Girth weld inspection acceptance criteria - B31.1, B31.3 1

[Bamend]

The API 1104 pipeline welding standard includes an option (Appendix A) for use of a fitness for service or fracture mechanics-based approach for evaluation of girth weld flaws in new construction. Is there any parallel option for new construction welds made in B31.1 or B31.3 piping? In my brief review of each standard it looked like both rely only upon workmanship stds. Perhaps one of the piping code experts out there knows of a code inquiry or code case or something else relative to this question. Thanks

 

[metengr]

I deal with B31.1, and other major Codes and Standards and there is no provision in the B31.1 Piping Code that I am aware of that deals with fitness for service flaws for new construction. Remember, you are dealing with a new construction mentality, and you have a threshold already established for injurious flaws (flaws that through experience have been deemed harmful to achieving long and safe design life of piping). So, why would ASME want to settle for anything less or better yet compromise on leaving flaws in new construction?

In addition, flaw analyses could add significant cost to the job and what about responsibility for the results? ASME Code and Standards have spent years developing minimum requirements for safe operation.

This is not necessarily my professional view as stated above but only a perspective from one who deals with various Codes and Standards.

 

[Bamend]

Metengr - The answer to your question regarding why ASME would settle for less quality is related to the whole premise behind "fitness for service" and is an easy one to answer. Workmanship criteria is a one-size-fits-all approach that sets acceptance standards based on an assumption that service conditions will be at your near the code allowable limits. Why should an operator or fabricator be held to the same quality standards for piping that will operate at much lower temperatures/pressures/stresses if larger flaws will still result in acceptable factors of safety, particularly when recognizing that repair can be costly and possibly result in inadvertant creation of more imperfections?

 

[metengr]

Why should an operator or fabricator be held to the same quality standards for piping that will operate at much lower temperatures/pressures/stresses if larger flaws will still result in acceptable factors of safety, particularly when recognizing that repair can be costly and possibly result in inadvertant creation of more imperfections?

Bamend;
I respectively have to disagree with the statement (in bold in your quote above). Larger flaws may not result in acceptable factors of safety in relation to unexpected service conditions, like over temperature or over pressure events or wrong design assumptions. Remember, the Code is not an engineering handbook or substitution for engineering experience. Leaving pre-existing flaws could result in low or high cycle fatigue crack propagation, or creep crack growth, or creep/fatigue crack propagation in service. Also, hydrostatic testing would either have to be waived or performed at a reduced pressure to assure no failure (local crack propagation or local gross deformation) occurs as a result of increased flaw size in girth welds.

Here is my take. We still safely operate aged boiler and steam lines with no serious consequences. Do you know why? It is in my technical opinion the result of generous design margins that were utilized to establish allowable stress values and stringent flaw acceptance criteria. I am still thankful to this day that the code requirements developed and implemented years ago used generous factors of safety and kept flaws to a minimum in pressure retaining welds for new construction.

You also did not answer my question about responsibility for this flaw analysis. New Codes and Standards are adverse to risk management, so this is a large obstacle to overcome (especially, if you wanted to relax flaw acceptance criteria by adopting fitness for service flaw analysis). In-service repair codes like the NBIC and API 579 have come a long way toward flaw evaluation for in-service conditions of pressure retaining items. But at this point in design life, the codes or standards of construction do not care because events can happen in service or poor design assumptions were used to result in a shortened design life.

 

[Bamend]

A proper fitness for service evaluation considers the potential for excursions in temperature and stress, and the potential for fatigue. Therefore, engineering judgement is an important contributor to successful application. In addition, the analyses include consideration of the tolerances on measured flaw sizes and the measured variabilities in pipe/weld properties.
The responsibility for the flaw analysis is the operator's although they may choose to supplement their technical capabilities with those of contractors or consultants with more expertise in that topic. My original question dealt with whether the B31.1 or 31.3 standards had either guidance or prescriptive procedures regarding application of a fitness for service approach in the same way some other new construction codes (such as API 1104) do. When that information is included in the code I believe it is the operator's responsibility to apply engineering judgement regarding the application of the procedures, while still meeting the minimum requirements of the procedure.


I don't agree with your statement regarding hydrotesting. Gas and liquid transmission pipeline operators routinely use hydrotest pressures that result in hoop stress of 100-105% of the specified minimum yield strength of the pipe even when a fitness for service criteria is used for flaw acceptance. That alternative approach typically results in allowable flaw sizes that are somewhat larger than the workmanship criteria, but still small enough to survive a test to SMYS.

In this case it appears a fitness for service approach is not an option for B31.1 or B31.3 so we will move on and use a conventional approach toward weld flaw assessment.

 

[ndeguy]

I would just like to add that the workmanship vs fitness-for-service approach has arisen in part due to the increased inspection measurement capabilities. Many new construction transmission pipeline welds are now evaluated on a fitness-for-service approach.

In the past the traditional inspection method for such welds has been radiography, an inspection with a high probability-of-detection (POD), for detection of volumetric flaws. However flaw orientation and its vertical height as a percentage of material through thickness are significant in the POD of RT for planar flaws. Unfavourably oriented flaws or those of very low percentage vertical height could easily be missed.

Many pipelines were welded manually using cellulosic rods for high productivity but with slag entrapment as a problem especially if interpass grinding was of poor quality. These slag deposits are clearly visible through radiography. The advent of mechanised GMAW as applied to pipeline girth welds greatly increased welding productivity. However the nature of expected flaws shifted away from volumetric slag to planar lack of fusion (LOF). The latter would often occur at fire-up positions. If these were not staggered and LOF was created at each fire-up then potentially interactive stacked multi-run flaws of short length but large through-thickness height would occur. Due to the geometries of the weld bevel and the radiation beam these are difficult to detect reliably and nearly impossible to dimension in the through-thickness dimension. Automated ultrasonic testing was developed to detect these and other flaws. As a corollary of the improvement of AUT systems and the reliability of through-thickness height measurements (previously very difficult with manual UT and next-to-impossible with radiography), the fracture mechanics based fitness for purpose acceptance criteria was developed. Now flaw vertical height, length and position (surface-breaking, surface interactive or embedded) are all used to determine its disposition. Many miles of pipeline has been laid both on land and sub-sea using the AUT/fitness-for-service approach. Both Owners and Contractors, originally cautious in adopting this method, accept it as the standard method for girth weld pipeline inspection. Now many flaws do remain in the welds, but the fracture mechanics analysis will deem them innocuous. Such examples are inter-run LOF of very low verticla height. Where previously these would have been rejected on length alone (50 mm max for most workmanship standard acceptance codes) now much longer lengths can be accepted provided the vertical height is low - as it is in this case).

The ASME BPV Code has Code Case 2235-7 which sets out the requirements for AUT and a generic acceptance criterion. I think that ultrasonic inspection based on fitness-for-purpose rather than the workmanship-based radiography will become the dominant inspection method, especially for large diameter transmission lines. The addition of phased-array and TOFD tools and the Health & Safety problems inherent in the radiographic process, will accelerate this changeover.

Regards



Nigel Armstrong
Karachaganak Petroleum
Kazakhstan