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Abstract Unpredictable coiled tubing (CT) service life is not acceptable in the CT industry and is not tolerated by the customer. Experience shows that service life becomes unpredictable without adequate tubing maintenance programs that include corrosion prevention. Therefore, monitoring and maximizing CT service life requires effective corrosion control on both inside and outside CT surfaces from the time the tubing string is milled until it is retired. Corrosive degradation of CT can result from contact with the atmospheric environment, pumped fluids, and production fluids. Corrosion, especially localized corrosion, must be prevented because it can greatly affect tubing life by initiating premature fatigue cracking and growth during cycling. Additionally, corrosion can reduce usable tubing strength and pressure integrity. This paper discusses effective preservation and inhibition programs instituted at the tubing mill, service centers, and in the field. In addition, electrochemical corrosion-rate data for as-milled and cycled CT is presented. Linear-polarization resistance and Tafel-curve generation were used to derive general corrosion-rate data for new and cycled CT-90 and CT-100 in various common oilfield fluids, including a stimulation fluid. These tests suggest that CT corrosion tendency is not significantly accelerated as a result of cycling, except at high temperatures. Additionally, the paper presents high-pressure fatigue data from tests performed on CT-90, comparing fatigue life in water and in inhibited 15% hydrochloric acid (HCl). Introduction Standard grades of CT (defined in API Specification 5LCP and covered in API Recommended Practice 5C7) are manufactured from low-carbon steels with limited alloy content. The alloy content generally does not increase the tubing's resistance to typical corrosion that occurs in oilfield environments. Although the tubing grades are considered weathering steels because they contain copper and other elements, these alloys do not significantly increase resistance to aqueous corrosion or other forms of corrosion typical of in oilfield operations. Corrosion can begin the day the CT is milled and spooled unless a suitable corrosion-inhibition program is implemented. If the tubing temperature reaches the dew point and moisture condenses on the tubing, rusting can initiate during the tubing's first night of existence. In this situation, hydrated iron oxides, Fe(OH)2 and Fe(OH)3, are formed. During a drying period, the hydrated oxides will lose water and form hematite (Fe2O3) and magnetite (Fe3O4). Additionally, the presence of contaminants, such as chlorides, sulfates, or carbon dioxide (CO2) will increase corrosion. Almost every environment to which the CT is exposed can be a potential source of corrosion. Corrosion can be encouraged by the atmosphere, produced fluids, injected fluids, and water used to flush the tubing after a job. However, CT can be protected from significant corrosion if proper maintenance procedures are followed. Examples of CT Corrosion-Related Failures The service life of a CT string can be greatly affected by corrosion. Many operators know that corrosion is the root cause of a significant portion of premature tubing-string failures. In 1999, service companies reported1 that 24 to 51% of all failures were caused by corrosion.
Abstract 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. Introduction 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.
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