Corrosion testing apparatus and methods

A system for sulfide stress cracking testing comprises an enclosed testing chamber including a fluid bath comprising a liquid saturated with hydrogen sulfide gas. In addition, the system comprises a test fixture disposed in the testing chamber and at least partially submerged in the fluid bath. The test fixture includes a housing having an internal chamber in fluid communication with the fluid bath and a test assembly disposed in the internal chamber. The test assembly comprises a first upper support and a second upper support, a first lower support and a second lower support, and a first platen engaging each of the upper supports and adapted to transfer an applied vertical load to the upper supports. Further, the system comprises a test specimen mounted in the test assembly between the upper supports and the lower supports.

Not applicable.

BACKGROUND

1. Field of the Invention

The invention relates generally to corrosion testing. More particularly, the invention relates to corrosion testing welded connections. Still more particularly, the present invention relates to testing welded steel joints for resistance to sulfide stress cracking.

2. Background of the Technology

Sulfide stress cracking (SSC) is a form of corrosive hydrogen embrittlement that can lead to weakening, fracturing, and cracking of susceptible metal alloys such as steel. This condition is called SSC because it requires the combination of both stress and hydrogen sulfide acting together on the susceptible metal alloy. Specifically, the metal alloy reacts with hydrogen sulfide (H2S) to form metal sulfides and atomic hydrogen as corrosion byproducts. The atomic hydrogen product combines to form hydrogen gas (H2) at the surface of the metal or diffuses into the metal matrix.

SSC has particular importance in the gas and oil industry since the materials being processed (e.g., natural gas and crude oil) often contain considerable amount of hydrogen sulfide. Specifically, exposure to hydrogen sulfide and associated SSC can cause catastrophic failure in otherwise high integrity steel.

To mitigate this problem, standardized testing procedures were developed by the National Association of Corrosion Engineers (NACE) and others. For instance, equipment that comes in contact with hydrogen sulfide gas can be rated for sour service with adherence to NACE MR0175 and NACE TM0177 for oil and gas production environments or NACE MR0103 for oil and gas refining environments. These standardized tests provide assurance that a given steel grade (and accompanying processing parameters) would be safe for use in hydrogen sulfide rich environments up to a particular stress level. A typical test includes subjecting a test sample or specimen to a high tensile load in a liquid saturated with hydrogen sulfide gas for 30 days. In general, a test sample is considered to pass the test if the sample survived the 30 day test without fracturing or showing visible cracking.

In the oil and gas industry, many types of steel tubulars designed for subsurface use (e.g., drill pipe) are welded together with friction-type welds. The area immediately surrounding each weld (approximately 0.50-0.75 inches laterally to either side of the weld) is now being required by some drillers to be demonstrated to be safe from SSC in service. Consequently, the integrity of weld areas of tubulars subjected hydrogen sulfide gas is now of principle concern in the oil and gas industry. Conventional test procedures and standards outlined by NACE do not adequately address or cover such friction-type welds. For example, NACE document TM0177 is the authoritative guideline providing specifications for SSC testing methods, and outlines specifications for several types of test fixtures as well as other parameters for carrying out SSC testing of steel. However, NACE document TM0177 does not specifically address SSC testing of welds. Further, NACE document MR0175 is the authoritative guideline for the use of various steel alloys and fillet welds in sour environments (i.e., hydrogen sulfide rich environments), but addresses only fillet-type and butt welds. Fillet and butt welds are sufficiently different from friction-type welds that the guidelines in NACE document MR0175 are generally not extended to friction-type welds.

Accordingly, there remains a need in the art for apparatus and methods for testing the durability of friction welds between steel components subjected to stress in hydrogen sulfide rich environments. Such testing apparatus and methods would be particularly well-received if they were relatively easy to implement, repeatable and reuseable, and accurately reflected conditions encountered in field.

