State-of-the-art underwater wet welding – offshore oil fields
C.E. Grubbs
As the number of offshore structures grow, and those in existence are continuously exposed to fatigue, corrosion and accidental damage, need for underwater structural repair increases
Offshore structures in place worldwide are an integral part of the oil and gas industry’s infrastructure. These structures provide strategic support for exploration, production and oil and gas transportation. Maintaining these structures is a challenging task faced by offshore operating companies, who must properly protect and repair vital structures after they have sustained: structural damage by accidents during and after installation, fatigue, corrosion, boat collisions and acts of nature. This article summarizes the most significant advancement made in underwater multiple temper bead (MTB) wet welding technology.
As a leader in underwater welding technology, Global Divers & Contractors (Global), along with the Center for Welding and Joining Research at Colorado School of Mines (CSM), leads a consortium of major offshore oil and gas companies and the Department of Interior’s Minerals Management Service (MMS) in developing improved underwater welding techniques and welding electrodes for use in constructing offshore structural steels.
In work with Edison Welding Institute, Global’s research also includes the development of underwater wet welding procedures on pipelines for Pipeline Research Council (PRC) International. This work is done at Global’s R&D center in New Iberia, Louisiana. This center includes hyperbaric facilities capable of simulating wet or dry welding environments for water depths to 1,200 ft (366 m).
UNDERWATER DAMAGE, REPAIR
Causes of underwater damage include:
* 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 material or equipment during off-loading, boat landings during installation, and pile driving adapter caps.
* Boat impact. Boats colliding with structures and repeated impact with through-water-line-members, boat landings and fendering systems have resulted in structural damage.
* Acts of nature. Hurricane Andrew did extensive damage to GOM structures including dragging of ships’ anchors causing several subsea pipelines to be displaced. Infrequent mudslides have also damaged structures and pipelines in the Gulf.
* Design engineering. While infrequent, design errors and unanticipated loads have resulted in offshore structure damage.
Repair procedure options 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 ANSI/AWS Specs 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 0.4 wt% (CE = C + Mn/6 + (Cr + Mo + V)/5 + (Cu+Ni)/15) due to hydrogen induced underbead cracking in the base metal heat affected zone (HAZ).
Underwater dry 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 structure loads. 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 member failure.
COMPARISON OF WET VS. DRY WELDS
Wet welding is done at ambient pressure with the welder-diver in the water and no mechanical barrier between water and 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, to 325 ft (100 m), further development of electrodes and welding processes will be required if satisfactory wet welded structural repairs are to be made at greater depths.
Dry hyperbaric welding is done at ambient pressure in a custom built chamber where water has been displaced with air or a gas mixture, depending on depth. Dry welds, when qualified in accordance with AWS D3.6 requirements for Class A welds, meet all weld requirements for above water. Several dry welded pipeline tie-ins have been made to 720 ft (220 m); in addition, one subsea tie-in was made at 1,012 ft (308 m). Repair cost and time for dry welded repairs are twice that for wet welded repairs.
AWS D3.6 definitions:
* Class A (Dry) welds. 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 spec combined with AWS D3.6 requirements for Class A welds.)
* Class B (Wet) welds. Underwater welds that are intended for less critical applications where lower ductility and greater porosity and other discontinuities [TABULAR DATA FOR TABLE 1 OMITTED] can be tolerated. AWS D3.6 states that the suitability of Class B welds for a particular application should be evaluated on a “Fitness for purpose” basis.
UNDERWATER WELDING DEVELOPING PROGRAM
The previously referenced 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.
Phase I objectives. These are: to improve wet weld properties to the highest practical levels, determine what those properties are and use them as fundamental engineering design principles for solutions to underwater repair and construction problems where wet welding vs. dry hyperbaric welding would result in significant time and cost savings.
Areas of expected improvements include increased ductility and toughness of weldments, reduced hardness and hydrogen cracking elimination in the heat affected zone (HAZ) of crack susceptible (CE[greater than]0.4) base metals.
How the program worked. Program work was guided by the Technical Activities Committee (TAC) which was made up of one member from each of the participating organizations, including Global and CSM. Phase I participants were: Amoco, Chevron, Shell Offshore, Marathon, Mobil, Exxon, the U.S. Navy, MMS and the UK Health and Safety Executive-offshore safety division.
