Abstract: Pipe-sticking occurred in a well drilled on semi-submersible platform in Western South China Sea during running the φ244.5 mm casing at 2 546 m,35 m away from the designed depth 2 581 m.Multiple measures were tried but failed,then the casing had to be cut and casing external tieback technique was used to tieback casing so that the casing hanger could land on the subsea wellhead and besealed.The tie-back process included:cut and retrieved casing and casing hanger at 40-60 m below the mud line,dropped mill shoe to mill 0.2-0.6 m on casing scarfing,dropped the tool of external casing tieback patch and made the initial casing into casing external tieback patch until the casing hanger land on the subsea wellhead so that the outer casing tieback patch could connect with original casing;applied pressure in the casing external tieback patch tool and tested the seal;cut at 203.02 m and retrieve nine φ244.5 mm casing joints and casing hanger,milling 0.24 m on casing scarfing,running casing external tieback patch to 205.40 m,pressure test at 20.69 MPa,butted and sealed casing smoothly.Field application showed that casing external tieback would not reduce the inner diameter of the existing pipe string so ensured casing drift,the sealing life could meet the requirements of oilfield development.It was able to solve the semi-submersible platform casing problem when casing could not reach the designed position.