BRIEF SUMMARY OF THE DISCLOSURE

These and other needs in the art are addressed in one embodiment by a system for sulfide stress cracking testing. In an embodiment, the system comprises an enclosed testing chamber including a fluid bath comprising a liquid saturated with hydrogen sulfide gas. In addition, the system comprises a test fixture disposed in the testing chamber and at least partially submerged in the fluid bath. The test fixture includes a housing having an internal chamber in fluid communication with the fluid bath and a test assembly disposed in the internal chamber. The test assembly comprises a first upper support and a second upper support, a first lower support and a second lower support, and a first platen engaging each of the upper supports and adapted to transfer an applied vertical load to the upper supports. Further, the system comprises a test specimen mounted in the test assembly between the upper supports and the lower supports. The upper supports engaging an upper surface of the test specimen and the lower supports engaging a lower surface of the test specimen. The test specimen has a longitudinal axis, a first end, a second end opposite the first end, and includes a weld and a heat affected zone axially disposed between the first end and the second end. The first upper support is axially positioned between the weld and the first end and the second upper support is axially positioned between the weld and the second end. The first lower support is axially positioned between the first upper support and the first end and the second lower support is axially positioned between the second upper support and the second end.

These and other needs in the art are addressed in another embodiment by a method for corrosion testing a weld. In an embodiment, the method comprises (a) providing a test specimen having a longitudinal axis, a first end, a second end opposite the first end, and a weld axially positioned between the first end and the second. In addition, the method comprises (b) mounting the test specimen between a pair of upper supports and a pair of lower supports. Further, the method comprises (c) subjecting the test specimen to a four point bending test with the upper supports and the lower supports to induce tensile stress in the specimen along a lower surface of the specimen during. Still further, the method comprises (d) exposing the weld to hydrogen sulfide gas during (c).

Thus, embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, systems, and methods. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings.

DETAILED DESCRIPTION OF SOME OF THE PREFERRED EMBODIMENTS

In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices, components, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to an axis (e.g., longitudinal axis of a body or a port), and the terms “radial” and “radially” generally mean perpendicular to the axis. The terms “lateral” and “laterally” generally mean to the side of another feature or object.

Referring now toFIG. 1, an embodiment of a test specimen or sample10including a weld20for SSC testing is shown. Sample10has an elongate body11with a central or longitudinal axis15, a first end11a, and a second end11bopposite first end11a. In addition, body11has a planar upper surface12extending between ends11a, b, a planar lower surface13parallel to upper surface12and extending between ends11a, b, planar end surfaces16,17extending vertically between upper and lower surfaces12,13at ends11a, b, respectively, and planar side surfaces18,19extending vertically between upper and lower surfaces12,13. Body11has a length L measured axially between ends11a, b, a thickness T measured perpendicularly from upper surface12to lower surface13, and a width W measured perpendicularly from front surface18to rear surface19. In this embodiment, body11has the general shape of an elongate rectangular bar since length L is greater than width W, and length L is greater than thickness T. For purposes of the four point bending tests described in more detail below, length L is preferably at least 20 times thickness T.

Sample10is formed from a first component21axially abutting and welded end-to-end to a second component22with weld20. In this embodiment, weld20is a friction weld. However, in general, other types of welded connections and joints may be tested in accordance with the principles described herein.

Components21,22, and hence sample10, are made from a material for which SSC testing is desired. Thus, for SSC testing of welds in steel, components21,22will comprise pieces of steel that are welded together. In general, heat from the welding process and subsequent re-cooling alters the microstructure and properties of the base material immediately surrounding the weld, often referred to as the heat affected zone (HAZ). Thus, sample10includes a heat affected zone23immediately surrounding weld20. Heat affected zone23extends along the entire length of weld20(i.e., between side surfaces18,19) and extends perpendicularly from weld20to heat affect zone boundaries23a, bpositioned axially (relative to axis15) between weld20and each end11a, b, respectively. For most welds (e.g., weld20), the heat affected zone (e.g., heat affected zone17) extends about 0.5 to 0.75 inches to either side of the weld. Thus, each boundary23a, bwill typically be positioned about 0.5 to 0.75 inches from weld20.

