Offshore Wind Power Foundations' Corrosion Protection Strategy: Anlysis remotely controlled corrosion protection system and comparison to traditional corrosion protection of offshore wind foundation

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I Operating and Maintenance Engineer Independent project

Offshore Wind Power Foundations’

Corrosion Protection Strategy

Analysis remotely controlled corrosion protection system and comparison to traditional corrosion protection of offshore wind foundations.

Mazen Alhamalawi 2021-04-19

Program: Operating and maintenance engineer Subject: Independent project

Level: 15hp

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Linnéuniversitetet

Kalmar Sjöfartshögskola

Utbildningsprogram: Drift- och underhållsteknik Arbetets omfattning: Examensarbete, 15hp

Titel: Korrosionsskyddsstrategi för havsbaserade vindkraftsfundament Författare: Mazen Alhamalawi

Handledare: Tobias Hedin Examinator: Stefan Adolfsson

Abstrakt

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När en metall är omgärdad av en elektrolyt, så som havsvatten, kommer det att byggas upp en naturlig potential. Det sker en elektronvandring mellan materialet och havsvattnet och ju större potentialskillnad desto större sannolikhet att metallen kommer korrodera. Korrosion är en stor och viktig fråga för offshorekonstruktioner och byggnader.

För att uppnå en konstruktions designade livslängd kan åtgärder vidtas med hänsyn till kapitalkostnader och drift- och underhållskostnader.

Denna studie syftar till att jämföra ekonomiska för- och nackdelar hos de två korrosionsskyddssystemen Galvanic Anode Corrosion Protection (GACP) och Impressed Current Cathodic Protection (ICCP) på havsbaserade vindkraftsfundament. Det förstnämnda systemet använder offeranoder och det sistnämnda är ett katodiskt korrosionsskydd med hjälp av påtryckt ström.

Studien bestod av flera steg av litteraturstudier där teori om korrosion och korrosionssystem användes för att till slut kunna jämföra valda korrosionsskyddssystem.

Resultatet visar att GACP har fler fördelar och färre nackdelar än ICCP och skulle därmed vara mer ekonomiskt fördelaktig i marina miljöer. GACP ger också önskad effekt direkt vid installation och behöver inte någon strömkälla, ICCP är mer komplicerat och är inte effektivt förrän hela systemet är monterat och i drift. Dessutom behöver ICCP extra strömkälla samt kablage.

Nyckelord

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Linnaeus University

Kalmar Maritime Academy

Education Program: Operating and Maintenance Engineering Scope of the Project: Graduation Project, 15hp

Title: Corrosion Protection Strategy for Offshore Wind Power Foundations

Author: Mazen Alhamalawi

Supervisor: Tobias Hedin Examiner: Stefan Adolfsson

Abstract

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When a metal is surrounded by an electrolyte, such as seawater, a natural potential will be built up. An electron migration between the material and the seawater will happen and the greater the potential difference, the greater the probability that the metal will corrode. Corrosion is an important issue when it comes to offshore structures. In order to achieve a structure designed lifetime, measures can then be taken with regard to capital costs and operating and maintenance costs.

This study aims to compare the economic advantages and disadvantages of the two, Galvanic Anode Corrosion Protection (GACP) and Impressed Current Cathodic Protection (ICCP), corrosion protection systems on offshore wind power foundations. The first mentioned system uses sacrificial anodes and the second is a cathodic corrosion protection by an applied current.

The study consisted of several stages of literature studies where theory of corrosion and corrosion systems was used to finally be able to make a comparison between selected corrosion protection systems.

The result shows that GACP has more advantages and fewer disadvantages than ICCP and would thus be more economical. GACP, for example, is efficient during installation and does not need an additional power source, but ICCP is more complicated and not efficient until complete assembly of the entire system and requires additional power source and cables. Right now, there is no design standard available with detailed requirements and advice has been given as for galvanic anodes systems.

Keywords

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Preface

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This project is one of the graduation projects for obtaining the bachelor’s degree in faculty of operation and maintenance engineering. It is a part of a huge study for Copenhagen Offshore Partners (COP) to develop the corrosion protection systems. The main aim of this graduation project is to make a simulation of the real practical life for the students.

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List of Figures:

FIGURE 1.AN OFFSHORE WIND TURBINE STRUCTURE OF MONOPILE FOUNDATION TYPE CONNECTED TO

THE SEABED BY A PRE-DRIVEN PILE, WITH A TOWER OF 60 METERS LONG AND ABOUT 150 TONS

WEIGHT LOADED WITH THE NACELLE IN WHICH THE WIND TURBINE GENERATORS IS ASSEMBLED

WITH OTHER COMPONENTS,SOURCE:(IABSE,2010). ... 3

FIGURE 2.DIMENSIONS FOR A TYPICAL 2MW OFFSHORE WIND TURBINE OF A MONOPILE FOUNDATION.

