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Corrosion Inside
Monopiles
Andy Duncan, Lead Consultant
Intertek Production & Integrity Assurance
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Monopile
Transition piece
Air tight deck
Monopile
Internal J-tubes exit
Source: Isaac Tavares – Centrica
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Monopile – internal zones
Atmospheric ZoneCoating
Monopile (internal)“Oxygen free” environment
Corrosion allowance
No coating
Mudline zone“Oxygen free” environment
Corrosion allowance
No coating
Monopile
• Designers have previously assumed that
by sealing monopile internals from
seawater & air, oxygen will be consumed
and corrosion will be supressed.
• However, experience has shown that
cable seals are difficult / impossible to
fully seal, so aerated seawater ingress
occurs.
• Result is corrosion of the internal
surfaces.
Source: Isaac Tavares – Centrica
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Monopile
Due to internal corrosion
the structural strength
and fatigue life may be
significantly reduced Log S
tress (
MP
a)
Log number of cycles
In air design curve
In seawater
free corrosion curve
With CP
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Corrosion Management Options
1. Do nothing – possible, but integrity assessment and regular monitoring is necessary to determine integrity of structure
2. Prevent corrosion by design or applied corrosion protection such as coating – only possible for new developments
3. Corrosion inhibition via chemical dosing – possible but is it feasible?
4. Corrosion protection methods such as Cathodic Protection (CP)
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Cathodic Protection (CP) options
The basic principle of CP is that the corrosion process is suppressed by theapplication of an opposing current which forces the potential of the metal tobe shifted from its active potential to its passive (protection) potential.
• Sacrificial (galvanic) systems
o A less noble metal (Al, Zn anode) is connected to the monopile with the Al, Zn corroding preferentially
• Impressed current system
o An inert anode is connected to the monopile via a DC current supply
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Small scale simulation project
Monopile simulation rig designed to
investigate:
1. Static environment conditions
2. Simulated tidal conditions
3. Sacrificial or ICCP systems
4. Mud zone
5. Changes in water chemistry with
elevation
We used real seawater
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Laboratory simulation
Objectives
• To reproduce, as far as possible, the conditions inside a
monopile.
• Monitor the cathodic protection response and any
environmental changes as a consequence of application
of CP.
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Parameters monitored
System specific parameters
• Potentials of anode and cathode
• Current
Monopile environment
• pH
• Oxygen
• Water analysis*, only Al and Zn concentration from dissolution (sacrificial system) are presented
• Free chlorine (ICCP system) as a result of the anode reaction
* Bicarbonate, iron
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Effects on the internal environment
(sacrificial – Al Anodes)
pH
/ A
l co
nce
ntr
atio
n (
ppm
)
Time
pH (no tide, under
tidal ingress, replenishment dependent)
Pote
ntial (m
V)
Time
+
-
Anode (Al)
Cathode
Oxygen (
ppm
)
Time
Oxygen ~5ppm (tidal ingress)
Al concentration (tidal ingress,
replenishment dependent )
Al concentration (no tidal ingress)
pH 7-8
pH <5
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Effects on the internal environment
(sacrificial – Zn Anodes)
pH
/ A
l co
nce
ntr
atio
n (
ppm
)
Time
pH (for both no tide, and
with tidal ingress)
Pote
ntial (m
V)
Time
+
-
Anode (Zn)
Cathode
Oxygen (
ppm
)
Time
Oxygen ~5ppm (tidal ingress) Zn concentration (tidal ingress)
Zn concentration
(no tidal ingress)
pH 6-7pH 7-8
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Effects on the internal environment (ICCP)
Fre
e C
hlo
rin
e c
on
ce
ntr
atio
n (
ppm
)Time
Tidal ingress
No tidal ingressPote
ntia
l (m
V)
Time
+
-
Anode (Ti-MMO)
Cathode
pH
/ O
xygen (
ppm
)
Time
pH between 7 and 8
Oxygen* ~8ppm (tidal ingress)
* Possible interference by free chlorine (oxidising agent)
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Conclusions and Recommendations
• The choice of CP system needs to be carefully considered
• The selection of CP system is highly dependent on the monopile design and internal environment (tidal ingress)
• Monopile designs vary, even within one windfarm
• For new monopile designs, industry must consider corrosion protection at the design stage
• Industry should collect and trend corrosion and integrity data to forecast when the future criteria of non-conformance will occur, e.g.:
i. Consumption of fatigue life
ii. Wall penetration by corrosion
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