Classification of CT Failures over a 9-year period (a) Undetected internal corrosion, especially at the 6 o'clock position, i.e. corrosion in and around the edges of the pool that forms under gravity when fluids drip down the inside of the tubing wall. Since most tubing contains an internal unremoved weld flash, and exhibits variable ID, the passage of wiper pigs does not always remove all the fluid. Surface tension also causes a layer to stick to the inner surface, and thus in a considerable volume of fluid can trickle down to the low point. (A 20-ft diameter tube section with ID = 1.5" had 3600 in 2 of surface area. If 0.001-in. were to adhere to the ID under surface tension, than 3.6 in 3 of fluid is available.) (b) Undetected corrosion on the bed-wrap from external storage in coastal areas. The attack here tends to be from salt-bearing rainfall, and the surface tension which holds fluids between the adjacent wraps. Such areas are almost constantly refreshed with water, and remain at high humidity. (c) General wall loss from letting the tube sit in the acids commonly used in well-servicing operations, e.g. 15% HCl.
Contexts in source publication
... order to systematically approach the problem of returned failures, an in-depth study of all failures was performed, including those already completed by metallurgical labs. The results are summarized in Fig. 3 using a simple classification scheme that had to include the potential for NDE 7,8,9 . Problems with tubing have now been generally determined to include the following: were to adhere to the ID under surface tension, than 3.6 in 3 of fluid is available.) (b) Undetected corrosion on the bed-wrap from external storage in coastal areas. ...
... Elongated gouges on the outer surface caused by dragging the tubing past some type of constriction in the well or injector. Figure 3 represents our attempt to classify known problems into broad categories based upon their origin, in order to determine cause and responsibility. The major categories are further discussed below. ...
The testing of coiled oilfield tubing is considered to be an important requirement prior to and during offshore servicing. One interesting method is that of wall thickness measurement of ferromagnetic tubes using a noncontact direct current magnetic technique. This paper covers some results obtained with this for carbon steel coiled tubing.