sided weld joint (Figure 17)
without post-weld heat treatment by controlled deposition
welding or temper bead techniques, proper training of welder
operators should be conducted to ensure the techniques are
implemented properly.
For the case study presented, standard methods of MPT, D-LPT,
and PAUT were used to ensure the quality of the weld. MPT &
PAUT were employed to ensure that no cracks were left behind
in the cone after the first weld pass and completion of the top
(final) layer as soon as cooldown allowed access. Throughout
the work regular hardness testing ensured the required
hardness’s were being met. NDT performed throughout the job
verified the repair work delivered on meeting the quality
requirements. Contractor experience along with the pre-build
mock-up demonstrated the ability to execute in the field and
maintain that same level of quality that was displayed in the
PQRs. The field work delivered on the quality standards
defined in the original scope.
Finally, with proper planning and execution, a code compliant
pressure vessel skirt attachment weld can be replaced in the
field without post-weld heat treatment. These technologies can
be applied to many Drums, Vessels, Tanks and Piping. The
Coke drum was placed back in service in the allotted time and
has operated for 12 months without issue at the time of writing.
Figure 17. Skirt to Knuckle Attachment Butt Weld Joint
The skirt windows were welded in place using GTAW-HP from
the OD only. The final weld was then ground smooth and final
surface and volumetric inspections were performed. The
completed skirt installation shown in Figure 18.
5 ACKNOWLEDGEMENTS
The authors wish to thank their respective companies for
permission to publish this paper.
6. REFERENCE
1
American Society of Mechanical Engineers, Boiler & Pressure
Vessel Code, Section VIII, Division I, “Rules for
Construction of Pressure Vessels”, ASME, New York, 2015.
2
American Petroleum Institute, Technical Report 934G , “Design,
Fabrication, Operational Effects, Inspection, Assessment and
Repair of Coke Drums and Peripheral Components in Delayed
Coking Units”, API, Washington DC. 2014.
3
WRC Bulletin 506”, Half Bead Temper Bead Controlled
Deposition Techniques for improvement of Fabrication and
Service of Cr-Mo Steels. 2005.
4
Weil, N.A., & Rapasky, F.S., “Experience with Vessels of
Delayed Coking Units”, Proceedings of the API Division of
Refining Vol. 38 No.3, American Petroleum Institute,
Washington, D.C., 1958
5
American Petroleum Institute, Technical Report 934A thru F,
H, “934-A, Materials and Fabrication of 2 1/4Cr- Steel
Heavy Wall Pressure Vessels for High- 1Mo, 2.25Cr-1Mo0.25V, 3Cr-1Mo, and 3Cr-1Mo-0.25V ”, API, Washington
DC. 2010 et.seq...
6
J.W. Thomas, “API Survey of Coke Drum Cracking
Experience”, American Petroleum Institute, Washington,
D.C., 1980.
7
API, “1996 API Coke Drum Survey - Final Report”, American
Petroleum Institute, Washington, D.C., 1996.
8
Antalffy, L. P., et. al., “Analyses of Alternate Skirt
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& Piping Conf., Vol. 315, American Society of Mechanical
Engineers, New York., 1995.
Figure 18. Completed Window Installation
4 CONCLUSIONS
This paper has provided discussions on the repair of pressure
vessel support skirt attachment welds by applying the TBW and
CDW techniques.
When designing a repair approach,
consideration should include material and aged condition,
extent and location of defects, welding process and
consumables, and codes, standards, and regulatory guidelines.
When repair by weld metal buildup to rebuild a skirt-attachment
weld configuration is considered, weld procedure qualification
and adequate mock-ups should be performed in order to ensure
a sound repair. Further, when invoking a code compliant repair
9
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9
12
Baxter, J. E., “Design Optimization of Coke Drum Support
Skirt”, Proc. Of the Pressure Vessel & Piping Conf., Vol. 359,
American Society of Mechanical Engineers, New York, 1997.
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Ramos, A., et. al., “Mechanical Integrity Evaluation of
Delayed Coke Drums”, Proc. Of the Pressure Vessel & Piping
Conf., Vol. 359, American Society of Mechanical Engineers,
New York, 1997.
11
Taagepera, J., & Kreiner, J., “Alternative Coke Drum Skirt
Designs”, Proc. Of the Pressure Vessel & Piping Conf., Vol.
430, American Society of Mechanical Engineers, New York,
2001.
American Society of Mechanical Engineers, Boiler &
Pressure Vessel Code, Section IX, “Welding, Brazing, and
Fusing Qualifications”, ASME, New York, 2015.
13
American Petroleum Institute, API 510, “Pressure Vessel
Inspection Code: In-service Inspection, Rating, Repair, and
Alteration”, API, Washington DC. 2014
14
James, M.R., “Relaxation of Residual Stresses an Overview,
Residual Stresses, Pergamon, 1987, Pages 349-365, ISBN
9780080340623.
15
Saenger, J.F. & Manz, A.F., "High Deposition Gas TungstenArc Welding", Welding Journal, American Welding Society,
May 1968, pp. 386-393.
10
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