Development of dynamic power cables for commercial floating wind farms

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Development of dynamic

power cables for

commer-cial floating wind farms

Literature Assignment

November 12, 2018

Ruben Weerheim

T ec hnische Universiteit Delft

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D

EVELOPMENT OF DYNAMIC POWER CABLES

FOR COMMERCIAL FLOATING WIND FARMS

L

ITERATURE

A

SSIGNMENT

N

OVEMBER

12, 2018

by

Ruben Weerheim

Faculty of Mechanical, Maritime and Materials Engineering Department of Marine and Transport Technology

Author: Ruben Weerheim (4358295) Supervisors: Dr. ir. X. Jiang

Dr. ir. H. Polinder Report Number: 2018.TEL.8288

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Preface

This report provides a literature study about the development of dynamic power cables for commercial float-ing wind farms. This literature study is part of the master track Transport Engineerfloat-ing and Logistics (TEL) at the faculty of 3mE. The main purpose of this report is to provide a thorough overview of the state-of-the-art of dynamic power cables and its ongoing challenges. Furthermore, this literature study was also done to ac-celerate the development of HV dynamic power cables in order to apply these to the first commercial floating wind projects within the next 5-10 years. In addition, I would to like to express my gratitude to Dr. Ir. X. Jiang and Dr. Ir. H. Polinder for their valuable advice and guidance during the literature study.

Ruben Weerheim Delft, October 2018

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i v 1

Summary

INTRODUCTION

Currently, both the climate change and the running out of fossil fuels like oil, gas and coal are demanding for new sustainable energy sources. Renewable energy is a big potential to provide this new type of energy with wind energy as one of the most promising energy source. However, wind energy is currently limited to land and offshore locations with a maximum depth of 50 m due to structural limitations. This leaves vast expanses of uninterrupted open sea unused, providing larger capacities compared to land-based and fixed offshore wind farms. This is because of more consistent and higher wind speeds compared to wind farms closer to shore and on land. Also larger wind turbines can be installed due to the reduction of size limitations. There-fore wind turbines on floating platforms provide the outcome to utilize these unused expanses. However, due to the immature development of floating wind energy, different challenges need to be faced with. One of the main challenges is the dynamic power cable connecting floating wind turbines, substations and sub-marine power cables with each other. Therefore it is of great importance to identify the challenges regarding dynamic power cables and its research gaps to accelerate the development of floating wind energy. The aim of this report is provide a thorough overview of the development of dynamic power cables for commercial floating offshore wind farms.

THE STATE-OF-THE-ART OF FLOATING OFFSHORE WIND FARMS

The main benefits of FOWFs are: stronger and more consistent winds resulting in higher capacity factors, more flexibility for installation locations, simplified installations and no constraints regarding the wind tur-bine size. However, drawbacks of FOWFs are: the technical challenges due to the immature technology, long distances from shore, higher cost and the need of on shore facilities for assembly, repair and maintenance. Floating platforms of wind turbines can be divided into three categories: tension leg platform (TLP), semi-submersible and spar-buoy and can be deployed in water depths ranging from 40 to 500 m. The leader in floating wind energy is Statoil (Hywind) and has the first FOWF worldwide in 2017. Larger FOWFs will have a substation for power transformation to reduce losses. While only the Fukushima project has installed a float-ing substation it is expected that substations up to 500 MW will be feasible. Currently, a celluloid layout has been proposed for FOWFs and seems to be a cost effective solution by sharing one anchor with a maximum of three mooring lines. However, there are also certain drawbacks regarding this design implying the need for improved layout designs.

THE STATE-OF-THE-ART OF SUBMARINE POWER CABLES

Each component of a submarine power cable showed that different designs are possible depending on its application. This shows that there is not one optimal submarine cable design. The export power cable of FOWFs is expected to be HVAC due to the reduced risk and cost compared to HVDC. Therefore, FOWF de-velopers will most likely locate their FOWF closer than 100 km to shore to avoid these risks and cost. In addition, there are many causes of submarine power cable damages and can be divided into three categories: installation, human activities and operational damages. Statistics have shown that human activities are the main source of damages which are mainly caused by fishing equipment and anchors. Improved mitigation strategies are necessary to avoid these damages in the future. Due to the immature stage of floating wind energy, designers and developers are unwilling to share their precious knowledge to competitors. To solve this problem, DNV-GL has already published two relevant standards, DNVGL-ST-0359 and DNV-OS-J103, for submarine and dynamic power cable design.

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CONTENTS iv

THE STATE-OF-THE-ART OF DYNAMIC POWER CABLES

MV dynamic power cables have already been applied to demonstration projects and are despite the limited applications a small step from the market. For HV dynamic power cables there is a gap on the market with voltages up to 220 kV . The Lazy-Wave and Steep Wave hanging configuration are the most suitable for dy-namic power cables. A W-shape configuration can be used for interarray purposes when water depths are too deep with respect to the wind turbines distance. The most critical fatigue points of the common Lazy-Wave configuration is at the hang-off and touch down point. Static and dynamic bend stiffeners as well as an Uraduct can be used to make the cable more resistant to fatigue. The most critical component of the dynamic power cable is the metallic sheath and can be replaced by a metallic corrugated tubular sheathing (MCTS) for improved fatigue life and flexibility. Dynamic power cable damages are expected to occur less than subma-rine power cables due to the elimination of human activity damages. This is because of the FOWF safety zones of 500 m and 2000 m as strong recommendation. The O&M challenges for dynamic power cables are mostly the same as for other wind turbine components. The main challenges include the accessibility and the cost reduction of O&M.

CONCLUSION AND RECOMMENDATIONS

Currently over 40 floating wind concepts are under development from which 27 are active and advanced concepts. In 2021 it is expected that 6 floating wind concepts have reached the pilot array stage (20-50 MW ) after which larger commercial projects can be started.

The main differences between submarine and dynamic power cables are the increased cross-sectional area of the conductor, double armoring, a friction reducing layer and a MCTS instead of lead sheath for improved fatigue life and flexibility. Currently, MV dynamic power cables have already been applied to demo projects and are a small step from the market. However, HV dynamic power cables up to 220 kV still need to be devel-oped. For submarine and dynamic power cables two standards can be used to provide criteria, requirements and guidance for the design and analysis of these cables which are DNVGL-ST-0359 and DNV-OS-J10. The main challenges of the dynamic power cable are: the configuration which influences the fatigue life of the cable, cable damages, sheath design, friction forces between cable layers, fatigue at deep waters and O&M. In this report different actions are described to mitigate these challenges, however some actions need more research and analysis. In order to accelerate the development of dynamic power cables as well as the commercialization of floating offshore wind farms it is recommended to research: the cable and in particular the MCTS for higher voltages, dynamic power cable damages, FOWF layouts, larger dynamic bend stiffeners, CM instruments to estimate the remaining lifetime and fatigue analysis at water depths up to 800 m.