MONOPILES’ DIAMETER CHANGES BETWEEN 4 AND 8 M,IN OFFSHORE WIND FARM, WIND

STRUCTURES ARE CONNECTED TO EACH OTHER AND TO BE CONNECTED FINALLY TO A STATION

ON THE LAND VIA CABLES,SOURCE:(MALHOTRA,2011). ... 4

FIGURE 3.THERE ARE DIFFERENT TYPES OF OFFSHORE WIND TURBINE FOUNDATIONS.SOME OF THE

MOST COMMON ARE MONOPILE (SECOND AND FOURTH FROM THE LEFT), GRAVITY-BASED

(THIRD AND FIFTH FROM THE LEFT) AND JACKET FOUNDATIONS (FIRST FROM THE LEFT).THE

CHOICE IS BASED UPON THE DEPTH OF THE SEAWATER AT WHICH THE WIND TURBINE WILL BE

INSTALLED,SOURCE:(DAVIDSEN,2016). ... 5

FIGURE 4.THE MINIMUM DISTANCES BETWEEN TURBINES IS TEN TIMES OF THE ROTOR DIAMETER.THE

TYPE OF THE STRUCTURE OF WIND TURBINE IS BASED ON THE DEPTH OF WATER AND AS THE

WATER DEPTH IS MORE THAN 60 M, THE STRUCTURE OF THE WIND TURBINE IS THE FLOATING

STRUCTURE,SOURCE:(USDEPARTMENT ENERGY,2014). ... 6

FIGURE 5.AJACKET FOUNDATION OF THREE JACKET LEGS, BRACES AND JOINTS WITH A TRANSITION

PIECE IN WHICH THE TOWER WILL BE INSTALLED AND TO BE ROTATED BY FEW MOTORS TO

EXPERIENCE THE WIND DIRECTION IN ADDITION TO A DAVIT CRANE TO LIFT UP TOOLS AND

SPARE PARTS FROM THE SHIP ON THE WIND TURBINE PLATFORM DURING MAINTENANCE AND

REPAIR WORK,SOURCE:(COP,2020). ... 7

FIGURE 6.THE DIFFERENT ZONES OF FOUNDATION BASED UPON THE SURROUNDING ENVIRONMENT

WHERE THESUBMERGED ZONEIS ALWAYS SURROUNDED BY WATER WHILE THEATMOSPHERIC

ZONESTAYS DRY.THE TIDALZONEIS THE AREA THAT IS ABOVE WATER AT LOW TIDE AND UNDER

WATER AT HIGH TIDE WHILE THESPLASH ZONEIS AFFECTED BY THE WAVE MOTION,SOURCE:

(COP,2020). ... 8

FIGURE 7.EFFECT OF THE VELOCITY AND CHLORIDE CONCENTRATION ON CORROSION OF CARBON STEEL

WHERE THE CORROSION RATE GRADUALLY INCREASES WITH THE INCREASE IN FLOW VELOCITY,

THEN IT GRADUALLY DECREASES AFTER A PEAK,SOURCE:(UHLIG,2011B). ... 12

FIGURE 8.EFFECT OF VELOCITY ON CORROSION OF STEEL AND CAST IRON IN SEAWATER.(A)CARBON

STEEL TESTED FOR 36 DAYS AT 23º C.(B)CARBON STEEL TESTED FOR 30 DAYS AT AMBIENT

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FIGURE 9.OFFSHORE FOUNDATION OF JACKET TYPE WITH FOUR JACKET LEGS LINKED TO THE SEABED

