Welding of Coiled Tubing
Introduction:
Coiled tubing is a unique product, because it is designed to retain pressure during and after undergoing fatigue cycling. It often becomes necessary to place butt welds into sections of coiled tubing. These butt welds must also be capable of withstanding fatigue cycling while providing continued pressure containment.
In order to assure the weld will provide the desired result in the field, a systematic approach to making good welds repeatedly must be adhered to. This systematic approach must consider the following individual phases to the system.
Qualified Welding Procedure Qualified and Competent Welder
Tubing condition at time of welding and welding preparation Welding environment conditions
Welding techniques
Weld finishing and Inspection
This Technical bulletin will review the considerations for individual phases of a well-planned coiled tubing welding system. Because every location and situation is different, not all of the comments included will be applicable all the time or at every location.
Qualified Welding Procedure
A welding procedure is a written procedure, or welding procedure specification (WPS), which provides the work instructions to the welder on how to make a weld. To assure the procedure is capable of making a sound weld, it is tested to verify welds made will adhere to specific mechanical property and inspection quality requirements. Separate WPS’s are required for each distinct grade of coiled tubing and they may be limited in application by code requirements. Procedures are unique to the specific company or location performing welding. The practices used and consumables employed are dependent on many factors, so "one size fits all" welding procedures are impractical, if not impossible. Therefore it is
difficult to determine if a procedure is correct just by reading it. Typical procedures reported in the literature or WPS’s for specific grades supplied by material suppliers may provide guides when preparing procedures.
The decisions leading to selection of welding variables which have produced acceptable results in WPS’s are reviewed in Appendix A
New or revised welding procedures are tested, mechanically and non destructively, to prove they can result in a sound weld. The results of these tests are recorded on a document supporting the procedure, called the Procedure Qualification Record (PQR). Coiled tubing undergoes physical distortion in the plastic regime, while retaining pressure, during normal operations. Most industry design and welding specifications or standards do not encounter this level of deformation in welded joints. Consequently, the level of
qualification and inspection of each weld made in coiled tubing must be, by necessity, in excess of most accepted national standards organizations, like ASME and API,
requirements and acceptance criteria. Liquid penetrant, radiography and or ultrasonic inspection are imposed on the as welded joint to identify any potential problems before mechanical testing is started. Non-destructive acceptance criteria are established at the limit of detection for the method being employed; due to the limited critical flaw size for plastically deforming coiled tubing. Transverse tensile testing requires failure in the parent material in addition to the strength requirement to assure a weak point will not be
introduced to the tubing. To assure the procedure does not create detrimental phases to achieve the strength requirement the weld is sectioned, metalographically inspected and the hardness in the weld, heat affected zone and base metal are verified.
Qualified and Competent Welder
Welders must demonstrate competency by passing a welder performance qualification test, before they can perform production welds. This test verifies they are capable of following the written WPS procedure and producing the same results obtained while testing the WPS. Before taking the qualification test, welders are trained in the specific skills required to weld coiled tubing. Experience has shown many welders qualified in other welding disciplines, including pipe fitters, may not make good coiled tubing welders. A welder qualification record defined by a specification like ASME Section IX is used to document the welding and testing. Welders qualifying to weld coiled tubing are tested more stringently than code requirements. Both non-destructive examination and mechanical testing are employed; and the weld, heat affected zone and base metal hardness are verified. Once qualified, a welder must maintain proficiency. Because of the unique skills and techniques required to weld coiled tubing, abstinence from practicing these skills can lead to unacceptable performance. Most coiled tubing welders maintain proficiency by regularly making welds on coiled tubing and logging their activity. When activity is slow, proficiency may be maintained by making practice welds on tubing ends, scrap tubing or samples procured from the manufacturer. When a welder does not make welds for an extended period, or there is reason to question his proficiency, he should be re-qualified. Welders making welds routinely inspected by radiography, need not re-qualify on periodic
timetables. All these welds are non-destructively tested to requirements in excess of code qualification requirements. This combined with monitoring field performance can assure welder proficiency is maintained.
