During a real-life pipeline repair, what sequence ensures both safety and weld integrity?
Isolating pipeline, cleaning joint, aligning, tack welding, final welding
For hot work on a pipeline the governing rule is that safety is secured before any welding, and then the joint is built up in the order that guarantees a sound weld.
The first step must therefore be isolating the pipeline, draining and depressurising it, so there is no flammable content or pressure during welding.
Next the joint is cleaned to remove rust, scale and grease that would otherwise cause weld defects.
The pipe ends are then aligned to the correct fit-up so the joint geometry is right before any metal is deposited.
Tack welds are made to lock the aligned pipes in position, and only then is the final full weld carried out to complete the joint.
Any sequence that welds before isolating the line, or tacks before cleaning and aligning, risks either an explosion or a misaligned, dirty joint, so those orders are unsafe or unsound.
Hence, the answer is Isolating pipeline, cleaning joint, aligning, tack welding, final welding.
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