BY A GROUTED CONNECTION TO FOUR PRE-DRIVEN PILES WITH THE BOAT LANDING

ASSEMBLY WHICH ALLOWS TRANSFERRING OF PERSONNEL FROM A SERVICE VESSEL TO THE

SUBSTRUCTURE AND VICE VERSA.THE BOAT LANDING FINDERS GIVE THE VESSEL A SOLID

STRUCTURE TO DOCK AGAINST,SOURCE:(PRI,2016) ... 15

FIGURE 10.THE POLARIZATION CURVE FOR THE SACRIFICIAL ANODE (BLUE) AND THE STEEL STRUCTURE

(RED) AS A RELATION BETWEEN THE POTENTIAL AND THE CURRENT DENSITY AS IT IS ON THE

LEFT SIDE AND BETWEEN THE POTENTIAL AND THE LOGARITHM VALUE OF THE CURRENT AS IT IS

ON THE RIGHT SIDE,SOURCE:(COMSOL,2014) ... 19

FIGURE 11.SIMPLE IMPRESSED CURRENT CATHODIC PROTECTION SYSTEM.A SOURCE OF DC ELECTRIC

CURRENT IS USED TO HELP DRIVE THE PROTECTIVE ELECTROCHEMICAL REACTIONS, THE

ANODES ARE CONNECTED ELECTRICALLY TO A DC POWER SOURCE, WHICH COULD BE A SOLAR

PANEL, A GENERATOR, OR RECTIFIED AC POWER. THE POWER SOURCE THEN CONNECTS TO THE

ITEM TO BE PROTECTED, LIKE THE HULL OF A SHIP OR AN UNDERGROUND PIPELINE, WHICH

ACTS AS THE CATHODE IN THE CIRCUIT,SOURCE:(NERVOSA,2012) ... 20

FIGURE 12.EXECUTION OF OPERATIONS AND MAINTENANCE BY A REMOTELY OPERATED VEHICLE, THEY

CAN BE USED IN CONJUNCTION WITH DIVERS, OR AS A REPLACEMENT TO THEM, WHEN THE

OPERATION’S ENVIRONMENTAL CONDITIONS ALLOW.FACTORS TO BE CONSIDERED IN DECIDING

WHICH CAPABILITY WILL BE USED SHOULD INCLUDE, BUT NOT BE LIMITED TO, THE NATURE OF

THE MISSION, THE REQUIRED DEGREE OF ACCURACY, AND THE ALLOWABLE TIME PLUS

RESOURCES AVAILABLE,SOURCE:(ECAGROUP, U.D.) ... 21

FIGURE 13.EXECUTION OF OPERATIONS AND MAINTENANCE BY DIVERS IS VERY IMPORTANT.THE MAIN

STANDARDS ARE THE HIGH LEVEL OF TRAINING AND QUALIFICATION.PROFESSIONAL DIVERS

HAVE WORKING WITH TRUSTED EQUIPMENT WHICH GUARANTEES THE HIGH SAFETY LEVEL

OPERATIONS UNDERWATER,SOURCE:(MARINEENGINEERING,2013) ... 21

FIGURE 14.QUOTATION PRICE OF ICCP SYSTEM FOR TWO DIFFERENT VIRTUAL PROJECTS 1 AND 2 FROM

TWO DIFFERENT SPECIALIZED COMPANIES A AND B IN COMPARISON WITH PRICE OF GACP

SYSTEM FROM CAPEX ASPECT. ... 24

FIGURE 15.THE CONNECTION BETWEEN THE PRE-DRIVEN PILES AND THE JACKETS LEGS IS A GROUTED

CONNECTION WITH HIGH PERFORMANCE BETWEEN THE THE OUSIDE OF THE JACKET LEGS AND

THE INSIDE OF THE PILES IN THE INTERFERENCE ZONES.EXTERNAL ANODES AS PART OF THE

CATHODIC PROTECTION SYSTEM ARE WELDED TO THE JACKET LEGS AND BRACES BELOW THE

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FIGURE 16.THE JACKET STRUCTURE IS TO PROVIDE STRUCTURAL SAFETY FOR THE NACELLE ASSEMBLY

AND TO TRANSFER THE TURBINE LOADS AND MULTIPLE LOADS ACTING ON THE FOUNDATION

INTO THE SEABED. ... 2

FIGURE 17.THE PURPOSE OF EXTERNAL J-TUBES IS TO PROVIDE PROTECTION TO THE CABLES FROM

SEABED TO THE UNDERSIDE OF TRANSITIONAL PIECE WHERE THE CABLES ARE PREPARED,

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List of Tables:

TABLE 1.SELECTION OF FOUNDATION BASED UPON THE WATER DEPTH AND THE ENTIRE LOADS AS PER

LICENGINEERING. ... 5

TABLE 2.APPLICATION STANDARDS AND REFERENCES ACCORDING TO (DNVGL,2016). ... 16

TABLE 3.ATMOSPHERIC CORROSIVITY CATEGORIES AND EXAMPLES OF TYPICAL ENVIRONMENTS AS PER

(ISO12944,2017) ... 17

TABLE 4.CATEGORIES OF WATER & SOIL AS PER (ISO12944,2017) ... 18

TABLE 5.ENVIRONMENT & CORROSION CLASS FOR A TYPICAL OFFSHORE WIND TURBINE FOUNDATION AS

PER (ISO12944,2017) ... 18

TABLE 6.ADVANTAGES & DISADVANTAGES FOR ICCP SYSTEM ... 23

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List of abbreviations & symbols

AC Alternating Current

C Concentration of dissolved oxygen [mol/L]

CA The corrosion allowance

CAPEX Capital Expense

D Diffusion coefficient for dissolved oxygen in water [cm2_/s]

DC Direct Current

F Faraday constant [C/mol]

GACP Galvanic Anodes Corrosion Protection

i Current density in the absence of diffusion barrier layer [A/cm2_]

ICCP Impressed Current Corrosion Protection

IM Immersion

M Atomic mass [g]

MO Alkalinity: Methyl Orange Alkalinity

n Number of electrons involved in the reaction

n΄ Numbers of electrons freed by the corrosion reaction

OPEX Operating Expense

P Density [g/cm3]

RH Relative Humidity

ROV Remotely Operated Vehicles TC The useful lifetime of painting

TD The lifetime from installation of jacket foundation to demolition

TP Transition Piece

Vcorr The maximum expected corrosion rate

δ Thickness of the diffusion layer [cm]

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1 Introduction

Corrosion is an important issue when it comes to offshore structures. Due to high costs of treatment and prevention of corrosion, it is of great importance to choose the most beneficial corrosion protection system (Vattenfall, 2020).

Offshore wind turbine is among those turbines where most companies working in this field have the corrosion problem in focus and they always searching for alternative solutions for reducing the maintenance costs (icorr, 2009).

1.1 Background

Wind power development arises generally from renewable energy production and climate change mitigation. The target of both European Union’s (EU) renewable energy and Swedish government of 100% renewable electricity production encourage to increase the amount of renewable energy sources which wind power is one of them (Energy, 2018).

Throughout the year 2003 to 2016, there has been a five-fold increase in the installation of wind power turbines in Sweden (see Figure 1). In 2018, 63% of renewable energy investments at the

EU-level comes from wind energy, which is an increase from 52% in 2017 (Grönlund, 2019).

To prevent the collapse of wind turbine foundations during its designed lifetime which is at least 25 years (Ingram, 2019). It is important to prevent the corrosion. As we learned during the study years in the university especially in maintenance engineering that there are two methods for maintenance, which are:

 Repairing of occurred defects: to treat the exist defect.

 Preventing occurrence of defects: to take some procedures which lead to prevent the defects where periodic maintenance is among of them.

1.2 Target & Questions:

The target of this study is to compare two different corrosion protection systems focusing on the operation and maintenance implications which the different offshore wind farm foundations have. This study will aim to suggest best corrosion protection strategy for different offshore wind foundations types.

1.2.1 Question 1:

What are the pros and cons for the different cathodic protection systems?

1.2.2 Question 2:

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1.3 Demarcation

Jacket foundation is one of many types of offshore foundations. Due to the long lifetime of offshore wind turbine, it should be protected against corrosion by using a corrosion protection system.

According to offshore design standards, the protection system should be a combination of two protection systems. These protection systems can be:

- Coating protection system with cathodic protection system.

- Corrosion allowance with cathodic protection system.

Coating protection and corrosion allowance system will be explained simply in this project but further focus to be taken on cathodic protection system. Cathodic protection system is either GACP or ICCP.

1.4 Ethics & sustainability

Both suppliers mentioned below of the ICCP system have been asked by the supervisor company of this project (study) to be mentioned with names and prices in this project (study). Both refused mentioning of name but approved the mentioning of prices.

1.5 Validity & reliability

Copenhagen Offshore Partners (COP) company is one of the long-established companies in offshore wind turbine field since many years, and they have many branches and projects in many countries in addition to the huge experience in this field which enhance the validity and reliability.

