Experimental Study on Strength and Ductility of Underwater Fillet Welds in Repairing Offshore Steel Structures

Underwater wet welding is one of the most common repair measures for corroded offshore steel structures. Few studies have been carried out systematically concerned with mechanical properties of such welds, thus current design provisions rely heavily on limited experimental data on welds made underwater and design properties for corresponding welds made in air. This paper presents a series of experiments on forty-five fillet welded specimens featuring welding both in air and underwater. Weld strength and ductility of fillet welds are examined through strength tests, which are also complemented by Vickers hardness tests and microstructure examination to better understand the weld details. The tested parameters include two welding environments, two weld orientations, two base structural types, and four base steels. Based on the tests, differences between underwater and in-air fillet welds are examined in terms of strength, ductility, and failure modes, underwater weldability of base steels is also evaluated. INTRODUCTION Many offshore steel structures, including marine platforms, trestles, pier piles, etc. have been suffering from increasing corrosion damages. These corroded steel structures are in urgent need of repairing and strengthening to retain or extend their service lives. Among all repair strategies available, underwater wet welding is one of the most common measures due to its high efficiency and relatively low cost (Coastal Development Institute of Technology 1997, Liu 2005). This technique has been adopted over years and gained popularity in offshore and maritime engineering for recent energy explorations into the sea (Wernicke and Billingham 1998). However, because of remarkable difference in welding environments, mechanical properties of welds produced underwater vary distinctly from those produced in air. Underwater welds have been studied a lot during past decades, most studies have focused on their metallurgical features (Kinugawa and Fukushima 1982, Ibarra et al. 1988), influence of quenching (Pope et al. 1996), development of new electrodes suitable for underwater wet welding (West et al. 1990), etc. Although some papers have studied mechanical properties of underwater welds (Akselsen et al. 2006, Zhang et al. 2003, Rowe et al. 2002), few studies have been carried out systematically for weld joint design. Due to a deficiency of test data, Japanese provision discounts strength of underwater welds by taking 80% of that of in-air welds irrespective of structural types, weld orientations, base steel types, and corrosion effects (Coastal Development Institute of Technology 1997). The main motivation of present study is to provide fundamental data on the strength of underwater fillet welds. In this study, forty-five fillet welded specimens are tested to failure. Weld strength, ductility, and failure modes are examined with respect to two welding environments: in-air and underwater, 1 Dept. of Civil Engineering, Nagoya University; Nagoya, Japan 2 two structural types: steel pipe and steel sheet pile, two weld orientations: transverse and longitudinal, and four base steels commonly used in offshore structures: SY295, SYW295, and corroded SY295 for steel sheet piles, and STK400 for steel pipes. SY295, SYW295, and STK400 are Japanese Industrial Standard (JIS) designations, and they are specified in JIS A5528, JIS A5523, and JIS G3444, with defined yield stresses 295 MPa, 295 MPa, and 235 MPa, respectively. Differences in mechanical properties between underwater welds and their counterpart in-air welds are investigated qualitatively as well as quantitatively. To understand weld features in detail, Vickers hardness tests and microstructure examinations are also performed to corroborate the mechanical features of underwater fillet welds. Based on experimental results, the paper ends with conclusions on weld strength and ductility in various cases. EXPERIMENT PROGRAM The main variables investigated in the fillet weld tests are welding environments, structural types, weld orientations, and base steel types. Table 1 presents the test matrix indicating different weld assemblies with their designations and parameters. Test Specimens and Material Properties Two configurations of specimens with different weld orientations are illustrated schematically in Fig. 1. In order for welds to fail before base or cover steels yield, the weld leg length is specified Table 1. Test matrix Assembly Designation No. of Specimens Structural type Welding environment Weld orientation Base steel Base thickness(mm) Weld length(mm) Cover steel 1 TYA 3 SY295 2 TWA 3 Transverse SYW295 3 LYA 3 SY295 4 LWA 3 SYW295 12.7 mm 40 mm 5 LCA 3 In air Longitudinal CSY295 6-8 mm 20 mm 6 TYW 3 SY295 7 TWW 3 Transverse SYW294 8 LYW 3 SY295 9 LWW 3 SYW295 12.7 mm 40 mm