Protecting the pipeline through its lifecycle

Lack of pipeline integrity can lead to failure with potentially catastrophic consequences.  Assuring the mechanical integrity of pipelines, whether in exploration, transmission or process plants is critical for ensuring complete plant safety. In order to maintain integrity and safety, accurate inspection methods must be employed at all stages of a pipeline’s life span. 

Many factors can affect pipeline integrity throughout its life cycle. From the original steel plates used to fabricate the pipes to the girth welds that join them together, pipelines need different types of inspections for different points throughout the lifecycle. As the pipe ages, the girth welds can be particularly vulnerable, and are often exposed to harsh conditions. These conditions can range from an exposed double joint yard to the back end of a lay barge in either very hot or very cold environments. Because some inspections take place in harsh environments, inspection equipment must be durable, accurate, and reliable. 

In-service inspections for integrity are usually centered on weld inspection, although another important damaging factor is corrosion/erosion. This type of damage can be caused by aggressive or abrasive fluids or environments. Environmental conditions can sometimes cause embrittlement and stress corrosion cracking of welds and base metals. Each inspection area poses its own particular challenges; and these challenges bring forth the need to develop application-focused solutions.

Inspection at the source

The quality of the welded pipe leaving the pipe mill is vital to the long-term integrity of any pipeline. For this reason, reputable pipe mills include some kind of inspection after virtually every stage of manufacturing. The supplier of the raw material inspects steel plates used in the manufacturing of pipes with documented results provided with each shipment of plate. Visual inspection is generally carried out after submerged arc welding (SAW) or electronic resistance welding (ERW) of the long seam to identify any obvious manual pipe weld repairs that need to be made. At the same time, the pipe may be inspected internally by a remote visual camera to check for any remaining flux or slag.

After this, typically, a first ultrasonic inspection of the weld is performed, where the weld seam is tested for any defects. Defective pipes are transferred to a repair station; and here they might be X-rayed to provide traceable documentation before being subjected to a hydrostatic test. The final ultrasonic inspection involves a complete examination of the welded seam together with the heat-affected zone and the circumferential pipe body of approximately 200 mm at each pipe end.

A final X-ray inspection of the weld seam at the pipe ends is usually carried out, and any suspected defects from the ultrasonic inspection are also documented via radiography. This final X-ray examination may be carried out using real-time digital equipment, and is sometimes extended over the full pipe length. The pipe is then inspected, measured and weighed and released for customer orders. Inspection documentation to this point is often collected and passed on to the next stage in the chain of integrity assurance.

Inspection during fabrication

Pipes are delivered in specified lengths to the fabrication yard, laybarge or the land-based pipe-laying location. They are then typically joined together by butt-welding to form piping systems. Inspection of these butt or girth welds is the next link in the integrity chain.

Historically, girth weld inspection has been performed using radiography.  This technique provides easy-to-interpret, two-dimensional grayscale images of the weld. With minimal training, an operator can interpret the image and determine the relative quality of the weld. Radiography is still widely accepted but, like any other technique, it does have its drawbacks and disadvantages, especially in terms of creating radiation hazards. 

Traditional radiography creates a two-dimensional image or picture of a weld, normal to the radiation source. As a result, weld cracks oriented perpendicular to the surface are often not detectable and present a possible failure mode if unchecked. Conversely, X-ray inspectors, simply because of a lack of adequate three-dimensional inspection data, sometimes reject perfectly acceptable welds.

Automatic welding and ultrasonics

Today, girth welding of large diameter pipe is more likely to be carried out by mechanized or “automatic” welding processes rather than by manual techniques. This conversion has also seen an accompanying conversion to automated inspection techniques and particularly to automated ultrasonic testing (AUT).

AUT systems employ an array of individual ultrasonic probes positioned on the upstream and downstream sides of a weld, with each probe focused on specific areas of the weld volume. A mechanical drive system provides controlled motion of the complete probe array, allowing the individual probes to scan the length of the weld to provide a comprehensive volumetric ultrasonic picture of the weld or pipe wall. 

Automated ultrasonic testing offers significant advantages over radiography. Specifically:

•  It does not pose a radiation hazard. 

•  Cycle times per weld, including acquisition and interpretation of data, are typically less than four minutes for large-diameter pipes. Girth welds may be inspected as soon as the weld is appropriately quenched, providing near real- time process feedback to the weld crew and dramatically reducing the cost of rework.

