Wet welding critical to
structural maintenance
State-of-the-art
work proving product
C.E.
Grubbs, Thomas J. Reynolds
Global Divers & Contractors
Offshore structures in place worldwide are an integral part of the
oil and gas industry' infrastructure. These offshore structures provide
strategic support for the exploration, production, and transportation of oil
and gas. Maintaining the structures is a challenging task.
Maintenance
divisions of offshore operating companies must properly protect and repair the
vital structures after they have sustained structural damage due to accidents
during and after installation, fatigue, corrosion, boat collisions, and acts of
nature.
Global
Divers & Contractors and the Center for Welding and Joining Research at the
Colorado School of Mines (CSM) lead a consortium of major offshore oil and gas
companies and the Department of Interior's Minerals Management Service in the
development of improved underwater welding techniques and welding electrodes
for use on structural steels used in the construction of offshore structures.
Working
with the Edison Welding Institute, Global's research also includes the
development of underwater wet welding procedures on pipeline steels for the
Pipeline Research Council (PRC) International. This work is done at Global's
Research and Development Center in New Iberia, Louisiana. The Center includes
hyperbaric facilities capable of simulating wet or dry welding environments for
water depths down to 366 meters.
As
the number of offshore structures grows, and those in existence continue to be
exposed to fatigue, corrosion and accidental damage, the need for underwater
structural repairs increases. This, of course, emphasizes the need for
continuing efforts to upgrade underwater repair technology.
Causes,
with typical examples, of underwater damage to offshore structures include the
following:
- Corrosion: Depleted sacrificial anodes, intermittent
operation of impressed current systems, inadequate design of cathodic
protection systems and improper grounding of barge/boat mounted welding
machines when welding on offshore structures.
- Skirt pile installation: Damage frequently occurs
when attempts to "stab" skirt piles into bell-guides are made
without a diver or video camera to provide underwater vision.
- Dropped objects: Objects dropped overboard have
included skirt piles, bundles of pipe and other items of material and
equipment during off-loading, boat landings during installation, and pile
driving adapter caps.
- Boat impact: Collisions involving boats and structures
are not uncommon and repeated impact with through the water line members,
boat landings, and fendering systems have resulted in gross structural
damage.
- Acts of nature: Hurricane Andrew did extensive
damage to Gulf of Mexico structures and the dragging of ship's anchors
displaced several subsea pipelines. Infrequent mud slides have also
damaged structures and pipelines in the Gulf.
- Design engineering: While infrequent, design errors
and unanticipated loads have resulted in severe damage to offshore
structures.
Repair options
Viable repair methods
include mechanical clamps, with and without grout, wet welding, and dry
hyperbaric welding.
Hundreds
of wet welded structural repairs have been made by welder/divers qualified in
accordance with the requirements of the ANSI/AWS Specification for Underwater
Welding (AWS D3.6), using qualified welding procedures, with no known failures.
However,
prior to developments during the Global/CSM Joint Industry Underwater Welding
Development Program (JIP), wet welds were not attempted on base metals with
carbon equivalents (CE) greater than 0.40 wt pct (CE = C + Mn/6 + (Cr + Mo +
V)/5 + (Cu + Ni)/15) due to hydrogen-induced underbead cracking in the heat
affected zone (HAZ) of the base metal.
Underwater
dry hyperbaric welds, qualified in accordance with requirements of AWS D3.6,
have mechanical properties equal to similar welds made above water. However,
under some conditions, installation of a dry weld chamber can impose
unacceptable loads on the structure. For example, a chamber installed on
structural members near the splash zone can be subjected to excessive loads
imposed by prevailing ground swells and wave action. Transfer of loads to
structural members can cause failure of the members.
Wet versus dry welds
Wet welding is done at
ambient pressure with the welder/diver in the water without any mechanical
barrier between the water and the welding arc. Simplicity of the process makes
it possible to weld on even the most geometrically complex node sections. While
wet welding procedures have been qualified, and used for underwater repairs,
down to 100 meters, further development of electrodes and welding processes
will be required if satisfactory wet welded structural repairs are to be made
much deeper than that depth.
Dry
hyperbaric welding is done at ambient pressure in a custom built chamber from
which the water has been displaced with air or other gas mixture, depending on
water depth. Dry welds, when qualified in accordance with the requirements of
AWS D3.6 for Class A welds, meet all the requirement for welds made above
water.
Several
dry welded pipeline tie-ins have been made down to 220m plus one subsea tie-in
was made at 308 m. Repair costs and time for dry welded repairs are usually at
least double that for wet welded repairs.
AWS
D3.6 defines Class A (dry) welds as underwater welds that are intended to be
suitable for applications and design stresses comparable to their conventional
surface counterparts by virtue of specifying comparable properties and testing
requirements. Class O welds are intended to meet requirements of some other
designated code or specification as well as the AWS D3.6 requirements for Class
A welds.
AWS
D3.6 defines Class B (wet) welds as underwater welds that are intended for less
critical applications where lower ductility and greater porosity and other
discontinuities can be tolerated, and states that the suitability of Class B
welds for a particular application should be evaluated on a "Fitness for
Purpose" basis.
