STRESS-CORROSION IN TRUNK PIPELINES: CURRENT STATUS AND SAFE OPERATION FORECAST

V.V. Kharionovsky (Gazprom VNIIGAZ),

V.N. Lisin, E.A.Spiridovich, A.F. Puzhailo (DOAO “Giprogaztsentr”)

1 Introduction

Stress-corrosion is a factor that shapes the operational status of the trunk pipelines. Being a complex and ambiguous phenomenon underlain by various environmental conditions, stress state, structure and metal-specific properties, stress-corrosion is known as one of the most dangerous types of damages affecting pipes (Figure. 1).

Figure 1. Factor set determining initiation and propagation of stress-corrosion in gas pipelines

According to recent findings, apart from linear part of trunk pipelines, underground flowlines (compressor stations process pipelines) are also affected by stress-corrosion.

In-pipe inspection is somewhat difficult to conduct on compressor station flowlines as soon as the pigs can not be run, therefore specific corrosion control and pipeline maintenance methods need to be developed.

These days, OAO Gazpom is implementing a plan package to mitigate risks of failures due to stress-corrosion. The root causes of stress-corrosion have been detected; a pipeline maintenance methods have been adopted (including methods of inspection, evaluation of strength properties and scope of repair); a pipeline re-insulation program has been launched (under this program a large variety of coatings have been developed to ensure reliable corrosion-proof operation of pipelines).

2 Focus Areas Due to the efforts of OAO Gazpom, its subsidiaries, specialised and research

organisations, operating reliability of gas transport objects is managed to maintain at the

Lateral SCC

Longitudinal SCC SCC

stable level and prevent avalanche increase of failures. The distinctive progress has been made in the lots of organisational and engineering issues associated with mitigation of stress-corrosion risk. At the moment, the major causes and conditions of stress-corrosion are identified and the strategy of maintenance service is developed for gas pipelines which are the subject of corrosion. This technique includes different diagnostic methods and technologies, strength assessment and repair of trunk pipelines exposed to SCC (Figure 2). Number of identified and repaired SCC defects is growing with the increase of maintenance overhaul; the production models of scanners able to detect SCC defects during re-insulation are developed along with the methods of SCC defects welding-up and maintenance of pipes with SCC defects by couplings.

Figure 2. Maintenance of trunk gas pipelines exposed to SCC

Evolution of the gas mains failure occurrences caused by SCC (Figure 3) in OAO Gazpom shows that the number of them tends to decrease due to the package plan implemented by OAO Gazpom, its subsidiaries, specialised and research organisations. The stable trend of reducing the pipelines accidents caused by SCC has emerged during the last 10 years, for the first time after the year 2004.

Progressing of non-destructive inspection facilities

Flaw detecting scanners

Working out methods and technologies for repair of pipes with SCC defects

Figure 3. Statistics of SCC-caused failures of trunk gas pipelines in OAO Gazpom during

1991-2008

The positive trends are observed in the dynamics of the amount of SCC defects, which are registered in the sections of the repeated in-line inspections (Figure 4). On the Figure 4 one can clearly see the decrease of SCC defects number registered by the repeated in-line inspections (ILI). This goes to prove that it is not a significant growth of existing SCC defects in the most sections of gas mains. A similar trend is generally observed in the overall dynamics (of primary and repeated inspections) to identify SCC defects

Figure 4. Comparison of the number of observed SCC defects while the repeated inspections

Pipeline protection projects may focus on the following areas: - regular inspections of trunk pipelines by running “Spetsneftegaz” magnetic flaw

detectors that are capable of detecting the longitudinal cracks; - development of production models of scanners to detect flaws during re-insulation;

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- development of regulations for inspecting and evaluating severity of stress-corrosion defects;

- research projects focusing on analysis of stress-corrosion in compressor stations flowlines;

- development of repair methods involving welding, application of pipe couplings, or removal of damaged sections.

Evaluation of stress-corrosion on the compressor station flowlines

Started in 2005-2008, re-insulation activities showed the high relevance of SCC challenge on the process pipelines of the compressor stations (CS). Opposite to assumptions, the pipelines of I and B classes occurred to be exposed to stress-corrosion among the others (Figure 5). The situation is worse in comparison with the linear trunk pipeline portions because the CS flowlines are still not available for in-line inspection whilst the consequences of the CS accidents may be incomparably greater. What is more, the existing regulations don't consider the possibility of pipes repair on the sections of I and B classes. The issue of SCC on the process pipelines of CS should be expanded.

Figure 5. State of SCC issue on the process pipelines of CS

During 2006-2008 and till now the comprehensive repair of the process

communications has been conducted on 75 CS compressor sections with total length of more than 90 km of flowlines. The examples of detected defects are presented along with their parameters on Figure 6.

