Development of Underwater Laser Wire-Filling Welding Device and Welding Maintenance Technology for Offshore Oil and Gas Pipeline
Authors: Zhu Jialei , Jiao Xiangdong , Wang Kai , Gu Yanhong , Cai Zhihai 
Published: November 2020

Abstract
In view of the problems of underwater wet laser welding, such as power attenuation, poor surface forming and narrow adaptability of welding water depth, the defects and causes of wet laser welding are expounded. The local dry underwater laser wire filling welding process is proposed to improve the quality of weld and increase the applicable of water depth. In order to verify the feasibility of the local dry underwater welding process, the quality and characteristics of the welds are compared and analyzed. The experimental results show that the local dry method can effectively increase the water depth and improve the welding quality, and the main mechanical properties of the weld are equivalent to that in the air welding.
With the increasing development of offshore oil and gas resources, there are more and more construction and maintenance of offshore oil and gas equipment. Underwater welding is one important part of underwater maintenance technology for offshore engineering equipments. At present, underwater welding uses mainly arc welding. With the increase of water depth, the environment pressure of underwater arc welding gradually increases, the reliability of arc welding gradually decreases, however, the welding quality and stability cannot be guaranteed [1]. As a new emerging technology of automatic welding maintenance, underwater laser welding is becoming a widely concerned underwater repair technology for offshore engineering. This is due to its advantages such as good accessibility of welding position, high welding quality and little impact of environmental pressure. Compared with the conventional underwater arc welding, the underwater laser welding has the advantages of little influence of environmental pressure, good accessibility of welding position, wide range of welding materials, high welding quality and low residual stress [2,3]. At present, there are two underwater laser welding methods: wet method and local dry method. The cost of wet underwater laser welding is low. This method is flexible, because the welding process is simple, and there is no need to add drainage device or protection device. However, due to the special underwater welding environment, there are complex factors such as high welding pressure, rapid cooling of weldments and high hydrogen content in welds. Therefore, it is difficult to obtain high-quality welds by wet welding [4]. Therefore, the local dry drainage device is designed and used to drain the water from the specimen surface in this experiment, and the underwater laser welding is carried out at the same time.
In order to study the quality and performance of underwater laser welding, the weld quality of wet welding and local dry underwater welding are compared and analyzed. Specifically, the macro morphology, microstructure, hardness, mechanical properties and fracture morphology, and the effects of water depth and local drying environment on weld quality are studied.