Increased demand for oil and natural gas, a rise in offshore production, and demand for cheaper methods of transportation have encouraged companies to accelerate the process of building subsea pipelines. Thus, the global offshore pipeline market is forecast to grow from US$11.5 billion in 2018 to US$15.8 billion by 2023, at a 6.54 per cent growth each year. Subsea pipelines are subject to several inherent challenges, some that are beyond the operator’s control.
More than just traversing thousands of kilometres, these pipelines often extend into extreme water depths (one of the deepest ever built was in the Gulf of Mexico, with a maximum depth of approximately 2,900 metres), which means they typically operate in a severe physically and technically demanding environment. Threats can range from being mechanically (i.e. manufacturing or installation-related issues) and operationally driven (i.e. operator neglect) to corrosion, natural hazards (seabed quakes, wave currents and sea-storms) or damage caused by a third-party (such as a shipping vessel damaging the pipeline when anchoring).
Oil and gas operators cannot afford to take these threats lightly since it can result in lost profits and expensive repairs, but can also have lasting impacts on coastal and marine environments and thus on the livelihoods of the fishing and tourism industries.
COMMON METHODS OF PIPELINE INSPECTION
By inspecting and monitoring subsea pipelines, oil and gas operators can make informed decisions regarding proactive measures such as repair or replacement. To realise greater cost and efficiency gains, the industry is working alongside researchers to develop and refine technology and methods in this space. Sensor technologies cover a wide range of physical principles, including electrical, optical, radiographic, chemical, and acoustic domains.
The following covers some of the most commonly used sensor technologies as well as recent research into sensor advancements. Electrical phenomena Magnetic Flux Leakage (MFL) is a widely used non-destructive testing technique which uses magnetic sensitive sensors to detect defects on both internal and external surfaces of pipelines.
For decades, Potential drop mapping techniques have been employed to monitor pipeline crack growth and wall thickness variations due to corrosion and/or erosion. This type of inspection is carried out by injecting currents in the specimen to be tested and measuring the arising electrical potential difference between two or more electrodes placed on its surface. The presence of a defect generally increases the resistance, and hence the measured voltage drop.
Inversion of this data can provide oil and gas operators with information in regards to the size and shape of the pipeline defect. Acoustics Ultrasonic testing is another important non-destructive testing technique. It uses ultra-high-frequency sonic energy to locate and identify discontinuities such as cracks in materials that are both on and below the surface of the material (such as metals or plastic, commonly used to make pipes). However, while ultrasonic testing offers excellent resolution in spot-checking pipelines for defects, the use of this technique for inspecting an entire pipeline is excessively time-consuming and is not cost-effective. This is typically where guided ultrasonic wave testing (or GUWT) is used to provide a much longer range (up to 100metres) inspection, although it typically comes at the cost of being at a lower resolution.
In-line inspection Over time, debris and precipitates (such as wax, hydrates, and asphaltenes) from the production fluid build up inside the pipeline. The build-up decreases the cross-sectional area and thus decreases flow rate, and in severe cases, completely shuts off flow when the deposits block the whole cross-section.
Pigging is an in-line inspection technique in which devices referred to as ‘pigs’ are inserted into pipelines to perform cleaning and inspection activities. Pigging can be conducted on a variety of pipelines sizes without having to stop the flow of material through the line.
Following the deployment of cleaning and gauging pigs, the more sophisticated intelligent pig can be launched into the pipeline to obtain more detailed information. Intelligent Pigging is a technique whereby an inspection probe, often referred to as a ‘smart’ pig, is propelled through a pipeline while gathering important data, such as the presence and location of corrosion or other irregularities on the inner walls of the pipe.
The main differentiator between the two is that smart pigs can perform advanced inspection activities as they travel along the pipe, in comparison to just cleaning it. Smart pigs can use non-destructive examination techniques such as ultrasonic testing and MFL testing to inspect for erosion, corrosion, metal loss, pitting, weld anomalies, and hydrogen induced cracking, among others.
Robotic vehicles As with other industries, when a task becomes either too expensive or dangerous for a human to carry out, robotic substitutes are used. Robotic substitutes are especially useful in subsea environments as they can be particularly inaccessible to humans.
Remotely operated underwater vehicles (or ROVs) are tethered robots that are operated by a crew in a safe environment, whilst the ROV performs tasks in the sea level below. The tether enables the robot to carry electrical power, real-time video and other data signals between the operator and vehicle. At a minimum, ROVs are fitted with a video camera and lights. However they can also include additional tooling such as magnetometers, sonars, still cameras, manipulators, cutting arms, water samplers, dredgers, cutting tools and cleaning tools – all to make pipeline inspection that much easier.
Autonomous underwater vehicle (or AUVs) are an advancement on ROVs. As their name suggests, these robots are self-guiding and can operate independently without human intervention.
There are AUVs available with the ability to conduct pipeline inspections in shallower water than boats can and deeper water than human divers or tethered vehicles can (sometimes up to 3,000 metres water depth).
According to the National Oceanic and Atmospheric Administration, most AUVs use specialised batteries, although some AUVs have used fuel cells or rechargeable solar power. Some AUVs (such as gliders) minimise energy demands by allowing gravity and buoyancy to propel them.
Each technology/method has its own strengths and weaknesses and often requires the simultaneous deployment of multiple technologies for a more comprehensive assessment of pipeline health.
Sources: Offshore Pipeline Market by Product (Oil, Gas, Refined Product), Line Type (Export Line, Transport Line), Diameter (Below 24, Greater Than 24), and Region (Americas, Europe, Asia Pacific, Middle East, & Africa) – Global Forecast to 2023, Markets and Markets 2018; Overview of Pigging, Inspectioneering 2019; Advances in potential drop techniques for non-destructive testing, Giuseppe Sposito; Applications of Ultrasonic Techniques in Oil and Gas Pipeline Industries: A Review, Wissam M. Alobaidi et al.; Inspection and monitoring systems subsea pipelines: A review paper, Michael Ho et al.; Practical long range guided wave testing: Applications to pipes and rail, Peter Cawley et al.