Together, weld20and heat affected zone23define an area of interest25in sample10to be SSC tested. As will be described in more detail below, embodiments of testing apparatus and fixtures described herein are employed to simultaneously subject area of interest25to stress and hydrogen sulfide gas to test its resistance to SSC. Results from such tests may be used to grade and/or qualify weld20and the associated area of interest25for use in sour environments (i.e., environments rich in hydrogen sulfide gas).

Referring now toFIG. 2, sample10is schematically shown being subjected to a four point bending test via a testing assembly30. Testing assembly30includes a pair of upper force transfer members or supports31a, b, a pair of lower force transfer members or supports32a, b, and a force or press platen35. Sample10is mounted between upper supports31a, band lower supports32a, b. Upper supports31a, bextend across upper surface12between surfaces18,19and are oriented parallel to weld20and perpendicular to axis15in top view. In particular, supports31a, bare evenly axially spaced (relative to axis15) to either side of weld20by a distance A measured perpendicularly from weld20. Supports31a, bare preferably positioned at or proximal heat affected zone boundaries23a, b. Thus, distance A is preferably equal to or within 10% of the distance measured perpendicularly from weld20to each heat affected zone boundary23a, b.

Lower supports32a, bextend across lower surface13between front and rear surfaces18,19and are oriented parallel to weld20and upper supports31a, b. Lower supports32a, bare evenly axially spaced (relative to axis15) to either side of weld20by a distance B measured perpendicularly from weld20. Distance B is greater than distance A previously described, and thus, lower supports32a, bmay be described as being positioned “outside” supports31a, brelative to weld20. In this embodiment, lower supports32a, bare positioned proximal ends11a, b, respectively. The difference between distance A and distance B defines a distance C equal to the distance measured axially (relative to axis15) from each upper force transfer member31a, bto its corresponding lower force transfer member32a, bon the same side of weld20. Each force transfer member31a,31b,32a,32bis configured and arranged to contact sample10along a line. Specifically, in this embodiment, each force transfer member31a,31b,32a,32bis an elongate cylinder that spans the entire width W of sample10and is oriented parallel to weld20.

Referring now toFIGS. 2 and 3, planar surfaces36,50are configured to apply loads to sample10via supports31a,31b,32a,32bto generate stresses within sample10. In general, purely vertical forces are preferred for four point bending tests. Thus, in this embodiment, a vertical downward load F is applied to platen35. In addition, load F is centered lengthwise and widthwise relative to platen35. Further, load F is axially centered relative to upper supports31a,band lower supports32a, b, laterally centered between sample sides18,19, vertically aligned with weld20. In other words, a projection of load F passes through weld20and is laterally centered between sides18,19. Such particular positioning and orientation of load F is preferred as it does not result in the generation of any rotational torques or moments on platen35or sample10.

Load F is transferred through platen35to upper surface12of sample10via upper supports31a, b. Due to the orientation of applied load F relative to the two supports31a, b, each force transfer member31a, bapplies one-half of load F to sample10. The total load F is transferred through sample10to lower supports32a, band surface50. However, since lower surface50is rigid and non-deformable, it exerts an equal and opposite reactive upward vertical load F that is shared and divided between lower supports32a, b, and applied to lower surface13of sample10. Thus, when vertical downward load F is applied to platen35, each upper support31a, bapplies one-half of load F to sample10, and each lower support32a, bapplies one-half of load F to sample10as shown inFIG. 3. Due to the positioning of supports31a, band location of application of load F to platen35, sample10is subjected to static conditions (i.e., sample10does not experience any moments, torques, or acceleration). Further, since upper supports31a, bare positioned between lower supports32a, b, the loads applied to sample10by supports31a,31b,32a,32bseeks to bend or urge ends11a, bupward relative to area of interest25, and bend or urge area of interest25downward relative to ends11a, b. As a result, stresses arise within sample10.

