Comparison of GWFAC for Various NDE Methods for Pipeline Installation

Dr. Kapil Mohan and Dr. Shashi B. Kumar DNV GL Singapore Pte. Ltd.

#16 Science Park Drive, Singapore 118227 E-mail:kapil.mohan@dnvgl.com, Shashi.bhushan.kumar@dnvgl.com

Abstract In order to ensure safe, reliable and optimized (cost efficient) design, installation and operation of pipelines, all relevant limit states (failure modes) should be considered and evaluated during design phase or all relevant phases and scenarios listed in design codes (e.g. DNV-OS-F101, API 1104, ASME B31.8 etc.). Unstable fracture and fatigue is considered one of the more important failure modes for design which is primarily governed by materials, welding and NDT aspects of the project specifications. For safe design, installation and operation of pipelines, welding defects need to be inspected by reliable NDT methods and scrutinized/ accepted/ rejected based on most relevant weld flaw acceptance criteria. In this paper, girth weld flaw acceptance criteria (GWFAC) for installation and operation of pipelines with various NDT methods, such as RT, UT, AUT based on applicable standards e.g. API 1104 and, DNV-OS-F101, are compared and presented for a real project pipeline. This paper also highlights the solutions for challenges involved in various NDT methods and acceptance criteria used in assessing the acceptance/ rejection of weld imperfections during pipeline-laying. The ECA-based acceptance criteria will also be presented here to cover various scenarios such as S-lay methods etc. as per DNV-OS-F101. Keywords: MUT, RT, AUT, Girth weld, Flaw acceptance criteria, Engineering critical assessment, Pipeline installation Introduction: There are typically various phases in a pipeline’s life as shown in Figure1. These phases are essential for different scenarios such as onshore/ offshore etc. or various laying method such as S-lay, J-lay, for all pipelines. Various activities take place under each phase.

Figure 1: Various phases for entire pipeline life Pipes are welded such that a complete circumferential weld joining pipe-end to pipe-end during the installation/ laying of a pipeline takes place during the construction phase. These welds, called girth welds, are usually made up of a number of weld passes beginning with the root pass or stringer bead and completed with the cap pass.

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Non-destructive testing is the testing of materials, for surface or internal flaws or metallurgical condition, without interfering in any way with the integrity of the material or its suitability for service. So NDT with relevant acceptance criteria are used to accept or reject the weld imperfections in girth welds of a pipeline during construction (at the lay barge for S-lay, J-lay installation method, at the spool base for Reel-lay installation method, alongside the Right of Way (RoW) for onshore pipeline) in order to achieve reliable pipeline installation and operation. Various NDT methods such as RT, UT and advanced NDT such as PUTR/ ToFD, AUT can be used for the same. Figure 2 provides some basic information about RT, MUT and AUT. Figure 3 presents the strip chart generated during the AUT scanning of a girth weld and an illustration of how to interpret the results.

(a) (b)

(c) Figure 2: Illustration of (a) RT [1] (b) UT [1] and (c) AUT

Figure 3: Example of AUT strip chart and evaluation of flaws RT has various advantages such as pictorial presentation, permanent record, suitability of any material however its disadvantages are inability to cope with thick sections, health hazard, need to direct the beam accurately for two-dimensional defects, necessity of film processing and viewing facilities, not suitable for surface defects, no indication of depth of a defect below the surface and not suitable for automation unless the system incorporates fluoroscopy with an image intensifier or other electronic aids. UT has various advantages such as Thickness and lengths up to 30 ft can be tested, position, size and type of defect can be determined, instant test results, portable, extremely sensitive if required, capable of being fully automated, access to only one side necessary, no consumables required. However, its disadvantages are operator dependent, no permanent record available, indications require interpretation (except for digital wall thickness gauges), considerable degree of skill necessary to obtain the fullest information from the test and very thin sections can prove difficult. Various pipeline welding codes such as DNV OS F101 [2] and API 1104 [3] and defined ‘workmanship based’ acceptance criteria for sentencing of the girths welds of pipeline exist. This paper comprises the acceptance criteria for various types of defect in the girths welds of pipelines provided by these standards using various NDT methods such as RT, UT, AUT. However, the use of workmanship based flaw acceptance criteria can lead to a higher rate of repair and increased costs, mainly arising from the delay to installation, but without any significant benefit to the integrity of the girth welds. Engineering critical assessment (ECA) is Fracture mechanics based assessment procedures commonly used to define flaw acceptance criteria for girth welds, which is an alternative to ‘workmanship based’ criteria. ECA procedure is detailed in BS 7910 [4], DNV-OS-F101 [2], ASME 31.8 [5] and API 1104 [3] etc. The ECA-based acceptance criteria is also being presented in this paper to cover one from numerous laying scenarios, such as S-lay, J-lay, Reel-lay methods etc. as per DNV-OS-F101. An example of a vessel laying a pipeline using S and J-lay are shown in Figure 4 (a) and (b) respectively.

