Through-wall Defects Research Articles

Analysis of the adhesive thickness effect on failure pressure of metal/composite hybrid joints in pressurized blister tests

Composite repairs are increasingly being used to restore the work capability of pipelines with through-wall defects by corrosion. In this context international standards, such as ISO and ASME are the references to design this kind of repair and to qualify such systems. The referred standards do not take into account the adhesive layer thickness (h) on the model for the calculation of the failure pressure (Pcr), as well as for the calculation of the energy release rate (γ). In this work an experimental investigation evidences the influence of the adhesive thickness on the failure pressure. In order to achieve these results, pressurized blister tests were performed in which the geometrical parameter (h) varied from 0,5 mm to 2,0 mm. Ultimately, an empirical model to calculate Pcr as when the critical energy release rate (γLCL) is a function of the adhesive layer thickness was proposed.

A noise-based high-resolution time-reversal method for acoustic defect localization in water pipes.

Pressurized water supply pipeline systems (PWSPS) are quintessential to human development and sustenance, but suffer a multitude of unresolved defects (i.e., leaks, blockages, etc.) due to aging and inaccessibility. In this paper, the ubiquitous high-frequency background noise in PWSPS is harnessed to introduce a high-resolution, passive defect detection technique. The relation between the acoustic Green's and cross correlation functions for a pressurized water pipe is derived for the case of high frequency waves, i.e., acoustic wavelengths smaller than the pipe diameter. This relation is subsequently used to formulate a time-reversal technique for localizing anomalies such as small variations in pipe wall impedance and through-wall defects (i.e., leaks). It is shown that the derived relationship between the cross correlation and acoustic Green's functions enables very accurate defect detection and localization by measuring the background noise at two locations along a pipe. This is an important result given that (i) there is limited access to buried PWSPS to conduct high-frequency active defect detection, and (ii) traditional methods to actively probe pipes (e.g., valve maneuverer) are low-resolution (tens to hundreds of meters) and often result in pipe overloading and fatigue.

Integrity increasing of damaged transporting pipelines using fiber reinforced polymer composite wrap systems

The main objective of the ongoing research is data collection for integrity management tasks, helping the transporting pipeline operators in different decision situations. On the one hand, can be used the composite repairing and reinforcing systems for a long-term; or there are only appropriate for a short-term maintenance period. On the other hand, can be identified the reserves of the girth welds after a long-term operation; or the girth welds with different defects should be repaired and/or replaced. The paper introduces a full-scale pipeline investigation program. Fatigue and burst tests were executed on repaired pipeline sections containing artificial surface and through-wall defects, modelling gouges and leakages, respectively. Both separate and interacting flaws were investigated. Different composite reinforcing methods, in other words wrap systems, were applied, investigated and compared. Furthermore, long-term operated and cut gas transporting pipeline sections containing girth welds were investigated, too. The testing results and the damage histories were compared with each other; safety factor was defined and calculated for the assessment of the effectiveness of the reinforcing systems and the reliability of the girth welds.

Three-Dimensional Reconstruction of Complex Defects in Thin Steel Strips with the Imaging Method of Magnetic Field Distortion

ABSTRACT In the study, the three-dimensional (3D) morphology of defects in thin steel strips with a thickness of 0.8 mm was determined with the imaging method of magnetic field distortion (MFD). First, MFD imaging of through-wall defects and wall-thinning defects in the samples collected from inline products was performed in our laboratory. The actual profiles of the defects were measured with a digital microscope. Second, MFD imaging results were compared with the actual shapes of the defects in order to develop a proper 3D reconstruction method of complex defects. Finally, the analysis results demonstrated that the high reconstruction accuracy of opening contours of complex defects could be realized with the carefully selected threshold value of MFD-induced voltage amplitude. The good linear dependency of MFD-induced voltage amplitude on the depth of complex wall-thinning defects was confirmed. Therefore, MFD imaging method is a promising method for accurately reconstructing 3D shape of complex wall-thinning defects in thin steel strips.

