NDT of coiled tubing has become increasingly popular in the past 3 years. This paper outlines the progress made in setting a standard for coiled tubing inspection. It outlines the common coiled tubing service problems, the equipment available for both electromagnetic full body inspection and all of the prove-up techniques for seam welds, bias welds, and butt welds.
Progress in the development of an API Standard for "Care, Maintenance and Inspection" of coiled tubing, including the difficulties in setting derating criteria are discussed.
Coiled tubing (CT) and coiled line pipe (CLP) are both defined as carbon steel API products made by the high frequency induction process. Short seamless products joined end-to-end are not so classed. API coiled tubing falls under the OCTG committee of API, while CLP falls under the Line Pipe committee.
CT grades follow a 10kpsi increasing sequence from 60 ksi in initial yield strength.
CLP grades follow API 5LCP, which was based on 41st-ed of API 5L (Line Pipe), and carry their yield and tensile strength designations. They thus have increasing importance, because the are cycled at most 2–3 times, while CT is cycled to failure or withdrawn from service at some theoretical value of life consumed in bending on the CT rig.
Both are made from ASTM A606-607 steels. Some lower CT grades (YS < 80 kpsi) can be classed as CLP grades.
API has one document covering CT. This is API RP 5C7, which is more of a user document than a manufacturer document. RP 5C7 does contain information required of the manufacturer, should a purchaser wish to set a standard which requires tubing to be manufactured to this document. Customers, however, continue to order CT by generic names.
A major problem with setting specifications for CT concerns the wall thickness, and more importantly, the remaining wall thickness at the root of removed flaws. Two purchasers have now set specifications for remaining wall at flaw removal locations.
Coiled Line Pipe (CLP), however, is different. CLP is an API product covered under API 5LCP. (Specification for Coiled Line Pipe). For this document, all API criteria must be met on each order. In supplying CLP to the N Sea, Charpy impact tests in the strip weld, the seam forged "weld", and any pipe-to-pipe welds must be taken.
Skelp is not traditionally inspected. Some skelp manufacturers claim "visual inspection", but electromagnetic and/or ultrasonic inspections are needed to detect certain minimum-sized flaws in skelp.
Skelp is joined end to end. The skelp-weld is inspected by radiography, and more recently by ultrasound. Radiography, as is well known, is relatively useless on tight planar flaws, while shear wave UT is excellent for this. SWUT at 5 MHz with an entry angle of 60 deg to the weld is ideal.
Milling of strip involves bending it into a cylinder and forging the edges while hot. Inspection has been by eddy current (ET), but almost all OCTG mills now use ultrasound.