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Nomenclature

1C Single core

3C Three core

AC Alternating Current

ACDP Acoustic Doppler Current Profiler

CLV Cable Laying Vessel

CM Conditional Monitoring

DC Direct Current

DP Dynamic Positioning

DSS Distributed Strain Sensing

DTS Distributed Temperature Sensing

EHV Extra High Voltage

FEA Finite Element Analysis

FOWF Floating Offshore Wind Farm

FOWT Floating Offshore Wind Turbine

GPS Global Positioning System

HDPE High Density Polyethylene

HV High Voltage

HVAC High Voltage Alternating Current

HVDC High Voltage Direct Current

LCOE Levelised Cost of Electricity

LDPE Low Density Polyethylene

LV Low Voltage

MCTS Metallic Corrugated Tubular Sheathing

MV Medium Voltage

MVAC Medium Voltage Alternating Current

O&M Operations and Maintenance

OWF Offshore Wind Farm

OWT Offshore Wind Turbine

PDM Partial Discharge

ROV Remotely Operated Vehicle

SCC Stress Corrosion Cracking

TLP Tension Leg Platform

ViV Vortex Induced Vibrations

VMS Vessel Monitoring System

WTG Wind Turbine Generator

XLPE Cross-Linked Polyethylene

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1 |

Introduction

Currently, both the climate change and the running out of fossil fuels like oil, gas and coal are demanding for new sustainable energy sources. Renewable energy is a big potential to provide this new type of energy with wind energy as one of the most promising energy source. However, wind energy is currently limited to land and offshore locations with a maximum depth of 50 m due to structural limitations. This leaves vast expanses of uninterrupted open sea unused, providing larger capacity factors compared to wind farms on land and closer to shore. This is because of more consistent and higher wind speeds. Also larger wind turbines can be installed due to the reduction of size limitations. Therefore wind turbines on floating platforms provide the outcome to utilize these unused expanses. However, due to the immature development of floating wind energy, different challenges need to be faced with. One of the main challenges is the dynamic power cable connecting floating wind turbines, substations and submarine power cables with each other. Therefore it is of great importance to identify the challenges regarding dynamic power cables and its research gaps to accelerate the development of floating wind energy.

The aim of this report is provide a thorough overview of the development of dynamic power cables for com-mercial floating offshore wind farms. This was achieved through the collection of different literature sources and by implementing the relevant information into the report. Because floating wind energy is an immature technology, there is a lack of reports and papers and therefore websites were consulted more frequently than usual to obtain a complete literature report. The main and sub research questions are formulated to provide guidance during the collection of the different literature sources and writing the report. The main research question that will be answered in this literature study is as follows:

• What is the state-of-the-art of dynamic power cables for commercial floating wind farms?

The sub research questions that will support the answering of the main research question are the follow-ing:

• What is the state-of-the-art of floating offshore wind farms?

• What are available submarine and dynamic power cables for power transmission?

• Which material and design choices are made for both submarine and dynamic power cables?

• What standards are associated with submarine and dynamic submarine power cables?

• What challenges are faced by the application of traditional submarine power cables as dynamic power cable?

• What actions are being taken with respect to these faced challenges?

The structure of the report is as follows: Chapter 2 starts with a complete description of the state-of-the-art of floating offshore wind farms. This includes the need for floating offshore wind farms and ongoing floating wind projects, but also the electrical system design which is key for an optimal floating offshore wind farm performance. Subsequently, Chapter 3 follows with the state-of-the-art of submarine power cables. This chapter provides insight into the mature development of these power cables and basic knowledge needed for the development of dynamic power cables. Lastly, the report ends with Chapter 4 about the state-of-the-art of dynamic power cables. This chapter discusses, amongst others, the challenges faced by dynamic power cables which are cable damages, fatigue, metallic sheath design, installation and operations and mainte-nance.

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2 |

The state-of-the-art of floating offshore

wind farms

Due to the demand of renewable energy, new energy sources are needed to satisfy this demand. Floating wind energy seems to be a promising source and is expected to become a major participant in providing this type of clean energy. Despite floating offshore wind farms (FOWFs) are at an immature stage, the development of floating wind energy is going rapidly with approximately 40 ongoing projects[1]. In this chapter the state-of-the-art of FOWFs is discussed to give an insight into the current stage and the path which still need to be covered. The first four sections will discuss the needs, benefits, drawbacks, floating platforms and current status of FOWFs. The last three sections cover the state-of-the-art of the electrical system for a FOWF. This includes the electrical collector design, floating substation and a FOWF layout proposal for the 1 GW Dogger Bank project.

2.1. T

HE NEEDS FOR FLOATING OFFSHORE WIND FARMS

Due to the climate change and the fact that fossil fuels like oil, gas and coal are running out, which are the main sources of energy, an alternative energy source has to be found. The answer to these problems is re-newable energy which is a type of energy that will never run out with minimal impact on the environment. The EU has set targets to accelerate the development of renewable energy in order to fully rely on renewable energy sources in the future. On the 14th of June 2018, the Commission, the Parliament and the Council of the EU have made an agreement that 32% of EU’s final energy consumption, which covers all energy supplied to the final consumer for all energy uses[2], has to be renewable (wind, solar, hydro, tidal, geothermal, and biomass) with a revision in 2023[3]. To realize this amount of renewable energy, wind energy is a promising source. EWEA claims that the offshore wind capacity could reach 460 GW by 2050 and that the total wind en-ergy will contribute to Europe’s electrical power supply for 50%[4]. However, this wind energy supply can only be realized when wind energy locations move to water depths greater than 50 m due to the limited shallow waters for fixed offshore wind farms.

Another reason to move to increased water depths is the larger offshore capacity factors compared to shore located wind farms[5]. The larger offshore capacity factor can be reached by higher and more consistent wind speeds that occur further from shore. Currently, fixed OWFs are limited to water depths up to 50-60 m and have never been built in increased water depths due to structural limitations. Figure 2.1supports this fact, which shows that only a few fixed offshore wind projects have gone beyond 40 m.

Figure 2.1: Average water depth per offshore project[5].