(a) (b) (c)

Figure 4: An example of vessel laying pipeline using (a) S-lay method (b) J-lay method (c) Side Boom Method This paper also demonstrates the comparison of girth weld flaw acceptance criteria (GWFAC) using ECA and workmanship for various NDT methods such as RT, UT, AUT using standards such as API 1104 [3] and DNV OS F101 [2] with real project pipeline data under S-lay installation. Figure 5 presents a flow diagram showing the approach for ECA based Acceptance criteria for girth welds.

Figure 5: Flow diagram showing the approach for ECA based Acceptance criteria for girth welds Comparison of Flaw acceptance criteria Procedure to carry out ECA (as shown in Figure 5) is described in DNV OS F101 [2], API 1104 [3] and BS 7910 [4]. Input or design data to carry out ECA for a real project pipeline data under S-lay installation is described in Table-1.

Table 1: Input data for real pipeline for installation and operation phase: Description Value Pipe Dimension, OD x WT 16” or 406.4 mm x 15.88 mm

Design standard DNV-OS-F101: 2013

Type of Service Non-sour/sour Linepipe Material Grade API 5L X 60 (Seamless) Actual yield strength 522 MPa at RT Actual tensile strength 614 MPa at RT Maximum Design Pressure 3000 psig/20.68MPa Maximum operating Pressure 810 Psig/5.58MPa Max. Operating Temperature 195 oF/90.56 oC Min. Operating Temperature 62 oF/16.67 oC Max. Pipeline Design Temp. 200 oF/93.33°C Min. Pipeline Design Temp. 0 oC Design life 25 years

Material Tensile Properties

Installation phase: YS = 522 MPa, TS = 614 MPa ,Lr cut off = 1.176 Operation phase: YS = 499 MPa, TS = 591 MPa, Lr cut off = 1.184 de-rating of 23 MPa has been applied based on maximum operating temperature of 90.56 deg C (as per DNV-OS-F101: 2013)

Primary membrane stresses

Peak Strain Pm (MPa) 0.167% (operation) 351.6 96% installation stress (Installation) 398.4

Cycling membrane and bending stress ranges and number of cycles

Total no. of shut downs during design life:1 cycle per year (i.e. 25 for whole design life) Operation fatigue stress (188+80= 268 MPa) + 2 hydro test cycles 30% of Hoop Stress (90% SMYS) = 112 MPa

Fatigue Crack Growth Rate as per BS7910: 2005, Mean+2SD

Installation Phase: In air condition: for surface breaking flaw at ID, OD and embedded: K range 63.0 – 144.0 Nmm-3/2 : m = 5.1, A = 2.1000E-017 K range >144.0 Nmm-3/2 : m = 2.88, A = 1.2900E-012 Operation Phase: In marine with Cathodic Protection (CP-850 mV): for surface breaking flaw at OD: K range 63.0 – 290.0 Nmm-3/2 : m = 5.1, A = 2.1000E-017 K range >290. 0 Nmm-3/2 : m = 2.67, A = 2.0200E-011 In air condition: for embedded flaws K range 63.0 – 144.0 Nmm-3/2 : m = 5.1, A = 2.1000E-017 K range >144.0 Nmm-3/2 : m = 2.88, A = 1.2900E-012 In non-sour environment: Assumed 1xin-air for surface breaking flaw at ID K range 63.0 – 144.0 Nmm-3/2 : m = 5.1, A = 2.1000E-017 K range >144.0 Nmm-3/2 : m = 2.88, A = 1.2900E-012 In sour environment: assumed 50xin-air for surface breaking flaw at ID K range 0 – 144.0 Nmm-3/2 : m = 5.1, A = 1.05E-015 K range >144.0 Nmm-3/2 : m = 2.88, A = 6.45E-011

