Optical in-line inspection tool for internal monitoring of pipelines

The types of anomalies detected and sized with precision include geometric flaws as well as internal corrosion. The data collected provides valuable information, especially for the integrity assessment of gas pipelines.

Introduction
The requirement for accurate and reliable inspection systems for pipelines is growing. Pipeline operators need to have access to precise information regarding the state of their networks in order to optimize maintenance procedures and usage of their pipeline systems. A critical set of data is the geometric information regarding possible anomalies and flaws in the wall of the pipe, including internal metal loss. Measurements of length, depth, width and location are needed in order to perform integrity assessment and fitness-for-purpose studies.

Growing demands include:

• Extending the use of existing pipelines beyond their original design life,
• Upgrading existing pipelines in order to increase throughput,
• Inspection needs regarding the combination of older and newer pipelines of different sizes in a wider infrastructure such as the North Sea,
• The inspection of previously termed "unpiggable" pipelines,
• Precise internal inspection data for offshore pipelines,
• Inspection of telescopic lines, i.e. pipelines of dual- or multi-diameter design.

Pipelines with varying diameter or nonconstant diameter have emerged as a result of "smart" design, i.e. allowing changes in size for one or the combination of the following reasons: smaller size platform risers on offshore systems in order to save weight and costs, smaller size main valves in a large diameter line in order to save weight and thus costs and step changes in the diameter of gas pipelines to account for the inverse relation between volume and pressure.

A large variety of in-line inspection tools has been developed over the years. Table 1 provides an overview about available technologies and the physical principles they are based on. Further introductary and advanced information can be found in the literature [1 - 6].

This paper will focus on introducing a novel optical in-line inspection tool incorporating a laser-based 3-D imaging technology. The tool design also includes the capability to inspect multi-diameter pipelines, i.e. pipelines with a variable diameter. Further information on the issue of multi-diameter inspection technologies can be found in [7].

Inspection of offshore gas pipelines
The tool being introduced here can be applied in a large variety of different types of pipelines, however initially the major focus regarding its application were offshore gas pipelines. The total length of offshore pipeline infrastructure is growing continuously. In the Norwegian sector of the North Sea alone over 7000 km of high pressure pipelines were taken into operation since the 1980's, mainly in the diameter range from 16“ to 42“. These pipelines are inspected regularly and maintained with greatest care. The range of specialized equipment used for this task, usually referred to as in-line inspection tools, includes technology utilizing a variety of non-destructive testing principles, for instance magnetic flux leakage and ultrasound.

Besides these more "classical" technologies, optical methodologies are also being used. One particular inspection tool, an opto-electronic laser based intelligent pig will be introduced in the following.

Principle of operation
The opto-electronic tool has been designed to provide an accurate and cost-effective solution for the inspection of the internal surface of pipelines and consists of a highly compact optical system built into a suitable mechanical pig frame. As with other in-line inspection tools the mechanical components must incorporate a drive unit, house the energy supply and provide room for the measurement and data storage devises.

The development originates from an earlier attempt to visually inspect the interior of risers and shorter sections of pipe by conventional video recordings. However, the use of video proved to be impractical for other than very short lengths, due to severe power and data storage limitations. The solution to these challenges came with the introduction of a novel line scan camera and laser illumination technology.

Figure 1 shows the principle applied. A continuously moving laser line illuminates the inner pipe wall. The reflected high intensity light is projected onto camera/optical sensors which are aligned at angles between 30º and 60º to the wall.

By utilizing a triangulation principle on a two-dimensional sensor, the system can both measure the position of an anomaly and its intensity, as can be seen in Figure 2. By combining these two measurements, a three-dimensional image of the pipe surface can be displayed.

In order to compensate for any disturbing vibrations during the ride through the line, the tool and for smaller diameters the individual tool bodies are supported by spring-loaded wheels. The tool consists of an inspection (e.g. laser/ camera) unit and a carrier unit. The inspection unit, for instance for the 42“ version, consists of eight lasers and eight cameras and is mounted on shock absorbers inside the carrier body. To simplify data and image processing, an antirotation facility is built into the carrier.

The inspection tool is fully autonomous, carrying onboard battery power and all necessary computer hard- and firmware. The laser based line scan technology is extremely power and data friendly making it possible to cover inspection ranges of up to 1000 km for the larger size tools, traveling at 5 to 10 m/s. The general mechanical design of the tool makes it suitable for use in telescopic lines, i.e. pipelines with varying diameter.

