Corrosion detection in aluminium alloy 1

[markjames]

I am a Production Engineer working for an underwater systems company. We currently have aluminium alloy parts come in for repair with various degrees of corrosion. Does anybody know of any corrosion detection equipment that can accurately acertain the degree and depth of corrosion?

 

[rustbuster]

Mark,

Depending on geomitries, automated ultrasonics may be a good option.

 

[ugwuegbuchima]

NDT.net - November 1998, Vol. 3 No. 11
Table of Contents ECNDT '98
Session: Pipeline Industry Ultrasonic Inspection of Risers a New and Simple Approach
R. van Agthoven
Röntgen Technische Dienst bv
P.O. Box 10065, 3004 AB Rotterdam, The Netherlands
telephone +31 10 2088208, fax +31 10 4158022
Email: rtd@rtd.nl , URL:

http://www.rtd.nl

TABLE OF CONTENTS
Abstract
Introduction
Problem Description
Approach With Umbilical Operated Tools
Riser Inspection Tools
Compact Riser Inspection Tool
Tailored Solutions
Conclusion
References



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Abstract
A riser forms the connection between the topside of an offshore platform and a pipeline on the seabed. Such pipeline systems may be equipped with launch and receive facilities for cleaning and general intelligent pigging of the entire pipeline. The riser may be of different material or of heavier wall thickness, making self contained intelligent pigging of the riser difficult, less effective or cost prohibitive. Today, offshore operators are strongly committed to protect the environment and pigging and riser inspection will assist in determining the condition of the installation and the riser in particular. In this description a new approach for a detailed ultrasonic inspection of the risers only is explained, based on the application of a compact umbilical (cable, tether) operated tool. This requires that the riser is bled down and liquid filled. The inspection tool provides on-line presentation of results for immediate evaluation. The system uses straight beam ultrasonic sensors for general corrosion survey in the pipe material and a novel ToFD ("Time of Flight Diffraction&quot technique for additional inspection of the circumferential weld area.

Keywords: offshore, risers, environment, inspection, ultrasonic, pigging, cable tools, corrosion, condition monitoring, evaluation, ToFD.

Introduction
For risers, which are usually encased or not accessible from the outside, inspection from the outside is impractical and often impossible. In this case, inspection from the inside is more practical and can provide full coverage over the entire length or height. Single side access from the platform must be arranged to utilize umbilical operated tools. This requires shutting down, depressurization and opening of the riser. Umbilical operated tools can now be used for ultrasonic corrosion detection and weld area inspection with the ToFD (Time of Flight Diffraction) technique.

Problem Description
A typical example of a riser is illustrated in Figure 1. Any inspection tool would have to enter through the launch and/or receive trap, used for cleaning pigs, or a specially made entrance point. The risk of a free-swimming intelligent pig getting stuck in the riser or pipeline may be considered too great or cost prohibitive. Risers and pipeline systems are often old with unknown obstructions or features. Moreover, the prime area of interest for inspection is often the "tidal zone" only in the heavy wall thickness riser. Different riser configurations exist, being J-tubes on the inside of a platform leg or on the outside, or risers inside a pull tube. As the pull tube may be open at the bottom water is present between the pull tube and riser, this is an area that needs much attention as external corrosion may occur in this area.

Fig. 1: Riser Configurations

Approach with Umbilical Operated Tools
Fig. 3: Electrically driven crawler for pipeline inspection between 16" and 48" diameter
Before discussing the special tools for riser inspection the experience with umbilical operated tools is presented: for the inspection of (off-)loading pipelines special cable-operated ultrasonic pigs [1] are available and thanks to ongoing developments more and more complex geometries can be inspected. Today, even distances up to 10 miles can be covered with an umbilical operated tool. As Figure 2 and 3 show, a motor driven crawler is used for diameters over 16". This self-propelled tool makes the operation independent of pumping facilities and other (expensive) assistance such as divers for offshore assistance. An array of ultrasonic probes is mounted on the front of the inspection tool.

To deploy the tool, the liquid filled pipeline must be interrupted and opened to insert the tool. Apart from power supply and hoisting equipment no other facilities are needed. The umbilical allows full control over the tool speed and performance and above all on-line presentation of the inspection results. These are very attractive features for pipeline owners as this vastly decreases the risks.

Fig. 2: Principle of (off-) loading pipeline inspection system. Fig. 4: Block diagram of liquid propelled tool for inspection of diameters between 16" - 14".

For diameters below 16", differential pressure is used to move a bi-directional tool in and out of the pipeline, still making use of the umbilical with the same advantages as mentioned earlier. See Figure 4 and 5. This tool is, similar to self-contained pigs, propelled by differential pressure over its propulsion discs. To retrieve the tool, the pressure difference and flow has to be reversed. For proper sealing of the umbilical at the launch/retrieve end of the pipeline a special closure head with a feed-through seal (the stuffing-box) has to be installed.

