## 2 Design of Robotic Laser Welding Process for LNG Cargo
### Overview of LNG Cargo Hold Welding Procedure Using Mobile Robot  
The automation of the entire LNG cargo-hold laser welding process is currently hindered by technological limitations. To overcome this, we have devised a robot-based laser welding procedure that is divided into segments that can be automated and those that require human intervention. The LNG cargo holds typically consist of 7-8 layers, and inter-floor movement is facilitated by remote control of robots navigating elevators. Even in traditional manual operations, a heavy-duty elevator with a capacity of approximately 1 ton is installed for equipment transfer. In the working environment of LNG cargo bays, various cables, including laser wires, power cables, and water tubes for chillers, required for laser welding, are pre-positioned on each floor and connected to the robot using reels. Upon reaching the target floor, the robot connects the necessary cables using reels and then moves to its initial position. To address the challenges associated with the mobility of the robot, a one-degree-of-freedom scanner of [13] was attached to the robot end effector to reduce positional errors in confined spaces. The welding procedure for the robot operation was established as follows:  
1. The mobile robot moved to the initial position of the target layer. To minimize possible positional errors, markers, such as QR codes, were pre-installed on the membrane.
2. In a stationary state, the robot identifies weldable seams on the membrane considering the robot's working area. The pre-installed information about the membrane is stored in a database (DB) format and includes the path information for the left, right, bottom, and top lines of the membrane.
3. The welding sequence for the identified weldable seams was determined by considering factors such as overlapping areas between membranes, constraints such as one-time operations considering the characteristics of welding, and other limiting conditions.
4. The robot moves to the next pre-planned stopping position, and Steps 2 and 3 are repeated. The stopping positions for the robot movement are preset, considering the robot's working area and safety margins. Precision markers were installed at stopping positions.  
Once the task for a specific layer has been completed, the robot will move to the next layer. This movement was facilitated manually. Initially, the thick power cables and coolant lines attached to the robot were disconnected. Subsequently, the robot arm was remotely folded using a control system, allowing the robot to move through a relatively narrow elevator. An elevator was then utilized to transport the robot to the next floor, where the robot cable bundle was reinstalled. The subsequent steps were identical to those described in Step 1.  
As previously stated, an advanced software program has been developed to streamline the welding process for LNG cargo bays. This software employs robots in the work area equipped with laser welding capabilities and offers real-time monitoring and control of the work status. Furthermore, it utilizes a pre-configured Membrane DB to generate precise robot movements.