Butt welding of coiled tubing performed routinely in both new and used tubing extends life or increases application versatility of existing strings. Welds are commonly made to replace overly fatigued sections, remove mechanical damage, extend string length, and attach special application strings or tools to work reels and repair tubing imperfections. Properly designed and executed butt welds provide the same load bearing properties of the surrounding tubing. The weld integrity is verified by non-destructive testing, insuring sound joints. The fatigue properties of the weld must be understood and properly managed. This has resulted in an extremely successful record for butt welds placed in coiled tubing for field application.
The paper documents the steps required to insure the best weld quality is being placed into a string of coiled tubing. Data on properties of welds from manual and machine welds, in both new and used tubing verify the load carrying capability of the welds. The down rating of fatigue life and potential corrosion implications are handled through continuous string management. The field experience of butt welds performed in both factory and field environments are reviewed. Results indicate the clear cost effectiveness of currently available, properly installed butt welds combined with systematic monitoring by the relevant service companies.
The success of a weld in coiled tubing is dependent on a number of preconditions including the welding procedure, the welder's ability and proficiency, inspection, and field management. The potential to make a less than acceptable weld exists in every weld made. Each of these components is like a link in the chain, it is only as good as the weakest link and all the links must be used. Placing proper assurances in place at one tubing manufacturer has produced 500 welds in with only limited problems in the field. Each of these problems can be traced to one or more specific missing links in the chain of assurances.
Coiled tubing undergoes physical distortion in the plastic regime during normal operations. Most industry design specifications or standards do not encounter this level of deformation in welded joins. Consequently, the level of qualification and inspection of each weld made in coiled tubing must be, by necessity, in excess of most codes and standards.
Butt welds are placed in coiled tubing strings for various reasons. Operating conditions and the need to optimize the investment in coiled tubing are the essential drivers in most butt welds.
Economic advantage of adding a short section to an existing string opposed to procuring a complete second string.
Butt welds are used to attach down hole devices and fittings to the coiled tubing.
Designed velocity or siphon strings are attached to the end of work reels for transport to the field.
When lifting capacity on offshore rigs is restricted, the tubing may be lifted in two or more sections, which can be welded together on sight.
Sections of coiled tubing can be rejoined after separation to perform shallow, high-pressure operations, where friction loss induced pressure drops are critical.
Sections of flow-line strings are joined together in the field to form continuous pipelines and umbilicals
Sections of tubing may be welded together after removal of segments containing mechanical damage, high cycled areas, or manufacturing or field operations related imperfections.