Table 2.1shows that in Europe 80% of the available offshore wind sources are at depths greater than 60 m with

a potential floating wind capacity of 4000 GW . EWEA claims that this floating wind capacity of 4000 GW can provide four times Europe’s electrical energy consumption[4]. Figure 2.2supports this fact by showing that most of the surrounding seas in Europe are at depths greater than 60 m.Table 2.1shows that besides Europe

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2.2.THE BENEFITS AND DRAWBACKS OFFOWFS WITH RESPECT TO LAND-BASED AND FIXED OFFSHORE WIND

FARMS 3

also the USA and Japan have large market potentials for floating wind energy. Fixed OWFs are infeasible at the coast of the USA and Japan due to continental shelfs that drop off too fast. All in all, it can be said that there is a promising future for floating wind energy and that research and development are essential to realize this.

Table 2.1: Offshore wind resource in deep water locations and potential floating wind capacity in Europe, USA and Japan[4].

Figure 2.2: Sea depth around Europe[6].

2.2. T

HE BENEFITS AND DRAWBACKS OF

FOWF

S WITH RESPECT TO LAND

-BASED AND FIXED OFFSHORE WIND FARMS

This section shows an overview of the benefits and drawbacks of FOWFs with respect to land-based and fixed offshore wind farms. The drawbacks mainly originates from the immature stage of floating wind energy and the remote offshore locations of FOWFs. The main benefits of installing FOWFs over land-based and fixed offshore wind farms are:

• Visual and noise annoyances avoidance: Since fixed OWFs are installed in shallow waters (<50 m), they

may disturb the view when installed too close to shore. Therefore, FOWFs provide a solution for visual annoyances by installing them beyond the view of beachgoers. At the same time, noise annoyances are also avoided by placing wind turbines offshore[7].

• Stronger and more consistent wind: InFigure 2.3, a large difference can be observed between the mean annual wind speed at an altitude of 80 m between land and offshore. This means that offshore wind provides stronger and more consistent wind inducing more electricity with a higher reliability. The evidence is shown by the first wind farm of Hywind consisting of five floating offshore wind tur-bines (FOWTs) of 6 MW each. [8] says that a capacity factor of 65% was achieved from November to January, while for land and fixed offshore wind farms capacity factors between 26% and 52%, and 32% and 45% are reached respectively[9]. Future demonstrations of floating wind energy will show whether this remarkable capacity factor of 65% is true for all seasons of the year and whether other floating wind turbine projects can confirm this number. For now, the difference in the previous mentioned capacity factors show the big potential of floating wind energy.

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2.2.THE BENEFITS AND DRAWBACKS OFFOWFS WITH RESPECT TO LAND-BASED AND FIXED OFFSHORE WIND

FARMS 4

Figure 2.3: Annual European mean wind speed at a 80 m height[10].

• Installations in deeper water: The concept of FOWFs opens up new opportunities to go further from

shore where water depths are increasing above 50 m. Since vast expanses of uninterrupted open sea are available[7], the flexibility of installation locations for FOWFs increases enormously. Furthermore, due to the flexibility of FOWF locations, FOWFs can also be installed in regions with narrow continental shelves. These regions include the west coast of the US and Japan[6].

• Simplified installations: Installation of a FOWT is much simpler compared to a fixed-bottom wind

tur-bine, since FOWTs can be assembled on shore and floated into place. Due to the assembly on shore, expensive and heavy vessels which are used to construct fixed offshore wind turbines (OWTs) are re-dundant and low-cost ships can be used instead[6]. InTable 2.2this difference in vessel cost can be seen which shows the typical day rates of installation vessels for fixed-bottom and floating wind instal-lations.

Table 2.2: Typical charter day rates for installation vessels[11].

• No wind turbine size constraints: On land, wind turbine sizes are mostly limited by road or rail

logisti-cal constraints[7]. However, the offshore wind turbine size is not limited by logistical constraints, since they can be assembled on shore and floated to the destination. This opens up opportunities to increase wind turbine sizes for increased power generation.

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2.3.AVAILABLE FLOATING PLATFORMS FOR WIND TURBINES 5

• Easing repair, maintenance and decommission: Due to the floating capabilities of a FOWT, heavy

maintenance, repair and decommission can be done by towing the FOWT to shore[6]. This replaces difficult repairs, maintenance and decommission on sea where weather circumstances are often un-favourable and time windows are small.

• Environmental impact[12]: FOWFs located far from shore are more environmentally friendly than fixed OWFs located close to shore. The reason for this is that seabird nesting sites are close to shore and seabird collisions with OWTs are more likely to occur. Also, FOWTs do not have long lasting con-struction activities as well as fixed foundations disrupting sea life of whales and birds for example. However, there also drawbacks regarding FOWFs and are as follows:

• Technical challenges[6]: FOWTs bring new technical challenges due to its immature stage. These tech-nical challenges include mooring and anchoring systems, dynamic power cables and the floating sub-station. Despite the mooring and anchoring system are well-established from the oil & gas industry, it needs to be optimized for FOWFs due to the large number and smaller sizes of floating platforms for cost reduction. New mooring line materials need to be developed such as nylon and polyester which has strong fatigue properties and is light weighted resulting in easier large scale installations. More at-tention need to be paid to these challenges to ensure that all unforeseen issues have been tackled and included in the design. Currently, these technical challenges seem not to be a bottleneck in the future, but before large FOWFs can be deployed more research and development need to be done with respect to these technical challenges.

• Shore - offshore distance[13]: For repairs and O&M, the large distance from shore to the installation site of the FOWF imposes challenges. Due to the large distances, sometimes over 100 km, the travel time to arrive at the FOWF will be considerably (5 hours or even more). Therefore a high quality O&M plan and well designed components is of great importance to decrease the number of visits for main-tenance and repair as much as possible.

• Cost[14]: Due to the immature stage of floating wind energy, the associated costs are reasonably high. Therefore, political support is crucial for the development of FOWFs. For example, the Hywind project is almost fully subsidized by the Scottish government. However, it is expected when FOWFs are largely deployed the cost will decrease substantially and the levelised cost of electricity (LCOE) will be more or less equal to conventional fixed OWFs. The expected LCOE of FOWFs, estimated by Carbon Trust and ETI, is between 85 and 95 pounds/MWh mid 2020s. Despite it is expected that the floating substructure will still be more expensive than a fixed foundation in the future, aforementioned advantages of FOWFs will level out this increased cost.

• On shore facilities[15]: For assembly, repair and maintenance of FOWTs, on shore facilities along with dedicated equipment are essential. This dedicated equipment includes cranes, dock space, repair boats, towing channels, laydown lots and staging areas. However, due to limited towing distance of FOWTs, each region of FOWTs need his own facility. This means that a strategic and available loca-tion needs to be found to build an on shore facility that can provide assembly, repair and maintenance activities for FOWTs.

