Research programmes to enhance the assessment of this type of damage have led to a new approach in the detection of inner diameter anomalies by means of the latest generation of high-resolution geometry inspection technology.

Based on experimental data obtained in the course of several research projects, this article presents the benefits of high-resolution geometry tools in conducting baseline surveys for the purpose of subsequent assessment of mechanical damage. The operating benefits gained thereby are explained in the context of other in-line inspection technologies.

The nature of ovalities and other ID anomalies

Being very complex in structure, ID anomalies can appear in a wide range of forms. The American Society of Mechanical Engineers (ASME) defines a dent as ‘a gross disturbance in the curvature of the pipe wall’.

The most important type of anomaly for the purposes of the present investigation are ovalities. Defined as ‘a deviation of the circular shape of the cross section of the pipeline’, an ovality affects the entire circumference of the pipeline cross section. Ovalities usually appear in combination with a dent making them part of a more complex ID anomaly.

A new approach to the detection of ID anomalies

To ensure full compliance with all requirements governing the wide variety of ID anomalies, a new approach consisting of two main features is now taken. Firstly, a new sensor has been developed which combines the proven mechanical caliper arm system with a ‘touchless’ eddy current device. Secondly, a new arrangement of these sensor devices with a second sensor ring, circumferentially offset from the first, ensures complete coverage of the internal pipeline circumference. In combination, the new sensor systems and their innovative arrangement have significantly improved probability of detection. Figure 1 shows how this new approach is implemented in a single tool.

Since it is often impossible to give a full description of the depth, length and width of many ID anomalies including dents, an accurate assessment of the properties of such anomalies based on reliable high-resolution geometry data leads to more detailed knowledge and consequently more reliable analysis of the pipeline contour. In combination, pipeline curvature and dent shape provide an important parameter for the calculation of dent strain, also known as ‘local strain’: abrupt changes in pipeline curvature are more severe and result in a higher dent strain value than plain dents. The new RoGeo·Xt makes an invaluable contribution to local strain assessment.

In addition, if the RoGeo·Xt is equipped with an Inertial Measurement Unit (IMU); it can also measure regional strain resulting from the displacement of pipeline segments due to external forces such as landslides. Whereas the displacements are detected with the IMU, all subsequent bending strain assessment decisions are as a rule supplemented by data obtained during the same inspection run on the basis of internal pipeline contour sampling with ID mapping sensors.

Exceeding regulatory requirements

Adopting a modular inspection approach whereby the newly developed high-resolution geometry technology is combined with other ILI methods such as magnetic flux leakage (MFL), geometry mapping and, as in this case, an IMU to form so-called ‘multi-purpose tools’, enables merging and mapping data sets to provide a more complete picture of the pipeline as well as a more thorough assessment of specific anomalies. Such multi-purpose tools incorporating a variety of ILI methods have been developed further to accommodate multi-diameter pipelines.

Difficult operating conditions in deep water, high-pressure or heavy wall pipes as well as challenging pipeline design features such as wyes have always placed great demands on the design of ILI tools and runs. The highly accurate data furnished by the enhanced geometric inspection system is therefore a great step forward in assessing the condition even of assets posing great inspection challenges.

In addition to the codes and regulations governing pipeline inspections, pipeline manufacturers must meet specifications for the production of pipeline joints and bends, for example by documenting in detail the manufacture of submerged arc-welded longitudinal seam pipes and stating the rating of manufactured bends to ensure satisfactory fitting of pipeline joints. An accurate baseline survey firstly confirms that these various pipeline elements perfectly fit each other following installation, and secondly provides an ideal database for analysing the pipeline’s ability to resist buckling due to environmental loading.

These strict requirements have led to greater interest in ID anomalies. In line with this interest, tools with improved performance have been developed, and this has in turned resulted in a significant increase in the number of features reported to an average of 1.5 dents per kilometre. The DOT guidelines for dents without any stress riser use as a critical benchmark a dent depth of 2 per cent and above in outer diameter. The first Rosen statistics based on ID anomaly data gathered with the RoGeo·Xt show that more than 80 per cent of detected ID anomalies are shallower than 2 per cent, meaning that the tool not only meets but by far exceeds current regulatory requirements.

Measuring ovality and welded areas including long seams as part of baseline surveys

Regulators specify that ovality (or ‘out-of-roundness’) is an applicable value for assessing the quality of manufactured pipeline joints. High-resolution geometry tools are a suitable instrument for assessing recent as-built pipelines in baseline surveys, which specify an ‘out-of-roundness’ value for all manufactured pipeline joints. ILI methods are then used to prove or disprove this value.

Figure 2 shows an example of a data set recorded during a geometric ILI of an offshore pipeline. The long seams (green appearance) around the 12 o’clock line (red) are very clear and distinctive; their position alternates between 2 o’clock and 10 o’clock with minimal scatters. This geometric inspection data allows a more comprehensive feature assessment, for example of dent curvature associated with seam welds.

Pipelines are usually welded with the long seams offset, typically at 10 and 2 o’clock. Such a pipeline layout scheme is shown in Figure 3.

A detailed comparison of the data sample and the actual pipeline layout highlights the sensitivity of the RoGeo·Xt ILI tool. This high sensitivity enormously benefits integrity management programmes, as the detection of welded areas including long seams forms a vital source of information for several defect assessment methods.

The unprecedented sensitivity in the sub-millimetre range of geometric ILI tools such as the RoGeo·Xt extends their range of applications, since internal wall thickness loss can be treated as ID changes. This means that geometry tools can now be used to support wall thickness recordings obtained from metal loss mapping tools.

Internal corrosion

The RoGeo·Xt is highly sensitive to features affecting the whole circumference of the pipeline. The sensitivity of geometric ILI tools to local features can be enhanced by combining high-resolution geometric technology with an ultrasonic (UT) device. As the distance between the UT sensor and pipe wall is known, UT not only measures wall thickness but also the stand-off signal. Since the two technologies use different measurement methods to supply the same type of information, their combination permits reliable feature assessment based on a comparison between UT stand-off and the lift-off measurement (eddy current sensor) of the RoGeo·Xt.

Due to the high sensitivity of the lift-off sensors, the RoGeo·Xt can even be used to search for shallow internal corrosion. This means that the extended tool can be used to detect and size shallow internal corrosion in exceptionally challenging conditions, for example where complete magnetisation of the pipe wall is extremely difficult or using UT is altogether impossible.

Conclusion

Rosen’s RoGeo·Xt has been proven in field applications to fulfil the requirements of standard codes and regulations. Based on a new sensor combining a mechanical calliper arm with a ‘touchless’ eddy current device as well as a novel arrangement of these sensor devices with a second sensor ring to ensure 100 per cent internal coverage, the new approach to the detection of ID anomalies provides a more complete picture of the condition of pipelines. Due to the enhancement of the tools’ geometric inspection capabilities, even marginal ovalities can be detected and sized.