Tubing condition at time of welding and welding preparation
Coiled tubing is new on the day it is made. From that point on, it begins undergoing physical, dimensional and mechanical property changes each time it is spooled. Coiled tubing strings undergo strain softening due to repeated application of stress in the plastic region during coiling operations. Used tubing exhibits strength properties lower than those of the new tubing. Studies have shown that welding procedures developed for new tubing can supply adequate mechanical properties on used tubing.
Welds in used tubing lead to additional challenges. Tubing can become deformed, typically oval in cross section, as a result of spooling. This presents alignment problems during fit up. If mismatch is allowed it could generate potential stress risers in service. Used tubing can contain residues of produced well fluids, injected fluids, inclusive of acids or water trapped in surface oxides or pits. Tubing which contained wireline or other galvanized coated material may be zinc coated. Any of these could contaminate the weld if not
properly removed before welding. Used tubing can be magnetized, requiring demagnetizing or magnetic countermeasures to be employed to prevent magnetic arc deflection during welding. Trained and proficient welders recognize these potential problems and take preventive action to preclude their becoming a cause for a weld imperfection.
Preparation for welding is vital to good finished welds and coiled tubing performance. The tubing must be aligned, so the weld will pass through injectors and over guide arch without imparting unnecessary stress to the tubing. The internal and external surfaces must be clean to prevent contamination of the weld. The tubing seam weld, internal flash must be removed to prevent it acting as a localized heat sink during welding and prevent
entrainment of the gas or oxides in the weld. Both the internal cleaning and flash removal must be done without leaving circumferential grinding marks that could result in stress risers. External chill blocks must be applied to the tube before welding to remove heat from the base metal. If the excess heat were not removed properly, the heat-affected zone could experience excessive grain growth and loss of mechanical properties.
Welding Environment
Welds can be made at the factory or in service centers in nearly ideal environments. Welds made at service company’s district yards; at field camps; outside operations and on site operations can present far less than ideal welding environments. Welds made in non-ideal environments may be the need to have a safety factor or mechanical property derating applied to the tubing. This derating should be considered independently from fatigue derating (if any) and only to the section as defined by coiled tubing management program containing the butt weld. The magnitude of any derating is the responsibility of the coiled tubing user. An example might be to treat QT-800 and QT-1000 butt weld mechanical properties as equivalent to QT-700 properties. The butt welds made in less than ideal environments, should be removed at the earliest opportunity and replaced by a butt weld made in a near ideal environment. The replacement weld should be capable of carrying all intended loads of the parent tubing.
Weld finishing and Inspection
After welding, the weld crown required to prevent weld bead cracking must be removed to allow the tubing to pass through the injector blocks. Removal is done carefully with tools,
which do not leave circumferential scars to a diameter several thousandths of an inch above the tubing wall. This is adequate to pass through the injectors while not risking material loss in the surrounding tubing wall.
After cooling to ambient temperature, the weld is dimensionally inspected to assure it meets all tubing requirements for diameter and ovality. The weld is liquid penetrant inspected, with special attention to the weld centerline and fusion lines, where defects detectable by this method are likely to occur. The weld, heat affected zone and base metal are hardness checked to assure no hard or excessively soft areas are present. In shops using multiple filler metal grades, this can provide a check that the correct one was used. Radiography is then used to volumetrically inspect the weld and surrounding area. The acceptance criteria are established at the limit of detection for the smallest visible hole or wire listed in the ASME code for the thickness being radiographed. Welds passing the inspection acceptance criteria are released for use and their location in the coiled tubing string documented in the string management records, allowing monitoring of the weld during coiled tubing operations.
CTES, L. C., Slimhole and Coiled Tubing Standards Phase 1 – Weld Technology, Gas Research Institute Drilling and Completion Group Final Report GRI-95.0500.1
Van Arnam, W. D. & Smith, D. Good Tubing Welds, Properly Managed, Do Not Break, SPE Paper 60694
Appendix A
Review Of Welding Parameters For Coiled Tubing
There are a number of manual GTAW welding procedures, documented in the literature, as being successful in welding coiled tubing. The following is a discussion of the individual welding variables considered important to preparation of a welding procedure.