Welding process GMAW Weld misalignment (Hi-Lo) 2.0 mm

Welding Residual Stress Installation: 522 MPa, Operation: 499 MPa CTOD Fracture Toughness 0.54 mm for installation case, 0.2 mm for operation case Weld Attachment Length Weld Cap: 15.1 mm, Weld Root: 5.5 mm AUT PoD (90% PoD at 95% confidence Level) 1 mm flaw height

AUT sizing error (5% error Fractile) ±1 mm in height an d ±3 mm for length

Flaw acceptance criteria for various types of defect as described in DNV-OS- F101 for RT, UT and AUT are shown in Table-2, Table 3 and Table 4 respectively. Table 2: Acceptance criteria for radiographic testing of welds as per DNV-OS-F101:2013 [2]

Type of defect Acceptance criteria 1) 2) 3)10)11) Individual discontinuities Maximum accumulated size of in any 300 mm

weld length for each type of discontinuity Porosity1) 2) Scattered Cluster 5)

Diameter: < t/4, but max. 3 mm Individual pore: <2 mm, cluster diameter max.

See Note 4 One cluster or total length < 12 mm

Wormhole 12 mm 2 wormholes or total length < 12 mm Hollow bead Isolated 6) On-line 7)

Length: t/2, but max. 12 mm, Width: t/10, but max. 3 mm Length: t, but max. 25 mm, Width: max. 1.5 mm

Length 2 t, but max. 50 mm - Length 2 t, but max. 50 mm

Diameter: < t/4, max 3 mm Diameter: <2 mm group length: 2t, but

max. 50

mm Slag 1) 2) 3) 8) Isolated Width < 3 mm Length 12 mm, but max. 4 off separated by min Single lines Width: max 1.5 50 mm Parallel lines Individual width: max 1.5 Length 2 t, but max. 50 mm

Length 2 t, but max. 25 mm Inclusions Tungsten Copper, wire

Diameter < 0.5 t, but max. 3 mm Not permitted

Max 2 off separated by min 50 mm -

Lack of penetration 1) 2) 3) 8)

Not permitted for welds in duplex stainless steel, CRAs and clad/lined steel

-

Root Length: t, but max. 25 mm Length: 2t, but

Length t, but max. 25 mm Length 2 t, but max. 50 mm Embedded 9)

Lack of fusion1) 2) 3) 8) Not permitted for welds in duplex stainless steel, CRAs and clad/lined steel

-

Surface Embedded

Length: t, but max. 25 mm

Length t, but max. 25 mm Length 2 t, but max. 50 mm

Cracks Not permitted - Shrinkage cavities and crater pipes

Not permitted -

Root concavity Length: 2t, but max. 50 mm Length: 2 t, but max. 50 mm Root undercut Excess penetration Burn through

See Table D-6 Appendix-D, DNV-OS-F101

See Table D-4 Appendix-D, DNV-OS-F101

Total accumulation of discontinuities (excluding porosity)

— Maximum accumulation of discontinuities in any 300 mm weld length 3 t, max 100 mm. — Maximum accumulation of discontinuities: 12% of total weld length. Any accumulation of discontinuities in any cross sections of weld that may constitute a leak path or may reduce the effective weld thickness with more than t/3 is not acceptable.

Notes:

1) Refer to the additional requirements in B903 for welding methods that produce welding passes exceeding 0.25 t. 2) Volumetric imperfections separated by less than the length of the smallest defect or defect group shall be

considered as one imperfection. 3) Elongated imperfection situated in a line and separated by less than the length of the shortest defect shall be

considered as one imperfection. 4) For single layer welds: 1.5% of projected area, for multi layer welds with t < 15 mm 2% of projected area, for multi

layer welds with t ≥ 15 mm 3% of projected area. 5) Maximum 10% porosity in cluster area. 6) “Isolated” pores are separated by more than 5 times the diameter of the largest pore. 7) Pores are “In a line” if not “Isolated” and if 4 or more pores are touched by a line drawn through the outer pores

and parallel to the weld. “On-line” pores shall be checked by ultrasonic testing. If ultrasonic testing indicates a continuous defect, the criteria for lack of fusion defect shall apply.

8) Detectable imperfections are not permitted in any intersection of welds. 9) Applicable to double sided welding where the root is within the middle t/3 only. 10) Acceptance criteria of Table D-4 shall also be satisfied. Systematic imperfections that are distributed at regular distances over the length of the weld are not permitted even if the size of any single imperfection meets the requirements above.