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2 Theory

2.1 Wind Turbine

A machine for converting the wind kinetic energy to a mechanical energy which being later exploited to produce electricity.

Offshore wind turbine structures consist of wind turbine generators with foundations to support the wind turbines in the submerged zone and into the seabed, as shown in figure 1 and 2.

Figure 1. An offshore wind turbine structure of monopile foundation type connected to the seabed by a pre-driven pile, with a tower of 60 meters long and about 150 tons weight loaded with the nacelle in which the wind turbine generators is assembled with other components, Source: (IABSE, 2010).

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Figure 2. Dimensions for a typical 2 MW offshore wind turbine of a monopile foundation. Monopiles’ diameter changes between 4 and 8 m, In offshore wind farm, wind structures are connected to each other and to be connected finally to a station on the land via cables, Source: (Malhotra, 2011).

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The wind turbine consists of the rotor, nacelle and tower. The support structure (foundation) can be one of different types depending on the water depth and loads on the entire structure, as figure 3:

Figure 3. There are different types of offshore wind turbine foundations. Some of the most common are monopile (second and fourth from the left), gravity-based (third and fifth from the left) and jacket foundations (first from the left). The choice is based upon the depth of the seawater at which the wind turbine will be installed, Source: (Davidsen, 2016).

Most common foundation types are jackets or monopiles, but other foundations types as suction bucket foundations and/or gravity-based foundations are also known from the industry (COP, 2020). Table 1 shows criteria for selecting foundation type based on water depth according to (Davidsen, 2016). More pictures for jackets foundation are listed in Appendix.

Table 1. Selection of foundation based upon the water depth and the entire load (Davidsen, 2016).

Type Water depth [m]

Concrete gravity Up to 27

Steel monopiles Up to 30

Steel tripods Up to 30

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The selection between foundation types is based on evaluation of the water depth as in figure 4:

Figure 4. The minimum distances between turbines is ten times of the rotor diameter. The type of the structure of wind turbine is based on the depth of water and as the water depth is more than 60 m, the structure of the wind turbine is the floating structure, Source: (US Department Energy, 2014).

The internal components of wind turbine, such as generator, transformer and switchgear, are supported on different layers of platforms and nacelle are placed at the top of the tower.

Further to the above, the foundations which is the main focus area of this project will contain different substructures. Figure 5 shows the external setup on a jacket foundation. The foundations normally include external platform used for laydown area and access to the wind turbine. Further to the external platform, a boat landing will be used for approaching service boats. Number of rest platforms will be externally included to the foundation structure based on the climbing height from the service vessel to the desired place. Cables are furthermore protected from the harsh environment via J-tubes from seabed and all the way up to the transition piece (COP, 2020).

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Figure 5. A Jacket foundation of three jacket legs, braces and joints with a transition piece in which the tower will be installed and to be rotated by few motors to experience the wind direction in addition to a Davit crane to lift up tools and spare parts from the ship on the wind turbine platform during maintenance and repair work, Source: (COP, 2020).

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The foundations are divided into three different zones namely atmospheric, splash and submerged zone, as it is shown in figure 6 below.

Figure 6. The different zones of foundation based upon the surrounding environment where the submerged zone is always surrounded by water while the atmospheric zone stays dry. The tidal zone is the area that is above water at low tide and under water at high tide while the splash zone is affected by the wave motion, Source: (COP, 2020).

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2.2 Corrosion in Water

This corrosion cannot be completely prevented, but it is possible to reduce the corrosion rate by applying corrosion protection. However, reducing the corrosion rate to negligible values such as 0.01 mm / year is possible (COP, 2020).

2.2.1 Theoretical Corrosion by water

Corrosion in water, wet corrosion, is an electrochemical process where the metal surface, electrode, and the liquid, electrolyte, makes a chemical reaction. All liquids that have electrolytic characteristics, can cause wet corrosion. Wet corrosion is also caused by moisture and condensation in air or soil (Uhlig, 2011b).

The theory for wet corrosion must be divided into fresh water and seawater, because of the presence of chloride ions in seawater. The chloride ion has a high influence on the film that can be formed on metal surfaces. Corrosion in both fresh water and seawater is highly depended on oxygen (C. Vogel, 2001).

2.2.2 Fresh water

The chemical reactions can be simplified to equation (3-1) (C. Vogel, 2001).

Ion + oxygen + water = corrosion (3-1)

Corrosion of steel in freshwater is an electrochemical process. The electrochemical reaction, using dissolved oxygen, can be seen in equation (3-2), where the reaction between steel and freshwater is shown.

Fe + ½O2 + H2O Fe (OH)2 (3-2)

The reaction can be split up so that the electron migration can be seen. The electron moves from iron as it is in equation (3-3) and to be collected by water and oxygen as it is in equation (3-4) which makes the new base react with the iron ion. The reaction to the steel can be explained from the anode reaction. The cathode reaction functions as an oxidation reaction (Uhlig, 2011b).

Anode reaction Fe Fe2+_{+ 2e¯} _(3-3)

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The theoretical current density for steel in stagnant air saturated freshwater can be determined by equation (3-5)

𝑖 = . . . 𝐶 . 10 (3-5)

Where:

i: Current density in the absence of diffusion barrier layer [A/cm2_]

D: Diffusion coefficient for dissolved oxygen in water [cm2_/s]

n: Number of electrons involved in the reaction F: Faraday constant [C/mol]

δ: Thickness of the diffusion layer [cm] C: Concentration of dissolved oxygen [mol/L]

When the current density is known, the corrosion rate can be calculated by using equation (3-6) (Uhlig, 2011b).