Fig. 5: Tool for inspection of diameters 6" to 14", from R to L: propulsion unit, odometer, 64 transducer inspection module, three electronic modules, cable termination, cable.

Riser Inspection Tools
At the end of 1991, the first umbilical operated riser inspection tool was completed. The Spider tool, named because of its appearance - is intended to inspect risers, which are unpiggable with commercially available intelligent pigs. Its first application, for which it has been tailored, was on a 20" gas riser in the Norwegian section of the North Sea. The riser and entire pipeline system was filled with sea water to allow ultrasonic inspection.
The 500 mm riser contained several 3D bends. The internal diameter of the riser pipe varied by more than 15%, and some of the diameter changes were sudden. The Spider tool could cope with these extremes and perform an inspection with full coverage due to the unique transducer lay-out. By swiveling the transducer array, which is equipped with 10 transducers, this full coverage is achieved. The data display computer produces a colour enhanced C-scan thickness image.

A derivative and compact version of these tools is developed to inspect small diameter thick-wall risers. This is further discussed in this document.

Compact Riser Inspection Tool

Fig. 6: Block diagram Riser Inspection Tool.

Fig. 7: Complete Compact Riser Inspection System.

Fig. 8: Riser Inspection Tool ready for deployment, adapted for 12" diameter.

Fig. 9:
Riser Tool being lowered in riser through ho
Fig. 10: Principle of ultrasonic stand - off technique.
Fig. 11: Procedure for inspection of gas risers.
Fig. 12: Black & White example of colour enhanced computer data system.
Fig. 13: Detection capabilities with transducers for corrosion detection
Fig. 14: Principle of ToFD technique.
Fig. 15: Typical ToFD image of weld with external corrosion.
To inspect the vertical sections of a riser a new compact umbilical operated ultrasonic tool is developed. The tool, a two-unit articulated device is lowered in the open riser by gravity using a winch assembly. A separate umbilical connects the ultrasonic electronics inside the tool with the ultrasonic unit and data presentation computer on the platform. This is illustrated in Figure 6. The entire system is shown on Figure 7.
The compact set of equipment consists of:


Riser inspection tool
Umbilical, tailored to inspection requirement,
Electric winch unit
Odometer assembly
Multi channel ultrasonic unit
Data storage and display computer
The tool can easily be adapted to inner riser diameters of 6" to 12", Figure 8 shows the tool prepared for an inspection of a 12" riser, Figure 9 shows the tool being lowered just above a riser. The Ultrasonic Stand-Off Method: The most suitable method for quantifying and differentiating between internal and external corrosion is the stand-off technique, as illustrated in Figure 10. The system is also capable of measuring wall thickness. A circular array of transducers is located at some distance from the inner pipe wall and the liquid in the pipe, usually crude oil or sea water, acts as the essential acoustic medium. In this way both internal corrosion as well as the pipe wall thickness, can be measured simultaneously to achieve coherent results with an accuracy much better than 1 mm. Moreover, the stand-off technique allows passage of severe dents and other geometrical obstacles.

Cleaning And Preparation: The inspection system can be used to inspect both gas and crude risers, provided the riser is filled with a suitable liquid during the inspection. In principle, a crude riser can be inspected with the riser filled with crude. This only requires depressurization and opening of the riser. For safety reasons the riser may also be filled with sea water, in that case the crude is displaced according to a similar procedure as used for inspection of gas risers:

To prevent that the entire gas pipeline and riser system needs to be filled with liquid a special procedure has been developed. This consists of the use of one or more high friction sealing pigs, which are pumped down the riser under water pressure, see Figure 11. The pigs have enough friction to hold the head of water above them, which can then act as a suitable acoustic medium. By balancing the (reduced) line pressure the riser can be safely opened and inspected. Upon completion of the inspection the riser can be closed and by increasing the gas pressure behind the sealing pig, it will be pushed back up, the water can be drained and the sealing pig retrieved via the pig-trap. This procedure is also recommended if crude risers have wax deposits on the inner surface. The ultrasonic method is sensitive to wax or dirt on the inner pipe wall. A maximum thickness of only up to 1 mm is acceptable. Excess wax can be removed by applying the above described technique with a high-friction scraper/sealing pig.

Electronic Equipment And Report: The entire inspection system consists of:
A Riser inspection tool, a special umbilical, a multi channel ultrasonic unit and a computer for data recording and display. This compact system, see Figure 7 allows easy handling and transport even by helicopter. The system provides instant results. Several modes of on-line (instant) data presentation are used. An example in black and white is given in Figure 12. These modes include colour-enhanced C-scan images, supplemented by analog presentations. All collected data, including tool position and time, are tape and disc recorded (CD-ROM) for later retrieval and off-line evaluation. This approach allows the furnishing of a preliminary site report before de-mobilization. A final report including an integrity map is supplied after evaluation of results at RTD's headquarters in The Netherlands.