Table 3: Acceptance criteria for manual ultrasonic testing of welds as per DNV-OS-F101 2013 [2]

Base material thickness 8 mm t < 15 mm Base material thickness 15 mm t 150 mm Max. echo amplitude Corresponding acceptable

indication length, L (mm) Max. echo amplitude

Corresponding acceptable indication length, L (mm)

Reference level (DAC) L t (but max. 8 mm) DAC + 4 dB L 0.5t (but max. 12.5 mm) DAC – 6 dB L > t (but max. 8 mm) DAC – 2 dB 0.5 t < L t (but max. 25 mm) - - DAC – 6 dB L > t (but max. 25 mm in both outer t/3)

- - DAC – 6 dB L > t (but max. 50 mm in middle t/3) Cracks are not permitted. Transverse indications: Indications shall be considered as transverse if the echo amplitude transversely exceeds the echo amplitude from the same indication longitudinally with more than 2 dB. Transverse indications are unacceptable unless proven not to be planar, in which case the acceptance criteria for longitudinal indications apply. For indications approaching the maximum permitted length: It shall be confirmed that the indication height is less than 0.2 t or maximum 3 mm (see B903 Appendix-D, DNV-OS-F101). If an embedded defect is located close to a surface, such that the ligament height is less than half the defect height, the ligament height between the defect and the surface shall be included in the defect height. Total accumulation of discontinuities: The total length of acceptable indications with echo amplitude of reference level – 6 dB and above shall not exceed 3 t, maximum 100 mm in any weld length of 300 mm nor more than 12% of total weld length. Any accumulation of defects in any cross section of weld that may constitute a leak path or reduce the effective thickness of weld more than t/3 is not acceptable. If only one side of the weld is accessible for testing 6 dB shall be subtracted from the maximum echo permitted above.

Notes:

1) Reference level is defined as the echo amplitude corresponding to the echo from the reflector in the reference blocks described in Figure 1, Figure 2 and Figure 3 of Appendix-D, DNV-OS-F101, or equivalent reflector.

2) All indications exceeding 20% of the reference level shall be investigated to the extent that the operator determines the shape, length and location of the imperfection.

3) Indications that cannot be established with certainty shall whenever possible be tested with radiography. Indications that are type determined in this way shall meet the acceptance criteria in Table D-5 Appendix-D, DNV-OS-F101.

4) Longitudinal imperfections where the echo height intermittently is below and above the acceptance level shall if possible be investigated with radiography. Indications that are determined in this way shall meet the acceptance criteria in Table D-5. If radiography cannot be performed, the length shall not exceed 3 t, maximum 100 mm in any weld length of 300 mm.

5) Length and depth shall be determined by an appropriate method, see B335 and B336 Appendix-D, DNV-OS-F101. 6) Detectable imperfections are not permitted in any intersection of welds. 7) Systematic imperfections that are distributed at regular distances over the length of the weld are not permitted even if the size of

any single imperfection meets the requirements above. Table 4: Acceptance criteria for automated ultrasonic testing of girth welds as per DNV-OS-F101 2013 [2]

Base material Defect location Acceptance criteria 1) 2) 3) 4)

Individual defect indications Maximum accumulated length of defect indications in any 300 mm weld length

C-Mn and low alloy steels 6,8)

Root Height: the lesser of weld pass height and 0.2 t, but max. 3 mm; Length: t, but max. 25 mm 5) Surface

Embedded 7) Height: the lesser of weld pass height and 0.2 t, but max. 3 mm; Length: 2t, but max. 50 mm

Cracks are not permitted. Porosity: Accumulated length of porosity signals exceeding 20% FSH amplitude in the volumetric channels: For single layer welds: 1.5% of the full circumferential length, for multi-layer welds with t < 15 mm 2% of the full circumferential length, for multi-layer welds with t 15 mm 3% of the full circumferential length. “Isolated” pores are separated by more than 5 times the diameter of the largest pore. Notes:

1) Volumetric defects separated by less than the length of the smallest defect or defect group shall be considered as one imperfection. 2) Planar defects interaction shall be assessed according to the criteria of BS 7910. Interacting planar defects shall be attributed

equivalent defect dimensions according to BS 7910, and the equivalent defects so obtained shall meet the above acceptance criteria.

3) Detectable imperfections are not permitted in any intersection of welds. 4) Systematic imperfections that are distributed at regular distances over the length of the weld are not permitted even if the size of

any single imperfection meets the requirements above. 5) Acceptance criteria of Table D-4 Appendix-D, DNV-OS-F101shall also be satisfied. 6) This table is not applicable for C-Mn and low alloy steels in sour service. 7) If an embedded defect is located close to a surface, such that the ligament height is less than half the defect height, the ligament

height between the defect and the surface shall be included in the defect height, and the defect shall be considered as a root or surface defect, as applicable.