Welding Joint
Several joint designs have been used on coiled tubing. They include double "V" grooves, with and without a land, "J" and "U" bevels and occasionally the square butt joint. Diagrams of these bevels and the terminology used in bevel design are shown in figure 1.
"V" grooves employ bevel angles to allow introduction of filler metals and distribute welding stresses developed due to thermal contraction of the weld bead. Included angles in the bevel can vary from 30° to as much as 90° , dependent on local needs. Studies on piping components have shown that 37 ½° ± 2 ½° per side or 75° included angles distribute these stresses and minimize distortion in tubing welds effectively. "V" grooves may have
featheredges of 1/16" to 1/8" root faces or lands on the root of the bevel. Featheredges are easier to prepare by hand grinding, which may be the only option in some locations.
Consistent lands are best prepared by weld beveling tools, but can be made by experience hand grinding operator.
"J" and "U" bevels must be prepared by weld beveling machines. They are used to reduce the amount of filler metal required in a weld groove. In coiled tubing welding of thicker and
higher strength materials, the dilution of too much filler metal can have an adverse influence on the mechanical properties of the welded joint.
Square bevels have been used for relatively thin wall coiled tubing and are normally welded autogenously (without filler metal additions). These joints are no longer recommended for welding coiled tubing. These close fit joints have no root spacing and the base metal provides the metal for the weldment. Weld penetration to fuse the inner root must be carefully controlled and may be limited by the maximum welding amperage. Without filler metal the bead can be concave, which can in turn, lead to weld cracking.
Weld joint preparation requires removal of internal flash, if present, from the weld area. If left in place, the flash could crate an artificial heat sink or source for weld contamination, effecting welding characteristics and quality. Joint preparation must leave the weld joint within tubing tolerances while not creating any local stress risers, such as visible to the naked eye, circumferential grinding marks, which could effect finished fatigue and mechanical properties.
Base Metals
Coiled tubing grades are predominately made from high strength low alloy strip, like coiling operations. This can significantly increases the tendency for coiled tubing to ASTM A606 Gr4 (Mod.). Tubing manufacture forms the tubing, seam anneals, stress relieves and spools this material, making each grade of coiled tubing a unique base material. This generally leads to the requirement that each separate grade of coiled tubing requires separate procedure and qualification.
Much welding is performed on tubing that has been used. Provisions must be made to handle residual bending, ovality, diameter and wall thickness differences in fitting up the tubing for welding. Contingency plans for handling contaminated, corroded or magnetized tubing should be in place as well.
Filler Metals:
There are no known filler metals with chemistry and welded mechanical property results matching coiled tubing grades. Filler metals must be selected from the available
commercial grades primarily designed for welding carbon and low alloy steels.
GENERALLY, AWS A5.18 ER70S-2 or ER70S-6 filler metals are used for grades up to and including QT-900. Though listed as having comparable mechanical properties to coiled tubing grades, many ER80X-X and ER90X-X grades contain chromium or other elements are not recommended. These filler metals are hardenable during air-cooling from welding temperatures and require preheat and or post weld heat treatment to control hardness and cracking. These thermal treatments are capable of severely reducing the strength of the surrounding coiled tubing base metal. AWS A5.28 ER120-S-1 is normally used for joining QT-1000.
In most cases the filler metal is "under matched " to the base metal. That means it either contains fewer alloying elements, which will not develop the same as welded strength as the material being welded. To compensate for the under match, dilution of the base metal chemistry and reproduction of the grain size are important to the finished weldment
under matched filler metal large enough to create a weak zone in an other wise uniform, strong coiled tubing string.
Filler metal sizes or GTAW rods are normally 1/32" to 3/32", based on the thickness of the bead and welder control required. 1/16" diameter filler metal is most often used.