Alternative methods for fracture energy acquisition in the qualification of composite repair system

Composite repair systems for metallic pipelines presenting a through-wall defect must be qualified in accordance with either ISO 24817 or ASME PCC-2 standards. This qualification method requires a number of hydrostatic tests to obtain the failure pressure. In the oil and gas industry, the failure pressure estimation is often performed using a linear fracture mechanics analysis described in both ISO and ASME standards. These standards require the determination of a constant fracture energy, also called the energy release rate. In the case of monotonically increasing loading histories, in the framework of linear fracture mechanics, brutal interfacial debonding occurs when the fracture energy reaches a critical value. This study is an attempt to show that a simpler test, such as, shaft-loaded and pressurized blister test can be employed as an alternative to hydrostatic tests in the qualification of repair systems. The goal is to obtain experimentally the critical fracture energy using blister tests. Results show a reasonable similarity between the critical energy values found using both pressurized and shaft-loaded blister test with the ones obtained with the standard required hydrostatic tests.

Parametric optimization for rehabilitation of pipes with adhesive bonding

This work deals with the optimization of process parameters for rehabilitation of the pipe through-wall hole defects using adhesive bonding. Epoxy-based nonstructural adhesive (NSA) and structural adhesive (SA) combination, precure temperature, bond length, and surface preparation were considered as the parameters. The optimization of parameters directly on pipes is an expensive and time-consuming process. Hence, stainless steel and carbon fiber-reinforced polymer (SS-CFRP) composite single-strap joint was considered. The equivalent loads that were acting during the hydrostatic pressure test were applied to the SS-CFRP joint and its parametric effect on bond strength was studied. The stress distribution along the adhesive layer length has been derived analytically. The surface roughness of prebond surfaces was measured using a 3-D microscope. The quality of the adhesive bond was evaluated using nondestructive testing (digital radiography). The damaged pipe was rehabilitated with optimized parameters and its hydrostatic pressure resistance was tested according to ISO/TS 24817 standards. From the results, it is observed that the rehabilitated pipe with optimized parameters was able to sustain a maximum of 79% of its allowable pressure.

Longitudinal Guided Wave Inspection of Small-Diameter Elbowed Steel Tubes with a Novel Squirrel-Cage Magnetostrictive Sensor

In the study, ultrasonic longitudinal mode guided waves were employed to detect defects in elbowed tubes (without welds) with a diameter of 10 mm. Finite element simulation results highlighted that the emitted L(0,1) mode guided waves experienced strong reflection and mode conversion at the elbow region to generate F(1,1) mode, followed by slow and weak F(2,1) mode. The guided wave reflected from the elbow with a through-wall defect was manifested as two overlapped wave packets, which were good indicators of a defective elbow. To conduct L(0,1) mode guided waves inspection on the small-diameter elbowed tubes, a novel tailored squirrel-cage magnetostrictive sensor was employed in the experiment. The new sensor employed the configuration of segmental iron-cobalt strips and small-size permanent magnet arrays. The entire sensor is composed of two identical C-shaped sensor elements and can be recycled and installed conveniently. Experimental results obtained from healthy and defective tubes were consistent with the conclusions obtained from finite element simulations. An artificial through-wall defect at the elbow and a notch defect at the straight part of the tube could be simultaneously detected by L(0,1) mode guided waves through comparing experimental signals with simulation results.

Comparison of the crack-growth rates for full-encirclement and patch composite repairs

In the United States, full-encirclement bonded composite repairs have been an allowable repair method for pressure vessels since 1991. These repairs are used to reinforce damaged pressure vessels by wrapping composite completely around the vessel wall. Installing a full-encirclement repair becomes expensive and cumbersome for larger diameter vessels. And so, a patch size repair becomes preferred for use on bigger vessels. This composite patch bonds over the desired area on the damaged wall instead of completely wrapping the circumference of the vessel. In this paper, we investigate the crack propagation behavior of two repair types used to repair through-wall defects on a pressure vessel. An optical method has been adopted which monitors the crack propagation by observing the out-of-plane deflection of the blister area. It was found that the two repair types have similar crack propagation behavior during fatigue testing.

