Pad-Weld Repairs of in-Service High-Strength Steel Plate used in Seawater Environments

Abstract

In seawater environments, naval platform hulls made from high-strength steel plate may be susceptible to pitting corrosion. This form of corrosion can occur as a result of inadequate cathodic protection, coating degradation and the presence of sulphate-reducing bacteria in seawater. Generally, there are two options available when in-service steel plate succumbs to major pitting corrosion. The first option, which forms the basis of this report, is to repair the plate by pad welding. Important considerations for undertaking successful pad-weld repairs include both repair-area dimensions and the weld procedures used. An alternative option to pad-weld repair is to completely or partially replace the plate, which requires careful evaluation of the potential risks to the integrity of the platform. This alternative option is not discussed in detail in the current report. An investigation was undertaken to determine possible limitations on the extent and frequency of pad-weld repairs in order to avoid introducing any adverse effects on the structural integrity of naval platforms. Relevant welding-quality standards, plate-delivery standards, and the general literature were reviewed in order to develop guidelines to ensure that the structural integrity of naval platforms was maintained after pad-weld repairs were carried out. Discussions with experienced welding engineers indicated that the solutions to this investigation could be extracted from the relevant plate-delivery standards. However, it was determined that the maximum allowable repair area (~2–5% of the plate area) in plate-delivery standards is based on economic rationale and customer expectations, which are significantly different from the pad-weld repair of a plate in an existing naval platform. It was concluded that the ultimate decision on the maximum allowable repair area and number of repairs should be based on whether the size or number of repair welds will compromise the mechanical properties of the steel plate and the dimensional stability of the hull. The overarching structural integrity (e.g., residual strength or buckling strength) of the platform could be compromised if the dimensional tolerances of the platform after pad welding were out-of-specification, or if the quality of the pad-weld repairs is in doubt. Pad welding undertaken in accordance with qualified weld procedures (and with qualified welders) will ensure that the mechanical properties of pad welds are within specification. Additionally, the through-life management of pad welds in critical locations should ensure that the integrity of the platform is maintained through-life. This process would mitigate the risk of propagation of hydrogen-assisted cold cracks, stress-corrosion cracks or corrosion fatigue cracks. The likelihood of such defects generally increases as hardness/yield strength increases or when there is cathodic ‘over’ protection. Moreover, it is important to avoid regions of high hardness (or yield strength) in pad-weld repairs. This can be achieved by ensuring that the minimum repair area does not result in the deposition of short (< 75 mm) weld beads.