Figure 6. The main features of detected SCC defects of CS processing pipelines

One may state that processes evident now due to SCC on the CS processing

communications are the same as on the linear trunk pipeline portions 10-15 years ago. The SCC defects were detected as a result of examination in the pipes with diameter 720, 1020, 1220 and 1420 mm. For the listed pipes SCC defects occurs at wall thickness from 9 to 23.3 mm, it confirms that SCC process can occur on the heavy walled pipes of I and B classes. So, it is evident today that that SCC defects occurrence on the CS flowline is not a random event. Obviously, the fact of detection of SCC defects on the CS flowline shows that the extent of SCC on the gas transportation facilities is really much more considerable than it was previously expected.

3. Research Methods 3.1.Comprehensive research.

Pipeline sections potentially vulnerable to stress-corrosion are detected by implementing a set of services that identify favorable stress-corrosion conditions and drivers stimulating its propagation. The set of services includes analysis of design and operation documentation, field studies, and data processing. The algorithm of services is shown on the following flowchart (Figure 7). The flow chart includes design stage measures that help mitigate stress-corrosion risks by selecting the appropriate route sections. During the operation stage the in-pipe inspection and coating inspection data is used.

• Max length of SCC zone — all the pipe (up to 11m)

• Max width — up to the half round of pipe (mostly on the ground part)

• Max depth of cracks — up to 5 mm

• Most of SCC zones include the area of thermal impact near the welding

joint (pipelines Du 720, Du 1020 and Du 1420 mm)

Figure 7 Potentially vulnerable sections distribution flow chart. Based on the result of research general drivers are identified – a stress state,

insulation status, soil corrosion activity, ground water occurrence, that are evaluated by using weight metrics.

On the next stage a severity level is evaluated, and the framework for the next inspection is provided.

3.2. Detecting potentially vulnerable sections of compressor station flowlines. As soon as inspection of process flowlines provides certain challenges, an individual

method was required for detecting potentially vulnerable sections and ranging compressor stations based on the priority of inspection and technical maintenance. The method suggests doing the following:

detect potentially vulnerable sections; evaluate stress-corrosion defects in sampling pits; evaluate operational reliability of the flowlines; provide forecast of the stress-corrosion state. This methodology is shown on the following flow chart, fig. 8. The main challenge with

the compressor station flowlines is that its underground sections need to be uncovered first

(the pits should be made) so that the inspection could be carried out. Then the workflow follows the research algorithm specified under para 3.1.

Interpretation of design, construction, and operational specifications for

compressors\boosters process flowlines

Inspection of pipelines to detect potentially vulnerable pipeline sections

Processing of inspection results and calculation of integral SCC risk index

SCC database

Data card

Scheduling inspection of pipelines in pits to detect potentially vulnerable sections

Inspection plan

Lithology evaluation

Evaluation of insulation status

Assessment of

ground water occurrence

Identifying sections

subject to cyclic wetting

Detecting magnetic

signatures by remote sensing

Detecting flaws and

defects by in-pipe inspection

Stress-stateevaluation

Evaluation of soil corrosion activity

Inspection of pipelines in pits

Conclusion on technical status

Figure 8 The workflow of identification of potentially vulnerable sections and inspection of

compressors process flowlines in pits.

3.3. Repair methods Based on the inspection results, the following repair methods to remove stress-

corrosion defects were developed: * crack grinding and a follow-up welding repair technique; * application of pipe couplings; * removal of a damaged section.

The applicable technique is selected based on the flaw severity and the priority status of a pipeline. Fig. 9 shows the repair technique selection algorithm for process flowlines of a compressor station that are deemed to be top priority facilities of a trunk pipeline. The flow

chart shows a number of selection scenarios that vary depending on the crack depth and length, physical properties of a pipe metal, and the degree of stress-state.

Figure 9 Selection of a method to repair pipes with crack-like defects

4. Providing a stress state forecast for a pipeline sections During the last years the significant amount of researches was carried out to develop

the new methodological approaches of detection of pipeline sections, which are potentially vulnerable to stress-corrosion; pipes of gas mains were ranked by their vulnerability to SCC etc. Figure 10 presents the methodological approach for scheduling the next in-line inspection on the trunk gas pipelines sections exposed to SCC. This approach is implemented in the Gazprom 2-2.3-095-2007 «Guidelines for diagnostic examination of linear portion of the trunk gas pipelines» standard statement document. Timing for the next in-line inspection of trunk gas pipelines is defined on the base of evaluation of the expected increase of detected defects number; statistic processing of ILI results are used for this

Repair method applicability check: controlled grinding for I & В pipelines.

yes

no

Controlled grinding repair

method

Repair method applicability check: pipeline couplings

yes

no

Application of pipeline couplings Lр ≤ [Lм]

Rт ≤ 1

Rc ≤ [Rg]

Rk ≤ [Rg]

Setting initial metrics:

category of compressor station process flowline;

pipe metrics (diameter Dн, wall thickness δ) and pressure p;

physical material properties (KIC, σв, σт);

real defect size (�, Lр, Wр)

coupling workload, ∆p

Repair method applicability check: controlled grinding and application of pipeline

couplings for I pipelines.