2.3. A

VAILABLE FLOATING PLATFORMS FOR WIND TURBINES

In this section the available floating platforms for wind turbines are discussed which are used by ongoing floating wind projects. The floating platforms can be divided into three categories and are shown in

Fig-ure 2.4: tension leg platform (TLP), semi-submersible and spar-buoy. Additionally, multi and hybrid

plat-forms are also available offering more wind turbines on one platform and desired platform characteristics. The available floating platforms provide installations of wind turbines in greater water depths from 40 m to almost 500 m. InFigure 2.5, an overview is shown of each floating platform category and the expected opera-tional water depth including uncertainty. Besides the large range of applicable water depths, the ideal depth is between 100 and 150 m which is a trade-off between buoyancy, length and cost of the mooring lines.

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Figure 2.4: Three dominant floating wind typologies: tension leg platform (TLP), semi-submersible and spar-buoy[16].

Figure 2.5: Application of floating platforms by water depth[6].

2.3.1. T

ENSION LEG PLATFORM

( TLP)

A tension leg platform (TLP) is a semi-submerged buoyant structure with tensioned mooring lines and is shown on the left inFigure 2.4. The tensioned mooring lines provide stability and are anchored through suction or piled anchors to the seabed. The operational depth of the TLP is between 50 and 420 m which is an advantage since the TLP can be deployed in shallow and increased water depths. Currently, the TLP structure is the least advanced design and has not been deployed yet. The strengths and weaknesses of TLPs can be found inTable 2.3:

Table 2.3: Strengths and weaknesses of a tension leg platform (TLP)[6][17]. Strengths:

- Low structural mass - Onshore turbine assembly - Few moving parts

- Can be used in depths from 50 m

- Excellent stability due to tendency for lower critical wave-induced motions Weaknesses:

- High loads on the mooring and anchoring system - Higher cost for installation of mooring lines

- Uncertainty about impact of possible high-frequency dynamic effects on turbine - Challenging installation process

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2.3.2. S

EMI

-

SUBMERSIBLE PLATFORM

While the TLP is stabilized by its tensioned mooring lines, the semi-submersible platform is stabilized by its platform and is shown in the middle inFigure 2.4. The platform consists mostly of 3 to 5 cylindrical platforms interconnected by tubes. The catenary or taut spread mooring lines of the semi-submersible platform are anchored through drag anchors to the seabed to make sure that the platform stays in place. Because the semi-submersible platform is stabilized by its platform, the platform requires a large and heavy structure. However, due to the shallow draft, meaning that the submerged bottom of the platform is not far below sea level, this floating platform is flexible for application and simple to install[6]. The operational depth is expected to be between 40 and 370 m which allows for a large range of deployment like the TLP. Both the strengths and weaknesses of semi-submersible platforms can be found inTable 2.4:

Table 2.4: Strengths and weaknesses of a semi-submersible platform[6][17]. Strengths:

- Flexible application due to the ability to operate in shallow water depths up to 40 m - Low vessel requirement (only basic tug boats required)

- Onshore turbine assembly

- Amenable to port-side major repairs - Lower installed mooring cost

Weaknesses:

- High structural mass to provide sufficient buoyancy and stability

- Complex steel structures with many welded joints can be difficult to fabricate - Potentially costly active ballast systems

- Tendency for higher critical wave-induced motions

2.3.3. S

PAR

-

BUOY

The spar-buoy stabilizes itself by having the center of gravity below the center of buoyancy. By this principle the spar-buoy stabilizes itself when it turns over. The center of gravity is shifted below the center of buoyancy by fabricating a heavy structure at the bottom of the spar-buoy. The spar-buoy is a relatively easy design and can be found on the right inFigure 2.4. However, the long submersed structure of the spar-buoy makes it challenging for assembly, transportation and installation[6]. Also, the spar-buoy needs to be installed at water depths larger than 75-100 m depending on the size of the wind turbine. The spar-buoy is kept in place by the attachment of catenary of taut spread mooring lines and anchored through drag or suction anchors. Both the strengths and weaknesses of spar-buoys can be found inTable 2.5:

Table 2.5: Strengths and weaknesses of a spar-buoy[6][17]. Strengths:

- Simple design

- Few moving parts (no active ballast required) - Lower mooring installation cost

- Tendency for lower critical wave-induced motions - Excellent stability

Weaknesses:

- Constrained to deep water locations larger than 100 m

- Offshore turbine assembly requires dynamic positioning vessels and heavy-lift cranes - Large draft limits ability to tow the structure back to port for major repairs

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2.4.OPERATIONAL AND PLANNED FLOATING WIND PROJECTS 8

2.4. O

PERATIONAL AND PLANNED FLOATING WIND PROJECTS

In this section the operational and planned floating wind projects will be discussed. An overview of all oper-ational demo projects and planned pilot floating wind projects, made by Carbon Trust, can be found in

Ap-pendix A. Carbon Trust claims that approximately 40 concepts are under development currently.Appendix B

shows a list of 27 concepts that are most active and advanced. In this figure all stages of development of these concepts can be found including the year of readiness. In 2021 it is expected that 6 concepts have reached the pilot array stage (20-50 MW ) after which the commercial project can be started with capacities larger than 50

MW . Below the most mature and promising floating wind projects will be discussed to give an insight into

the current development of floating wind energy.

2.4.1. S

TATOIL

(H

YWIND

)

Hywind is a floating wind project conducted by Statoil/Equinor a Norway’s government-owned oil company. Hywind has developed the first full-scale grid-connected floating wind turbine of 2.3 MW in 2009 and was called Hywind Demo[18]. Hywind Demo has a spar buoy as buoyant substructure and was installed at a water depth of approximately 200 m. In 2017, the Hywind Scotland Pilot Project was installed, which is a successor of the Hywind Demo and is shown inFigure 2.6. The Hywind Scotland Pilot Project is the first global floating offshore wind farm and consists of 5 FOWTs each with a capacity of 6 MW providing 20,000 households of electricity[1]. The pilot park is installed at a water depth of 95-120 m and located near Buchan Deep which is 25-30 km off the coast of Peterhead in Aberdeenshire (UK)[6]. The measurements obtained from the Hywind Demo project was used to optimize and develop the Hywind pilot project. More data and a comparison of the Hywind Demo and Pilot project wind turbine can be found inAppendix C.

Figure 2.6: Hywind Scotland Pilot Project (2017)[19].

2.4.2. F

UKUSHIMA

FORWARD

The Fukushima Project is conducted by the Ministry of Economy, Trade and Industry, Furukawa Electric and VISCAS Corporation. In 2013 the first FOWT of 2 MW was deployed along with the first floating substation (25 MV A) worldwide and can be seen inFigure 2.7. The floating substation is installed near the FOWT to transform the voltage in order to reduce energy losses when the generated electricity power is transmitted to shore. This substation is the second project that uses a spar as floating platform in addition to the Hywind project. In 2015 a larger FOWT of 7 MW was installed at the coast of Fukushima with a top blade height of 200 m above the waterline[20].

