Preliminary Investigations on the Effect of Short Circuit Variables on Metal Transfer Above 60 bar abs.
Authors: Dos Santos J.F. , Szelagowski P. , Schafstall H.G. , Dobernowsky A. 
Published: October 2008

This work presents the preliminary results of a comprehensive and systematic evaluation of the variables affecting short circuit metal transfer behavior under hyperbaric conditions using an advanced welding power supply. The results presented in this study indicate that for a given welding condition (i.e., reference voltage, slope of the static characteristic, short circuit curve form, etc.) the appropriate selection of current rise and fall rates can minimize +6the effects of arc plasma instabilities casued by high ambient3 pressures. All the experiments described in this work have been carried out in the 10m3 test chamber 08 of the GKSS Underwater Simulation Plant (GUSI).

1.INTRODUCTION

For much of the world, especially in the Gulf of Mexico and Brazil the best potential for major new oil and gas reserves lies beneath the deep waters of the continental slopes, or in other words, at depths below 450msw. This is a significant depth since it is about the present limit for ambient pressure diving. Eventual welding operations at such depths (Le. below 50Omsw) can only be carried out by mechanized or fully automatic systems. Oue to the operational nature of such systems (Le. basically, unmanned operations) an optimum process behavior is required to avoid possible weld defects or inconsistent bead formation. Considering the use of consumable welding processes, the necessary control over arc stability and bead geometry can only be achieved through a basic understanding of the factors affecting metal transfer. Additionally, taking into account the usual underwater repair and maintenance applications (i.e circumferential joints), particular attention should be given to those transfer modes with positional capabilities

Positional underwater welding with consumable processes has been traditionally performed using the dip transfer mode or intermediate transfer forms consisting of varying open arc periods. More recently, the developments in transistorized welding power supply technology have allowed a better control over metal transfer particularly through pulsed current techniques. Such developments have led to an extended range and quality of the GMAW process application at atmospheric conditions which have been subsequently applied to hyperbaric environments, offering therefore, an alternative to the conventional dip transfer technique. Unfortunately, the published work presently available on the application of pulsed current technique under hyperbaric conditions has been limited to the flat position at relatively high heat inputs. Therefore, it seems that although pulsed GMAW is potentially one of the most promising processes for hyperbaric applications, some of the fundamental aspects of metal transfer under pulsed conditions at elevated pressures still have to be clarified before its actual use.

Although, as mentioned above, it is widely accepted that presently only dip transfer would provide the required positional capabilities in underwater welding applications, most of the literarture on the hyperbaric use of the GMAW process has concentrated its attention on the open arc transfer modes (Le., globular, spray, etc.). In general, it could be said that on those reports in which the dip transfer has been systematically studied, the obtained results described an unstable process prone to a series of operational defects, with excessive spatter generation and associated with unaceeptab1e weld bead geometric characteristics.