8) POD and sizing accuracy of the AUT system shall be according to Appendix E, clause E401 Appendix-D, DNV-OS-F101.

Flaw acceptance criteria for various types of defect as described in API 1104 for RT and UT are shown in Table-5 and Table 6 respectively. No workmanship flaw acceptance criteria for AUT is available in API 1104.

Table 5: Acceptance criteria for RT of welds as per API 1104 2016 [3] Type of defect Acceptance criteria Individual discontinuities Maximum accumulated size of in any 300 mm

weld length for each type of discontinuity Surface breaking flaw as Inadequate penetration without High-low (IP), Incomplete Fusion (IF)

Length< 25 mm Length< 25 mm or 8% of the weld length

Surface breaking flaw as Inadequate penetration with High-low (IPD)

Length< 50 mm Length< 75 mm

Embedded flaw as Inadequate cross penetration (ICP)

Length< 50 mm Length< 50 mm

Embedded flaw as Incomplete Fusion due to cold Lap (IFD)

Length< 50 mm Length< 50 mm or 8% of the weld length

Table 6: Acceptance criteria for UT of welds as per API 1104 2016 [3]

Type of defect Acceptance criteria Individual discontinuities Maximum accumulated size of in any 300 mm

weld length for each type of discontinuity Linear Indication (including IP, IPD, ICP, IF, IFD, ESI, EU, IU, HB)

height < one quarter of wall thickness Length< 25 mm or 8% of the weld length

height < one quarter of wall thickness Length< 25 mm or 8% of the weld length

Linear buried indication height < one quarter of wall thickness Length< 50 mm or 8% of the weld length

height < one quarter of wall thickness Length< 50 mm or 8% of the weld length

Note: IP- Inadequate penetration without High-low, IPD- Inadequate penetration with High-low, ICP- Inadequate cross penetration, IF- Incomplete Fusion, IFD- Incomplete Fusion due to cold Lap, ESI- Elongated slag inclusions, EU- Undercutting adjacent to the cover pass, IU- Undercutting adjacent to the root pass, HB- Hollow bead porosity Comparison of workmanship flaw acceptance criteria using RT, UT and AUT Technique as per various coeds including DNV-OS-F101 and API 1104 with ECA based flaw acceptance criteria for 16” ODx15.88 mm WT CS pipeline with Sour and Non-Sour service considering surface breaking flaw at ID and embedded flaws are carried out and demonstrated in graph format in Figure 6-9. For good comparison purposes, the same flaw acceptance criteria used for flaw type or NDT Technique or code are put under the same legend in each graph for comparison purposes. The reflector size was kept as a 3mm side drill hole and DAC was established for UT procedure used in this work. ABBREVIATIONS used in graphs and texts are as per below: IP- Inadequate penetration without High-low, IPD- Inadequate penetration with High-low, ICP- Inadequate cross penetration, IF- Incomplete Fusion, IFD- Incomplete Fusion due to cold Lap, ESI- Elongated slag inclusions, EU- Undercutting adjacent to the cover pass, IU- Undercutting adjacent to the root pass, HB- Hollow bead porosity

(a) (b)

Figure 6: Comparison of flaw acceptance criteria for using RT, UT and AUT for various coeds as DNV -OS-F101 and API 1104 with ECA based flaw acceptance criteria for 16”OD x 15.88 mm WT CS pipeline Non-Sour service considering surface breaking flaw at ID (a) single flaw (b) accumulative flaws

(a) (b) Figure 7: Comparison of flaw acceptance criteria for using RT, UT and AUT for various coeds as DNV -OS-F101 and API 1104 with ECA based flaw acceptance criteria for 16”OD x 15.88 mm WT CS pipeline Sour service considering surface breaking flaw at ID (a) single flaw (b) accumulative flaws

(a) (b) Figure 8: Comparison of flaw acceptance criteria for using RT, UT and AUT for various coeds as DNV -OS-F101 and API 1104 with ECA based flaw acceptance criteria for 16” OD x15.88 mm WT CS pipeline Non-Sour service considering embedded flaws (a) single flaw (b) accumulative flaws

(a) (b) Figure 9: Comparison of flaw acceptance criteria for using RT, UT and AUT for various coeds as DNV -OS-F101 and API 1104 with ECA based flaw acceptance criteria for 16” ODx15.88 mm WT CS pipeline Sour service considering embedded flaws (a) single flaw (b) accumulative flaws Discussion: There are lots of differences observed in flaw acceptance criteria depending upon the NDT technique, type of flaw and code. Few important points from this comparison work to take note are described as below:

1. Flaw acceptance criteria found to be depending upon the version of code so latest version of published codes (DNV –OS-F101- 2013 and API 1104 -2016) are used for this work.