Welding program
The Global/CSM JIP program
started in 1993. Phase I of the program was completed in 1995. Phase II, with
the objective of increasing the depth at which code quality (AWS
"Specification for Underwater Welding" D3.6) welds can be made, is
ongoing.
Objectives
of Phase I of the program were to improve the properties of wet welds to the
highest practical levels, and to determine what those properties are so they
may be used as fundamental engineering design principals for solutions to
underwater repair/construction problems where wet welding versus dry hyperbaric
welding, usually results in significant savings in time and costs.
Areas
of expected improvements included increased ductility and toughness of
weldments and the reduction of hardness and elimination of hydrogen cracking in
the HAZ of crack susceptible (CE.40) base metals.
Program
work was guided by the Technical Activities Committee (TAC) which was made up
of one member from each of the participating organizations, Global and CSM.
Phase I participants were: Amoco Research Center, Chevron Research &
Technology Company, Shell Offshore Engineering Research Department, Marathon
Oil Company, Mobil Research & Development Corporation, Exxon Production
Research Company, the US Navy, US Offshore Minerals Management Service
(Department of the Interior) and the UK Health and Safety Executive-Offshore
Safety Division.
Global
provided management, welding engineering, technicians, welder/divers,
hyperbaric facilities, welding/diving equipment and materials. CSM provided
scientists, a graduate research engineer dedicated to the program, welding
electrode formulations, analytical equipment and technical reports on their
research tasks.
Matrix and base metal
The test Matrix for Phase
I of the program included the following tasks:
- Refinement of the multiple temper bead (MTB) wet welding technique used for the prevention of hydrogen cracking and reduction of hardness in the heat affected zones of crack susceptible base metal.
- Selection of optimum welding power source and
auxiliary equipment for underwater wet welding.
- Development of improved electrodes through
reformulation of flux coatings and selection of core wires.
- Qualify welding procedures for all position wet
fillet and groove welds at 1 meter and 10 meters and make groove welds at
1, 10, 20, 30 and 50 meters with the improved electrodes.
The test matrix for Phase II concludes with 19 mm groove welds
made at depths of 21, 43, 61 and 91 meters, with electrodes formulated for
welding at those depths.
ASTM
A537 Class 1 19-mm steel plate was selected as the program base metal because
of its proven propensity for hydrogen induced cracking, and excessive hardness,
in the heat affected zone, when welded with conventional wet welding
procedures. The carbon equivalent of the A537 material was .462 including .20
wt pct carbon. The specified minimum yield and tensile strength were 50 ksi and
70 ksi, respectively.
Multiple temper bead
The unique and proprietary
multiple temper bead (MTB) wet welding technique involves three essential
variables which were methodically investigated and are described as follows.
- Toe-to-toe distance: The distance between toes of primary weld beads that tie in to the base metal and toes of temper beads is one of the variables that govern temper bead heat input to the crack susceptible HAZ. During this part of the program, multiple temper bead welds were made on the A537 material with toe-to-toe distances of 1.59, 2.38, 3.175, and 22.22 mm. Results of microscopic (250x) examinations, and Vickers 10 kg (VH 10) hardness tests of the heat affected zone were used to determine unacceptable, acceptable, and optimum toe-to-toe distances.
- Time intervals: For the prevention of HAZ hydrogen
cracking, it is essential that we know how long it takes for HAZ hydrogen
cracks to develop, such as the maximum allowable time between deposition
of primary weld beads and temper beads. Based on the data from five
experiments using electrodes other that the Program Ex 7 electrode, on the
A537 material, a baseline crack initiation time was determined to be 3-10
minutes.
To determine the maximum acceptable time between deposition of
primary and temper beads, welds were made with the Program Ex 7 electrodes with
the time intervals reported below. The following are time intervals and results
based upon microscopic (250 x) examination of the HAZ:
- 4-10 minutes with 30-second intervals - no cracks.
- 10-60 minutes with 10-minute intervals - no cracks.
- 1.0-1.5 hours with 30 minute intervals - no cracks.
- 2.0-4.0 hours with 30 minute intervals - all specimens
had typical HAZ hydrogen-induced cracks.
For validation of the highly desirable results (1.5 hours with no
cracks), additional experiments were conducted. The Ex 7 electrodes were used
to make an untempered in 19 mm by 305 cm) groove weld on ASTM A516 Gr. 70 (CE
.44) material. Previously, when this material was welded with commercially
available wet welded electrodes, HAZ cracks developed within 10 minutes. After
burning the third electrode, the welder/diver observed cracks in the HAZ of
weld metal deposited with the first electrode.
When
welding with Ex 7 electrodes, the welder saw no cracks, and when the weld was
completed, none were detected with magnetic particle examination. Later, one of
four cross sections showed no cracks when examined at 250x.
A
second weld was made on the same material with the Ex 7 electrodes utilizing
the MTB technique. For this MTB weld, HAZ hydrogen cracking was eliminated.
Knowing
the maximum time interval between deposition of primary weld beads and temper
beads is essential to the selection of the most efficient sequence for
deposition of filler metal.