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Figure 2.7: 2 MW FOWT prototype and 25 MV A substation of the Fukushima project[21].

2.4.3. P

RINCIPLE

P

OWER

( W

IND

F

LOAT

)

The WindFloat project is conducted by Principle Power Inc. The first floating wind turbine was deployed in October 2011 and was a full-scale 2 MW prototype[22]. The WindFloat prototype can be seen inFigure 2.8

and is the first deployed semi-submersible FOWT. The floating platform consists of three cylindrical plat-forms. The wind turbine is installed on one of these cylindrical platplat-forms. This prototype was installed at 5

km from the shore of Aguçadoura in Portugal at a water depth of 40-50 m[22]. The WindFloat turbine was successfully connected to the grid and valuable data was obtained during operation for further development. In July 2016, the FOWT was decommissioned and towed back to shore being the first decommissioned float-ing offshore wind turbine. Due to the success of the WindFloat prototype, plans are befloat-ing made to scale up projects in the coming 5-10 years in the US (Oregon, 30 MW ;Hawaii, 2 x 408 MW ), Scotland (Kincardine, 48-50 MW ), and Portugal (Aguçadoura, 25 MW )[6].

Figure 2.8: Floating wind turbine design of WindFloat[18].

2.4.4. I

DEOL

(F

LOATGEN

)

Despite France is not a major participant in offshore wind energy, France has two operational and one planned floating wind projects[23]. All three projects are based on the ’damping pool’ system that uses water inside the platform square to damp its motions. The floating wind turbines will be installed at water depths greater than 35 m. The Floatgen project led by Ideol is the first floating wind project and is a concrete semi-submersible 2 MW wind turbine shown inFigure 2.9. This wind turbine is installed in 2018 at the coast of Le Croisic at a

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water depth of 33 m. The second project from IDEOL is the Japanese demonstrator with a capacity of 3 MW . The Japanese demonstrator is operational at Kitakuyshu in Japan at a water depth of 55 m. The third project that will be led by IDEOL is the Eolmed project which will be constructed in 2020-2021. This project consists of four floating wind turbines each generating 6 MW at Occitanie at a water depth of 55 m. If these projects have been successful, IDEOL is planning to scale up to a FOWF of 500 MW in France[6].

Figure 2.9: IDEOL’s first floating wind project Floatgen[23].

2.4.5. H

EXICON

Hexicon is the global leader amongst multi-turbine platforms. These multi-turbine platforms are semi-submersible and has distributed columns in a truss structure to provide sufficient buoyancy. In addition, a mooring system is installed to the platform to enable alignment with the wind to avoid wake losses. The main project called Dounreay Tri is shown inFigure 2.10and is planned to be deployed at the North coast of Scotland with two 5 MW turbines installed on the platform[6]. The construction started in March 2017, but has stopped temporarily. It is expected that the demonstrator will be deployed in the first quarter of 2020[24].

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2.4.6. GICON

The SOF project led by the engineering company GICON is the first mover of the TLP. The GICON-SOF is equipped with a 2.3 MW wind turbine and has not been installed yet. This project has started due to the drawbacks of large manufacturing sites for semi-submersible platforms and the deep drafts of spar-buoys[26]. A TLP does not have these challenges and is therefore preferred with in addition low structural weight and high stiffness. The GICON-SOF has four vertical taut-leg mooring lines and four support mooring lines all connected to the four columns of the platform to provide stability. The operational depth of the GICON-SOF is expected to be between 40 and 250 m offering flexibility for installation sites[6].

Figure 2.11: GICON-SOF 2.3 MW wind turbine with TLP[26].

2.4.7. P

ELASTAR

The Pelastar project is conducted by Glosten a company specialized in naval architecture & marine engi-neering services. Glosten believes that TLP is the clear leader among the other platform types due to its low structural weight, an in-harbor system assembly method, potential to access relatively deeper sites, and su-perior dynamic responses to sea conditions[27]. The water depth for the Pelastar ranges from 70 to 200 m and the TLP is anchored to the ground with five tensioned cables as can be seen inFigure 2.12[6]. Despite of the promising future of Pelastar no prototype has been installed yet due to planning and consenting delays. Glosten expects to deploy its prototype of 6 MW later in 2018[6].

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2.5.ELECTRICAL SYSTEM DESIGN 12

2.4.8. C

ONCLUSION

As conclusion, it can be said that many floating wind projects are currently under development with promis-ing results and prospects. Despite only one wind farm from Hywind has been installed yet, it is expected that in 2021 6 pilot farms will be deployed and that in the coming 5 – 10 years even bigger steps to large scale floating wind farms will be made. However, the technology needs to be prepared for these developments which is currently not the case. For example the dynamic power cable, which is not designed yet to export the generated power from large scale FOWFs to shore. Further discussion of the state-of-the-art of FOWFs is provided in the following three section and will cover different aspects of the electrical system and its short-comings.

2.5. E

LECTRICAL SYSTEM DESIGN

The electrical system design is an important element of a FOWF, since it highly influences efficiency, cost, reliability and performance of a FOWF[28][29]. In this section the most important aspects related to the electrical system of FOWFs will be discussed.

2.5.1. E

LECTRICAL INFRASTRUCTURE

At the start of the electrical system design, it is important to know how the electrical infrastructure of the FOWF looks like. Currently there are three common types of electrical infrastructures applied to OWFs and can be distinguished by the installation of a substation and/or AC/DC converter. InFigure 2.13a schematic overview of these infrastructures is shown, where the two overlapping circles represent the substation and the squares with a wave and equal sign represent the AC/DC converter. The different infrastructures have each their own purposes and depending on the size and location of the FOWF, the right infrastructure can be chosen. A short description of the three common electrical infrastructures can be found below:

• No substation, no AC/DC converter: The first electrical infrastructure inFigure 2.13has no substation and no AC/DC converter. This infrastructure is preferred when the (F)OWF is installed closer than 10

km to shore and the installed capacity is low, because of the lower cost. The wind turbines can be either

connected individually or in series. MVAC cables are used for electrical power transmission.

• Substation, no AC/DC converter: The second electrical infrastructure inFigure 2.13has a substation at the (F)OWF site, but no AC/DC converter. This infrastructure is preferred when the (F)OWF is installed between 10 and 50-100 km from shore and the installed capacity is high. The substation is installed at the (F)OWF site to step up the voltage in order to reduce energy losses and cost. MVAC cables are used for interarray purposes and a HVAC export cable is used for power transmission to shore.