1𝑚𝐴/𝑐𝑚 = 3.28 . 𝑀

n΄. 𝑃 𝑚𝑚/𝑦𝑒𝑎𝑟 (3-6)

Where:

M: Atomic mass [g]

n΄: Numbers of electrons freed by the corrosion reaction P: Density [g/cm3_]

The conversion for steel Fe is 1mA/cm2_{equal to 11.6 mm/year while n is equal to two, M is equal}

55.85g and the density is 7.88 g/cm3_{(CorrosionDoctors, 2014).}

The corrosion allowance at the different corrosion zones to be calculated according to DNV-OS-J101 (May 2014) by equation (3-7)

𝐶𝐴 = 𝑉 x (𝑇 − 𝑇 ) (3-7)

Where:

CA: The corrosion allowance

Vcorr: The maximum expected corrosion rate

TD: The lifetime from installation of MP to demolition

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Several parameters influence the corrosion factor for natural freshwater (CorrosionDoctors, 2014). They are:

 pH

 Dissolved oxygen

 Calcium hardness (Ca) (Amount of CaCO3 in water)

 Total alkalinity (MO)  Total dissolved solids  Chloride ion (Cl-₎

 Sulphate ion (SO42-)

Dissolved oxygen and pH are fundamentals, but usually they do not change the corrosion rate of steel in natural freshwater. The pH value is normally between 5 and 8.5 and dissolved oxygen is 8-10 ppm at ordinary temperatures (Uhlig, 2011b).

Ca hardness, pH, MO alkalinity and the total dissolved solids determine if a CaCO3 diffusion barrier

film is formed on the metal surface or not. The CaCO3 film is less protective at high temperature and

high levels of salts (Uhlig, 2011b).

The Saturation Index is dependent on pH value, Ca hardness, MO alkalinity and the total amount of dissolved solids and can be calculated by using equation (3-8). CaCO3 is deposited when the saturation

index is higher than one (Uhlig, 2011b).

𝑆𝑎𝑡𝑢𝑟𝑎𝑡𝑖𝑜𝑛 𝑖𝑛𝑑𝑒𝑥 = 𝑝𝐻 − 𝑝𝐻 (3-8)

Where pHs can be calculated in equation (3-9).

𝑝𝐻 = −𝑙𝑜𝑔[𝐻 ] (3-9)

When the saturation index is:

 Positive: there is a tendency to form CaCO3 diffusion-barrier film.

 Negative: No CaCO3 diffusion-barrier film is formed.

The corrosivity is not defined by the saturation index alone. The corrosion can be different even when the value of the saturation index is the same. The pH, Ca hardness, MO alkalinity and the amount of dissolved solids also define corrosivity (Uhlig, 2011b).

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Chloride and sulphate ions are always present in natural fresh water, they increase the conductivity which affects the penetration rate of localized corrosion. The ions also affect the critical concentration of oxygen and the critical water velocity where the passivation of steel occurs. The chloride and sulphate ions contribute little to the corrosion rate when the corrosion rate is governed by dissolved oxygen (Uhlig, 2011b).

Chloride ions and velocities influence the corrosion rate for steel as it is seen in Figure 7, where the velocity is a main factor. The corrosion rate will continue to increase when the velocity increases until the critical velocity is reached, after which it will decrease (Uhlig, 2011b).

Figure 7. Effect of the velocity and chloride concentration on corrosion of carbon steel where the corrosion rate gradually increases with the increase in flow velocity, then it gradually decreases after a peak, Source: (Uhlig, 2011b).

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2.2.3 Seawater

The important variables for corrosion in seawater is salinity, the concentration of dissolved oxygen, temperature, pH, carbonate, pollutant and biological organisms (Uhlig, 2011c).

The parameters affect differently. The dissolved oxygen concentration has a high influence on the corrosion rate. Other factors influence the amount of oxygen or the carbonate film that can be formed on the metal surface to protect the steel (Uhlig, 2011c).

Velocity influences corrosion in seawater. The increasing of corrosion rate is proportion directly with the increasing of the velocity until reaching the critical velocity. Upon reaching the critical velocity, the corrosion rate will never be decreased with the velocity increasing, but it will be at a lower rate as it is shown in Figure 8 (Uhlig, 2011c).

Figure 8. Effect of velocity on corrosion of steel and cast iron in seawater. (a) Carbon steel tested for 36 days at 23º C. (b) Carbon steel tested for 30 days at ambient temperature. (c) Cast iron tested for 7 days at 25º C, Source: (Uhlig, 2011c).

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2.3 Carbon Dioxide Corrosion

Corrosion with carbon dioxide is seen in the offshore industry where CO2 is found naturally in gas and

oil pipelines (Uhlig, 2011a).

2.3.1 Theoretical corrosion by carbon dioxide

When carbon dioxide and water corrode iron, it happens by carbon dioxide change into the form of carbon acid (H2CO3). Equation (3-10) shows the carbon dioxide dissolved in water (Uhlig, 2011a).

These reactions are based on (Uhlig's Corrosion Handbook, Chapter 19, 2011a).

CO2(g) + H2O CO2 (dissolved) (3-

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Equation (3-11) shows the migration of electron being absorbed by carbonic acid and becoming bicarbonate (HCO3¯) and a charged hydrogen atom.

CO2 (dissolved) + H2O H2CO3 H+ + HCO3¯ (3-11)

In equation (3-12), electrons are used, and the electrons can come from an electron migration from a metal.

2H2CO3 + 2e¯ H2 + 2HCO3¯ (3-12)

The reaction of the iron is a migration of an electron as it is seen in equation (3-13), where the electrons leave iron, and the iron becomes charged.

Fe Fe2+_{+ 2e¯} _(3-13)

Combining equations (3-10) to (3-13) produces equation (3-14). This equation shows the start and the end of the reaction that occurs in the corrosion of iron with carbon dioxide and water. The reaction forms iron carbonate, FeCO3 and dihydrogen, H2.

