Wet welding critical to structural maintenance

State-of-the-art work proving product

C.E. Grubbs, Thomas J. Reynolds
Global Divers & Contractors

Multiple temper bead wet welding groove weld 34,920 bytes]

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 inTable 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.