Self-cleaning system
In order to make sure that all the optical lenses and orifices are kept clean, a permanent flow of gas is induced across the length of the tool due to the differential pressure across the tool. Any dirt or debris, due to dust or sediment in the gas transported is thereby kept from settling on the tool.

Data visualization
Data recorded during a survey can be displayed immediately after a run has been completed and are easy to interpret. What you see is what you have, as can be seen clearly in Figure 3a and Figure 3b. This technology provides a very direct and quick means to assess the state of the internal surface of a line inspected. The data analysis software includes a variety of viewing options.

Any anomalies are initially viewed by simply "surfing" through the pipeline. Viewing options include 2D- and 3D-representation as well as wire-frame configurations. The latter enable a detailed inspection of the pipe and the sizing of any flaws found. Figure 3a shows a regular photograph of an anomaly on the surface with a diameter of 25 mm. Figure 3b shows the same defect as seen by the tool and presented in a 2D mode.

Figure 3c shows the 3D visualization of that anomaly and Figure 3d the corresponding wire frame model. The pictures clearly show that the resolution of the optical system is such that the viewer/data analyst obtains a "photographic" image of any anomaly detected.

Technical specifications
The optical and electronic system of the tool works with a depth resolution of 1 mm and a minimum defect size (area) of 2 x 2 mm. The optical resolution of the image is thereby better than 1 x 1 mm. The available mechanical adaptation kits enable the tool to be used for any nominal pipeline diameter from 10“ through to 52“. The design incorporates the ability of the tools to inspect multi-diameter pipelines.

Inspection speeds of up to 5 m/s in gas and operational pressures of up to 200 bar can be accommodated. The tools are fairly light weight. For instance the weight of a 40“ configuration for the inspection of a 100 km pipeline is only approximately 70 kg. Figure 4 and Figure 5 show a CAD drawing and a photograph of the 42“ configuration of the tool.

Acceptance tests
The opto-electronic tool has been used for a number of inspection projects to date including a positive "Factory Acceptance Test" (FAT) by DNV. The tool fully complies with a number of technical requirements placed upon it:

• high sensitivity and measurement accuracy regarding internal anomaliesand flaws,
• internal corrosion/metal loss as well as any type of internal anomaly including geometric changes are detected and sized with a very high resolution,
• data access and data review are possible at site immediately after an inspection run,
• the tool has a full multi-diameter capability including the ability tonegotiate diameter reduction (bore restrictions) of up to 50 %.

Inspection of non-standard pipelines
The design of the device makes it well suited also for the inspection of non nonstandard pipelines. These types of lines, which are not normally suitable for the inspection with free-swimming tools are also sometimes referred to as "non-piggable" lines. Due to its mechanical flexibility the opto-electronic pig introduced here can provide valuable information for the integrity assessment of such pipelines.

The tool can be applied as a standalone inspection device or in combination with the use of other in-line inspection tools complementing the information collected during metal loss and crack detection surveys.

References
[1] Cordell, J., Vanzant, H., All About Pigging, On-Stream Systems, Circencester, 1995

[2] Skerra, B. et al., Handbuch der Molchtechnik, Vulkan-Verlag, Essen, 2000

[3] In-Line Nondestructive Inspection of Pipelines, prepared by NACE International Task Group 039, NACE International Publication 35100, NACE international, 2001

[4] Pipeline Pigging & Integrity Technology, 3rd edition, Editor John Tiratsoo, Scientific Surveys and Clarion, 2003

[5] Specifications and requirements for intelligent pig inspection of pipelines, Pipeline Operator Forum, Shell International Exploration and Production B.V., EPT-OM 1998

[6] Reber, K., Beller, M., How To Get The Most Of The Regular And Repeated Inspection Of Pipelines Using In-Line Inspection Tools, Proceedings of the Sixth International Conference, Pipeline Maintenance & Rehabilitation, PennWell, Berlin, 2003

[7] Neestas, Ola, New Condition Monitoring Tools For Cost Effective Pipeline Integrity Assurance, Proceedings of the 2nd Petro- Min Pipeline Conference, PetroMin, Singapore,2003

[Source: 3R INTERNATIONAL: Journal for Piping, Engineering, Practice
Special Edition 13/2005]