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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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End Slide