Removal of sections in I & B pipelines

yes Controlled grinding and application of pipeline

couplings Lр ≤ [Lм]

Rk ≤ 1

Rм ≤ [Rg]

dр ≤ [d]

no

purpose. This figure shows also predicted dependence of mean time between trunk gas pipeline failures on the amount of the maintenance works after the in-line inspection.

Processing results of in-pipe inspection

Identifying flaw distribution parameters

σ(t)

53

The number of pipes that require maintenance

Next in-pipe inspection date

and time recommendations

Flaw depth distribution after

maintenance

Flaw depth distribution before

maintenance

Time

Defect propagation forecast for a

pipeline section

Expected number of

faulty pipes (before

inspection), n

Distribution

parameter σo

Maintenance

)d(f

oσ

n =356

m=123

d= /dd

Evaluating current status of pipeline sections

Inspection costs

Effective number of

faulty pipes detected

during inspection, m

Maintenance

costs

Processing results of in-pipe inspection

Identifying flaw distribution parameters

σ(t)

53

The number of pipes that require maintenance

Next in-pipe inspection date

and time recommendations

Flaw depth distribution after

maintenance

Flaw depth distribution before

maintenance

Time

Defect propagation forecast for a

pipeline section

Expected number of

faulty pipes (before

inspection), n

Distribution

parameter σo

Maintenance

)d(f )d(f

oσoσ

n =356

m=123

d= /dd d= /dd

Evaluating current status of pipeline sections

Inspection costs

Effective number of

faulty pipes detected

during inspection, m

Maintenance

costs

Figure 10 The stress-state forecast for a trunk pipeline section provided on the basis of in-

pipe inspection results and definition of the scope of repair

The surface technique for stress-corrosion diagnostics has got a practical application. It is based on the design and as-built documentation and was developed in “Giprogaztsentr” research institute. The prediction methodology for identification of potentially vulnerable stress-corrosion sections was developed on the base of this technique (see Figure 11). Scheme shows that it is possible to detect the sections with the probable stress-corrosion on the base of soil conditions analysis, hydrogeological data, design and construction solutions and take appropriate actions on the design stage.

Figure 11. Prediction technique for identification of potentially vulnerable stress-corrosion

sections

Hydrogeology data — areas of pipes cyclic wetting

Indications of SCC vulnerability

III-IV class section Ph – soil Film insulation

Potentially vulnerable sections (PVS)

PVS lists for enterprises

SCC forecast with map-making

Operational planning and optimization of in-line

inspection

Long-term planning Design preventive maintenance; Diagnostics; Repairs

The new technology recently developed in VNIIGAZ Ltd. is rather prospective for identification of pipe steels vulnerability to SCC. This technology is based on the laboratory tests of the model x-shaped samples and allow to simulate cracking process in the short time (Figure 12). The technology implemented in Gazprom 2-5.1-148-2007 «Test methods for steel and welded joints for stress-corrosion cracking” document allows to estimate the influence of forces of metallurgic and engineering nature that are responsible for SCC occurrence. According to this technique the vulnerability to SCC of number of Russian and imported pipes is evaluated. Figure 13 presents the results of stress cracking resistance examination of 6 pipe types that was conducted together with «JFE STEEL CORPORATION» (Japan). Figure 13 shows that the examination allowed to rank the pipes by their vulnerability to stress-corrosion and detect the correlation between structure and strength of the steel and vulnerability to cracking.

Figure 12. The new techniques of pipe steels examination for resistivity to SCC

Testing for SCC with biaxial loading

Samples cracking while testing X-shaped samples

Figure 13. The results of evaluation of resistance to SCC for pipe steels.

Investigation of VNIIGAZ-JFE (Japan)

At present, the most part of scientific and engineering problems related to the applied components of the SCC challenge are solved as a part of yearly R&D Programs of OAO Gazprom.

For the last three years VNIIGAZ Ltd. has developed five regulatory documents (Figure 14) relating to SCC effect in diagnostics, strength calculation, testing and improvement of repair methods.

Figure 14. Regulatory documents considering SCC effect.

Temporary instruction for identification of stress-corrosion vulnerable sections and technical

diagnostics of gas flowlines on compressor stations

Temporary instruction for rejection and repair of gas flowlines on

compressor stations

Technology of classification of SCC defects that are subject to

repair with pipe couplings on the gas pipelines sections of the first

and B classes

5. Conclusion In Russia a well-established comprehensive system is implemented that enables detect, repair and provide forecast for stress-state defects propagation and applicable methods of repair. The scope of stress-corrosion field research is required to include the following:

- the study of impact of pipe structure and factory defects on physical properties of the pipes, stress cracking resistance, and operational reliability.

- development of methodical framework and tools enabling control of physical and chemical environment metrics that stimulate stress-corrosion, general, and sub-coating corrosion during pipeline construction and operation.