The stresses induced by the four point bending test shown inFIG. 2and associated loads shown inFIG. 3include compressive stress parallel to axis15in the upper portion of sample10, and tensile stress parallel to axis15in the lower portion of sample10. The compressive stresses induce compressive strain in the upper portion of sample10, and the tensile stresses induce tensile strain in the lower portion of sample10. Without being limited by this or any particular theory, the compressive stress in sample10decreases moving perpendicularly downward from upper surface12, and the tensile stress in sample10decreases linearly moving perpendicular upward from lower surface13. In particular, the compressive stress in sample10decreases to zero at a “neutral plane” parallel to and positioned between surfaces12,13, and the tensile stress in sample10decreases to zero at the neutral plane. Thus, the compressive stress and associated strain in sample10are maximum at upper surface12, and the tensile stress an associated strain in sample10are maximum at lower surface13. The maximum tensile stress in sample10during the four point bending test shown inFIGS. 2 and 3can be calculated according to equation 1 described in more detail below.

For purposes of SSC testing, the combination of tensile stress and exposure to hydrogen sulfide gas presents the most common failure mode to friction welds, and thus, the tensile stress and strain at lower surface13of sample10in area of interest25are of primary concern and interest. As shown inFIG. 4, the tensile stress in sample10at lower surface13is constant and at a maximum between supports31a, b(i.e., in area of interest25), and tapers off linearly to zero moving axially (relative to axis15) from support30ato support31aand moving axially from support30bto support31b. Without being limited by this or any particular theory, and as is known in the art, the maximum tensile stress induced sample10at lower surface13between supports31a, bby the four point bending test shown inFIG. 2may be calculated as follows:

ST=3⁢CFLT2(equation⁢⁢1)
where:ST=the maximum tensile stress in sample10at lower surface13(i.e., between upper supports31a, b);C=the distance C between each outer transfer member32a, band the nearest inner transfer member31a, b;F=the load F applied to the force plate (e.g., platen35);L=the length L of sample10; andT=the thickness T of sample10.
Thus, for a given test apparatus (e.g., assembly30), once distance C, specimen length L, and specimen thickness T are established, a specific tensile stress STmay be induced in sample10at lower surface13by simply adjusting the applied load F.

Referring now toFIG. 5, an embodiment of a testing apparatus100for SSC testing a sample with a weld (e.g., sample10previously described) is shown. Apparatus100includes a testing chamber110and a test fixture120disposed in testing chamber110. In this embodiment, testing chamber110comprises a generally box-shaped base111having an enclosed bottom111aand an open top111b, and a removable lid112that closes off and seals top111b. Lid112is removed from base111to position fixture120within testing chamber110. In this embodiment, lid112includes a vent113and a valve114that controls fluid flow through vent113. When valve114is open, vent113allows fluid communication between the inside and outside of testing chamber110.

Testing chamber110is partially filled with a testing liquid116to a liquid level117, thereby defining a fluid bath115within which test fixture120is partially disposed. Then, with lid112closing off open top111bof base111, hydrogen sulfide gas118is pumped from a gas tank119through a valve119ainto fluid bath115. Hydrogen sulfide gas118bubbles through liquid116and fills the portion of testing chamber110between liquid level117and lid112. A portion of the hydrogen sulfide gas118in testing chamber110diffuses into and completely saturates liquid116. As desired, valve114may be opened to bleed remove some of the hydrogen sulfide gas118from testing chamber110through vent113. Otherwise, testing chamber110is generally maintained at ambient temperature and pressure.