Positions
Most procedures are developed in the ASME Section IX defined 5G position with the tubing horizontal. This requires the weld bead to start at the bottom of the weld joint and progress upward. Welds made in the factory, service centers base camps and even field locations are made in this position. Occasionally there are requirements for welds to be mad in the 2G position with the pipe vertical. In ASME Section IX qualifications, both welding
procedures and welders qualified in the 5G position qualify to weld in the 2G position, but not vice versa. Field experience has indicated the same is true for coiled tubing
qualifications. There are instances in operations where welding in the 6G position or between 15° of vertical and 15° of horizontal. If the tubing cannot be positioned within 15° of vertical or horizontal, the codes require separate qualifications. If performed, the
qualification in 6G position, then qualifies the procedure or welder for the 5G and 2G position also.
Preheat
Coiled tubing is made from high strength low alloy steels designed to be welded without preheat to prevent thermal cracking. Experience has shown preheating existing coiled tubing grades can reduce mechanical properties of the heat-affected zone. In fact, the use of chill blocks to remove heat and preserve properties is well established in welding all grades of coiled tubing. To assure mechanical properties in thicker walls, requiring multi-pass welds; it may be necessary to provide auxiliary cooling to the chill blocks between passes. There have been no reported incidences of weld cracking due to lack of preheat. Heat may be applied to drive off absorbed surface moisture from sources including over night condensation or contaminates carried by used tubing. When this is done, it is advisable to cool the joint to approach ambient temperature before making the weld. Historically carbon equivalent equations have been used to predict the need for preheat in carbon and low alloy weldments. The chemistries of the higher strength grades of coiled tubing suggest preheat should be employed. The carbon equivalent equations were developed to include much thicker welds and highly restrained weld joints. Today’s coiled tubing wall thickness and weld fixturing allow more latitude. Coiled tubing grades, primarily due to their high strength low alloy, fine grain structure; appear more tolerant to welding without preheat then their low alloy counterparts with comparable carbon equivalents. Post Weld Heat Treatment
Existing procedures do not utilize post weld heat treatment. Welding procedures developed have shown the mechanical properties and base material hardness requirements can be meet without thermal treatment after welding.
The shielding gas will usually be either welding grade Argon or 75% Helium, 25% Argon. It is important that the gas be welding grade or better to assure it is not contaminated with elements such as oxygen. Since the shielding gas protects the tungsten electrode and weld puddle from high temperature oxidation, the purity is important.
The selection of welding gas may first be dictated by availability, particularly in remote locations. Argon tends to have a cooler welding arc with less penetrating power than
helium. Mixed gases of argon and helium will tend to have hotter arcs and more penetrating power as the concentration of helium increases. A second benefit is helium, being lighter than air, tends to rise from the torch flow, while argon, being heavier than air, tends to fall. Mixed gases entering the inside diameter of the tubing are believed to provide better overall protection of the root when backing gases are not used.
Gas flow rates are important to provide continuous shielding of the weld pool. The minimum flow rate must protect the weld pool, while overcoming disruptive influences of arc heating or cross drafts. If the flow rate is too high, the gas flow can become turbulent and inspirited air with its oxygen to the weld pool. To allow higher flow rates, gas lenses are used in the torch nozzle to keep flow laminar at higher flow rates. Flow rates should be measured with a flow meter calibrated for the molecular weight of the shielding gas. The difference in molecular weight between helium and argon means the flow rates required to provide equivalent protection for helium are normally higher than for argon.
An important step in assuring the effectiveness of shielding gas is the protection of the welding area form strong cross drafts. When welding in exposed areas, it is often
necessary to employ barriers to wind flow. Area or cooling fans should be turned off during the time actual welding is performed. The welding shielding cannot compromise the safety of the welder and the quality of his breathing air.
Electrical characteristics
The GTAW process uses direct current, straight polarity in the welding arc. This makes the tungsten electrode negative in the circuit.
The tungsten electrode is normally 3/32" diameter 2% Throated Tungsten. The electrode is ground to a point with a controlled included angle, typically 60 degrees. The tip is then truncated, with a small flat place on the end of the point to prevent the tip melting off into the weld puddle.
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