Quiz- Annex A Alternative Acceptance Standards for Girth Welds- CWI Part C- 10 Questions
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Annex A: Alternative Acceptance Standards for Girth Welds
A.1 General
The acceptance criteria for welds in Section 9 are based on the length of defects and are typically conservative. Annex A offers an alternative way to determine acceptance criteria by means of fracture mechanics analysis and fitness for purpose considerations (alternatively called engineering critical assessment for ECA). These alternative acceptance criteria permit larger imperfections but require additional testing. These alternative criteria apply only under the following conditions:
(a) Circumferential (girth) welds between pipes of equal specified wall thickness.
(b) NDT performed for essentially all welds.
(c) No gross weld strength undermatching.
(d) Maximum axial design stress no greater than the SMYS.
(e) Maximum axial design strain no greater than 0.5%.
(f) Welds in pump and compressor stations, repair welds, fittings, and valves in the mainline are excluded.
A.2 Stress Analysis
In order to use Annex A, a stress analysis must be performed to determine the maximum axial design stress anticipated during construction, installation, and operation. This analysis must include consideration of potential dynamic loads on girth welds, such as loads from the closure of check valves. Paragraph A2.2.2 addresses environmental effects on fatigue, such as CO2 and H2S. Paragraph A2.3 addresses sustained-load cracking and include stress corrosion cracking, failures that have occurred, and reference to NACE MR01 75.
A.3 Welding Procedure
Qualification of welding procedures to be used with this annex shall be in accordance with Section 5 or 1 2, with the additional mechanical property testing in accordance with A.3.4. The essential variables for welding procedures to be used with this annex, however, are very different and have more restricted qualification ranges than those listed in Section 5 or 1 2. They are listed below:
(a) A change in the welding process, mode of arc transfer, or method of application.
(b) A change in the grade, source/mill, raw material processing facility, pipe manufacturing facility, pipe manufacturing process, or compositional limits, including any significant increase in the carbon equivalent.
(c) A major change in joint design.
(d) A change in position from rolled to fixed, or vice versa, or from vertical to horizontal, or vice versa.
(e) A change in the specified qualified wall thickness of more than +/- 0.1 25 inches.
(f) A change in the size, type, heat/lot number, or manufacturer of the filler metal and flux.
(g) An increase in the time between the completion of the root bead and the start of the second bead.
(h) A change in welding progression (from uphill to downhill or vice versa).
(i) A change from one shielding gas or mixture to another.
(j) A change in the nominal qualified flow rate of more than +/-1 0%.
(k) A change in the shielding flux, including a change in the manufacturer.
(l) A change in the type of current (AC or DC) or polarity.
(m) A change in the preheat temperature requirements.
(n) A decrease in the inter pass temperature or an increase of 45 °F or more in the inter pass temperature.
(o) A change in the post-weld heat treatment requirements.
(p) An increase in the specified pipe OD of more than 50% from that qualified or a decrease in the specified pipe OD of more than 25% from that qualified.
(q) A change in the heat input of more than +/-1 0% from that qualified for each pass.
Paragraph A.3.3 provides conditions under which multiple pipe sources may be qualified.
Paragraph A.3.4 lists the mechanical property tests required for procedure qualification. The tension test specimen geometry shown in Figure A.1 is the same as that required in Figure 4 b) in Section 5. The acceptance criteria for tension tests are similar to that in Section 5, except that failures in the base metal may be acceptable if the observed tensile strength is no less than 95 % of the SMTS of the pipe material and additional test requirements are met.
Charpy V-notch impact testing of both the weld metal and the heat-affected zone is also required. Six specimens are required from each of the 1 2, 6, and 3 or 9 o’clock positions, for a total of 1 8 specimens per procedure. For each location in the weldment, three specimens shall have the notch located in the coarse-grained heat-affected zone and three shall have the notch located on the weld centerline. All specimens shall be tested at the lowest design temperature. The average absorbed energy for each set of three specimens shall be no less than 30 ft-lbs and the minimum absorbed energy for each set of three specimens shall be no less than 22 ft-lbs. These criteria apply to both full-sized and subsidized specimens.
Fracture toughness testing of the weld and heat-affected zone in accordance with BS EN ISO 1 5653 is required as well. The location and orientation of the crack-tip opening displacement (CTOD) specimens required for this test are shown in Figures A.3 and A.4 on page 88. Both the weld and the heat-affected zone must be tested. Two specimens are required from each of the 1 2, 6, and 3 or 9 o’clock positions, for a total of six specimens per procedure. For each location in the weldment, fatigue pre cracks shall be located in the center of the weld and in the coarse-grained heat-affected zone. All specimens shall be tested at the lowest design temperature in accordance with BS EN ISO 1 5653. The qualification criteria in paragraph 1 2.4 of BS EN ISO 1 5653 shall be met. The minimum CTOD value of all six specimens must be greater than 0.002 inches to use this annex.
A.4 Qualification of Welders
Welders must be qualified in accordance with Section 6. For mechanized welding, the welding operators must be qualified in accordance with subsection 1 2.6.
A.5 Inspection and Acceptable Limits
To locate imperfections, whether planar or rounded, use inspection methods capable of determining an imperfection’s length, height, and depth. This will typically require the use of ultrasonic testing. Regardless of the NDT method employed, its accuracy must first be established (see paragraph 1 1 .4.4).
Paragraph A.5.1 .2 addresses three options for an engineer to determine the maximum acceptable planar imperfection size. Option 1 is described in paragraph A.5.1 .3 along with an example to show the interaction of pipe diameter and wall thickness, CTOD, axial strain, and calculation allowance for inspection error.
Planar imperfections have sharp ends that can easily propagate to failure in the presence of transverse tensile stress, particularly if the stress is cyclic in nature. These flaws are the most critical type in any pressurized pipe. Paragraph A.5.1 .6 states that the height of imperfections that are indicative of stacked weld bead starts and stops shall not exceed the lesser of ¼ inch or 50% of the wall thickness.
Volumetric imperfections, addressed in paragraph A.5.2, are not as critical as planar imperfections because they do not have sharp ends. Buried volumetric imperfections, such as slag or porosity, contained in high-toughness material are less likely to cause a catastrophic failure than are planar imperfections. These buried volumetric imperfections can be treated conservatively (like they are planar and more dangerous) or by the simplified method of Table A.4 on page 99. In this table, limits are given for the height or width and length, for both porosity and slag, for pipe of a given wall thickness, t.
Paragraph A.5.3 addresses arc burns, which typically result from inadvertent arc strikes or improper grounding. The acceptance criteria for unrepaired arc burns are given in Table A.5 on page 1 00. Arc burns that contain cracks visible to the eye or on conventional radiographs are not covered by this annex and shall be repaired or removed.
When multiple imperfections exist in close proximity, they may behave like one. Figure A.1 1 on page 101 provides criteria for determining whether one imperfection will interact with another to create a more serious condition. If a repair is indicated, any interacting imperfections shall be repaired in accordance with A.7.
A.6 Record
The type, location, and dimensions of all accepted imperfections must be recorded. This information must be stored with radiographs or other pipeline inspection records.
A.7 Repairs
Imperfections that violate the rules of this annex shall be repaired or removed according to Sections 9 and 1 0.
A.8 Nomenclature
Subsection A.8 defines the terms and variables used in this annex.