The composition of liquid116is preferably selected to be the same or very similar to the downhole liquids expected to contact the welds for which the test is being conducted. Thus, the composition of liquid116may be varied for different tests. For example, to SSC test steel welds for use in offshore environments, liquid116is preferably sea water or synthetic sea water. Examples of suitable compositions for liquid116include, without limitation, an acidified and buffered aqueous brine solution (e.g., 5.0 wt % sodium chloride and 0.5 wt % glacial acetic acid dissolved in distilled or deionized water; 5.0 wt % sodium chloride and 2.5 wt % glacial acetic acid and 0.41 wt % sodium acetate dissolved in distilled or deionized water) and a buffered aqueous brine solution with a chloride content (e.g., distilled or deionized water containing 0.5 g/L sodium acetate and chloride). In addition, to facilitate the sulfide stress cracking phenomenon, liquid116preferably has an acidic pH between 2.2 and 6.0.

Referring now toFIGS. 5 and 6, fixture120comprises a housing130and four point bending test assembly30previously described disposed within housing130. In particular, housing130includes a through passage131extending horizontally through housing130and defining an internal chamber132within housing130. Since passage131extends completely through housing130, chamber132is in fluid communication with liquid116and hydrogen sulfide gas118in bath115. Chamber132includes a lower portion133defined by vertical parallel walls133aand an upper portion134defined by vertical parallel walls134aextending vertically from lower portion133. Lower portion134is wider than upper portion134. As will be described in more detail below, test assembly30is disposed in chamber lower portion133.

A load screw135is threaded into bore134and has a central axis136, a first or upper end135aexternal housing130, and a second or lower end135bextending into chamber upper portion134. In this embodiment, upper end135acomprises a head137having a textured (e.g., knurled) outer surface and lower end135bcomprises a cylindrical tip138. The textured outer surface of head137enhances frictional engagement and gripping of upper end135aby a tool (e.g., wrench). Load screw135is rotated and vertically advanced into and out of bore134by applying rotational torque to screw135at upper end135avia head137.

In general, housing130may be made of any suitable material(s), but preferably comprises a durable, rigid material capable of withstanding the loads applied by load screw135to test assembly30, which may exceed 2,000 lbs. Further, since housing130is partially submerged in liquid116, which is saturated with hydrogen sulfide gas118, housing130is preferably made from a low alloy steel that is SSC resistant. In this exemplary embodiment, housing130has a cylindrical outer surface with a diameter of about 7.0 inches, and has a wall thickness of about 1.5 inches as measured between the outer and inner surfaces of housing130.

Test assembly30is disposed within chamber lower portion133and includes platen35, upper supports31a, b, and lower supports32a, bas previously described with reference toFIG. 2. Test specimen10is mounted between upper supports31a, band lower supports32a, bas previously described. The lower surface of chamber132is planar and supports lower supports32a, bin the same manner as surface50previously described.

Referring still toFIGS. 5 and 6, in this embodiment, fixture120also includes a thrust bearing140, an upper platen150, and a load cell160arranged in a vertical stack between screw135and test assembly30. As will be described in more detail below, bearing140, platen150, and load cell160transfer vertical load F applied by load screw135to lower platen35of test assembly30. As previously described, application of load F to lower platen35subjects sample10to a four point bending test and induces internal stresses in sample10(e.g., tensile stress in sample10at lower surface13). To minimize and/or eliminate the application of any rotational moments to sample10, upper portion134, lower portion133, thrust bearing140, upper platen150, load cell160, lower platen35, and sample10are configured, sized, and positioned such that each is centered relative to load screw135within housing130. In other words, a projection of load screw axis136passes vertically through the center of bearing140, upper platen150, load cell160, lower platen35, and sample10in top view.

As best shown inFIG. 6, in this embodiment, sample10is centered within housing130with a centering assembly170including an elongate alignment member171and an alignment plate172. Alignment member171is a rectangular beam that has a height H171less than height H of sample10and a length equal to length L of sample10. Alignment member171is placed between upper supports31a, band lower supports32a, band bears against side surface19of sample10. Alignment plate172is used to urge alignment member171and sample10through internal chamber132until sample10is centered within fixture120and housing130. Specifically, alignment plate172has a width W171selected such that sample10is centered within fixture120and housing130when alignment plate172comes into contact with the back of housing130. Once sample10is centered, alignment plate172may be withdrawn from housing130and alignment member171may be removed from chamber132.