An assessment of composite repair system in offshore platform for corroded circumferential welds in super duplex steel pipe

The main aim of this study is to assess the effectiveness of a composite repair system in severely corroded circumferential welds in super duplex stainless steel pipes as a preventive measure against the premature corrosion damage at the welds. Artificial defects were fabricated on the super duplex steel tube in order to reproduce the localized corrosion damage defects found in real welded joints. Three kinds of through thickness defects were considered: 25%, 50% and 96% of the perimeter of the pipe. The performance of the repaired pipe was assessed by hydrostatic tests as per ISO 24817 standard. The results showed that the composite repair system can sustain the designed failure pressure even for the pipe damaged with through-wall defect up to 96% of the perimeter of the pipe. Hence, the composite repair system can be used as a preliminary tool to protect the unexpected or premature failure at the welds and maintain an adequate level of mechanical strength for a given operating pressure. This composite repair system can assure that the pipe will not leak until a planned maintenance of the line. Nevertheless, further work is still desirable to improve the confidence in the long-term performance of bonded composite.

Polymer-based composite repair system for severely corroded circumferential welds in steel pipes

Abstract In offshore installations, the use of piping is crucial for transporting different kinds of fluids. In this kind of environment, corrosion is one of the major damage mechanisms of the metallic pipelines, due to aggressive fluid and atmosphere. Duplex and Super duplex stainless steel pipes are increasingly being used in offshore platforms due to the excellent material's resistance to corrosion combined with good mechanical properties. However, the welding process is not simple and a high susceptibility to corrosion may occur if some parameters are not very well controlled. Severe corrosion damage in some weld beads eventually impairs the serviceability, mainly through-wall defects that cause leaks in the pipelines. The main motivation is the duplex and super duplex stainless steel pipes that are increasingly being used in offshore platforms due to the excellent material's resistance to corrosion combined with good mechanical properties. However, the welding process is not simple and a high susceptibility to corrosion may occur if some parameters are not very well controlled. Severe corrosion damage in some weld beads eventually impairs the serviceability, mainly through-wall defects that cause leaks in the pipelines. Examples concerning the use of composite repair systems in different damage situations (artificial defects aim at reproducing localized corrosion damage defects found in real welded joints) show the possibility of effective use of the proposed alternative procedure as a reliable tool for lifetime extension and to reduce the pipeline downtime. This methodology is conceived to adequately repair weld joints presenting damage up to 80% of the wall thickness and through-wall defect up to 50% of the perimeter of the pipe.

Modelling brittle fracture propagation in gas and dense-phase CO2 transportation pipelines

The development and application of a fluid–structure interaction model for simulating the transition of a through-wall defect in pressurised dense (150bar, 283.15K) and gas phase (34bar, 283.15K) CO2 pipelines into a running brittle fracture is presented. Given the economic incentives, the fracture model is employed to test the suitability of the existing stock of natural gas pipelines with the relatively high ductile to brittle transition temperatures of 0 and −10°C for transporting CO2 in the terms of their resistance to brittle fracture propagation. The hypothetical but nevertheless realistic scenarios simulated involve both buried and above ground 10km long, 0.6m i.d. pipelines. Based on the assumption of no blowout of the surrounding soil upon the formation of the initial leak, the results show that the transition of the leak into a running brittle fracture in buried CO2 pipelines is far more likely as compared to above ground pipelines. In addition, gas phase pipelines are more prone to undergoing a propagating brittle fracture as compared to dense phase pipelines despite the lower operating pressures of the former. Furthermore, counter-intuitively, isolation of the feed flow following the discovery of a leak is shown to facilitate brittle fracture failure. The initial defect geometry on the other hand is shown to have a profound impact on the pipeline's resistance to propagating brittle fractures.

