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Customer required design anodes can also be manufactured and
supplied.
Designing a System
Before designing a cathodic protection system certain data will
be required:
Customer preferences in system
design
An estimate of all surface
areas to be protected
The configuration of all structural
steel work to ensure adequate distribution of anodes with
particular reference to highly stressed areas such as nodes.
Details of pipeline risers,
conductor tubes, wells, piles, pipelines and all other steelwork
above and below seabed level.
Details of internal flooded areas
including fresh water, drill water, ballast and fuel tanks.
Details of steelwork conduits,
grouting tubes, pipe guides, launch pads, and floatation chambers.
Details of chains, fenders, anchor
wires, propulsion units.
Nature of any coatings to be applied and extent of
application.
Period of time for which protection is required, together with
estimates of coating performance.
In this case of existing
installations, age and condition of the structures steelworks and
coating.
Availability of electrical
power supplies and location of electrically hazardous areas.
Location of structure, weather
cycles, wave heights, water depth, tidal flow, nature of seabed
and probability of scour.
Water resistance and temperature
range.
Design Limitations, especially with
respect to weight loadings if structures, particularly during
float.
Details of existing or proposed
adjacent or linked structures and type of cathodic protection
installed or planned.
The first step in the design is to calculate the surface area of
the structure exposed to an electrolyte, making due allowance for
small items such as scaffolding clamps. ladders, etc, which are part
of the structures and a further allowance for surface roughness.
The current density for protection varies around the world and
indeed will vary locally in one particular area dependent on a
variety of conditions. The actual current density is not normally
known, but the appropriate range is known for most parts of the
world where there is offshore activity. The actual method used to
calculate the protection requirements will vary as different
classifying authorities use different basic rules. The table below
gives guidance on design current densities for different parts of
the world.
Guidance on minimum
design current densities (mA.m2) for cathodic protection:
| |
Initial |
Mean |
Final |
|
North Sea
(Southern) |
130 |
100 |
90 |
|
North Sea
(Northern) |
150 |
115 |
100 |
|
Gulf of
Mexico |
65 |
- |
- |
|
US West
Coast |
87 |
- |
- |
|
Cook Inlet |
440 |
- |
- |
|
Persian Gulf |
130 |
70 |
90 |
|
Indonesia |
65 |
- |
- |
|
Australia
(Southern) |
130 |
90 |
85 |
|
Brazil |
130 |
90 |
85 |
|
Stagnant
seawater (Initial) |
75 |
50 |
40 |
|
Saline mud
(ambient temp) |
20 |
20 |
20 |
|
Once the current requirement for the structure is known, a choice
between impressed current and sacrificial anode system may be made. The
choice is often complex and may be based on economies over the lifetime
of the system, float out weight, and the availability of power and past
experience in similar conditions.
Sacrificial Anode System:
If a sacrificial anode system is chosen, the weight of material
required to provide the protection current for the protected lifetime of
the structure is calculated from a knowledge of the current demand and
also the specific electrochemical properties of the anode alloys. Once
the total weight has been calculated, then the optimum weight is known.
The MASS of sacrificial anode alloy required will be given by:
W = Y x 8760 x A x C
1000 x Z x U
where Y = Design Life (Years)
A = Surface
Area (m2)
C = Current
Density ( mA/m2)
U = Anode
utilization factor
Z = Capacity
of material ( Amp Hrs/Kg)
The capacity of an anode alloy is a measure of the quantity of
electricity; which the material will give as useable protection current
per unit weight of alloy corroded in unit time. The utilization factor
is a measure of the proportion of the anode, which can be expected to
deliver adequate current at the end of the system's lifetime and is
related to the reduced cross section and length of an almost fully
consumed anode. For most long offshore platform anodes, this is 0.9 and
for complex shapes, e.g bracelet, 0.85 or even lower is used.
The calculated weight of anode alloy cannot be installed all in one
piece but must be distributed over the structure in the form of smaller
anodes to ensure uniform distribution of current. In order to select the
best size and shape of anode, the total current demand of the structure
both at the beginning and end of its life must be considered.
The anode must deliver adequate current to polarize the structure and
build up cathodic chalks, but also must be capable of delivering the
required mean current for the structure when 90% consumed. Moreover, the
system as a whole must be adequate excess current demand over that
originally catered for in the design so as to ensure that the system is
not limited by the current output characteristics of the anodes.
ETC's Bracelet Anodes
ETC has extensive experience in submarine pipeline protection using both
half shell and segmented bracelet anodes. Sizes regularly produced range
from 4 inches to 48 inches diameter and 10 kg to 800 kg in weight. A
number of standard moulds are therefore available to meet short lead
time production requirement. In addition, bracelet anodes can be
produced by practical design. It is ETC's recommended practice that
adequate steelwork forms an integral part of anode to ensure that the
anodic material is well supported in later life, ensuring continued
protection of the pipeline.
The outer diameter of the bracelet anode is normally designed to
coincide with the outer diameter of the pipeline's concrete weight
coating. When this is not possible, bracelet anodes are produced in a
tapered form to facilitate their movement over the stinger during the
laying operation. When a retrofit system has to be fitted to a pipeline,
a number of solutions are available to the pipeline engineer; and
specific systems can be designed to suit the particular problem.
1. Structural
2. Sub sea
3. Customization
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