Global provided management, welding engineering, technicians, welder-divers, hyperbaric facilities, welding and 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.
Test matrix and base metal. Phase I of the program included:
* Refinement of the multiple temper bead (MTB) wet welding technique used for prevention of hydrogen cracking and hardness reduction in the HAZ 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 3 ft and 33 ft and make groove welds at 3, 33, 66, 95 and 165 ft (1, 10, 20, 30 and 50 m) with improved electrodes. (Test Matrix for Phase II concludes with 3/4-in. (19-mm) groove welds made at 70, 140, 200 and 300 ft (21, 43, 61 and 91 m), with electrodes formulated for welding at those depths.)
ASTM A537 Class I, 3/4-in. (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 HAZ when welded with conventional wet welding procedures. The carbon equivalent (CE) of the A537 material was 0.462 including 0.2 wt% carbon. The specified minimum yield and tensile strength were 50 ksi (345 MPa) and 70 ksi (483 MPa), respectively.
[TABULAR DATA FOR TABLE 2 OMITTED]
MULTIPLE TEMPER BEAD WET WELDING (PHASE I)
All of the program objectives were achieved; however, this report addresses the most significant advancement in state-of-the-art of wet welding; the MTB wet welding technique, Fig. 1. It should be noted that details proprietary to participants in the program are not divulged in this report.
The unique and proprietary MTB wet welding technique involves three essential variables, which were methodically investigated: 1) toe-to-toe distance, 2) time interval, and 3) temper bead heat input for prevention of the heat affected zone (HAZ) hydrogen cracking.
Toe-to-toe distance. Toe distance between primary weld beads that tie into the base metal and toes of temper beads is one of the variables that governs temper bead heat input to crack susceptible HAZ. During this part of the program, multiple temper bead welds were made on A537 material with toe-to-toe distances of 1/10, 3/32, 1/8 and 3/16-in. (1.59, 2.38, 3.175 and 22.22-mm). Microscopic (250x) examinations and Vickers 10 kg (VH 10) hardness tests of HAZ were used to determine unacceptable, acceptable, and optimum toe-to-toe distances.
Time intervals. For HAZ hydrogen cracking prevention, it is essential that we know how long it takes these cracks to develop; and what is the maximum allowable time between deposition of primary weld beads and temper beads. Based on data from five experiments using electrodes other than the Program Ex 7 electrode, on A537 material, a [TABULAR DATA FOR TABLE 3 OMITTED] baseline crack initiation time was determined to be 3 to 10 minutes.
To determine the maximum acceptable time between deposition of primary and temper beads, welds were made with Program Ex 7 electrodes with varied time intervals. The results are based on microscopic (250 x) examination of HAZ, Table 1.
Supplemental welds for time interval evaluation. For validation of the highly desirable results (1 1/2 hr with no cracks), additional experiments were conducted. Ex 7 electrodes were used to make an untempered 3/4×12-in. (19×305-mm) groove weld on ASTM A516 Gr. 70 (CE 0.44) material. Previously, when this material was welded with commercially available wet welded electrodes, HAZ cracks developed within 10 min. (after burning the third electrode, the welder-diver observed cracks in HAZ of weld metal deposited with the first electrode). When welding with Ex 7 electrodes, no cracks were observed; 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 Ex 7 electrodes using MTB techniques. For this MTB weld, heat affected zone (HAZ) hydrogen cracking was eliminated.
Knowing the maximum time interval between deposition of primary weld beads and temper beads is essential to selection of the most efficient sequence for deposition of filler metal.
Temper bead heat input for reduction of heat affected zone (HAZ) hardness. Throughout 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 exception of a very small area ([less than]1/8×3/16 in.) (3.175×4.76 mm) in HAZ beneath the toes of cap passes – maximum hardness of weld metal and HAZ was well below the Vickers 10 kg (VH10) specified by AWS D3.6 for Class A (Dry) welds. Because of high carbon equivalent (0.462) and especially high carbon content (0.2), hardness in the small areas in the HAZ beneath the toes of the cap passes ranged from 400 to 442. To meet 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 cap passes. For these welds, optimum heat input reduced the aforementioned hardness range of 400-442 to 252-300.