Local and Global Reference Stress for Circumferential Irregular-Shaped Defects in Pipes

Numerical procedures for calculation of reference stresses for pipelines with circumferential defects, based on simulation of global and local ultimate plastic states, are proposed. There are some peculiarities of proposed procedures. First, the schematic analysis of the deformation process of pipe with surface defects is suggested, based on which two critical cases have been specified: Global (the net-section-collapse (NSC) criterion) and local, which are applicable for a very wide surface defect and for a sharp crack, respectively. The global solution also describes behavior of through-wall defects. Second, within the framework of the available NSC criterion the unified algorithm for determination of reference stresses (global solution) for irregular-shaped circumferential defects under multifactor loading (internal pressure, axial force, bending moment) is proposed. Third, according to the local modeling, the restricted capability to resist plastic deformations is takes into account by inserting of artificial symmetrical defect. The unified procedure for calculation of reference stresses (local solution) for pipe with irregular-shaped circumferential defects under multifactor loading is developed. Finally, the results obtained from the proposed solutions are compared with the ones from full-scale burst tests.

Comparison of coupon and full-scale determination of energy release rate for bonded composite repairs of pressure equipment

Bonded composite repairs of pressure equipment intended to rehabilitate a through-wall defect are governed by the fracture resistance of interface between the composite and the substrate. This fracture-dominated failure mode requires a detailed understanding of the fracture energy of the composite–substrate interface. Current approaches for measuring this interfacial fracture energy use simulated repairs on small-scale pressure vessels. In this paper we describe a simple coupon-based test using a width-tapered-double-cantilevered beam specimen. Fracture energies determined using the coupon-based approach were in close agreement with values back calculated from full-scale testing performed on simulated through-wall defects.

Global limit load solutions for thick-walled cylinders with circumferential cracks under combined internal pressure, axial force and bending moment − Part II: Finite element validation

Global limit load solutions for thick-walled cylinders with circumferential internal/external surface and through-wall defects under combined positive/negative axial force, positive/negative global bending moment and internal pressure have been developed in Part I of this paper. In this Part II, elastic-perfectly plastic 3-D finite element (FE) analyses are performed for selected cases, covering a wide range of geometries and load combinations, to validate the developed limit load solutions. The results show that these limit load solutions can predict the FE data very well for the cases with shallow or deep and short cracks and are conservative. For the cases with very long and deep cracks, the predictions are reasonably accurate and more conservative.

Global limit load solutions for thick-walled cylinders with circumferential cracks under combined internal pressure, axial force and bending moment – Part I: Theoretical solutions

Global limit load solutions for thick-walled cylinders with circumferential internal/external surface and through-wall defects under combined positive/negative axial force, positive/negative global bending moment and internal pressure are derived based on the von Mises yield criterion and the net-section collapse principle. Solutions for cases both with/without considering crack face contact when all or part of the crack is located inside the compressive stress zone are obtained. For some limiting cases, the solutions are compared with available solutions and with some published finite element (FE) data, showing good agreement. Systematic validation of the solutions using a wide range of elastic–perfectly plastic 3-D FE data is described in part II.

Qualification of a composite system for pipeline repairs under dry, wet, and water-submerged conditions

A new glass/epoxy prepreg system has been developed as a solution to a long-standing challenge of corrosion and other damage, such as gouging and denting, sustained by piping, pipelines, and risers. The system has been designed to be applicable in the majority of operational conditions encountered in the oil and gas sector, encompassing onshore as well as offshore environments. This paper discusses the comprehensive qualification process undertaken to enable the repair of wall-thinning defects (Type A) and through-wall defects (Type B). The results show that the composite system meets the requirements of ISO/TS 24817 and so also concurrently complies with ASME PCC-2.

