Definition - What does Field Joint mean?
A field joint is a place where two pipeline sections meet and are welded together.
When steel pipelines are laid down, individual small pipeline sections are placed in their respective pipeline locations (either underground or on supports) and then welded or joined together with either a single or a double joint segment, usually using butt welding. The locations on the sections where the butt welding is performed are called field joints.
Corrosionpedia explains Field Joint
Because pipelines extend for long distances there will be many field joints where the pipeline sections are joined and welded together. Proper field joint anti-corrosive liquid coatings such as epoxies, urethanes, heat shrinkable sleeves and sometimes fusion-bonded epoxy are required for the field joints.