PRODUCTION OPERATIONS
Retrofit designs help extend life
of offshore structures
Several offshore installations original- ly designed for 20 to 30 years in the North Sea have now been in service for over 40 years. The number of ag- ing offshore structures and pipelines
grows each year as more operators look to
extend their life due to the development of
new practices and technologies. Complex
engineering solutions are required to ensure continued asset integrity. This requirement was initiated and is supported in the
United Kingdom by the Health and Safety
Executive under Key Programme 4 (KP4)
Aging and Life Extension (November 2012).
Most structures or pipelines were originally protected using sacrifcial anodes, and
many of these systems are now either approaching depletion or beyond their original
design effcacy.
While most certifying authorities and standards organisations provide CP guidance and
recommendations for the design of systems
for new offshore structures and subsea pipelines, there is currently little or no data available for designing and implementing CP upgrades to existing assets.
There is a growing demand to design CP
systems that can be retroftted to enhance
an existing system, in order to continue corrosion mitigation throughout the extended
life of the structure or pipeline.
Collating historical data of the original CP
design, including operation and maintenance
records, is essential. In some instances, a permanent CP monitoring system comprising a
number of reference electrodes and monitored anodes that provide real-time data may
have been installed. However, because these
systems are not mandatory, they are in the
minority. Without a permanent monitoring
system, it is necessary to carry out regular
CP surveys during routine structure inspections, typically using an ROV.
Traditionally, ROV operators record struc-ture-to-electrolyte potential data at stab points
that are rarely repeated from one survey to
the next. Unless care is taken to establish and
repeat the point at which the measurement is
taken, it is diffcult to make any prediction on
future performance of the CP system.
Frequently, ROV operators report visually
on the condition of anodes and remaining alloy in terms of percentage wastage, such as
15%, 50%, 75%, etc. This provides only an es-
timation of anode consumption and is some-
times made even without knowledge of the
original anode dimensions; or without having
frst cleaned the anode to remove any ma-
rine growth or bi-products from the surface.
Cleaning of the anode is essential to obtain
an accurate assessment of wastage unless the
anode is totally consumed and only the steel
insert is visible.
By careful analysis of available CP data and
review of ROV inspection measurements,
trends in structure potentials and accurate
anode consumption rates can be evaluated to
establish when it is necessary to implement
a CP upgrade. Ideally, this should be under-taken before this data indicates the structure
or pipeline is no longer fully protected.
The associated graph shows recorded structure potential measurements at predetermined
locations at - 4.5m horizontal elevation. The
graph indicates a dimensioning trend in protection levels and initiating, together with the
evaluation of other detailed survey data, the requirement for a CP upgrade design and installation of a retroft CP system.
Cathodic protection systems are designed
to provide suffcient electrical current to es-
tablish protection at installation; maintain
protection for the duration of the design life;
and retain the ability to re‐establish protec-
tion even at “the end‐of‐life.”
The amount of electrical current required to
achieve this objective on a carbon steel surface
in seawater varies with the rate of availability of
oxygen at the surface. This is determined by
factors such as water depth, fow rate, and re-
sistivity. The latter is controlled by salinity and
temperature. It is also known that if a structure
has been polarized for a long time, calcareous
deposits form on exposed steel surfaces and
act in a similar manner to protective coatings by
reducing the oxygen availability at the steel sur-
face, thereby reducing the amount of current to
maintain polarization.
Environmental conditions vary depend-
ing upon the offshore location. It is possible
to select criteria for CP design parameters
but this must be carefully assessed depend-
ing upon the structure location. Of course,
condition in various locations can vary con-
siderably, and result in a degree of excess
anode material – or worse, not enough.
The retroft CP design typically considers
only the platform structure, including pile
guides, piles and, where applicable, any con-
ductors or other appurtenances. Allowances
for other appurtenances such as risers, fow-
lines and other subsea structures should
also be considered if electrical continuity
exists with the platform structure.
The retroft CP design uses conventional
and modifed CP criteria to maintain exter-
nal structure potentials within the range of
‐800 mV to ‐ 1,100 mV with respect to silver/
silver chloride (seawater) reference elec-
Mike Moffat
Chris Lynch
Corrpro Companies
