The challenges

Kjell Wold, Emerson Process
Management, Norway,
examines the different
variables behind corrosionin the oil and gas sector andreviews how these challenges
are being addressed.

Corrosion and material degradation is one of the biggest
challenges in the upstream, midstream and downstream
oil and gas sectors.
From the dangers of increased water production
upstream (necessitating the injection of corrosion inhibitors)
through to corrosion in cooling water systems, flare systems or
distillation units at the refinery, corrosion is a continuous costand drain on resources. It can also have a major impact on the1Figure 1. Emerson`s Roxar CorrLog Wireless transmitter for ERand LPR probes.

cost effective flow of hydrocarbons from reservoir torefinery.
Another major area of corrosion is between thereservoir and the refinery in the long distance pipelinesthat characterise so many onshore and offshore fields
today. In such cases, the real-time flagging of corrosion
issues to ensure that effective remedial action is taken
immediately – without any negative impact on pipeline
flows – is vital.

Key factors behind corrosion
Many factors can have an impact on corrosion. Age, original
construction specifications and pipeline processes can varysignificantly as can the different pipeline materials beingused. In upstream oil production and in the pipeline flowfrom reservoir to refinery, corrosion is also often related tothe presence of water, carbon dioxide (CO2), and hydrogensulphide (H2S).
In refineries, corrosion might also include general
corrosion in carbon steel lines and vessels; localised
corrosion in stainless steel units; or naphthenic acidcorrosion in high temperature distillation units and transferlines. In addition, changes in operational temperatureand velocities can also influence plant performance andpipeline corrosion.
It is clear that in all cases the results can be devastatingwith a highly negative impact on the economics of theupstream, midstream and downstream processes, potential
obstructions to flow, as well as concerns over the safety
and maintenance of infrastructure and personnel.

Intrusive corrosion monitoring
One of the most common corrosion monitoring
technologies today are intrusive probes and coupons.
In pipelines and upstream oil and gas production, highpressure retrievable style probes are most commonlyused, whereas a retractable style probe is more common
in refineries. In particular, there are weight loss coupons,electrical resistance (ER) and linear polarisation resistance(LPR) probes. Figure 1, for example, shows Emerson’s
CorrLog wireless corrosion transmitter for ER and LPR
probes.

2 World Pipelines / SEPTEMBER 2015

Weight loss coupons are the simplest devices for
intrusive corrosion monitoring. Pre-weighed steel samples
are inserted into the pipe or vessel for a given period, thencleaned and weighed after retrieval. Based on the weight
difference, corrosion rates, deposits and the tendency tolocalised attacks on the exposed coupon are calculated
and reported.
LPR probe measurements are based on calculating thecurrent response to a small polarisation of the probe’s
working electrode in a 2- or 3- electrode configurations.
The corrosion rate is proportional to the current
response measured, and utilisig theoretic and empiric
factors, corrosion rates can be determined from only
one measurement. Other electrochemical methods likeelectrochemical noise may also be applied in pipeline
corrosion monitoring. However, a limitation withelectrochemical methods is that they require a water phasein order to give good data.
ER probes are based on measuring changes in theresistance of the probe’s measurement element. As themetal element corrodes, the thickness is reduced and theresistance increases. Reference elements inside the probe
are used to compensate for temperature fluctuations.
The advantages of ER probes are that they have a highresolution (down to tens of nanometres of metal loss) andare hence able to respond to changes in corrosion rates
quickly. The probes can also be used in most environments(oil, gas and water) and are available with a hightemperature rating. Intrusive probes can actually detectcorrosion rates changes and enable corrective actions to
be implemented and verified before such rate changes haveeven been detected by non-intrusive systems.
In summary, due to their high resolution and fastresponses, intrusive probes are important for active
corrosion management as well as the verification andoptimisation of corrosion inhibitor use and facilitation ofoil and gas flow.

The benefits of non-intrusive monitoring
For all the benefits of intrusive monitoring, however,measurements are still local. The probe is therefore most
likely to be useful if the corrosion is uniform. It is often
difficult, however, to pick up localised corrosion attacks,
such as pits or uneven forms of corrosion experienced in
naphthenic acid corrosion.
In addition, corrosion is often most severe at thebottom section of the pipeline because this is the location
where water is most likely to be present (often at low
spots of the pipeline). Monitoring such locations withtraditional probes necessitates access to the bottom of
the pipe, requiring big pits under the pipeline and space foraccess fittings and operating retrieval tools.
It is with these limitations in mind that non-intrusive
corrosion methods, directly installed on the pipe, have also
become attractive to operators. One of the most popular
non-intrusive monitoring techniques is electric fieldsignature monitoring (FSM).

