defect sizing accuracy of serimer dasa saturnax 5-user perspective

Det Norske Veritas Presentation

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Det Norske Veritas

An Investigation on the

“Defect Sizing Accuracy”of

RD-Tech Automated Ultrasonic Testing System on“Serimer Dasa Saturnax-5”

Automatic GMAW Welding Machine-User perspective

Presented byCharlie Chin-Chan Chong

ASQC-CQE, ASNT NDT Level III (JM1715), MICorrSenAWeldI., European Welding Inspector Level II.

NACE Senior Corrosion Technologist.


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Introduction:

CNOOC China Ltd. intends to develop the Dongfang1-1 Gas Field, which is locatedin the south-west part of the South China Sea, approximately 104 km offshore fromHainan Island, with a maximum water depth of approximately 70 meters. A 22” gasexport pipeline with API 5LX56 Grade approximately 105KM will be laid. Thewelding for the first 7KM from Shore Terminal was carried out using SMAW and therest of the pipeline will be welded by Serimer Dasa “Saturnax-5” automated GMAWwelding system and NDT using RD-Tech Automated machinated UT System.

DnV-OF-S101 “DnV Offshore Standard for Submarine Pipeline System” EngineeringCritical Assessment Criteria was used as an alternative acceptance for pipelinewelding.

Objective:

As both the pipeline welding system and the automated UT systems or systemscombination were first used by the Contractor (COOEC), it is important that thedefect sizing capability be demonstrated, adequate for detecting defects in compliancewith the specified acceptance code.

The investigation was carried out in the field to confirm the system capabilities asclaimed in the published technical data sheet of the UT system.

Methods:

To investigate the defect sizing capability of the automated UT system in combinationwith the proposed automated GMAW welding machine, following criteria shall besatisfied:

1. Sensitivity of detection2. Sizing of defect locations3. Sizing of defect Heights

To investigate the above criteria, sample of the pipe nipples were welded withintentional defects, scanned with the automated UT system and size & locationreported. The test nipples were either:

1. Grind to located the defect locations2. Manual UT to located the defect locations and rough sizing.3. Section and macro etched the defect for accurate defect sizing.

Radiographic testing was earlier proposed and not used for safety reason


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Result & Discussion

Three welded samples were scanned using the proposed automated UT system.Material : API 5LX 65Wall thickness : 12.7mmProcess : GMAW Twin Torches Automatic. 4 stations 8 passes

To determine the accuracy of detecting the locations of the defect, 3 samples weremanual UT to determine the length and size of the defects. The defective areas werelater disk ground and the location of the defect were located and compared with thereported location and these were found to be accurate.


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The proposed UT system can be concluded practically able to detect the locations(length and offset from point of origin) of the detected discontinuities accurately.

Macro-etched samples were also prepared from the test samples. Macro sections werecut at 2 to 3 locations from the reported samples with signal indications in 3 channels.

As an example, the above macros were section from the affected area, which indicateda lack of fusion on the above macros. The height of the sample as predicted from thegraph will be 2-3 channels height, correspond to a discontinuity height of 4mm to5.2mm or more. The actual measured height of the discontinuity was approximately2mm height.

From the graph above, the A-scan signals that appear in the 3 channels were practicalof the same shapes and vary in signal amplitudes only (Similar figure print). Thedefect may be interpreted of the same shape or morphology. And of one origin, i.e.one single defects. This was supported by the macro samples.

Like all NDT methods, for proper interoperation it is important to understand thegroove preparation and the welding process well. For Saturnax-5, the GMAW processwas welded indoors with no wind drought and the common defect is lack of side-wallfusion and occasionally root penetration defect in the form of incomplete penetration.


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In this experiment the welds were welded by 4 stations twin touches GMAW processwith air cool copper backing. All defects were of lack of side wall fusion type.

The weld passes and profile as seen from micro specimens and as measured from sitemay de generally dimensioned as follow with standard deviation of 1.2mm both forstation-1& 3 and 1.0mm for station-2 and 0.9 for station-4 welding:


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The automatic UT system in their inspection philosophy claimed that the machine wasable to perform “Zone Discrimination” in which the weld is divided into a number ofzones and the focused transducer inspects each zone. The beams are claimed to besufficiently small that each beam effectively interrogates its zone only.

Following are zone classification adopted by the UT system for this project:

However these were not the case as evidence from the macro specimens discussedearlier. The causes of discrepancy can be due to many reasons. The manufactureradvice should be sought for further clarification.

The ECA Acceptance criteria: 22”Diameter 12.7mm Wall Thickness 85% SMYS

Location Defect Height Maximum lengthRoot & LCP 1.15mm 315mmRoot & LCP 2.3mm 135mm

HP 2.4mm 50mmFill 2.66mm 40mm

From the table, it can be deduced that the sizing accuracy capability of the system wasfound to be not adequate for effective implementation of the pipeline welding work.


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Summary:

1. The automated UT system in combination with Serimer Dasa Saturnax-5 wasable to locate the defect accurately

2. The automated UT system was able to determine the length of the defectaccurately.

3. There were discrepancies in reporting the defect height of the weld. This can leadto repair of accepted weld and miss bad weld.

4. Comparing the actual measured weld bead profiles and the ultrasonic zoning,there were mark differences, which may or may not affect the efficiency of themachine.

5. For A-scan output signals of similar shape and location in one or more channels:

a. The Weld beads height for root pass and hot passes were both 2.2mm heightincluding re-melt area. These indicate that height of defects can not exceed thisfigure.

b. The height for Fill passes (Station-2, 3 and 4) were 4 to 4.5mm withapproximately 2mm re-melt on the previous bead. Which means that forVolumetric defects, the height of the defects might reaches 4 to 4.5mm heightand for fusion type defects.2.5mm height maximum.

6. Further clarification and study on defect height sizing from the equipmentsupplier may be necessary.

defect sizing accuracy of serimer dasa saturnax 5-user perspective

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