Plant safety continues to be of the utmost importance to operators and safety engineers. The prevention of corrosion/erosion through live process pipeline monitoring provides asset and integrity managers with a real time picture of how their facility is coping with the high demands placed upon it by corrosive fluids. This information assists in risk management and auditing.
By Kevin Clarke, Sales Director, Permasense
Steel pipework and vessels are always at risk of corrosion or erosion. Unless monitored there is a risk of failure which is both a safety and environmental hazard, quite apart from the financial costs of operational interruption, repairs and of reputational damage.
As oil and gas operators produce and process ever more corrosive and erosive hydrocarbon streams, and chemical/petrochemical producers push process operating envelopes, for example in pursuit of higher product purities, the demands on plant metallurgy steadily increase.
Permanently installed sensor systems deliver a continuous picture of asset condition over time, at a cost comparable to that of a single manual inspection. The integrity data can be correlated with key process condition drivers that may be causing corrosion or erosion, and with corrosion control strategies such as inhibitor use. This enables the asset manager to move beyond merely knowing whether corrosion or erosion is occurring to understanding why and at what rate. This understanding informs better quality decisions; resulting in improved plant profitability, cost-effective improvement of plant integrity and also, therefore, safety.
The need for continuous process pipeline monitoring
There are various established techniques for the periodic assessment of pipe and vessel integrity. The drivers of corrosion and erosion - process conditions, crude constituents and abrasive solids - and the inhibitors to hold corrosion rates in check are familiar to most operators.
Periodic inspections do not, however, deliver continuous pipework condition data that can be correlated with either corrosion drivers or inhibitor use to enable understanding of the impact of process decisions and the use of inhibitors on plant integrity. Manual acquisition of ultrasonic wall thickness data is also frequently associated with repeatability limitations and data recording errors.
Real-time sensor data from installations is showing that, where corrosion is taking place, it is often intermittent rather than continuous. In such cases it is particularly valuable to be able to correlate thickness data over time with the key operating or feedstock quality parameters. Moreover, the data can highlight which prevention or mitigation strategies are most effective.
Optimisation of prevention and mitigation strategies
Direct, accurate and sufficiently frequent measurement of pipework thickness to accurately identify trends is not practically feasible with manual inspection methods, when coupled with the challenges of accessibility, and avoidance of safety risks to personnel from high temperatures and difficult to reach locations.
Permanently installed sensor systems, on the other hand, deliver continuous data of high quality. Installed on pipes/vessels operating at up to 600°C (1100°F) sensors have been certified as intrinsically safe for use in the most hazardous of environments, and proven in operation over a number of years in refinery and petrochemical environments, as well as onshore and offshore upstream facilities.
The figure below shows the range of drivers that are motivating plant operators to invest in permanently installed corrosion monitoring systems.
Typically, investments in sensor technology are justified by safety and operational risk mitigation factors - assuring plant integrity, improving equipment availability, accessibility of measurement points from an inspector safety point of view, or changing the role of limited resources (like qualified inspectors) from making measurements in the field to analysing the causes and mitigation of corrosion issues.
However, once installed, these systems are highlighting where corrosion is taking place and whether it is intermittent or continuous. This information is proving particularly valuable in informing the production planning and operations decision making processes, to drive the plant towards improved profitability, by processing feedstocks with a more corrosive or erosive tendency, or operating the plant more severely.
At the core of these permanently installed systems is an ultrasonic sensor mounted on stainless steel waveguides. The waveguides isolate the sensor electronics from extreme temperatures and guide the ultrasonic signals to the pipe wall and back without excessive signal degradation and distortion.
The sensors can monitor pipe wall thicknesses of over 3 mm (1/8 in) in a wide range of steels and other alloys. The frequent measurement of corrosion allows for metal loss detection at the level of 10s of microns.
In the short-range configuration, each sensor is equipped with a radio and communicates with other sensors and a gateway (base station) within a 50m (55yd) range. The sensors form an independent mesh or wireless network and each sensor radio can act as a relay, or repeater, enabling the network to span hundreds of metres from the gateway.
In the long-range configuration the same ultrasonic wall thickness measurement sensors are utilised as for the short-range system, but sensors are cabled to a range extender which incorporates an antenna. This enables the extender to communicate directly with a gateway up to 25 miles (40km) away. Thus data can be retrieved even from the remotest monitoring locations, enabling systems to be configured to meet the needs of upstream facilities and above-ground pipelines.
In both configurations the data is channelled via the gateway to a database on a connected computer. If, as is usual, this computer is networked, browser-based visualisation software enables the corrosion/inspection engineer to view the data at their desk, which could be anywhere in the world.
Improved insight into the impact of feedstock decisions
Continuous corrosion monitoring is being used to support more strategic decision making processes such as feedstock selection and diversification. One refiner carried out a one month trial of a crude that it had not previously processed, to gain a better understanding of the potential integrity impacts. The crude was processed at 20-25% of the total slate during the test period. The sensors installed on the kerosene draw-off from the crude tower showed a marked increase in corrosion rate (below) - a valuable insight that is being used by the operator to develop operational guidelines and inhibitor strategy for future processing of the crude on an ongoing basis.
Processing of high acid 'opportunity' crudes is a key profit improvement strategy for many refiners in the Western Hemisphere. While naphthenic acid corrosion tends to be quite localised, Permasense is seeing refiners deploying arrays of sensors, making a series of point measurements to highlight where a significant increase in corrosion activity is being observed. This approach also enables an understanding of the effectiveness of inhibitor chemical injections in the 'at risk' locations.
On an offshore gas processing platform, sensors have been deployed in arrays around the gas risers to detect sand erosion (as shown in the photograph below).
The responsiveness of the Permasense sensor technology to changes in the wall thickness driven by sand breakthrough has enabled the operator to increase gas production by 10% with confidence, in the knowledge that equipment integrity was being closely monitored in real-time. In extreme environments, such as offshore oil and gas production facilities, the application of advanced sensor technology is being used to systematically limit the amount of inspector time required on the platform, to reduce costs of transferring staff from shore by helicopter and to add flexibility within constraints on the total numbers of staff offshore at any one time.
The installation of permanently installed sensors eliminates the cost of repeat measurements (for example, from requiring scaffolding to be built for access) and there is no personnel exposure to high-risk locations or adverse ambient conditions. However, operators using permanently installed continuous corrosion monitoring systems are finding additional benefits from having a more accurate and timely understanding of the corrosion and erosion rates occurring within their facility.
While often installed as part of safety or operational risk management programmes, the data from such systems is providing plant operators with valuable insight into the effect of changing operations on corrosion/erosion rates, and supporting more effective risk-based decision making around issues such as feedstock cost reduction, chemical inhibition strategy, shutdown timing and selection of metallurgy for plant upgrades. Permanently installed systems are also enhancing inspection strategies in locations where access is costly, dangerous or physically restricted, while the availability of wireless transmission of data significantly reduces the time needed to install such equipment in these tough environments.