Inspection Coverage for Corrosion Detection: To obtain a fine grid of data, small axial sampling intervals are applied. Sufficient circumferential coverage is achieved by applying a large number of ultrasonic transducers. The size of the corrosion pits that can be detected and quantified will depend on the type (beam geometry) and the number of transducers employed. The current riser inspection tool applies 32 transducers, which are equidistantly distributed around the circumference. The main area of interest when inspecting risers is the external surface in the tidal zone. In a vertical pipe section corrosion can occur at random locations on the circumference. Therefore a high surface coverage is required. The circumferential coverage can be established using the diagram in Figure 4 for transducers with a beam diameter of 10 mm. This means for example in an 8" riser, where 50% coverage is achieved, that defects of 20 mm cannot escape detection. Increase of coverage is obtained by inspecting during both the downward and upward movement, while some axial rotation occurs this leads to practically full coverage. To further increase the coverage a second inspection sequence can be performed. As the inspection speed can be as high as 500 m/hour and many risers are only a few hundred feet high the total inspection time is very short, even for a few complementary runs and easily performed within a one-day shut-down.

Detection Capabilities: The system is capable of detecting and quantifying corrosion, both internal and external corrosion can be sized. The system is capable of inspecting up to (no dead zone) the circumferential weld area, however the weld volume itself is not inspected, as illustrated in Figure 13, case #7. This also means that external corrosion in the weld area itself could not be sized. However, new techniques have recently become available to assess the weld, of which the most promising technique is the so-called ultrasonic ToFD technique. [3] With only a two transducer system the weld can be inspected where weld defects and depth of external corrosion attack can be accurately sized. The principle of the ToFD technique is based on mapping of the edges of defects by using diffracted signals. This is in marked contrast with conventional ultrasonic weld inspection, which relies on the amplitude of specular reflections received from defects. In order to determine the position of the defect edges, a wide-angle transmitter probe is placed on either side of the weld (Figure 14)

In case of a flawless weld the relevant ultrasonic signal will consist of a so-called "lateral wave", corresponding to the direct surface path between transmitter and receiver, an a "back-wall" echo. Between the lateral wave and back wall echo there will be no signals. If defects are present in the weld, however, the ultrasonic waves will be diffracted by the edges of the defect. This technique can be used to size weld-defects but also to quantify external corrosion. The latter is the main reason for implementing this technique in this system. The depth of the defect edge can be calculated from the time of flight of the corresponding ultrasonic wave. Defect height or corrosion depth can be readily measured. A typical figure for the accuracy that may be achieved in through thickness height measurement is 1 mm. A complete image over the length of the weld is achieved by recording the ultrasonic signals at regular intervals (typically 1 mm). The B-scan images thus obtained may be looked upon as a side view of the weld. A typical example of a weld with external corrosion is shown in Figure 15.

For a ToFD inspection of riser welds the transducer system needs to make a circumferential scan along the weld. Such a scan mechanism can be attached to the front of the riser inspection tool. The principle is shown in Figures 16 and 17. This system is successfully applied for a job offshore USA territory.



Fig. 16: ToFD module for weld area inspection. Fig. 17: ToFD module attached to riser inspection tool.

Tailored Solution
In the past 15 years, umbilical operated vehicles have been developed and used for the inspection of many large diameter (off-)loading pipelines, with lengths of over 10 km now. Gradually new solutions became available for inspection of pipelines and risers with more complex geometries and smaller diameters [2]. Moreover, by adding the newly developed ToFD technique an even more powerful tool became available. The experience with these systems and capability to tailor systems to particular requirements allowed us to modify our tools and design the compact riser inspection tool discussed in this paper.

Conclusion
It is proven that with this new umbilical operated small diameter riser pipe inspection tool a fast and complete inspection can be performed of specific areas of a riser including the welds. The reinforced umbilical allows safe and full control over the performance of the tool as well as instant results.

References
J.A. de Raad, "Cable and other ultrasonic pigs, Pipes & Pipelines International, March 1990, Vol. 35, 2.
J.A. de Raad, "Experience with cable-operated ultrasonic pigs for pipeline and riser pipe inspection", Paper No. 28, NACE Annual Conference and Corrosion Show, Corrosion 95, March 26-31, 1995 Orlando / Florida
F.H. Dijkstra and T. Bouma, "Inspection of Complex Geometries with Time of Flight Diffraction". Paper presented at the 14th International Conference on NDE in the nuclear and Pressure Vessel Industries, September 24-26 1996, Stockholm/Sweden.
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