Defects in Linepipe Welds and Pipeline Girth Welds

Linepipe is either made by welding plate into a cylinder shape, or by producing a seamless tube, the former being the most popular for larger diameter pipelines. This weld is either longitudinally orientated or is a spiral, depending on the linepipe manufacturing process.

These sections of linepipe are welded together to form the pipeline. The circumferentially -orientated welds that join each section are called girth welds.

Comment: Presumably ‘inches’

? Most have used mm. I’ll check instructions, but use of ‘’ is confusing

i. Linepipe welds - Longitudinal or spiral seam welds in linepipe are made under controlled, good conditions in a pipe mill, and will have survived the pipe mill test and pre-service hydrotest; therefore, new linepipe should not contain large (pre-service) defects in their welds. Many years ago, when weld quality control was not as good as today, and some of the welding processes in use were relatively new, these welds could be delivered to site with large defects. Additionally, some types of older welds (electric resistance or flash welded seam welds) are susceptible to preferential corrosion (the bond line corrodes preferentially to the surrounding steel) that can produce a ‘sharp’ V-shaped notch along the bondline.

ii. Girth welds - Girth welds are manufactured in the field, under more difficult conditions compared to linepipe welds, and the stresses created by the hydrotest will be <50% of that experienced by the linepipe seam welds.

Therefore, there is a chance of large, undetected, weld defects entering service.

4.5.9.1 Pipeline Welding Codes and Defects

All weld s will contain some defects, so pipeline welding codes allow reasonable levels of defects to remain. These levels are often called ‘workmanship’ levels – those expected when reasonable operators, materials and working practices are followed.

The codes allow some ‘planar’ and ‘non planar’ defects. Non planar defects are slag, porosity, etc., and planar defects are lack of side wall fusion, etc.:

i. Non planar defects are not usually a problem, but indicate poor quality, and they could mask the presence of more serious defects.

ii. Planar defects can act like cracks, and hence are limited in extent by the welding codes.

Typically, a surface breaking defect of 25mm (1in) in length²², or a 50 mm (2in) embedded defect is allowed to remain in girth welds by most weld ing codes. Cracks are not usually allowed; cracks can indicate bad welding, poor material, poor quality control, etc., and hence are usually repaired.

These codes, and their acceptance levels, have been shown to be effective; pipeline girth welds rarely cause problems during service. However, defects are sometimes detected during service, and they require assessment. Additionally, operators may wish to set different defect acceptance levels (based on fitness-for-purpose, rather than workmanship limits) at the construction stage of their pipeline.

4.5.9.2 New Acceptance Limits for Girth Weld Defects

The European Pipeline Research Group, EPRG has published guidelines on the assessment of defects in transmission, and they constitute the state of the art. The reader is directed towards these guidelines and their background literature [Hopkins,

22 Pipeline girth welding codes normally only quote defect length in their acceptance charts. This is because pipeline welds have traditionally been inspected using radiography, which will not give a measure of through -thickness depth.

1993, Knauf & Hopkins, 1996]. They can be used on new pipelines (to set weld defect acceptance levels), but have applicability to existing pipelines.

The EPRG guidelines present three ‘tiers’ or levels of defect assessment. The first tier is a simple workmanship level, where short defect lengths are quoted as acceptable.

This tier can be used on new pipeline constructions where there is no need for large defects to be either assessed or accepted.

The two higher tiers use fracture mechanics, supported by a large experimental programme, to give very generous defect limits for girth welds. These limits are not intended for new constructions; modern girth welds should easily meet current workmanship standards. It is recommended that the new limits in the EPRG guidelines be used for:

- special applications where longer defects are anticipated (e.g. new processes),

- as a concession by the pipeline operator to avoid unnecessary repairs,

- as an insurance policy for cases where a defect is detected during post-construction audit or during in -service inspection.

Figure 22 gives the Tier 3 defect limits from the EPRG guidelines. Note that the guidelines do not take account of fatigue loading, or environmental attack.

Figure 22. Defect Limits for Girth Welds in the EPRG Guidelines (note conditions apply)

4.5.9.3 Arc Strikes

Welding arc strikes/burns are often found on linepipe. These are areas of localised melting of the linepipe, usually causing:

- a small depression in the pipewall,

- an area of local hardness (this hardened layer may increase susceptibility to environmental cracking and possibly small cracks).

8 10 12 14 16 18 20 22 24

MIN. OR MEASURED WALL THICKNESS (mm) 0

10 20 30 40 50 60

DEFECT LENGTH (% pipe circumference)

Total - All Planar Defects

Interacting Planar DefectsIndividual Planar Defects Total - All Defects

Welding codes require arc burns/strikes to be removed – this is simple and quick when the pipeline is being welded, and crews are on site.

They are not considered a problem in service as:

- they are not known to be a cause of in-service failures, - the area hardened/cracked is very small and insignificant,

- the arc strike is usually coated after welding, so it is protected from the environment,

but good practice is to grind them away, ensuring that the local wall thickness is not reduced below the minimum specified or calculated by methods such as those in Section 4.5.3.