Ultrasonic NDE of Spoolable Composite Tubulars

OBJECTIVE: To develop a technology for extremely rapid, in-situ, ultrasonic non-destructive testing (UNDT) of spoolable composite tubulars. To develop associated acceptance/rejection criteria for these tubulars. This work we will build upon our early work on OTRC project [1].

INTRODUCTION: As we enter the twenty-first century it is envisioned that the existing steel tubulars will be increasingly replaced with composite tubulars. A unique aspect of these tubulars is that during repeated coiling and uncoiling, they will be subjected to extremely large strains. The evolution of damage caused by these large strains is not well understood. In order to ensure the reliability and safety of composite tubulars, technology for quality control/health monitoring must be developed. Since these tubulars are expected to be rather long (thousands of feet), an NDT method for a rapid inspection is needed. A regular inspection of the tubulars by the use of this technology will result in (a) significant cost savings to the industry, and (b) provide the MMC with a regulatory tool. Almost all existing NDT methods for periodic checkups of pipes are based on scanning of entire pipe surface using a "smart pig," a robotic device that crawls inside the pipe carrying hundreds of ultrasonic sensors each performing an independent pulse-echo measurement. Due to a lack of a comprehensive signal analysis this technique frequently give erroneous results [2]. Moreover, as it is presently practiced it cannot be applied to composite materials where multiple reflections from complex internal structure mask the reflection from a defect. Significant development of this technology is required before it can be used for the inspection of spoolable composite tubulars; this defines an objective of the proposed work.

APPROACH: Historically, UNDT techniques have been developed for situations where the object of inspection is stationary, and the inspection tool moves (e.g., a typical c-scan device); this limits the speed and/or quality of inspection. The situation here is exact opposite: the composite tubular is moving rapidly; the inspection station therefore may be chosen to be stationary. This opens up the possibility for the use of extensive and elaborate hardware, and of powerful computers and sophisticated data analysis.

A schematic of the proposed digital UNDT station is shown in Fig. 1. The arrow indicates the direction of motion of the tubular. The tubular is shown as it passes through an enclosure where it is immersed in water. The role of water is to acoustically couple the transducers and the tubular. In order to cover the entire surface of the pipe, scanning in axial and circumferential directions is required. The former is naturally provided by the axial motion of the pipe. The latter is accomplished by arranging a large number of transducers to cover the entire circumference.

Two of the (many) possible designs are shown in Fig.2. In the first, spherically-focussed piezoelectric transducers will be used in the pulse-echo mode (Fig.2A). In the second a cylindrically-focused phased-array transducer will be employed (Fig 2B). Phased array transducer has the capability of beam steering by introducing appropriate time delays in the transmitted and received signals. As a result, the same transducer can be used for normal as well as oblique incidence. Most commonly found defects in filament-wound composite pipes, and their characteristic effect on ultrasonic wave propagation are listed in Table 1 [3]. Inspection at normal incidence will be used to detect these defects. It is well known that transverse matrix cracks (i.e., cracks whose faces are parallel to the radial direction) cannot be easily detected by using normal incident waves; therefore, obliquely incident waves will be used [4,5].

Figure 1.

 

Figure 2

In actual practice a number (say, about 16) of transducer and associated electronics units working in parallel will be needed to cover the entire circumference. However, in the proposed work we will build and test only one unit. To achieve a high probability of detection with a small number of false alarms, we will use a two-step algorithm. In the first, signal at each point will be broken into a number of wavelets using simplified generalized matching and transversal digital filters [6]. The deviation of a signal from a reference signal beyond an experimentally established threshold will be used as a defect indicator. The potential defect location will be marked and the associated array of data will be downloaded to an external computer for further analysis. In the second step this data will be further analyzed by using more sophisticated pattern recognition procedures to discriminate various defects according to their characteristics.

Finally, in cooperation with the damage evolution modeling effort of team members Drs. Ochoa, Schapery, and Whitcomb, acceptance/rejection criteria for a tubular with known defects will be developed.

 

ANTICIPATED PROJECT DURATION: 3 years

PROJECT PLAN FOR 1999/2000:

SCOPE OF WORK:
a. Design and fabrication of a single channel prototype of NDE work station using mechanical scanning (instead of electronic scanning that will be developed and used in the final work station).
b. Development of signal analysis algorithms for defect identification.

ANTICIPATED RESULTS: Proof of concept of fully automated automatic high speed ultrasonic inspection of composite tubulars

PROJECT PLAN FOR FUTURE YEAR(S): 2000 /2001

SCOPE OF WORK:
a. Design, fabrication and testing of the multiple channel prototype of NDE work station.
b. Writing of computer codes for real time defect detection.
c. Start test on spoolable tubulars.

ANTICIPATED RESULTS: Working prototype of NDE work station.

PROJECT PLAN FOR FUTURE YEAR(S): 2001/2002

SCOPE OF WORK: Develop acceptance criteria for spoolable composite tubular. Develop a database for a variety of defects normally encountered in the spoolable tubulars. Identify critical defects. Develop procedures for UNDT of these defects.

ANTICIPATED RESULTS: A methodology for the detection of defects. Acceptance/rejection criteria for spoolable composite tubulars.

PRINCIPAL INVESTIGATORS: Vikram K. Kinra, One PhD student.

References

1. "Ultrasonic nondestructive evaluation of offshore structures with curved surfaces," Offshore Technology Research Center, Research grant 10/96-9/99.
2. Williamson, G.C., and Bohon, W.M. "Evaluation of Ultrasonic Intelligent Pig Performance: Inherent Technical Problems as a Pipeline Inspection Tool - part II," Corrosion Prevention & Control, 42:8-12, 1995).
3. "Immersion testing of composite materials; Test of composite tubing," in Nondestructive Testing Handbook; Vol. 7: Ultrasonic Testing, (ed. by P. McIntere , ASNT, 1991), 246-248.
4. "Testing composite at oblique incidence," idem, 534-537.
5. Maslov, K, Kim, R.Y., Kinra, V.K., and Pagano, N.J. "A technique for ultrasonic detection of internal transverse cracks in graphite/epoxy composite laminates," Composites Science and Technology, 59, 1999.
6. Habibi-Ashrafi, F, and Mendel, J.M. "Estimation of parameters in lossless layered media systems," IEEE Trans. Automat. Contr. AC-27, 31-48, 1982.

 

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