•  It provides a wealth of data, allowing accurate sizing and location of defects and facilitating the use of alternative acceptance criteria. For example, techniques such as engineering critical assessment reduce repair rates and speed up production, while maintaining weld integrity and providing overall cost savings to a project.

Advancements in AUT

AUT has been applied to girth weld inspection years, but its benefits have now been significantly improved with the introduction of the latest generation of AUT equipment.

For example, locating all the ultrasonics electronics on the scanning head and employing the latest in electro-magnetic interference (EMI) shielding design have now significantly increased resistance to EMI. This design element minimizes the potential for externally induced electronic noise from welding equipment and power lines, which can negatively impact the probability of detection (POD). This, in turn, reduces the possibility of missing indications or generating false calls. This also means that weld inspection can be carried out in real time, without the need to wait for the welding machine to be advanced further down the pipe or to cease operation.

Another area of significant technical advance is the introduction of phased array probes to reduce the complexity of system design. Until recently, AUT systems were built with numerous channels of conventional ultrasonics. Those systems are optimized using a variety of techniques to provide excellent inspection quality. The only drawback to this architecture is that each ultrasonic channel typically requires an individual ultrasonic probe, making the probe arrays somewhat large and bulky – especially for complex inspections that can require 30 or more probes to provide adequate weld coverage. 

Several years ago, phased array ultrasonics was introduced to AUT with the intention of simplifying probe arrays. In theory, phased array ultrasonics should allow a pair of probes to replace a complete array of conventional probes on an AUT scanner and, to a large extent, this is the case. However, specific critical ultrasonic shots require conventional probes for a proper girth weld inspection. Specifically, these are those required for time of flight diffraction (TOFD) and transverse defect detection.

In-service inspection

In-service inspection is fast becoming the area that is seeing the application of more “smart” technology. As raw materials increase in cost, and as the claims on skilled personnel become ever greater and more varied, oil companies are continuing to look at extending the operating life of their assets. Even this brings its problems, as there is a decrease in the number of skilled inspection technicians across the entire non-destructive testing industry. As a result, inspection technology has had to focus on developing equipment and systems that can inspect faster with an easy-to-use interface, without compromising accuracy and reliability.

These needs have led to the advancement of ultrasonic phased array technology, which is now available in portable flaw detectors. The portability saves time and improves the probability of detection. X-ray crawlers now use X-ray units with very high radiation outputs and small focal spots, allowing minimum exposure times and the ability to operate inside high wall-thickness pipes. Conventional flaw detectors are now even more robust and easy to use in applications ranging from weld inspection to corrosion monitoring. And it has meant embedded sensors are increasingly being used to provide remote monitoring of pipelines, from unmanned offshore platforms to land pipelines in hostile environments.

Remote corrosion monitoring

Corrosion and erosion account for a large portion of the damage costs associated with transmission pipe failures in the United States. Corrosion and erosion are caused by incidental but unwanted chemical, electrochemical or mechanical effects, which cause surface damage to materials and especially metals. Certain locations within pipelines and multi-phase piping systems are particularly susceptible to corrosion and erosion. These can include water drop-out zones, slugging areas, bends and other areas that stimulate turbulence within the product flow. Corrosion and erosion at these critical locations is usually apparent by a thinning in the wall thickness of the pipe or vessel. Wall thickness measurement is a tried and tested method of monitoring the effects of corrosion and erosion

Traditionally, manual ultrasonic inspection has been used to carry out corrosion monitoring of pipelines. However, to provide the necessary coupling, manual ultrasonic inspection often requires the removal of insulation or lagging, and the erection of scaffolding. It can even involve the excavation of pipelines and the shutdown of plants, as well as the not inconsiderable costs of transporting personnel to and from remote (and sometimes inhospitable) inspection sites. Various designs of embedded sensors have been used over the years; but today, solutions are available that allow for remote corrosion monitoring of pipelines and provides corrosion and process engineers with accurate and repeatable, on-demand wall thickness measurements.

Conclusions

Monitoring pipeline integrity at all stages of a pipe’s life is essential for ensuring productivity and guaranteeing safety. Inspection is often viewed as a cost, which, although necessary, does not make a quantifiable contribution to the bottom line.  Advances in technology and inspection equipment make for more cost-effective monitoring solutions that help to ensure integrity at every stage of a pipeline’s life.