INTEGRITY MANAGEMENT—1: Argentine NGL case study applies economic procedure

June 9, 2008
A simple economic procedure applied to NGL pipelines can help operators determine whether abnormalities detected during periodic magnetic-flux leakage pigging are large enough to shut the pipeline for repair or if operations can continue.

A simple economic procedure applied to NGL pipelines can help operators determine whether abnormalities detected during periodic magnetic-flux leakage pigging are large enough to shut the pipeline for repair or if operations can continue.

An Argentine NGL plant in Neuquén separates rich components of gas and pumps them 600 miles through a pipeline to the Bahía Blanca fractionating plant (Fig. 1). Pipeline construction used low-carbon API-5L-X65 steel, electric resistance welding, and an external three-layer extruded polyethylene coating to prevent external damage.

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In September 2004, after 4 years of uninterrupted operation, the operator used a magnetic-flux leakage intelligent pig1 to conduct internal inspection, fulfilling its maintenance plan. The final report showed some internal defects, only one of which was of major interest, having a 44% thickness reduction and a rectangular shape.

Pipeline failure would likely result in a stoppage of production or safety incidents, such as fire, explosion, environmental pollution, injury, or death. Identifying and recognizing defects and failures in hydrocarbon pipelines is important and necessary.

This first of two articles uses a simple economic procedure to help operators, engineers, and general managers make the right decision should a leak be detected, based on its behavior under different scenarios.

The procedure consists of three steps:

  • Identification. A magnetic-flux leakage intelligent pig report and ultrasonic Scan B testing located the defect.
  • Quantification. Ultrasonic thickness measurement and Scan B results use gammagraphic inspection to determine the defect’s size and location on the wall pipe.
  • Assessment. A simple finite element analysis model simulates the real defect and then corroborates results against API Recommended Practice 579 Fitness for Service, January 2000.

The concluding article next week will apply this procedure and finite element analysis directly to a fitness for service assessment.

Procedure

An MFL intelligent pig inspection conducted after 4 years of uninterrupted pipeline operation fulfilled the pipeline maintenance plan. The MFL pig inspection, though not the focus of this work, is an important step of a pipeline integrity management plan.

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The procedure presented here applies to all pipelines after an intelligent pig inspection report has been generated and consists of the three steps listed earlier (Fig. 2). Table 1 shows different scenarios analyzed in the defect zone.

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Identification

The pig inspection report showed a rectangular (61 × 90 mm) internal defect with 44% metal loss on the bottom pipe wall, near valve No. 5 (about 83 km from the NGL extraction plant). In this part of the procedure, identification in the field marked where excavation for nondestructive testing should occur (Fig. 3).

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Ultrasonic inspection introduces beams of high-frequency (0.1-25 Mhz) sound waves into materials for the detection of surface and subsurface flaws in the material.2 Sound waves travel through the material and are reflected at interfaces. The reflected beams detect and locate flaws or discontinuities.

Ultrasonic inspection preformed NDT on the NGL pipeline through its protective coating (three-layer polyethylene extruded, PE-3PL) without breaking it. Successful application of ultrasonic techniques requires good adhesion between the protective layer and steel. Extruded polyethylene coatings generally have optimum adherence to the steel and allow high-quality ultrasonic testing (Fig. 4).

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The coating helps prevent external damage due to soil interaction. Ensuring pipeline protective coating’s integrity during NDT requires ultrasonic equipment capable of performing an examination through the three-layer polyethylene extruded material (2.82 mm).

UT Scan B conducted on the pipeline exposed the defect profile and determined where the gammagraphic film should be located on the pipeline wall. Analysis of the UT Scan B screen images yielded the following results:

  • Axial scan. The defect resembled a keyhole; rectangular with sharp sides increasing the hoop stress on the pipe wall. Real internal metal loss equaled 1.2 mm, or 23%, instead of the 44% reported by the MFL pig inspection.
  • Circumferential scan. The screen image in this direction showed the internal metal loss as progressive on both sides.

Quantification

Following identification and location of the defect with UT Scan B, a gammagraphic inspection identified the defect’s real dimension and position on the pipe wall.

Radiography detects features of an assembly exhibiting a difference in thickness or physical density compared to surrounding material. Radiography generally only detects features with an appreciable thickness parallel to the radiation beam. Radiography can be used to inspect most types of solid material; metallic and non metallic. Inspection of this pipeline used gamma-rays.

Successful in-service pipeline radiographic inspection depends on knowledge of NGL’s absorption coefficient (the attenuation of electromagnetic radiation) because its density varies between water and gasoline.

Field-development of the radiographic film revealed a 40 × 60-mm defect, circumferentially oriented. The dimensions corroborated the rectangular defect predicted by UT Scan B and the pig inspection report, but revealed a surface area difference of 44.4% between the defect reported by pig inspection and radiographic examination. No cracks were found on the corner defect, which could have been created by the notch effect.