Comparison of weld properties. Tables 2 and 3 provide a practical comparison of mechanical properties of the state-of-the-art welds made during Phase I of the Underwater Welding JIP. The criteria for these tables were based on:
* Mechanical properties of wet welds with AWS D3.6 Underwater welding specification requirements for Class A (Dry) welds, Table 2.
* Wet welds with API Recommended practice for planning, designing and constructing fixed offshore platforms – Working stress design (RP-2A-WSD) for welds made above water, Table 3.
[TABULAR DATA FOR TABLE 4 OMITTED]
The mechanical properties reported are test results performed on welds made by Global divers in 1984 (prior to the JIP), and are provided as general information; notice the variation in mechanical properties of wet welds as depth increases, Table 4. When Phase II of the ongoing research program is completed, comprehensive mechanical test results will be available for wet welds made at depths of 33, 70, 140, 200 and 300 ft (10, 21, 43, 61 and 91 m), plus baseline information reference pressure/water depth induced changes in the chemistry and microstructure of wet weld metal deposited from 33 to 400 ft (10 to 122 m).
Charpy V-notch values of lip quenched and tempered wet welds were significantly greater than the AWS D3.6 requirements for Class A (Dry) welds, Fig. 2. During an Underwater Development Welding JIP, Sea-Con Services made a series of wet welds to determine fatigue properties of wet weldments and how they compared to welds made above water, Fig. 3.
Five dry-welded and nineteen wet-welded fatigue specimens were taken from 1-in. (25-mm) thick fillet welded T-plates. Wet welds were made at 33 ft (10 m). Specimens were tested in simulated seawater with fully reversible cantilever axial loads of 20-ksi (138-MPa) tension and 20-ksi (138-MPa) compression with 28,840 cycles until the first appearance of macro cracks and 29,635 cycles to failure. Fatigue properties of the heat affected zone (HAZ – the area most vulnerable to fatigue failure) of wet welds were equal to those of welds made above water, and significantly exceeded the minimum fatigue properties specified by API, Recommended practice for planning, designing and constructing fixed offshore platforms – Working stress design (RP 2A-WSD), Fig. 3.
Supplemental Information. In addition to the welding done during the JIP Underwater Welding Development Program, the following welding projects executed by Global are indicative of state-of-the-art underwater wet welding. Unless specified otherwise, welds were qualified in accordance with AWS Specification for underwater welding requirements. The projects include:
* Wet welding procedures were qualified, and used for repair of an offshore production platform, at a record depth of 325 ft (100 m). 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 (0.476 wt%) steel for repairs to an offshore structure (when wet welded with ferritic electrodes, base metals with CE[greater than]0.4 are subject to hydrogen induced cracking in the HAZ).
* Qualified underwater wet welding procedures on new micro alloyed high strength (TMCP) steels used in fabrication of deep water offshore structures.
* First to qualify underwater wet welding procedures on carbon steel with ferritic welding electrodes in accordance with requirements of ASME Boiler and Pressure Vessel Code for Underwater Welding, Section XI, Div. 1, code case N-516-1.
* Provided proprietary welding procedures, welding electrodes and technical consulting services to the repair contractor, plus project oversight for the offshore platform operator, for first underwater wet welded structural repair in the North Sea.
* First to qualify underwater wet welding procedures on stainless steel in accordance with AWS D3.6 Class O/ASME Sec. IX requirement.
* During JIP wet welding development, Sea-Con Services performed a fatigue test on a series of specimens taken from 1-in. (25-mm) thick fillet welded T-plate in simulated seawater with fully reversible cantilever axial loading (20-ksi tension, 20-ksi compression). The results significantly exceeded API RP 2A – WSD requirements for welds made above water, Fig. 3.
The authors
C.E. (Whitey) Grubbs is director of underwater welding R&D at Global Divers & Contractors. During his more than 27 years of dedication to underwater welding, he founded the AWS committee that developed the Specification for underwater welding, and served as it’s chairman for 15 years. He has authored more than 50 papers on underwater welding, received numerous awards for his contributions, and holds three patents. He is included in the 1998/1999 edition of Who’s Who in Science and Engineering.
Thomas J. Reynolds is manager of technical services at Global Divers & Contractors, Division of Global Industries. He has more than 15 years experience in planning and executing wet and dry hyperbaric offshore and nuclear industry welding projects. He has written numerous articles on the application of underwater wet welding, holds a BSME degree and is a registered professional engineer (Texas).
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