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2.4 Corrosion Protection Systems

In order to protect the foundation structure shown in figure 9 against galvanic corrosion, a corrosion protection must be used. This can according to offshore design standards be done by a combination of two protections systems. This can be one of the following:

- Coating combined with cathodic protection system. - Corrosion allowance / cathodic protection system.

Systems used currently for offshore cathodic protection systems are either GACP or ICCP system (Matcor, 2020).

Figure 9. Offshore foundation of Jacket type with four jacket legs linked to the seabed by a grouted connection to four pre-driven piles with the boat landing assembly which allows transferring of personnel from a service vessel to the substructure and vice versa. The boat landing finders give the vessel a solid structure to dock against, Source: (Pri, 2016)

2.4.1 Protective Coatings

The main aim of coatings is to provide a barrier between the metal and the environment in addition to the possibility to combine the protective function with aesthetic appeal. Coating can be classified into metallic and non-metallic coatings (Ahmed, 2006).

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The protection systems design for offshore structures is used in combination with a coating system. The use of coatings can significantly reduce the required current for cathodic protection. For the weight of the structure, the coatings help to reduce the numbers of anodes required to carry out the cathodic protection.

The minimum service life of coating systems to be selected for corrosion protection of offshore structures is 15 years (Ahmed, 2006).

2.4.1.1 Normative References

The specification, preparation, and application of coating systems for steel parts must comply with the latest version of these standards described in Table 2.

Table 2. Application standards and references for preparing procedures, (DNVGL C. P., 2016).

NORSOK M-501:2012 Surface Preparation and Protective Coating

ISO 20340:2009 Paints and varnishes — Performance requirements for protective paint systems for offshore and related structures

EN ISO 4628-1 to 6:2003

Paints and Varnishes – Evaluation of Degradation of Coatings – Designation of Quantity and Size of Defects, and of Intensity of Uniform Changes in Appearance

EN ISO 12944-1-8:1998 to 2007 Paint and Varnishes – Corrosion Protection of Steel Structures by Protective Paint Systems

EN ISO 14713:1999 Protection Against Corrosion of Iron and Steel in Structures – Zinc and Aluminum Coatings

EN ISO 2063:2005 Metallic and other Inorganic Coatings – Thermal Spraying – Zinc, Aluminum and their Alloys

ASTM A 123 Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products

ASTM A 153 Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware

ASTM G 102-89 Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements BS 5493:1977 Code of Practice for Protection of Iron and Steel Structures

Against Corrosion

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2.4.1.2 Corrosive Categories

The different corrosive categories for offshore wind turbines are defined in ISO 12944 part 2. Five corrosivity categories are defined from (C1) not corrosive interior atmosphere up to industrial and sea climate (C5 I + C5 M), I stand for Industry and M stand for Marine. Decisive point for the determination of the categories is the loss of mass of unprotected steel and galvanized steel at outdoor storage as it is shown in Table 3. IM1 to IM3 represents the load in water and soil as it is shown in Table 4.

Table 3. Atmospheric corrosivity categories and examples of typical, (Swedish Standards Institute [SIS], 2017) Corrosivity

2.4.2 Cathodic Protection

Cathodic protection is a method to reduce the corrosion by decreasing the potential difference between the anode and the cathode where a current to be applied to the structure from an external source.

The principle of cathodic protection is to connect an external anode to the structure to being protected where a direct current (DC) passes to the whole structure so that the structure becomes cathodic and therefore do not corrode. The external current source can be from an external anode where the current is a result of the potential difference between the two metals, or it may be an impressed current from external power source.

The two main types of cathodic protection systems are ICCP and GASP. Both types have an anode, a continuous electrolyte from the anode to the protected structure and an external metallic connection (wire). These items are necessary for all cathodic protection systems (Ahdash, 2010).

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2.4.2.1 Sacrificial Anodes

A sacrificial anode cathodic protection system makes use of the corrosive potentials for different metals. Without cathodic protection system, one area of the structure will be at a more negative potential than the other and results the occurrence of corrosion on the structure. On the other hand, if a negative potential metal such as Mg is placed adjacent to the structure to be protected and a metallic connection is installed between the object and the structure, the object will become the anode and the entire structure will become the cathode. A new object will sacrificially corrode to protect the structure, and that is why this protection system is called a sacrificial anode cathodic protection system because the anode corrodes sacrificially to protect the structure. Materials of anode in this system are usually made of magnesium, aluminum and zinc because of the higher potential of these metals compared to steel structures.

Applications of sacrificial anodes

The principle of GACP is that the sacrificial anode who is connected to the steel structure is more active than the steel i.e. has a high electrochemical potential which means that the electrons will be transferred from the sacrificial anode towards the steel structure which will be completely cathode. The electrochemical reaction between the electrodes is at the expense of the anodes which progressively dissolved while oxygen reaction takes place at the surface of the structure. The supply of oxygen available for the oxygen reduction is mostly what limits the current density (COMSOL, 2014).

Figure 10 below schematically shows the polarization of the sacrificial anodes and the oxygen reduction reaction taking place at the surface of the protected steel structure.

Figure 10. The polarization curve for the sacrificial anode (blue) and the steel structure (red) as a relation between the potential and the current density as it is on the left side and between the potential and the logarithm value of the current as it is on the right side, Source: (COMSOL, 2014)

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2.4.2.2 Impressed current, ICCP

In case of high electrolyte resistivity or for larger structure, galvanic anodes protection system cannot provide enough current economically to protect the structure. Thus, ICCP systems can be used in this case because the current provided from an external DC power source as shown in Figure 11 (Janowski, 2016).