Referring again toFIGS. 5 and 6, lower end135bof load screw135bears against thrust bearing140. In this embodiment, the upper surface of thrust bearing140includes a cylindrical recess141that slidingly receives cylindrical tip138of screw135. Tip138has an outer diameter that is substantially the same or slightly less than the diameter of recess141, thereby restricting and/or preventing thrust bearing140from pivoting or moving translationally relative to tip138and load screw135. Such mating engagement of screw tip138and bearing recess141helps maintain the vertical alignment of loading screw135relative to thrust bearing140, thereby reducing the likelihood of generating rotational moments that could unevenly load test assembly30. Further, in this embodiment, thrust bearing140and upper platen150are disposed within chamber upper portion134and slidingly engage vertical internal walls134adefining chamber upper portion134. Thus, as screw tip137engages and is rotated relative to thrust bearing140about axis136, walls134asimultaneously prevent thrust bearing140and upper platen150from rotating along with screw135and guide the vertical movement of bearing140and upper platen150within upper portion134.

Load cell160is positioned between platens35,150, and transfers and measures vertical loads therebetween. An electrical conductor161couples load cell160to an output device162that displays the vertical force measured by load cell160. In general, load cell160may comprise any suitable load cell capable of measuring the applied linear loads. Load cell160is preferably positioned above fluid level117so that it is not harmed by the corrosive fluids in bath111.

In this embodiment, lower platen35has an upper surface37including a recess38centered relative to screw axis136, vertically aligned with weld20, and centered between supports31a, b. Load cell160is seated in recess38, which aligns load cell160within fixture120and provides an opening for wire161to exit fixture120in route to output device162. Load cell160has an outer diameter that is substantially the same or slightly less than the width of recess38, thereby restricting and/or preventing lower platen35from pivoting or rotating relative to load cell160. Such mating engagement of load cell160and platen recess38helps maintain the vertical alignment of load cell160relative to lower platen35, thereby reducing the likelihood of generating rotational moments that could unevenly load test assembly30. Further, in this embodiment, lower platen35slidingly engage vertical internal walls133adefining chamber lower portion133. Thus, walls132aguide the vertical movement of lower platen35.

To apply load F to platen35for SSC testing of weld20and area of interest25of sample10, load screw135is rotated and advanced through housing bore134and into engagement with thrust bearing140. With screw tip138seated in bearing recess141, continued rotation and advancement of screw135applies a vertically downward load F on thrust bearing140. It should be appreciated that application of load F by rotation of screw135allows for smooth, controlled application and variation of load F. Rotation of screw135is achieved by application of rotational torque to head137, which may performed with a hand wrench. Thrust bearing140transfers load F to platen140, which transfers load F through load cell160to lower platen35and testing assembly30. Thus, in this embodiment, two platens35,150are employed to transfer vertical load F to testing assembly30.

Apparatus100includes several features that offer the potential to maintain purely vertical loads on sample10during application of load F, thereby enabling uniform, consistent application of forces to sample10, and minimizing and/or eliminating the application of rotational moments to sample10. Such features include the vertical alignment of screw135, thrust bearing140, platens150,35, load cell160and testing assembly30; the mating engagement of tip138and bearing recess141; the mating engagement of load cell160with platen recess38; the sliding engagement of bearing140and upper platen150with housing walls134a; the sliding engagement of lower platen35with housing walls133a; and the centering of bearing140, platens35,150, load cell160, and sample10relative to screw axis136and fixture120.

