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Site Survey
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Data Collection
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System Audits
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Turnkey Projects
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Consultancy Services
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Onshore Surveys
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Theoretical basis for CP Inspection |
The corrosion inspection of a
cathodically protected pipeline can be
conducted by measuring the following two parameters:
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Electrochemical Potential (CP) |
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The electrochemical potential of a steel
member is a parameter defining the protective level produced by the
cathodic protection system installed on the structure. It is
commonly accepted that a properly protected pipeline must have a
potential level between -800 mV and -1050 mV (with reference to
marinized silver/ silver chloride (Ag/ AgCl), if zinc sacrificial
anodes are installed and between -800 mV and -1100 mV for Aluminium
anodes. If the potential reading is more positive than -800 mV the
point of measurement is considered to be under protected creating a
possible environment for corrosion action on the steel surface.
Potentials measured between -800 mV and -900 mV although protected,
are considered, as only marginally protected. The marginal
protection band between -800 mV and -900 mV is caused by possible
seasonal variations in above -800 mV. In addition in all cathodic
protection survey techniques, an error margin caused by, for
example, distance of the probe from the metal surface etc., exists.
This needs to be considered when analyzing survey data, therefore
any potentials in the aforementioned range is considered as marginal
protection.
On areas of potentials more negative than -1050 mV /-1100 mV, the
production of hydrogen ions instead of hydroxyl ions may lead to the
possibility of hydrogen embrittlement of the steel surface occurring
and possible enhancement of fatigue cracking. Hydgrogen
embrittlement is a phenomenon where atomic hydrogen is absorbed into
the metal structure and interacts with the metal, this in turn
weakens the structure, making it susceptible to cracking. For
pipelines under unaerobic conditions i.e buried, it is generally
accepted that a potential more negative than -900 mV is required to
achieve satisfactory protection.
The unpolarised potential of Zinc and Aluminium in seawater is the
order if -1050 mV and -1100 mV respectively, depending on material
quality, surface condition and temperature.
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Electrical Field Gradients (FG)
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The measurement of cathodic protection
potentials will give an adequate indication of the protective
conditions set up by the CP system for that moment in time. Valuable
information can then be obtained for examination of the potential
profiles along a pipeline or over a structure, especially at anodes.
However more detailed information on anode output, current drain at
exposed steel and the general operating performance of the CP system
can be obtained by also measuring the Field Gradient. These readings
are obtained by measuring the IR drop between two half cells in
seawater and can be used in conjunction with data analysis to define
anode current output, remaining anode life, indication of marginal
protection and current drain caused by coating damage. The potential
profiles and field gradient readings obtained along the pipeline
give valuable information of the corrosion protection level and CP
system performance.
The potential gives general protection level information while the
field gradient may give more detail on anode activity and areas
requiring higher current distribution such as at damaged areas,
field joints, and spool pieces, etc. Also from field gradient
readings, anode current output can be calculated and the remaining
anode life can be estimated. Therefore from close relation between
potential and field gradient measurements, it is possible to compute
the level and distribution of potential and current density along
the pipeline. This gives a general representation of the efficiency
and effectiveness of the CP system and the quality of the pipeline
coating integrity.
Typical results can be summarized as follows:
A well protected pipeline is expected to have:
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Low electric field
strength, and thus low anode current output.
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General potential levels
between -900mV and -1050 mV against silver chloride (Ag, AgCl) for
zinc anodes, and between -900mV and -1100 mV for Aluminium alloy
anodes.
A poorly or marginally
protected pipeline is expected to have:
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High field strengths and
higher current output at anodes.
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Large drops in the
potential profiles at the anode due to high current output.
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High field strength
readings at local areas of high steel concentration
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General potential
levels more positive than -800 mV
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Subsea Surveys
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Equipment
CTC-2 system and anode continuity check |
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CTC-2 (stab probe)
Subsea digitizer c/w external plug-in calibration system for CDIS
system
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Laboratory matched silver/ sliver
chloride (Ag/Agcl) reference cells c/w weighed tow cable
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3 calomel cells for calibration of
the half-cells in accordance with DNV RP B403
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1 PC Computer
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ColS Survey software & COISP
post-processing software.
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LaserJet 4L Printer (if on-line
printing is required)
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Auxillary stabber
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Surface continuity interface unit
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3 x 12 V DC Batteries
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Interfacing |
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The CTC-2 probe is connected to
the subsea digitiser, which in turn is connected to the ROV
umbilical
via a subsea connector. The subsea digitizer data transfer system
requires a twisted shielded pair through the umbilical and a 24V DC
power supply from the ROY. The towed remote reference cell signal is
also incorporated in the twisted pair. If the COIS system is utlized
then the CTC-2 probe is hardwired up to the surface COIS box via the
RDV umblical which in turn is connected to the PC via a 50 way IDC
interface cable.