Figure 11. Simple impressed current cathodic protection system. A source of DC electric current is used to help drive the protective electrochemical reactions, the anodes are connected electrically to a DC power source, which could be a solar panel, a generator, or rectified AC power. The power source then connects to the item to be protected, like the hull of a ship or an underground pipeline, which acts as the cathode in the circuit, Source: (Nervosa, 2012)

Cathodic protection transformer-rectifier units are manufactured and equipped with many different features, including remote monitoring and control, current interrupters and different types of electrical devices. The negative terminal of the rectifier output DC is connected to the structure to be protected by the cathodic protection system, but the positive terminal output is connected to the anodes. The cable power from the external source AC is connected to the rectifier input terminals (Janowski, 2016).

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2.4.2.3 Inspections

The purpose of the inspections and maintenance of the corrosion protection system is to confirm that the foundation structures are protected during the lifetime of the foundations. Offshore operations are normally expensive and should be limited. However, Remotely Operated Vehicles (ROV) is reducing the risk of man operations under water, which can be result in fatality for the service diving crew (Janowski, 2016). Offshore Operation and maintenance can be performed either with an offshore ROV or by offshore divers as it is shown in figures 12 & 13 respectively.

Figure 12. Execution of operations and maintenance by a remotely operated vehicle, they can be used in conjunction with divers, or as a replacement to them, when the operation’s environmental conditions allow. Factors to be considered in deciding which capability will be used should include, but not be limited to, the nature of the mission, the required degree of accuracy, and the allowable time plus resources available, Source: (ECAGROUP, u.d.)

Figure 13. Execution of operations and maintenance by divers is very important. The main standards are the high level of training and qualification. Professional divers have working with trusted equipment which guarantees the high safety level operations underwater, Source: (MarineEngineering, 2013)

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3 Research Methodology

At the beginning, a contact was made with the company who the study is concerned and a general information regarding the target of the study, which was a comparison between the two cathodic protection systems, has been obtained.

To achieve the target of this study, a certain data had to be collected to be able to make a comparison between the two systems with all its equipment and requirements, and to achieve that, the following steps was made:

First, the keywords were used for searching on the internet to collect the required information about the corrosion protection systems for offshore wind turbine which represented in SACP and ICCP. Many sources such as websites and old research regarding the cathodic protection systems was found in addition to the documents obtained from COP and the information obtained on the telephone.

Second, the collected information from those sources were filtered and written as a draft at the beginning and then been read completely as one text to show if the text was in linear format and achieved the required target of this study or not.

The information obtained about the advantages and disadvantages of the two systems has been collected and decided based upon the financial and operating aspect where the positive thing was taken as an advantage and the negative as disadvantage.

For enhancing the results of this study, a feasibility study was made for virtual projects, by using quotations obtained from some suppliers. CAPEX was included in these quotations but OPEX wasn’t and been taken from the sources in addition to the wide experience of COP in this field.

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4 Results

By using the information obtained from the different sources such as energies, DNVGL, intertek websites and old research such as ABS, it became able to achieve the expected target of the study which was to answer the following:

 What are the pros and cons for the different cathodic protection systems?

Tables 6 and 7 are describing the advantages (pros) and disadvantages (cons) for ICCP and GACP systems, respectively.

Table 6. Advantages and disadvantages for ICCP system

ICCP

Advantages Disadvantages

Possible with fewer anodes (Energies, 2019)

Risk of over protection causing hydrogen embrittlement (DNVGL, 2015).

Purchase costs may be lower Requires control reference electrodes, for actual case at least two per foundation (DNVGL, 2015). Installation costs may be lower (less welding)

(ABS, 2018)

Requires external power source and rectifiers (OnePetro, 2004).

No retrofit if designed and installed correctly (i.e. 20-25 years life is possible)

(ABS, 2018)

Not effective until turbine and ICCP system is commissioned (Intertek, 2018).

Can be remotely controlled if required (ABS, 2018)

Generates more hydrogen than galvanic anodes (Intertek, 2018).

Need more maintenance / follow up (Intertek, 2018).

ICCP is lost if power source, rectifiers or cables fails

Table 7. Advantages and disadvantages for GACP system

GACP

Advantages Disadvantages

GACP is effective upon installation at yard (Intertek, 2018)

More anodes required compared with ICCP (Energies, 2019).

No need for external power source and wiring

(ABS, 2018) Uncontrolled (ABS, 2018).

Passive system is requiring no maintenance other than biannually

(OnePetro, 2004)

Short lifetime (Byrne, 2015). No experience needed for installation

(Byrne, 2015) Unknown degree of protection (Byrne, 2015).

No commissioning and tuning required (Intertek, 2018)

Cheaper than ICCP considering life cycle costs No risk of over protection and hydrogen embrittlement

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 How to select between the different cathodic protection systems?

Figure 14 shows the comparison between the prices of equipment required for both protection systems.

Figure 14. Quotation price of ICCP system for two different virtual projects 1 and 2 from two different specialized companies A and B in comparison with price of GACP system from CAPEX aspect.

The selection between the two systems has depended on the price of the whole system from the capital aspect and the operational aspect after installation over the lifetime.

In fig 14, both suppliers A and B has the same price for executing both projects 1 and 2 by using GACP system (201,679 Euro/jacket) as it is shown in the second column.

Supplier A has quoted a price of (126,154 Euro/jacket) for executing project 1 and (120,358 Euro/jacket) for executing project 2 by using ICCP system as it is shown in the third and fourth columns, respectively.

Supplier B has quoted the same price (114,974 Euro/jacket) for executing both projects A and B by using ICCP system as it is shown in the sixth column. Both suppliers quoted the same price (15,000 Euro/jacket) for ICCP design.

Mathematically, the execution of projects A and B by using ICCP system is economically in comparison with the GACP system, and that enhanced the results mentioned above in Table 6. The suppliers A and B have quoted just the capital expenditures for these projects without the operating expenditure.