HAZ hardness reduction
Throughout the many MTB
welding experiments, prevention of HAZ hydrogen cracking was consistently
accomplished without any deliberate action to increase temper bead heat input
by increasing welding amperage or reducing travel speed. For the same welds -
with the exception of a very small area (3.175 mm by 4.76 mm) in the HAZ
beneath the toes of cap passes - maximum hardness of the weld metal and HAZ was
well below the Vickers 10kg (VH10) specified by AWS D3.6 for Class A (dry)
welds.
Because
of the high carbon equivalent (.462) and especially the high carbon content
(.20), hardness in the small areas in the HAZ beneath the toes of the cap
passes ranged from 400 to 442. To meet the AWS D3.6 maximum hardness of 325 for
dry welds, a series of welds were made using progressively increased levels of
temper bead heat input in the cap passes. For these welds, optimum heat input
reduced the aforementioned range of 400-442 to 252-300.
Weld comparisons
Table 1 [139,813 bytes] and Table 2 [102,064 bytes] provide a practical
comparison of the mechanical properties of the state-of-the-art welds made
during Phase I of the Joint Industry Underwater Welding Program. Table 1
compares the mechanical properties of the JIP wet welds with the AWS D3.6
"Underwater Welding Specification" requirements for Class A (dry)
welds.
Table
2 compares the JIP wet welds with the American Petroleum Institute
"Recommended Practice For Planning, Designing and Constructing Fixed
Offshore Platforms - Working Stress Design" (RP-2A-WSD) for welds made
above water.
Mechanical
properties reported in Table 3 [153,425 bytes] are
the results of tests performed on welds made by Global Divers in 1984 (prior to
JIP), and are provided as general information reference the variation in
mechanical properties of wet welds as depth increases.
When
Phase II of the ongoing research program is completed, comprehensive mechanical
test results will be available for wet welds made at depths of 10, 21, 43, 61
and 91 meters, plus baseline information reference pressure/water depth induced
changes in the chemistry and microstructure of wet weld metal deposited from 26
meters to 122 meters.
Figure 1 [23,926 bytes] shows
that Charpy V-notch values of the JIP quenched and tempered wet welds were
significantly greater than the AWS D3.6 requirements for Class A (dry) welds.
During a Joint Industry Underwater Development Welding Program, Sea-Con
Services (later acquired by Global Divers) made a series of wet welds to
determine the fatigue properties of wet weldments and how they compared to
welds made above water (Figure 2 [15,467 bytes]).
Five
dry welded and 19 wet welded fatigue specimens were taken from 25.4 mm thick
fillet welded T-plates. Wet welds were made at -10 meters. Specimens were
tested in simulated sea water with fully reversible cantilever axial loads of
20 ksi tension and 20 ksi compression with 28,840 cycles until the first
appearance of macro cracks and 29,635 cycles to failure.
As
shown on Figure 2, fatigue properties of the heat affected zone (the area most
vulnerable to fatigue failure) of the wet welds were equal to those of the
welds made above water, and significantly exceeded the minimum fatigue
properties specified by the American Petroleum Institute, "Recommended
Practice for Planning, Designing and Constructing Fixed Offshore Platforms - Working
Stress Design" (RP 2A-WSD).
Other projects
In addition to the welding
done during the Joint Industry Underwater Welding Development Program, the
following welding projects executed by Global Divers are indicative of the
state-of-the-art of underwater wet welding. Unless specified otherwise, welds
were qualified in accordance with the requirements of the AWS
"Specification for Underwater Welding".
- Wet welding procedures were qualified, and used for the repair of an offshore production platform, at the record depth of 300 meters. Ferritic (mild steel) welding electrodes were used on carbon manganese structural steel.
- Wet welding procedures were qualified with nickel
welding electrodes on high strength, high carbon equivalent (CE .476 wt
pct) steel for repairs to an offshore structure. When wet welded with
ferritic electrodes, base metals with a carbon equivalent of more than .40
are subject to hydrogen induced cracking in the heat affected zone.
- Qualified underwater wet welding procedures on the
new micro alloyed high strength (TMCP) steels used in the fabrication of
deep water offshore structures.
- Global was first to qualify underwater wet welding
procedures on carbon steel with ferritic welding electrodes in accordance
with the requirements of ASME Boiler and Pressure Vessel Code for
Underwater Welding, Section XI, Div. 1, Code Case N-516-1.
- Provided proprietary welding procedures, proprietary
welding electrodes and technical consulting services to the repair
contractor, plus project oversight for the offshore platform operator, for
the first underwater wet welded structural repair in the North Sea.
- During a joint industry wet welding development
program, Sea-Con Services (later acquired by Global Divers) performed a
fatigue test on a series of specimens taken from 1-in. thick fillet welded
T-plates in simulated seawater with fully reversible cantilever axial
loading (20 ksi tension, 20 ksi compression). The results, shown in Figure
2, significantly exceeded the American Petroleum Institute RP 2A - WSD requirements
for welds made above water.
Copyright
1998 Oil & Gas Journal. All Rights Reserved.
06/01/1998
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