• Substation, AC/DC converter: The third electrical infrastructure inFigure 2.13has a substation and AC/DC converter. This infrastructure is preferred when the (F)OWF is located further than 50-100 km from shore and AC power transmission losses become too large. In this case the installation of a AC/DC converter is more cost effective and converts the power to DC after the substation. For this infrastruc-ture, MVAC cables are used as interarray cables and a HVDC export cable is used for power transmission to shore.

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Figure 2.13: Most common electrical infrastructures of OWFs [30].

The electrical infrastructure with substation and without AC/DC converter is the most convenient for FOWFs. The first reason is that FOWFs will most likely be located further than 10 km offshore due to the water depths greater than 40 m and large capacities. The second reason is that the installation of an AC/DC converter with floating capabilities brings too high risks and costs.

2.5.2. E

LECTRICAL COLLECTOR SYSTEM DESIGN

An important part of the electrical system design is the electrical collector system, which include the cable connections between wind turbines and the substation(s). The layout of a collector system mainly depends on the wind farm size and the desired reliability. InFigure 2.14four available basic collector system layouts for (F)OWFs can be found and are discussed below[31]:

• a) Radial design: The radial design is the simplest design consisting of separate arrays of wind turbines.

This design requires the least cable length and is easy to control. The wind turbines in these arrays are connected to one cable going from the last turbine to the front. The amount of wind turbines connected to one array is limited by the capacity of the power cable and the generated power of the wind turbines. However, a drawback of the radial design is poor reliability due to its simplicity. When one of the wind turbines close to the substation has a cable or switchgear fault, then all generated power from the subsequent wind turbines cannot be transmitted.

• b) Single-sided ring design: The single-sided ring design is basically an extension of the radial design.

It has an additional power cable running from the last wind turbine in the array to the substation. Also cables with higher capacities are needed in the array due to the power flow in both directions.

• c) Double-sided ring design: The double-sided ring design does not have an additional power cable

running from the last wind turbine to the substation, but is connected to the last turbine in a nearby array. This design has a smaller total power cable length, but the interarray cables need higher power capacities to cope with a double number of wind turbines. The double-sided ring design is like the single-sided ring design more reliable and therefore higher revenues can be achieved in case of cable or switchgear faults.

• d) Star design: The star design has a high reliability, since cable or switchgear faults do not influence

neighbouring wind turbines. However, the central location of the substation might impose some addi-tional challenges.

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Figure 2.14: Four basic electrical collector system designs[29].

2.5.3. L

OCATION AND NUMBER OF SUBSTATIONS

As explained before, the substation is the collection point of a wind farm to transform the voltage. The right location and number of substations is crucial for an efficient FOWF design. Below different design criteria for the number and location of substation is discussed[31]. Insection 2.6a more detailed discussion of the (floating) substation can be found including the state-of-the-art and its challenges.

• Since large distances of interarray cables increase power losses, the distance between a substation and a wind turbine has to be minimized. Therefore dividing the wind farm into more areas with its own substation and locating substations strategically, more efficient designs can be obtained.

• The capacity of the wind farm, transformer and HV cable all influence the number of substations. In general, a substation can deliver a maximum capacity of 500 MW . When the FOWF has a larger capacity than 500 MW , which is currently the limit, more substations need to be installed.

• External influences like offshore industries, shipping, subsea cables, pipelines, etc. also affect the num-ber and location of offshore substations. At the start of the FOWF design, these external influences have to be examined to avoid future complications.

2.5.4. I

NTERARRAY POWER CABLES

The interarray power cables, which connect the FOWTs and substation, have to be designed according to the maximum power transmission. As explained in the collector system design, the maximum interarray power transmission depends on the electrical collection system layout, since the maximum transmission power is the sum of all subsequent linked wind turbines. For example, inFigure 2.15a link between three wind turbines and the substation is shown transforming the electrical voltage from 33 to 138 kV . From this figure, it can be noted that the cross-sectional area of the power cables increases when coming closer to the substation. This is because the same voltage is preserved while the power transmission is increased, which requires larger cable capacities and thus larger cross-sectional areas. Cross-sectional areas of submarine power cables can be found in a wide range from 95 mm2to over 1000 mm2.

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2.6.OFFSHORE(FLOATING)SUBSTATION 15

2.6. O

FFSHORE

(

FLOATING

)

SUBSTATION

As discussed in the previous section, substations are crucial for an efficient electrical system design to save power transmission losses. In this section, the substation will be discussed in more detail and in particular for FOWFs. First the general layout of a substation will be discussed after which the challenges and the state-of-the-art of floating substations will be covered.

2.6.1. L

AYOUT OFFSHORE SUBSTATION

Offshore substations are crucial when the FOWF is installed at a distance greater than 10 km from shore. In general, the substation increases the voltage from 33 kV or 66 kV to 132-220 kV depending on the wind farm size. The cost of a fixed substation is 7% of the total wind farm cost and may increase for FOWFs due to a more complicated design[31]. Typically, one substation can be used for a wind farm as large as 500 MW . This means when a wind farm generates more than 500 MW additional substations are required. Also an additional substation might be added as back up when financially possible. An offshore substation can be divided into three classes and is also applicable to a floating substation[31]:

1. Components related to the electrical system 2. Components related to the facilities

3. Components related to the structure

A general layout of an offshore substation is shown inFigure 2.16. From the bottom 33 kV and 132 kV power

cables are collected by different modules and send to the power transformer. As can be seen inFigure 2.16, also a heli lift is present for helicopters, which offers a significantly faster arrival time at the substation than a vessel for repair and maintenance purposes. Furthermore, a back-up diesel generator is installed on the substation when there is loss of power.

The power transformer is the main part of the substation and is crucial to save losses and to maximize the profit. Since, (floating) offshore substations are located far from shore, it is essential that the transform-ers are well insulated. The insulation must be non-flammable like gas (SF6) or insulating liquid, since fire can be hardly detected and extinguished and consequences due to fire can be disastrous for the whole wind farm[31]. Switchgears are needed for control, protection and isolation of wind turbines. They can isolate a wind turbine when they are broken and/or maintenance is required while keep running other wind turbines. For switchgears, gas (SF6) insulation is preferred due to its compactness and safety level[31].

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2.6.2. F

LOATING SUBSTATION

Currently, offshore substations have been applied in shallow waters less than 50 m. However, due to the rise of floating wind energy at increased water depths, floating substations are necessary to transform the generated electrical energy at remote locations to save energy losses. Floating substations also bring new challenges compared to fixed offshore substations. Nonetheless, the substation from Fukushima project has already been deployed successfully and will be discussed at the end of this section.