Cathodic protection of offshore structures by galvanic anodes has well established and generally preferred for such structures. Use of ICCP for such structures may offer certain advantages but is largely unproven for offshore wind turbine structures and there is no design standard available with detailed requirements and advice has been given as for galvanic anodes systems.

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5 Discussion

GACP and ICCP are two types of corrosion protection systems for offshore wind turbine structures. My points in this study are to compare between those two types from CAPEX and OPEX aspect focusing on a certain type of foundations which called Jacket foundation, and how to select the appropriate protection system for the structure. Throughout the obtained result, we notice that the GACP system is more efficient than ICCP system from both CAPEX and OPEX side.

A feasibility study should be made to be able to compare between the two protection systems. This study was important to clarify the comparison from CAPEX and OPEX aspect. The results obtained from the feasibility study was very efficient for the two different virtual projects with the two suppliers despite the non-inclusiveness of estimation of spare parts and electrical failure of any component.

The result shows that the GACP system is more preferred than ICCP system where each project in Fig 14 includes 46 jacket foundations and total saving by using ICCP system for 46 jackets is about 4 million Euro. Despite ICCP system seems to be cheaper than GACP system from CAPEX aspect but it is ineffectual from OPEX aspect and still unpreferred because of the lack of the experience in that system in addition to the unexpected defects of it.

This result is similar to the results obtained from a previous study “A Comparison of Impressed Current and Galvanic Anode Cathodic Protection” where it is proved that GACP is superior to ICCP for Floating Production Storage and Offloading vessels (FPSO’s) (Corrosion, 2004).

Because of this study is a big study and is exceedingly difficult to cover all the concerned parts on the limited time for bachelor’s degree students, many parts of study have not been taken into consideration such as, but not limited to, the expected defects of the ICCP system.

An expected defect of the ICCP system can be, but not limited to, a current called for stray current. This is a current comes from unknown source and affect the protection system and must be treated. If the value of the current is constant, that means the source of the current is DC source but if the value is variable, that means the source is AC source.

Due to the extremely importance of this part of expected defects of ICCP system, I advise to study this part detailly in the future which will give us a complete picture side by side with this study about the actual different between the two cathodic protection systems.

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References

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Shipping. Retrieved from https://ww2.eagle.org/: https://ww2.eagle.org/content/dam/eagle/rules-and- guides/current/offshore/306-cathodicprotection-offshore-structures/cathodic-protection-offshore-gn-dec18.pdf

Ahdash, A. I. (2010). Design of impressed current cathodic protection forsteel immersed in freshwater. Malaysia.

Ahmed, Z. (2006). Principles of Corrosion Engineering and Corrosion Control. USA: Butterworth-Heinemann.

Byrne, A. (2015). State of the art review of cathodic protection for reinforced concrete structures. Norton: Ice.

C. Vogel, C. J. (2001). Metallurgy for Engineers. Viborg: Polyteknisk Company. COMSOL, M. C. (2014, October). Retrieved from

https://doc.comsol.com/5.3a/doc/com.comsol.help.corr/CorrosionApplicationLibraryManual.pdf COP, C. O. (2020). Changfang and Xidao Offshore Wind Farms. Thailand: Copenhagen Offshore Partners.

Corrosion. (2004). A Comparison of Impressed Current and Galvanic Anode Cathodic Protection. Houston, Texas 77060 USA: NACE International.

CorrosionDoctors. (2014, June 12). Retrieved from http://www.corrosion-doctors.org/Principles/Conversion.htm

Davidsen, F. J. (2016). Design of Offshore Wind Farms. Copenhagen: LICEngineering, A/S. Retrieved from https://docplayer.net/21559965-Design-of-offshore-wind-farms-prepared-by-flemming-jakobsen-andrass-ziska-davidsen-licengineering-a-s.html

DNVGL. (2015). Corrosion Protection of Floating Production and Storage Units. DNVGL. DNVGL, C. P. (2016, March). DNVGL-RP-0416. Retrieved from DNVGL:

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https://ec.europa.eu/energy/sites/default/files/documents/sweden_draftnecp.pdf). Government offices of Sweden . Retrieved from https://ec.europa.eu:

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icorr. (2009). https://www.icorr.org/how-does-cathodic-protection-work/. Retrieved from https://www.icorr.org: https://www.icorr.org/how-does-cathodic-protection-work/

Ingram, E. (2019, 09 20). www.renewableenergyworld.com. Retrieved from Renewableenergyworld: https://www.renewableenergyworld.com/om/how-to-extend-the-lifetime-of-wind-turbines/#gref Intertek. (2018, April 03). https://www.intertek.com/blog/2018-04-03-corrosion/. Retrieved from https://www.intertek.com/: https://www.intertek.com/blog/2018-04-03-corrosion/

Janowski, M. (2016). ICCP Cathodic Protection of Tanks with Photovoltaic Power Supply. ResearchGate.

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OnePetro. (2004, March 28). https://onepetro.org/NACECORR/proceedings-abstract/CORR04/All-CORR04/NACE-04095/115513. Retrieved from https://onepetro.org/:

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Appendix

Figure 15. The connection between the pre-driven piles and the jackets legs is a grouted connection with high performance between the outside of the jacket legs and the inside of the piles in the interference zones. External anodes as part of the cathodic protection system are welded to the jacket legs and braces below the lowest astronomical tide.

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Figure 16. The jacket structure is to provide structural safety for the nacelle assembly and to transfer the turbine loads and multiple loads acting on the foundation into the seabed.

Jacket brace

Jacket leg

Jacket joint

Sacrificial Anodes

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Figure 17. The purpose of external j-tubes is to provide protection to the cables from seabed to the underside of transitional piece where the cables are prepared, stored and connected to the inside the transitional piece.

Offshore Wind Power Foundations' Corrosion Protection Strategy: Anlysis remotely controlled corrosion protection system and comparison to traditional corrosion protection of offshore wind foundation