of inspection. The secondary purpose is to predict whether the CP
system will continue to perform adequately for some period (the next
three to five years, for example), or when retrofit would be necessary
for continued service.
Good data is essential to making this judgement. The classic ap-
proach to CP inspections is to take measurements on accessible parts
of the exterior envelope. This is of limited value, however. These test
points are typically on secondar y items, such as structural frames and
not on, or even near, the safety critical components. This happens
because of limited ROV dexterity in these complex and confined
spaces on subsea equipment.
So, what is good data? The best way to answer this question is to
first consider what the typical trend of CP measurements should be;
then, compare this to real data. For a sacrificial CP system, when
taking measurements with a silver/silver-chloride reference cell, the
timeline is something like this:
• The instant the equipment is submerged, the bare metals are
unprotected. The CP readings of bare carbon steel would typically
be -600mV to -650mV.
• As the CP electrochemical process occurs, the CP readings move
toward the protected range, which is more negative than -800mV.
• In time, perhaps a month or more from immersion, the CP reading becomes reasonable static. A well protected structure would
have a CP value around -950mV.
• This CP level remains reasonable stable over the bulk of the
life of the anodes - typically 15 to 20 years. There will be minor
changes, around +/- 25mV, caused by several factors that include
reading inaccuracies. These minor fluctuations are not serious
and to be expected.
• As the anodes reach the end of their lives, they put out less current
and, consequently, the CP levels reduce exponentially.
• As a rule of thumb, retrofit CP is needed once the CP level drops
to -850mV. The objective is to replace the anodes before the item
In summary, the data graph should show an initial rapid increase
in CP level, followed by a prolonged period of slightly fluctuating
readings, and ending with a decay curve that leads to under and
then no protection.
Now, when one looks at some real-life data, there can be challenges.
A set of readings taken on an item of subsea equipment using an
ROV fitted with an industry standard contact CP probe illustrates the
potential challenges. The problems included the following:
• At any given test point, the year-on-year data varied wildly – much
more than the generally acceptable +/-25mV fluctuation. The lines
of data should lie on top of each other, within a +/-25mV fluctuation,
or show a progressive decay from inspection to inspection point.
• It is permissible to have variations in CP from place to place on
one entity. CP is never totally uniform. If the variations are true
changes in CP, they should be reasonably consistent year on year.
• One dataset dropped dramatically and indicated under protection
at multiple points. Then, another dataset jumped back to the
general range of earlier sets. It is all too easy to say that one set
is the bad apple, but there’s no way of knowing for sure without
It is very likely that the inspections were performed by different
crews and ROVs. They may not have had the previous inspection
results available, leaving them without a reference point. In the ideal
world, this is not relevant. Good operators with good equipment
should achieve good data. The problem is that there are many factors,
beyond the scope of this article, that justify the data.
The main point is that data like this does not conclusively prove
that the corrosion risks are being well managed, and there must be
a better way of monitoring the CP status of critical and expensive
subsea infrastructure as part of a risk management plan.
Permanent CP monitoring
Permanently installed CP monitoring is not new. As early as 1972,
there have been permanent monitoring systems in offshore platforms. Deepwater Corrosion Services has been involved in providing
monitoring equipment since the mid-1980s, with permanently installed
zinc reference electrodes on Gulf of Mexico structures. These older
systems only worked on fixed or floating structures, as monitoring
was done topsides.
Since then, the company has developed a vast improvement to this
early technology with the solar-powered monitoring system that al-
lows for consistent and reliable measurements at the locations where
integrity risks are their greatest. The system only requires the ROV’s
lights and camera to take the measurements. This system has been
successfully installed at various locations, including Shell’s Olympus
field in the Gulf of Mexico.
The fully self-contained monitoring system consists of two primar y
elements: the reference electrodes and the test panels.
The reference electrodes are zinc because of their stability and
proven longevity. These are installed at points on the equipment/
structure where corrosion risks are at their greatest.
The test panels are fixed to the structure at locations accessible
to the ROV. They contain voltmeters, powered by the photocells, to
measure the CP potentials. In the event of electrical/electronic failure,
the test panel has stab plates where the ROV can take conventional
Once installed, taking readings is straightforward. As the ROV
approaches the test panel, its lights activate the solar panels under
the glass domes. The power generated brings the voltmeter and
ammeter to life. The CP potential and current are displayed on the
gauges at the top of the panel, which is captured by the ROV camera.
In addition to the consistency and repeatability of readings, this is a
fast scan system that significantly reduces ROV work time.
The benefits of the system seem to be self-evident. Even so, there
appears to be a need for a paradigm shift in the offshore industry
before this technology is widely accepted. •
Brian Gibbs has been involved with integrity management since the late 1970s,
predominantly in the offshore oil and gas business. He has worked in many parts
of the world including Asia Pacific, the Middle East, West Africa, the North Sea,
and the Gulf of Mexico. His work has included in-service inspection, life extension
of floating assets, risk assessments, and verification. Brian has presented papers at
many conferences, including NACE and OTC, and written articles for technical
publications. He is currently employed at Deepwater Corrosion Services, Inc. as the
senior integrity management consultant. Prior to this he worked at ABS Consulting
and Oceaneering International for a combined total of nearly 25 years. He gained
his degree in civil engineering from the Polytechnic of Wales in the 1970s.
Solar-powered test panels mounted on subsea equipment.