See Section 3.1 of API-650 (specifically 3.1.3.5 and Fig 3-3B).
Section 3.1.3.5 states:
"lap-welded joints, as tack-welded, shall be lapped at least five times the nomonal thickness of the thinner plate joined; however, with double-welded lap joints, the lap need not exceed 50mm (2in), and with single-welded lap joints, the lap need not exced 25mm (1in)."
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Regards,
Richard Ay - Consultant