New Underwater Welding Process Proved For Continental Shelf Depths

ABSTRACT A new flux-cored wire arc welding process has been developed for use in underwater welding inside ambient pressure dry welding chambers. The process offers a number of Advantages over current alternatives, exhibits good arc stability and fusion characteristics, high metal deposition rates and excellent mechanical properties as long as correct chemical conditions are provided at the arc. The process has been successfully used offshore, has been approved by the major Classification Societies, and represents a significant advance in the field of subsea welding. INTRODUCTION The two greatest factors opposing the usage of arc fusion welding processes for underwater operation are the high pressure affecting arc stability and the wet environment creating metallurgical difficulties. The problems associated with the wet environment can be eliminated by the use of a dry welding chamber enclosing the area to be welded but certain metallurgical problems will still exist. For example reactions between weld metal, gas and slag will vary with pressure, as will base metal and weld metal cooling rates. All of these factors will influence the chemical composition and mechanical properties of the deposited weld metal. The effect of pressure on the electric arc is now fairly well known in that the arc constricts and operates at successively higher voltages as pressure (water depth) is increased. The bare wire gas shielded arc welding processes such as G.M.A. (gas metal arc) suffer from arc instability problems at depths much in excess of 250 ft (9 bars abs). The instabilities take the form of arc " outages " and unusable forms of metal transfer across the arc column. By careful choice of shielding gas composition, wire diameter and process variables the bare wire processes can be operated at depths of 600 ft (19 bars) but the base metal fusion characteristics are so poor and tolerance to parametric deviations so low that they are impractical for underwater site operation. The use of flux wire combinations offer a number of advantages over the bare wire G.M.A. systems. The metal/flux combinations are more efficient thermally and fluxing ingredients can be added which improve arc ionisation and promote stable metal transfer. The continuously fed flux cored wires are an obvious improvement compared to the low duty cycle operation, manual metal arc (M.M.A.) or " stick " processes. The flux cored welding consumable in particular offers a number of significant advantages over other fusion welding processes at depths down to approx. 1000 ft water depth (31 bars abs).Highly stable metal transfer due to a balance of fluxing ingredients and gas shield.High heat input to workpiece offsetting heat losses due to pressure effect.High metal deposition rate in all welding positions.High duty cycle due to continuous nature of the process.Fairly low operational skill requirements.High tolerance to parametric variations.