FLOATING SUBSTATION CHALLENGES

Due to the required floating ability of the offshore substation, new challenges are faced with respect to fixed substations making the design of a floating substation costly and more complicated. The main challenge for floating substations is the resistance of the electrical equipment like the transformer and switch gears to

inclinations and vibrations due to wave motions[1]. The transformers are immersed in an insulating liquid through which wave motions can cause rolling. Therefore rolling knowledge from shipbuilding to anti-quake needs to be applied for a proper design[34]. InFigure 2.17a vibration and inclination test is shown executed by the Fukushima project. After successful tests, the floating substation was ready to be installed in June 2013.

Figure 2.17: Inclination and vibration test through shaking table tests[34].

Another challenge is the immense weight of the substation[6]. Because of the heavy weight of the substation a large and heavy floating platform is required which is expensive. Also, when a FOWF is deployed in waters beyond 100 km from shore an additional AC/DC converter is required to save energy losses. This means that due to a converter an even larger and heavier floating platform is required, which is more challenging. Therefore, it is expected that these converters will not be applied in the upcoming time to FOWFs due to the high associated costs and risks. Therefore FOWFs are expected to be within 100 km from shore. Furthermore, it is expected that FOWF sizes will increase in the future, which results in more dynamic power cables that

will be connected to the substation[6]. This means that the risk of entanglement and damages of dynamic power cables increases.

Currently, engineers think about fixed-bottom jacket foundation substation going to depths as high as 80-100 m to avoid all associated risks and challenges to floating substations. However, this solution is limited to depths up to 100 m. An alternative solution is a distributed transformer system and is being developed for fixed-bottom OWFs. This distributed transformer system consist of transformers located at the last wind turbine platform of each row of wind turbines. The concept that is being developed by Siemens for bottom-fixed OWFs is expected to save cost by an exceptional 40%. If this concept will be applied to FOWFs, the impact on the FOWT’s floating platform stability needs to be considered causing heavier platforms[6].

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THE STATE-OF-THE-ART OF FLOATING SUBSTATIONS

Despite the challenges and alternative proposed floating substation solutions above, [1] claims that through concept design and analysis substations up to 500 MW will be feasible[1]. For this analysis three floating plat-form concepts were developed to support the substation at increased water depths. The platplat-form concepts are TLP, semi-submersible and spar-buoy and are shown inFigure 2.18where the TLP and semi-submersible concepts are identical. The platform connections to the seabed are more or less the same as the floating platforms for wind turbines, where TLP has tensioned steel wires and the spar-buoy and semi-submersible platform have a four point catenary mooring system.

Figure 2.18: Concept substation hull designs for semi-submersible and TLP (left) and spar (right)[1].

The hydrodynamic analysis of the three platforms was done for four different sites shown inTable 2.6. This analysis showed that the platforms have an acceptable fatigue life and are able to cope with different wave motions and maximum accelerations. However, increased accelerations and excitations need to be avoided caused by large square platform designs.

Table 2.6: Water depth and wave characteristics at potential FOWF sites[1].

Although the analysis of supporting floating platforms of substations up to 500 MW shows its feasibility, only one 25 MV A substation has been installed yet. This floating substation of the Fukushima project is shown

inFigure 2.19and is installed on a spar floater in June 2013. The floating substation has a upper, middle and

lower hull and has a met mast and helicopter deck installed on top of the upper hull. The substation is located in the upper hull with all its components. Concrete is added to the lower hull in order to maintain stability. The motion of the floating installation can be controlled by the cob located underneath the middle hull. Also measurement devices are installed on the floating substation to measure wind velocities, waves and currents. Cup anemometers, wind vanes and sonic anemometers are installed on the met mast and a doppler radar on the main deck to measure wind velocities. A wave meter and ADCP are used to measure the wave and current at the floater. Also the floater motion is measured by means of an accelerometer, GPS and gyros to gain information about floater motion control.

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2.7.FOWFLAYOUT PROPOSAL 18

Figure 2.19: First floating substation from Fukushima project[35][36].

2.7. FOWF

LAYOUT PROPOSAL

Currently, the Hywind pilot park is the only existing FOWF consisting of 5 FOWTs without a substation. How-ever, due to the success of current demo projects and the promising future of FOWFs, efficient electrical sys-tem layouts are key to save cost, material, installation time, etc. for increased sizes. Commonly, fixed OWFs have a radial layout design like the Horns Rev 2 wind farm (Denmark) shown inFigure 2.20. The interarray cables are 22-33 kV connecting the wind turbines and substation. Fixed OWFs usually have one substation with 132-155 kV connected export cables.

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2.7.FOWFLAYOUT PROPOSAL 19

Despite the radial design is commonly used for fixed OWFs, a different layout for FOWFs may be more effi-cient. Currently, little is known about the most optimal FOWF layout, but a proposal of the 1 GW Dogger Bank project is available[38]. Despite the water depth at the Dogger Bank is between 20 and 60 m, an evaluation was done by EERA DTOC of the impact on a FOWF layout by surrounding fixed wind farms with the planned situation of the Dogger Bank[38]. The design is based on the fact that one anchor can hoist three mooring lines which offers a more cost effective solution. InFigure 2.21, this hypothetical proposal for an optimal FOWF layout of the Dogger Bank is shown. The layout consists of celluloid shapes with sides of 1600 m and a FOWT installed on each corner. A substation is located in the center and is connected to the 6 nearest FOWTs. The generated power from the other wind turbines enters the substation by passing at least one or more wind turbines. Furthermore, the substation is also connect to an onshore substation indicated by the bold lines. The Dogger Bank project consists of 104 FOWTs with a capacity of 10 MW and a total FOWF capacity of 1

GW . In addition, the FOWF is split into two areas due to the limited maximum substation capacity of 500 MW .

Figure 2.21: FOWF layout including substation, array and export cables[38].

On one hand the celluloid wind farm layout can cause cost reduction through less anchors, reduced installa-tion costs and less geotechnical surveys and inspecinstalla-tion requirements[1]. But on the other hand this celluloid layout makes implementation more complicated, while the benefits are expected to be insignificant accord-ing to [1]. The drawbacks that are related to this celluloid wind farm layout are[1]:

• Anchor positions are restricted to strict requirements

• Array layouts and mooring systems are less optimized due to the limited distances between FOWTs

• Seabed infrastructure, poor seabed conditions and complex bathymetry can lead to unfeasible celluloid wind farm layouts

All in all, it can be concluded that the optimal FOWF layout differs for each floating wind project. For the Dogger Bank project the celluloid layout seems to be the most optimal, while at locations where the seabed infrastructure and conditions are poor, another layout may be more suitable. Obviously, there is room for improvement to ensure efficient FOWF layout designs in the coming years.

