## 1 Introduction  
Robotic technology plays a crucial role in various industries, particularly in the automotive sector, where it aids in the manufacturing process by improving production efficiency and enhancing product quality. By executing repetitive and precise tasks in automated operations, robots reduce production costs and enhance the safety of production lines. The use of robots in laser welding has been extensively researched due to its numerous advantages. Laser welding, which involves welding from a distance, has a reduced dependence on the shape of the object, making it an advantageous method for welding complex or angular objects. Furthermore, it offers the benefit of faster execution than conventional spot-welding methods as depicted in [1].  
This study shifts the focus from the prevalent application of laser welding with robots in automobile manufacturing to the welding of LNG vessel cargo holds. The LNG cargo tank, a significant compartment found in LNG carriers, typically measures over 20 m in height, width, and depth, and may exceed 30 m in depth for specific vessels as shown in Figure 1. The inner surface of the LNG cargo tank is constructed through the welding of membrane modules, as depicted in Figure 2. The size and shape of the membranes vary based on their attachment location and specifications, and corrugations are incorporated into the membrane to accommodate temperature-induced volume fluctuations for safety during transport. Approximately 2000 membranes, each with dimensions of 3000 mm by 1000 mm, contribute to the construction of a single LNG cargo tank.  
The fabrication of an LNG cargo tank, due to its considerable size and intricacy, involves a labor-intensive process that requires much of the work to be performed manually. The production of a single LNG cargo tank can take several months to complete. Currently, the interior of the LNG cargo hold is welded together sequentially, similar to the construction of a building, using scaffolding to partition the floors and makeshift elevators to facilitate movement. The goal of this study is to increase automation by utilizing a mobile robot for laser welding throughout the entire process.  
While prior research has explored offline programming methods for laser welding on complex objects [2; 3], the application of offline programming software for mobile welding robots in the automation of expansive LNG cargo tank welding on a global scale has yet to be addressed. Ref. [4] utilized intelligent welding with robots through Virtual Reality technology, but the system relied on conventional fixed robotic systems and required frequent human intervention, rather than full automation. Similarly, ref. [5; 6] conducted research on welding automation using vision sensors, but the resulting welding precision was lower than conventional methods and the application was limited to targets within a restricted working area of fixed robotic systems.  
This research presents an autonomous laser welding approach for multi-layered structures, such as those found in LNG cargo holds, utilizing mobile robots to operate in expansive spaces. In order to achieve this goal, we have developed detailed welding procedures that can be executed by mobile robots, while also considering the limitations of complete automation. As such, the process has been divided into automated and manual components, with restrictions on the areas of the target that can be accessed. The welding process for LNG cargo holds utilizing mobile robots is programmed using the main offline programming (OLP) software, allowing for flexible application to a variety of task configurations.