In the manner described, vertical load F is applied to testing assembly30to place sample10in a four point bending test and induce internal stresses in sample10. During application of load F, load cell160and output device162enable real time measurement and monitoring of the actual value of load F and the ongoing SSC test to alert the operator of a failure (specimen cracking or fracture). In addition, load cell160enables accurate, precise control of the load F and associated stress induced in the sample (e.g., sample10) during SSC testing with apparatus100.

As previously described, the particular load F necessary to achieve a desired stress in sample10at lower surface13may be calculated. Depending on the desired stress and corresponding load F (necessary to achieve the desired stress), screw135may be smoothly and controllably rotated in a first direction to increase load F and rotated in a second direction opposite the first direction to decrease load F. Thus, fixture120enables controlled application of load F and inducement of stress to sample10. Further, load F and associated stresses induced in sample10can be maintained constant in a particular region of sample10(e.g., area of interest25and weld20) for an extended period of time.

In some conventional bent-beam tests, the induced stress is calculated based on the measurements of sample bending or deflection. Consequently, the samples used in such tests are typically thin (e.g., 0.062 inches thick) in order to exhibit a sufficiently large deflection that can be measured accurately. However, inclusion of load cell160enables simple calculation of the induced stress without the need to accurately measure deflection or bending, thereby eliminating the need for thin specimen. Without being limited by this or any particular theory, as compared to thin testing samples, thicker testing samples more accurately reflect the behavior of welds in downhole equipment used in the field.

During application of load F, test assembly30and sample10are positioned below fluid level117, and thus, are exposed to liquid116and hydrogen sulfide gas118. Thus, sample10, weld20, and area of interest25are simultaneously subjected to hydrogen sulfide gas118and stress for SSC testing. In general, sample10may be SSC tested with apparatus100for any desired period of time. However, consistent with other standardized SSC testing standards, sample10, weld20, and area of interest25are preferably tested for a period of 30 days.

In this embodiment, apparatus100does not include any strain gages mounted to sample10, however, in other embodiments, one or more electronic strain gages are affixed to the sample (e.g., sample10) to measure and monitor stress induced in the sample.

As previously shown and described, testing sample10is a rectangular bar having orthogonal, planar surfaces. However, in the field, the downhole steel tubulars subjected to stress and hydrogen sulfide gas have a cylindrical geometries. Thus, an SSC test specimen or sample that includes a cylindrical surface offers the potential to more accurately reflect the effects of SSC on downhole tubulars and associated welds.

Referring now toFIG. 7, an embodiment of a test specimen or sample210that offers the potential to more accurately reflects tubular weld performance in the field is shown. InFIG. 7, sample210is shown upside down to highlight the features on the lower surface of sample210.FIG. 8illustrates the preferred orientation of sample210mounted between upper and lower supports31a, b,32a, b, respectively, of test assembly30previously described, andFIG. 9illustrates sample210being subjected to SSC testing with test apparatus100previously described.

As shown inFIG. 7, sample210has an elongate body211with a central or longitudinal axis215, a first end211aand a second end211bopposite first end211a. In addition, body211has a planar upper surface212extending between ends211a, b, a lower surface213extending between ends211a,2b, planar end surfaces216,217extending vertically between upper and lower surfaces212,213at ends211a, b, respectively, and lateral or side surfaces218,219, respectively, extending between upper and lower surfaces212,213.

Similar to sample10previously described, sample210is formed from a first component221axially abutting and welded end-to-end to a second component222with a friction weld220. Components221,222, and hence sample210, are made from a material for which weld SSC testing is desired (e.g., steel). A heat affected zone223extends the length of friction weld220and immediately surrounds friction weld220. Heat affected zone boundaries223a, bdefine the extent to which heat affected zone223extends from weld220. Together, friction weld220and heat affected zone223define an area of interest225in sample210to be SSC tested.

