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

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

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 3/4x12-in. (19x305-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.