Underwater Wet-Welding Techniques for Repair of Higher-Carbon-Equivalent Steels

Abstract

ABSTRACT Underwater wet welding repair techniques using ferritic electrodes are limited to steels with carbon equivalents below 0.40% because the fast quench that occurs after welding produces hard heat affected zones susceptible to hydrogen cracking. This paper describes the problems and options for wet welding of offshore structures. It compares the advantages and disadvantages of using ferritic and nickel base wet welding electrodes. Recommendations are made for producing crack free wet welds in higher strength steels including temper bead techniques and the use of electrodes with thermit fluxes. INTRODUCTION The construction of offshore structures in depths greater than 100 meters or in the hostile environment of high wave actions, such as in the North Sea, requires greater load carrying capacity which can be satisfied with either larger steel sections or with steels of greater strengths (1). Neither of these options are without major fabrication and integrity concerns. Heavy steel sections will increase the weight of the structure which can limit its full utilization and also increase the cost of materials and fabrication. The use of higher strength steels, especially steels with a carbon equivalent (CE) greater than 0.40 wt. pct. raises the susceptibility of the weldments to hydrogen cracking (2). The carbon equivalent is typically given by the expression. (mathematical equation)(available in full paper) It has been found that wet welding of higher strength steels with ferritic electrodes results in hydrogen cracking in the heat affected zone (HAZ) of the weldment. This type of defect has been described as an "underbead crack". Unfortunately most of the platforms that are used in the North Sea are made with thick wall steels that have carbon equivalents greater than 0.40 wt. pct. In the Gulf of Mexico, wet welding techniques have been very successful because the thinner wall and low carbon equivalent of the steels make repair by wet welding relatively easy. However, in the Gulf region some of the mudslide platforms at the mouth of the Mississippi have the thicker wall and higher carbon equivalent steels which could result in future repair problems if wet welding is needed. Even in platforms with steels that one would normally expect to have low CE, the inability to conduct a composition check could make a wet weld repair risky because of the risk of excessive hardening in the HAZ is not known. The susceptibility to hydrogen cracking is a concern for surface welding as the strength increases above 350 Mpa (50 Ksi), but it can be controlled by the proper selection of welding procedures and consumables. Since moisture drastically increases the availability of hydrogen, wet underwater welding of higher strength steels isvery susceptible to hydrogen cracking. Underwater wet weld repair is an essential and economical way to maintain and to extend the service life of these marine structures. Both ferritic and nickel base welding electrodes are being evaluated for underwater wet welding repairs of higher carbon equivalent steels. Other methods of repair include mechanical clamping or underwater dry hyperbaric welding.