Field signature measurements
Field signature measurement (FSM) corrosion monitoring isa non-intrusive method for monitoring corrosion, erosion,or localised attacks.
FSM is based on the feeding of an electric current
through a selected section of the structure to bemonitored. This is achieved through non-intrusive sensing
pins distributed over the areas to be monitored.
By inducing an electrical current into strategicallylocated pipe sections, the induced electric current createsa pattern determined by the geometry of the structure andthe conductivity of the metal.
Voltage measurements on each pin pair (up to 384 pins
can be applied in a matrix) can then be compared to the‘field signature’ that provides the initial reference, andchanges in the electrical field pattern can be monitored.
Conclusions can then be drawn relating to the general wall
thickness and the initial signs of metal loss.
FSM’s ability to distinguish localised attacks and general
corrosion in real-time as well as the ability to detectcorrosion rates much earlier than traditional corrosion
methods is a significant benefit. This allows corrective
action to be taken before any damage occurs.
Furthermore, in response to operator needs for realtime measurements, Emerson has also introduced an online
system and online data logger used with a wide range of
wireless communications solutions.
The result is higher data collection frequencies, thereby
increasing the accuracy of the system; and an online systemthat allows remote and wireless data communications
direct to the operators’ offices.

Applications across the industry
Today, intrusive and non-intrusive corrosion monitoring
are being seen in all areas of the oil and gas sector –upstream, midstream and downstream – and are being usedworldwide to detect corrosion in a wide range of criticalpipeline applications and facilitate flow.
In the Jamnagar refinery in India, for example, the Roxar
FSM is helping the operator visualise localised corrosion
in 3D format and providing metal loss in detail. The
results illustrate how FSM can monitor and control hightemperature corrosion accurately and efficiently, optimise
production, and guarantee integrity management. The FSM
online systems used in Jamnagar has a resolution within therange of 0.1% of the wall thickness.1
Emerson has also secured a number of repeat orders
for land-based field development corrosion monitoring
projects in East Asia that combine non-intrusive andintrusive monitoring. For those repeat orders, Emerson
provided corrosion monitoring systems to track corrosion
in surface pipeline gathering facilities in a high H2Senvironment.
The system consisted of corrosion coupons, ER andLPR probes, and corrosion monitoring/data collection
communications systems – all helping the operator track
any incidents of hydrogen induced cracking. A central

element of the system was also the non-inserted Roxar
FSM online system.
As well as the monitoring solutions, injection systems
based on hydraulic access fitting connections wereincluded in the deliveries. The complete system monitors
outward transportation acid pipes between each gas
gathering station and the internal and external corrosion of
the acid gas gathering pipes.

Selecting the right solution
A corrosion monitoring system should be based on thespecific corrosion and integrity management strategy ofeach individual case and often comprises both intrusive andnon-intrusive methods. The goal is therefore to provide thebest total monitoring solution for both intrusive and nonintrusive applications.
To this end, in addition to Emerson’s non-intrusive
FSM technology and upstream high-pressure technologies,
Emerson has brought to market a new range of retractablestyle (refinery), high temperature probes.
The intrusive range of probes are available withWirelessHART communications, making online monitoring
available at an affordable price as well as enabling thelinking in of other field applications related to pressure,temperature and vibrations. WirelessHART is widely
accepted in the industry with integration between multiple
monitoring and control applications now possible within
the same network.
On the software side, the Roxar Fieldwatch software
also offers advanced tools for data management and thereporting of corrosion monitoring data. Integration with
Emerson’s asset management software (AMS) or essential
asset monitoring (EAM) software is also possible.
Other advantages of refinery probe monitoring are thatthe instrument reads both ER probes and LPR probes as well
as being able to read probes from other suppliers; and thefact that the wireless instrument allows up to 20 m of cablebetween probe and instrument. This provides the necessary
flexibility for finding the best location with respect tomaintenance and line of sight for optimised wireless radio
signal routing.
Corrosion is a continuous challenge for pipelines
and other infrastructure in upstream, midstream anddownstream operations today.
From intrusive probes and coupons through to thelatest field signature method (FSM) developments andWirelessHART technology that is providing a catalyst for
integration, operators are finally starting to get the choices
they need.

References
1.

WOLD, K., JENSSEN, H., STOEN, R., SIRNESS, G., SHINDE, V., ANAND, U.,“On-line, Non-Intrusive Corrosion Monitoring Based on Electric Field
Signature Technology - an update on Installation Experience and Typical
Field Data”, 13th Middle East Corrosion Conference, Bahrain, 2010.

SEPTEMBER 2015 / World Pipelines

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