Definition - What does Mid weld mean?
Mid welds are performed during pipe installation on site. After one section of pipe has been successfully installed into a hole the other end is left above ground. The next section of pipe is then moved into position and carefully aligned to weld the two sections together. The weld is still properly treated and sealed as it would be in a continuous string installation.
Trenchlesspedia explains Mid weld
Traditionally, all sections of the pipe are welded before installation begins. Although it is sometimes necessary, mid welds should be avoided when possible. Mitigating factors like weight and space sometimes make a smooth pullback operation impossible. Even if mid welds are to be used, it is important to limit their number as much as possible. Stopping during the pullback operation leaves the door open for complications such as a cave-in ahead of the previously installed pipe or drill fluid thickening that will prevent the pullback from continuing.