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This part of the procedure completely quantified the real defect, combining the UT Scan B and radiographic examination results. Fig. 5 clearly shows the three-dimensional defect image.

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Table 2 shows the difference between the defect reported by MFL pig inspection and that reported by ultrasonic and radiographic examination.

Defect origin

Fitness For Service assessment requires identification of the damage mechanism. Skipping this critical step can lead to failure; creating false conclusions from a stress perspective.

Identifying the damage mechanism requires the appropriate NDT method, an estimate of the future damage rate to find remaining life, and proper monitoring and mitigation methods.

NGL’s non-corrosive nature makes it unlikely the defect occurred in service, raising the probability it occurred during manufacturing. Electric-resistance welded line pipe consists of plates, longitudinally butt welded by heat from electric current, without filler metal and rolled until uniform in their OD. It is unlikely either rolling or storage created the defect. Quality inspection at the steel manufacturer likely failed.

This type of damage is not accepted by the international standard API 5L Specification for Line Pipe.3

Assessment

This part of the procedure ensures the mechanical integrity of the pipeline under different loading scenarios based on the real defect quantified. The need for more accurate and reliable assessment results forces use of both analytical and numerical methodologies. This analysis method allows assessment of whether the pipeline can keep operating at normal working pressure and still handle other potential loads (such as surges or liquid slugs) or if it is necessary to take the pipeline out of service to repair the damage.

A simple finite-element model performed the stress analysis on the damaged zone. It first considered the highest loads (Table 1), then corroborated and validated the numerical result using Recommended Practice API-579, Fitness For Service, Section Five.4

Surge pressure

A sudden change in flow velocity and pressure can cause a surge, potentially creating large forces and overpressure. Fig. 6 shows a simple view of this phenomenon.

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If a liquid flowing steadily at a velocity v and pressure P from a reservoir or pump station through a valve finds the valve closed, the liquid comes to a stop, with v = 0 and pressure increasing to P + ΔP. Both a pressure wave and a reflected wave subsequently travel through the fluid, up and down the pipe at the velocity of sound in the medium.

Flow stops in the vicinity of the valve, even as material continues to enter the pipeline at the pump station. The resulting surge wave begins at the valve and travels upstream, reflecting back downstream, and oscillating back and forth until its energy is dissipated by pipeline wall friction. The amplitude of the surge wave (dP), or the magnitude of pressure surge (Psurge), is a function of change in velocity and the velocity of sound in NGL.

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This case assumed the mainline block valve (LVS 5) closed instantaneously, using a simple hand calculation to determine the pressure reachable in the damage zone by a surge event (Equation 1).

Equation 2 calculates the amplitude of the wave pressure, while Equation 3 calculates surge pressure.

Two cases considered stress analysis due to transient fluid, first when the pipeline was subject to normal working pressure and second at maximum expected pressure during normal operation. If valve LVS 5 suddenly closed, the surge pressure calculated by Equations 2 and 3 would equal 85 bar (73 + 12) in normal conditions and 92 bar (80 + 12) undergoing maximum expected pressure.

This calculation only approximates surge pressure magnitude for limited cases by the stated time closure criteria (T < 2L/v, where T is the valve closing time and L is the distance between valve and pump station). Dynamic simulation can yield detailed results if required. Friction attenuates the surge and the surge pressure arriving at the damaged zone is less than the surge pressure at the origin point.

Acknowledgments

The authors acknowledge the support of Laza Krstin from ABB Eutech as well as Antonio Baptista and Alberto Sanches.

References

  1. Tiratsoo, J., Pipeline Pigging Technology, 2nd Ed., Houston: Gulf Professional Publishing, 1992.
  2. ASM Metals Handbook, Nondestructive Evaluation and Quality Control, Vol.17, pp. 486-592, 1992.
  3. American Petroleum Institute Specification 5L: Specification for Line Pipe, 42nd Ed., Washington, July 2000.
  4. American Petroleum Institute RP- 579: Fitness for service, 1st Ed., Washington, March 2000.

Based on presentation to the Pipeline Rehabilitation & Maintenance conference, Manama, Bahrain, Dec. 11-13, 2007.

The authors

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Fernando Vicente ([email protected]) is maintenance-integrity engineer at ABB Full Service, Neuquén-Loma La Lata, Argentina. He has also served as maintenance engineer for Petrogas in the Repsol-YPF oilfield. Vicente holds a mechanical engineering degree (2003) from Universidad Tecnologica Nacional-Regional General Pacheco, Argentina.

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Eduardo Risso ([email protected]) is engineering department chief at ABB Full Service, Neuquén-Loma La Lata. He holds a mechanical engineering degree (1981) from Universidad Tecnologica Nacional-Regional Cordoba, Argentina.