State-of-the-art underwater wet welding - offshore oil fields

World Oil, July, 1998 by C.E. Grubbs, Thomas J. Reynolds

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.