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There are a number of methods that are commonly used for the assessment of a girth weld containing a ‘fabrication’ defect. These range from the more generic workmanship limits through to more complex pipeline specific Engineering Critical Assessment (ECA) methodologies. The workmanship limits stipulated in pipeline design codes can be very conservative, resulting in un-necessary and costly repairs. The ECA approach is being increasingly used to derive girth weld defect acceptance limits specific to a pipeline. These limits have been derived using either semi-analytical methods or from the results of large-scale tests conducted on pipeline girth welds. However, at present there is no one standardized method. The guidance produced by the European Pipeline Research Group (EPRG) is an example of an established methodology based on the results of large-scale tests, while commonly used pipeline specific semi-analytical assessment methods include API 1104 and CSA Z662. Other commonly used analytical methods, which are more generic in application, include BS 7910 and API 579-1/ASME FFS-1. Application of these methods to girth welds in grade X100 pipelines has not yet been validated. The US Department of Transportation, Pipeline and Hazardous Materials Safety Administration (PHMSA) commissioned Electricore, Inc and GL Noble Denton to investigate the applicability of these ‘commonly used’ girth weld assessment procedures to grade X100 pipelines. To facilitate this project, BP provided 10 girth welds from a full-scale operational trial of two grade X100 48in diameter pipeline test sections, following completion of the trial at GL Noble Denton’s Spadeadam test facility, Cumbria, UK. The girth welds were selected to enable the effects of material variability between abutting pipes, different heats and different manufacturers (pipe was sourced from two world class pipe mills, with the plate supply for one mill coming from two sources) to be investigated. A substantial test program has been undertaken to fully characterize the mechanical properties of each girth weld, comprising curved wide plate (CWP), tensile, Charpy impact and fracture mechanics tests. The results from the CWP tests have been analyzed using the procedures given in API 1104 (Option 2), EPRG, CSA Z662, BS 7910 and API 579-1/ASME FFS-1. This paper presents an overview of the tests undertaken and a comparison of the actual test results with the predictions from the assessment methods.

Reliable predictions of the remaining strength of corroded pipelines are dependent on the assumptions made during the defect assessments, and the availability of an accurate description of the defect shape that is detected by in-line inspection (ILI) tools. If only the maximum defect dimensions are recorded and subsequently used in a defect assessment then a low remaining strength is likely to be predicted for the corroded pipeline. However, if the defect profile had been more accurately mapped then improved predictions of the remaining strength can be determined from the subsequent defect assessment. This improved assessment could be enough to justify safe operation of the corroded pipeline rather than to undertake expensive repairs. For these reasons, PRCI has sponsored Advantica to investigate aspects of both ILI data accuracy and assessment methodology to determine the benefits gained in the remaining strength predictions. This has been undertaken using a combination of in-house specialist computer software and finite element modelling. Work has been undertaken to determine the benefits of an improvement in the measurement accuracy of the three characteristic flaw dimensions (depth, length and width) when determining the remaining strength of a pipeline, and to determine the effect that changes to the defect interaction criterion, used in the assessment, has on remaining strength predictions. Case studies using example ILI data from two pipeline operators are used to determine how measurement accuracy affects remaining strength predictions. Using deterministic assessment methods, such as RSTRENG, it is concluded that sizing accuracy for the defect depth and length are most critical in predicting remaining strength predictions. The defect width is used only to determine interaction of closely spaced defect clusters. In addition, it is concluded that the interaction criterion generally used in industry is overly conservative and could be relaxed.

PT Transportasi Gas Indonesia (TGI) own and operate a 536 km long natural gas pipeline in South Sumatra, Indonesia which transports natural gas from ConocoPhillips’s gas field in Grissik, South Sumatra to Chevron Pacific Indonesia’s station facilities in Duri. On 29th September 2010, an event occurred that resulted in a release of gas from the pipeline. In response to the emergency, the affected section of the pipeline was isolated by closing block valves upstream and downstream of the leak. The incident was brought under control by TGI on the same day and there were no reported injuries or fatalities. Failure was located in a girth weld on a 28 inch diameter pipe section, which had spread into the adjacent pipe material. Subsequently a failure investigation was requested by TGI. The investigation included a fracture examination and materials testing of the failed girth weld and parent pipe; a geotechnical investigation; and an engineering critical analysis (ECA) of the failure. This paper describes the multidisciplinary works undertaken to investigate the cause of the incident. The primary observation of this failure investigation is that no single factor contributed to the failure that occurred. The pipeline at the rupture location had been subjected to high bending stresses when the pipeline was laid and the stresses were exacerbated following consolidation and creep settlement of the underlying swamp material. The field joint coating had been compromised, leading to the formation of near neutral stress corrosion cracking (SCC). Initial cracking from the SCC had then extended to the point where the remaining ligament then failed by plastic collapse. It was judged that the settlement may also have been enhanced by a recent earthquake.

Situations can arise where the condition of a pipeline system is poorly known. This may be due to a variety of operational or commercial reasons. Failures will eventually occur if time dependent degradation mechanisms are active. While an appropriate response may be to inspect or hydrotest, this is generally not feasible within a short time frame and integrity assessments or replacements must therefore be prioritized. This paper looks at an ageing upstream pipeline system subject to internal corrosion. A case study is presented in which a system approaching its original design life is required to carry fluids from reservoirs now forecast to be productive for another 50 years. Fluids include sweet or sour gas, crude oil and injection water. Design data are available but inspection information is sparse with less than 10% of lines inspected by ILI; coupon data and well production forecasts are available. The challenge was to prioritize line replacements according to the remnant life of each pipeline, based on the limited available data. Current condition was measured for lines where ILI data were available. A corrosion risk assessment was conducted to identify credible degradation mechanisms. The pipelines were then grouped according to the fluids being transported. This enabled an estimate of current condition for all pipelines based upon the limited inspection and coupon data. In order to predict the remnant life it was necessary to estimate the future corrosion rates, again for all lines. A number of approaches could be used for estimating future corrosion rates. These include basing the rates on historical inspection data or using corrosion modeling techniques. The paper describes a hybrid method that synthesises these two approaches to allow a corrosion rate distribution to be postulated for calculating remnant life. In addition, the options for future corrosion rate estimation are described and the advantages and disadvantages of each one discussed.