Unlike sample10previously described, lower surface213and side surfaces218,219of sample210are not planar. Specifically, in this embodiment, lower surface213includes a first lateral or outer section213a, a second lateral or outer section213b, and an intermediate section213cpositioned between sections213a, b. Each section213a, b, cextends axially (relative to axis215) between ends211a, b. In addition, first lateral section213aextends between intermediate section213cand side surface218, and second lateral section213bextends between intermediate section213cand side surface219. In this embodiment, each lateral section213a, bof lower surface213is planar, however, intermediate section213cof lower surface213is arcuate. In particular, intermediate section213cis concave and cylindrical. In this embodiment, intermediate section213chas a constant radius of curvature. In this embodiment, the radius of intermediate section213cof lower surface213is sufficiently large that it has little to no impact on the maximum tensile stress calculations. In other words, even though sample210does not have a uniform thickness (due to the curvature of intermediate section213c), equation 1 previously discussed may still be used to calculate the maximum tensile stress induced in sample210at lower surface213. Each side surface218,219extends axially (relative to axis215) between ends211a, b. In addition, side surfaces218,219extends between upper surface212and lower surface sections213a, b, respectively. In this embodiment, each side surface218,219is convex.

Side sections213a, bof lower surface213are flattened to reduce the likelihood of stress concentrations when sample210is mounted in testing assembly30previously described and lower supports32a, bbear against sections213a, b. In addition, the intersection of each side section213a, bwith intermediate section213cis rounded or radiused to reduce stress concentrations, and the intersection of each side section213a, bwith its corresponding side surface218,219, respectively, is rounded or radiused to reduce stress concentrations. Rounding the intersections between each side section213a, bwith its corresponding side surface218,219, respectively, also limits hydrogen access and diffusion to a single surface as opposed to two distinct intersecting surfaces.

Referring now toFIGS. 8 and 9, sample210is mounted in test assembly30and SSC tested in apparatus100in the same manner as sample10previously described. Namely, sample210is mounted between upper supports31a, band lower force supports32a, b. Sample alignment assembly170previously described may be used to center sample210relative to screw135and housing130. Supports31a, bextend across upper surface212between surfaces218,219. Upper surface212is planar, so each support31a, bcontinuously contacts surface212between side surfaces218,218. In addition, supports31a, bare oriented parallel to friction weld220and are evenly spaced to either side of weld220by distance A measured perpendicularly from weld220. Distance A is equal to or within 10% of the distance measured perpendicularly from weld220to the boundary of heat affected zone223. Thus, supports31a, bare positioned to extend along heat affected zone boundaries223a, b, respectively.

Lower supports32a, bextend across lower surface213between side surfaces218,219and are oriented parallel to friction weld220. Supports32a, bengage planar lateral sections213a, bof lower surface213, but do not contact intermediate cylindrical section213csince it is recessed relative to surfaces213a, b. Supports32a, bare evenly spaced to either side of weld220by lateral distance B measured perpendicularly from weld220. Distance B is greater than distance A.

Supports31a, bapply forces to sample210along upper surface212, and lower supports32a, bapply forces to sample210along lower surface sections213a, b. In this embodiment, the primary focus of the SSC test is area of interest225along the curved, cylindrical intermediate section213cof lower surface213.

During application of a load F applied by load screw135, test assembly30and sample210are positioned below fluid level117, and thus, are exposed to liquid116and hydrogen sulfide gas118. Thus, sample210, weld220, and area of interest225are simultaneously subjected to hydrogen sulfide gas118and stress for SSC testing. In general, sample210may be SSC tested with apparatus100for any desired period of time. However, consistent with other standardized SSC testing standards, sample210, weld220, and area of interest225are preferably tested for a period of 30 days.

In the manner described, embodiments of testing apparatus100provide a system for use in SSC 30-day corrosion testing of steel welds (e.g., friction welds). Such testing assures the steel welds can survive under a prescribed stress in a liquid environment with hydrogen sulfide gas exposure for a duration of at least 30 days. In addition, embodiments of apparatus100provide a relatively simple, low cost, easy to use system for frequent and/or repeated testing of welds and associated heat affected zones.

While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims.