A new type local dry underwater flux-cored arc welding system based on a min-cap was developed. This paper describes the details about the design of the mini-cap underwater welding system. Underwater welding based on the mini-cap is carried out using a variety of selfshielded and gas-shielded flux-cored wires in a pressure cabin. Microstructure, hardness, diffusible hydrogen content and toughness of the mini-cap underwater welds have been studied. The results show that the weld quality carried out by this novel system is better than the quality of wet welding. To guarantee the underwater welding penetration, an online penetration control system was also developed. This system uses a laser stripe to detect the weld width and can recognize the welding penetration based on a support vector machine (SVM) model. It then controls the welding current so as to adjust the welding penetration. Because the size of the mini-cap is very small, the underwater welding system is suitable to most conditions of offshore structure building and underwater repairing.
Underwater welding has been one of the most important repair techniques for offshore structure fabrication. As the number of offshore oil and gas structures grows, and those in existence are continuously exposed to fatigue, corrosion and accidental damage, the demand for underwater structural repair increases (Grubbs and Reynolds, 1998). Underwater welding techniques can be divided into three types: wet welding, dry hyperbaric welding and local dry welding (Labanowski, 2008). Wet welding is carried out directly at ambient water pressure with the welder/diver in the water and without any physical barrier between water and welding arc. Dry hyperbaric welding is done at ambient pressure in a custom built chamber from which the water has been displaced with air or other gas mixture, depending on water depth (Shi et al, 2006).
