Improved design bases of welded joints in seawater

The presented work aims to investigate and establish a precise, thorough and detailed database from series of experimental testing of submerged arc welded, SAW, specimens of various thicknesses typically applied in offshore structures and foundations. Additionally, the testing was performed in two different environments, i.e. under in-air conditions and in a corrosion environment.

Welded structures of all sizes and shapes exhibit fatigue failure primarily in the welded region, rather than in the base material, due to imperfections and flaws relating to the welding procedure. The welded region has therefore received much attention from universities, research institutions along with industry as it is of significant practical importance for all fatigue loaded structures, such as e.g. marine structures.

As-welded SAW specimens of three different thicknesses, manufactured by Lindoe Welding Technology A/S and Bladt Industries A/S, were subjected to uni-axial tension loading at relatively high R-ratios in order to simulate tensile residual stresses of yield magnitude. The main goal was to confirm the thickness effect for the specific case of large butt joints in the as-welded condition as well as to validate whether the thickness correction factor according to recommendations, codes and guidelines is too conservative when it comes to butt-welded joints.

A conservative thickness effect factor results in larger, heavier and more expensive structures. The thickness effect considers the influence of the plate thickness on the fatigue resistance of welded joints and is generally included in design rules by scaling the fatigue strength with a recommended factor.

The existing database of experiments that relate to the thickness effect is comprehensive and the effect is well proven experimentally and theoretically for various types of welded joints. However, in the case of large butt-welded joints there is room for improvement as details, quality and precise data which can influence the fatigue life of the welded joint is often lacking and severely lacking in truly thick joints.

Additionally, as-welded SAW specimens were tested in a corrosion environment with cathodic protection. The specimens were subjected to high fatigue loading at the samestress ratio as the tests performed in-air. A direct comparison to the specimens testedunder in-air conditions was performed in order to evaluate the effects of the corrosion environment on fatigue resistance.

Furthermore, novelty 25 mm thick steel laser-hybrid welded specimens in the as welded condition were subjected to experimental testing. A fatigue resistance S-Ncurve was established for the laser hybrid welded joints in addition to a more detailed analysis. The laser hybrid welded joints were thereafter compared directly to the traditional SAW specimens in order to investigate the two different welding techniques.

The laser hybrid welding technique offers great potential in lowering the cost of energy associated with offshore structures.