New CP monitoring tool
helps reduce corrosion risks
Solar-powered subsea system said to provide better data
The increasing demands on subsea infrastructure to work at greater depths, in harsher conditions, and for longer design lives requires a new look at the tools and techniques used for cathodic protection (CP) monitoring. This is increasingly important in today’s world, where regulatory oversight and
the need to effectively manage risks is paramount.
The conventional way of verifying that a CP system is working
effectively is to measure the electrochemical potential, referred to
commonly as CP readings. This technology was developed for use on
shallow-water fixed platforms and other large structures, such as ship
hulls, where the technology works well. Complex subsea equipment
presents major challenges for gathering good CP data when using
the tools developed for large simple structures. Yes, CP readings
are recorded, but their value can be limited if they are not taken
consistently and at the locations where the corrosion risk is greatest.
The consequence of poor data can result in additional inspections,
premature retrofit of anodes, or unexpected failure.
Permanently installed monitoring tools have been in use for decades
and generally these require cables to be brought up above water to
electrically powered meters. This approach is totally impractical for
remote deepwater infrastructure. Deepwater Corrosion Services has
addressed this impediment by incorporating solar panels into the CP
monitoring system. This new product – the SunStation – is a solar-
powered monitoring tool that has many advantages over the traditional,
and often difficult, approach to taking contact probe CP readings.
The illumination provided by the ROV lights is sufficient to activate the solar panels. This compact, stand-alone monitoring system
requires only the ROV’s lights and video camera to gather the vital
CP data from the strategically positioned CP measurement probes.
This solution to CP monitoring provides the operator and the
regulatory bodies with consistent and reliable data, which is vital for
an effective risk mitigation plan. Additionally, it reduces ROV operational time and, therefore, operating costs. The technical advantages,
the reliability of the data, and the reduced operational cost by far
outweighs the cost of supplying and installing the monitoring system.
The goal here is to show how bad corrosion data hampers the
ability of the integrity engineer to offer sound judgement on the
condition of the subsea infrastructure, and how new technology can
help mitigate this risk.
The key to successful integrity management is to demonstrate that
risks are managed to acceptably low levels. Key corrosion data provides
this assurance. For the integrity management authority, the challenge is
dealing with the uncertainty of verifying that the materials of fabrication
are performing well over the expected service life; often, even longer
because of the need to continue extending the operating life of fields.
In complex subsea equipment, we mix a variety of metals to meet
specific design and operating needs. We often build increasingly
complex systems, many of which need protection from the natural
process of corrosion. In doing so, we mix corrosion resistant alloys and
a range of carbon steels, with the possible addition of other materials
such as titanium. These materials are electrochemically incompatible,
meaning that one metal will cause another to corrode. While this approach creates an effective mechanical design, it presents challenges
from an integrity management standpoint.
Carbon steels require CP to limit the effects of seawater corrosion.
The effective solution to address the corrosion issues and electrochemical imbalance is, and probably will always be, sacrificial anodes.
This generates two main challenges for the designer: where to attach
the anodes, and how to monitor their effectiveness.
Attachment of the anode is usually confined to structural components, such as the frames, and the design relies on a series of
mechanical (bolted) connections to create the electrical continuity
required for the anodes to protect safety-critical components.
In deepwater environments, verifying the ongoing in-service ef-
fectiveness of CP comes down to two simple techniques. The first is
visual inspection to see that the anodes are present, functioning, and
to look for signs of corrosion. The second is CP measurement, which
typically employs a simple probe that contacts steel somewhere on
the perimeter of the equipment.
This simple CP measurement probe, intended for checking CP on
offshore structures, only tests the location where the measurement
is made. It does not examine the safety-critical (usually pressure
containing) components, and it can be concluded that taking CP
measurements at non-critical locations does not provide the sound
data needed to verify that corrosion risks are being well managed.
The primary purpose of CP inspection is to confirm that the metals
at risk of corrosion are protected by cathodic protection at the time
Deepwater Corrosion Services, Inc.
ROV interrogating the CP monitoring panel. (All images courtesy Deepwater Corrosion Services, Inc.)