2. Cracks are not permitted for UT, AUT and RT as per DNV-OS-F101 however a shallow or star crack of 4 mm length is permitted for RT as per API 1104.

3. AUT flaw acceptance criteria described in DNV-OS-F101 are not applicable to Sour service as per note mentioned in code, however no separate criteria is mentioned for other NDT techniques or all NDT Techniques for other codes. The impact of sour service on flaw acceptance criteria can be significant as seen in comparing the ECA based flaw acceptance criteria developed based on fracture and fatigue assessment using relevant input parameters. The parameters not having impact due to presence of sour/ non-sour conditions are kept same in these two scenarios. It indicates that the flaw acceptance criteria defined in various codes cannot be fit for purpose for certain situation such as sour and high pressure/high temperature.

4. Most of the workmanship flaw acceptance criteria defined in various codes for non-sour service is found to be conservative compared to ECA based flaw acceptance criteria which are more justified based on fracture and fatigue assessment using relevant input parameters.

5. For shorter flaw height, such as 1 mm, ECA based acceptance can accept flaw length up to 100 mm or significantly higher compare to all the workmanship flaw acceptance criteria defined in various codes for various NDT Techniques.

6. ECA based flaw acceptance criteria based on fracture and fatigue assessment using relevant input parameters varies with stress/ strain conditions or laying method, material properties used for pipelines and environmental conditions applied for material however

workmanship flaw acceptance criteria defined in various codes are not linked with any of these parameters. Thus sometimes workmanship flaw acceptance criteria could be highly conservative however in other cases it could be non-conservative. It is difficult to judge unless some sort of assessment is carried out.

7. It has been observed that radiography testing performs well for volumetric defects/ flaws however it is not so good for critical planar defects/flaws. Furthermore concern about the radiation generated during RT affect the health of operators.

8. For Manual Ultrasonic testing, it is good for detecting both volumetric defects/ flaws as well as planar defects/ flaws however it is operator dependent, the results are non-recordable and possible to have significant errors in measurements of height and length of all types of flaws.

9. On the other hand, not only both volumetric defects/ flaws as well as planar defects/ flaws can detect by AUT but also it is not so much operator dependent. There is no health hazard for operators however it does require more competent operators and more rigorous qualification/ validation procedure such as that given in DNV-OS-F101 Appendix-E.

Conclusions: It has been observed that sometimes workmanship flaw acceptance criteria could be highly conservative however in other cases it could be non-conservative, since workmanship flaw acceptance criteria defined in various codes are not developed for specific stress/ strain conditions or laying methods, material properties used for pipelines and environmental conditions applied for material. On the other hand, ECA based flaw acceptance criteria using fracture and fatigue assessment considering relevant input parameters, is more justified for accepting or rejecting the girth weld during the laying of pipelines. ECA based flaw acceptance criteria can be developed using DNV-OS-F101, API 1104, ASME 31.8 and BS 7910. There are a few gaps in workmanship flaw acceptance criteria defined in various codes for different NDT techniques and each NDT Technique has its own advantages and limitations. Considering the advantages, AUT may be a better NDT Technique compare to others, however it requires more competent operators and more rigorous qualification/ validation procedures such as given in DNV-OS-F101 Appendix-E. References: [1] A Brief Description of NDT Techniques, A Paper By Mark Willcox & George Downes, Insight NDT, [2] DNV Offshore Standard DNV-OS-F101: 2013: “Submarine Pipeline Systems”, Det Norske Veritas. [3] API 1104, “Welding of Pipelines and Related Facilities” 21st Edition, September 2013, ERRATA-1,2 3 and 4, JULY 2014 ADDENDUM1- July 2014 and ADDENDUM2, May 2016. [4] British Standards, BS 7910: 2005, “Guide on methods of assessing the acceptability of flaws in metallic structures”, July 2005. [5] ASME 31.8, Gas Transmission and Distribution piping System (ASME code for Pressure piping, B31) -2014.