Performance of a carbon-fiber/epoxy composite for the underwater repair of pressure equipment

The use of composite materials to rehabilitate damaged piping and pressure vessels has been growing in recent years. Most of these repairs occur onshore to either aboveground or buried pressure equipment. Recently, there has been increasing demand for these systems to repair damaged systems offshore or in submerged environments. This paper details the test and performance of a composite repair system that has been installed and cured while submerged. Testing was performed on a coupon-level to investigate the impact of an aqueous submersion on materials properties. Additionally, full scale testing on three pipe geometries, straight, elbow, and tee, with through-wall defects were also performed to determine the influence of water contact on repair efficacy. Results indicated that the short-duration water exposure for the coupons has little statistically significant effect when compared to samples cured under ambient conditions. Failure pressures of the pipe specimens repaired underwater were lower than specimens that had been repaired dry.

Composite repair of through-wall defects in pipework – Analytical and numerical models with respect to ISO/TS 24817

A major problem for the oil and gas industry is the repair of corroded steel piping. As a replacement of affected sections usually results in high expenses for the operator, rehabilitation methods which can be implemented on an operating pipeline are favoured by the industry. The application of overwraps made of Fibre-Reinforced Polymer systems offers such a promising method. In this work it is assumed that an overwrap is already installed, however, ongoing internal corrosion leads to a through-wall defect in the pipe. Hence, the fluid pressurises the repair and a detachment of the overwrap from the steel substrate can occur. This can even result in external leakage. In order to avoid this case of damage an analytical assessment approach is developed by means of Linear Elastic Fracture Mechanics and Finite Element Analysis. Subsequently, this approach is compared to the one given in the Code ISO/TS 24817 which is currently the mainly used code for the qualification and design of composite repairs for pipework.

Level 2 analysis to estimate LERF risk from a thermally induced rupture of a steam generator tube defect

This paper outlines the Level 2 portion of a methodology for determining the incremental induced steam generator tube rupture large early release fraction caused by an actual through-wall defect. This defect was responsible for the minor steam generator tube leak that occurred in September 2002 at the Comanche Peak Steam Electric Station Unit 1. In order to quantify the performance of the defect over the operating cycle, a range of defect lengths were input to the PROBFAIL computer code [Kenton, M., 2001. PROBFAIL: A Computer Code for Evaluating the Likelihood of Steam Generator Tube Rupture in Severe Nuclear Power Plant Accidents, CREARE TM-2138], using appropriate boundary conditions derived from MAAP4 [Henry, R., et al., May 1994. MAAP4—Modular Accident Analysis Program for LWR Power Plants, Computer Code Manual, EPRI Research Project 3131-02] runs. From the analysis of the calculated times of burst for each assumed defect length, the minimum through-wall defect length necessary for tube burst to occur prior to hot leg or surge line creep rupture was calculated. The probability that the defect would actually have this length was then estimated by determining the fraction of the cycle for which the defect would be at least that long. The methodology development and implementation relied on MAAP4 runs, which are discussed extensively in connection with their role in: (1) guiding the construction of the accident progression event tree, (2) generating relevant information for probability assignments in the various underlying fault trees and (3) obtaining boundary conditions of pressure and temperature for use in PROBFAIL. The overall increment in LERF due to the existence of the defect was calculated to be approximately 4E−08.

Evaluation of the burst characteristics for axial notches on SG tubings

Some events of steam generator tubes have been reported in some nuclear power plants around the world. Main causes of the leakage are from various types of corrosion in the steam generator (SG) tubing. Primary water stress corrosion cracking (PWSCC) of steam generator tubing have occurred in many tubes in Korean plants, and they were repaired using sleeves or plugs. In order to develop proper repair criteria, it is necessary to ascertain the leak behavior of the tubings. A high-pressure leak and burst testing system was manufactured. Various types of electro-discharged-machined (EDM) notches having different lengths were machined on the o.d. of test tubes to study SG tube behavior. Leak rate and ligament rupture pressure as well as the burst pressure were measured for the tubes at room temperature. Rupture pressure of the part through-wall defect tubes depends on the defect depth and length. Water flow rates after the rupture were independent of the flaw types; tubes having 20–60 mm long EDM notches showed similar flow rates regardless of the initial defect depth. A fast pressurization rate generated a lower burst pressure than the case of a slow pressurization.