A KP and real time fix is supplied by the navigation computer
via an RS232 link with cycle time of approximately 3 seconds.
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Operating Procedures |
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The CTC-2 and subsea digitizer is fitted to the ROV via a twisted
shielded pair in the umbilical to the topside computer logging
system. The towed reference cell is deployed from the starboard side
of the ship at approximately 20-30 meters water depth. Additional
weight is mounted on towed line to keep the cell at a fixed depth.
The CP measurements are logged on the CP survey computer with
reference to the KP value and the real time supplied from the
navigation computer.
The reliability and accuracy of the 'cell to cell' technique
normally depends on the ability to obtain the absolute potential for
calibration by means of metallic contact on bare steel or anodes at
regular locations along the lines.
On pipelines with adequate CP design and good coating quality, the
general CP level will be close to anode potential with low field
gradients and anode outputs recorded. Buried anodes in this
condition are difficult to detect.
Anode stabs are made where possible in order to maintain an accurate
CP profile. On lines where anodes or bare steel cannot be stabbed
due to burial it is not possible to fix the absolute potential
level. It should be noted that the largest change in the CP levels
are normally measured within 1m from the pipe surface, i.e there may
often be little difference in CP readings between 2m and 5m.
Therefore, local variations in potential and field gradient, may
not be observed due to attenuation of the potential gradient,
dependent on the pipe to probe distance.
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ANALYSIS OF
CATHODIC PROTECTION SYSTEM & ANODES DISTRIBUTION (Offshore
Structures)
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We (Emirates Techno Casting LLC) can
conduct the analysis of Sacrificial
Cathodic Protection System for the
Platforms and also the anodes distribution
using the Boundary Element Method using
the Beasy Software – UK.
BEASY predicts how effective proposed
corrosion control strategies are at
protecting structures and how they will
perform over a structure’s life cycle.
Also, how they will interfere with nearby
systems
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Simulating
galvanic corrosion;
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Optimize CP
system design;
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Reduce post
commissioning costs;
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Reduce
interference;
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Electric and
magnetic field prediction;
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Marine
application;
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Underground
infrastructure applications.
Simulate Galvanic
Corrosion
BEASY can be used by the designer to
simulate the electric fields and
electrochemical reactions (polarisation)
present in corrosion applications.
Potential levels and currents can be
predicted on metallic structures in the
surrounding electrolyte.
Interference
Reduction
Models can include adjacent structures which
may interfere with the operation of the CP system. BEASY
quantifies the interference and enables easy modification of the
CP design to reduce these effects. Detailed data on
potential shifts and current demand are computed.
Optimize CP System
Design
BEASY provides
the corrosion engineer with the ability to model the performance
of a CP system and to modify the key parameters to achieve maximum
protection of the structure.
Marine Applications
Predicting the performance of cathodic protection systems in the
marine environment is a major application area of computer
modelling. Both sacrificial and impressed (ICCP) systems on
oil and gas structures, ships and boats can be modelled.
Underground
Infrastructure Applications
It is difficult to imagine how many different kinds of electrical
impulses surge through today’s underground environment.
Power lines, transit rail lines, communication cables and
industrial plant foundations are just a few of the contributors to
this buried infrastructure nervous system. BEASY provides
the tools to understand this complex system and predict how
pipeline and storage tank cathodic protection systems will
perform.
Electric and
Magnetic Field Prediction
When used in conjunction with the BEASY CRM software the corrosion
related electric and magnetic fields can be predicted (eg, UEP,
CRM etc).
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Feasibility Studies |
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ETC-CPS has the appropriate engineering expertise and resources to
conduct feasibility studies on behalf of Clients who may wish to
determine the viability
of providing corrosion protection to their plant facilities.
The study would address one or any of the
following parameters: -
Confirm by field investigations if a corrosion
environment exists.
* Establish the type of corrosion and it's effects.
* Determine methods of mitigation.
* Evaluate costs
* Develop technical specification
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MATERIAL SELECTION
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Consistent with
the activities of the design function, ETC-CPS 's engineering
personnel has at its disposal a comprehensive supply of product
literature
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INSTALLATION SUPERVISION |
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As part of its
comprehensive services, ETC-CPS is able to offer installation
services dependent on the nature of the work to be executed and the
resources it has available. In most cases, the Client opts to have
the materials installed by its own Contractor, but to engage the
services of ETC-CPS to provide installation supervision and
technical support. Either way, to satisfy the exacting needs of
a project, ETC-CPS is willing and able to offer whatever the service
the Client requires
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COMMISSIONING |
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ETC-CPS stock a range of test
instrumentation to accommodate all types of commissioning tests
associated with cathodic protection systems.
ETC-CPS 's support
services are locally available to meet the Client's specific
requirements
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MAINTENANCE |
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Such maintenance
services include full status reports and recommendation
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