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2.8.CONCLUSION 20

2.8. C

ONCLUSION

Due to the climate change and the running out of fossil fuels, the European Commission has made agree-ments that 32% of EU’s final energy consumption in 2030 has to be from a renewable energy source. To realize this goal, floating wind energy seems to be a huge potential, because of the vast expanses of unin-terrupted open sea at increased water depths, where fixed foundations are structural infeasible. By using these expanses, EWEA claims that a total offshore wind capacity of 460 GW can be reached in 2050 and that Europe’s total electrical power supply will dependent on wind energy for 50% at that time.

The main benefits of FOWFs are stronger and more consistent winds with respect to fixed offshore and land-based wind farms resulting in higher capacity factors, more flexibility for installation locations, simplified installation and no size constraints regarding the wind turbine size. However, drawbacks of FOWFs are the technical challenges due to the immature technology, long distances from shore, higher cost and the need of on shore facilities. Despite the drawbacks of FOWFs, it can be said that the benefits dominate the drawbacks considering the 40 ongoing floating wind projects. Among these projects, Statoil (Hywind) is the current leader with the first installation of a 30 MW pilot wind farm. It is expected that in 2021 6 of these pilot wind farms (<50 MW ) will be deployed and that in the coming 5 - 10 years several large scale FOWFs will be deployed (>50 MW ). However, to realize these large scale FOWFs, different challenges associated with the increased wind farm sizes need to be tackled which include the substation, dynamic power cables, mooring lines, anchors, but also financial support due to the high cost caused by the immature technology.

The FOWF infrastructure has most likely a substation without AC/DC converter. AC/DC converters convey too high risks and costs and are only beneficial from a distance of 100 km from shore. Therefore it is expected that wind farm developers are not taking the risk and will install the FOWF closer than 100 km to shore. MVAC dynamic power cables will be used as interarray cable and HVAC submarine and dynamic power cables as ex-port cables. Currently one available proposal of a FOWF layout is available and comprises of FOWTs in cellu-loid shapes with a central located substation. However, the benefits of this FOWF layout seem to be marginal and therefore more research and development need to be done to find optimal FOWF layouts.

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3 |

The state-of-the-art of submarine power

cables

This chapter aims to give an insight into the state-of-the-art of submarine power cables as principle for a bet-ter understanding of dynamic power cables, which will be discussed inchapter 4. To obtain this knowledge, firstly the general structure of submarine power cables will be discussed with an explanation of each com-ponent. Subsequently, generic submarine power cables will be shown with their characteristics. Then the consideration between HVAC and HVDC export cables for FOWFs will be reviewed by a cost comparison. Af-ter that, common damages of submarine power cables with mitigation strategies will be discussed to identify the damages of dynamic power cables inchapter 4. Lastly, applicable standards for static as well as dynamic power cables are shortly reviewed for the design of these cables.

3.1. D

ESIGN ELEMENTS OF SUBMARINE POWER CABLES

In this section the design elements of submarine power cables will be discussed and are mainly based on [39]. These design elements are kept as general as possible and can be applied to most submarine power cables. The layering of an one core (1C) and three core (3C) HV submarine power cable can be found in

Figure 3.1and shows a general structure of a submarine power cable. The only difference between a 1C and

a 3C cable is the integrated optical fibres and the PVC/PE added as filling to create and maintain a circular cross-section[40].

Figure 3.1: Composition of a static HV submarine power cable [40].

3.1.1. C

ONDUCTOR

In general, the conductor is the core of each submarine power cable. As the name reveals, the conductor conducts the electrical current from one point to the other. The conductor is made from either copper or aluminum depending on the application. Despite copper is more expensive than aluminum, copper is mostly used in submarine power cables due to a better current-carrying capacity. This is because copper allows a smaller conductor cross-section resulting in less material needed for the outer layers. However, it is possible to use copper as well as aluminum in the same cable connected with joints. For example, aluminum has

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3.1.DESIGN ELEMENTS OF SUBMARINE POWER CABLES 22

better properties for deeper cold parts in water while copper prevails in warmer parts close to shore[41]. Currently, many shapes of conductors are available with the most common presented inFigure 3.2. The shape depends on different factors like the amount of voltage, cross-sectional area, water tightness, hole required, etc. A more detailed explanation about each conductor design can be found in [39].

Figure 3.2: Available conductor designs [39].

3.1.2. D

IELECTRIC SYSTEM

The dielectric system contains the conductor screen, insulation and insulation screen. The insulation is the most important layer of these three, since it is a barrier for potential differences to prevent electrical leak-age in the cable. Nowadays, cross-linked polyethylene (XLPE) is the most dominant material for the insula-tion in submarine power cables and has been used since 1973. XLPE consists of cross-linked long molecular chains of LDPE forming a three dimensional network. The cross-linking is irreversible and melting is pre-vented when the XLPE insulation is exposed to high temperatures. Due to the rough conductor surface, local stresses will develop between the conductor and insulation, resulting in dielectric strength losses. To pre-vent local stresses on the insulation a semi-conductive XLPE conductor screen is added as layer between the conductor and insulation. The conductor screen will remove these local stresses by means of its extremely smooth surface. Also, an semi-conductive XLPE insulation screen is added as layer between the insulation and swelling tape to protect the insulation for outer layers and to preserve its stable dielectric surface. The three layers of the dielectric system are manufactured simultaneously by triple-extrusion resulting in a high quality insulation system.

3.1.3. S

WELLING TAPE

InFigure 3.1, it can be seen that the swelling tape is placed between the insulation screen and metallic sheath.

Swelling tapes may be added to the submarine power cable when moisture diffuses into the cable because of longitudinal welding seams. Also a swelling agent in powder and yarns can be applied to suck moisture in the cable. Swelling agents can also be inserted between the conductor layers to provide longitudinal water tightness.

3.1.4. W

ATER

-

BLOCKING SHEATH

A water-blocking sheath is added to the submarine cable to prevent water ingression into the dielectric sys-tem and conductor. MV submarine power cables have a polymeric sheath with a water absorbing agent

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un-3.1.DESIGN ELEMENTS OF SUBMARINE POWER CABLES 23

derneath. The water absorbing agent is added since water vapor may diffuse through the polymeric sheath. A submarine power cable without a impermeable metallic sheath is called a ’wet’ cable design. HV submarine power cables have a metallic sheath to prevent water ingression and is called a ’dry’ design. HV submarine power cables have mostly a smooth sheath, whereas a corrugated sheath is preferred for HV dynamic power cables and will be elaborated inchapter 4. Furthermore, a metallic sheath provides protection against Teredo which descends from the aggressive "shipworms". Below a short description can be found of each sheath material which can be applied to submarine power cables:

LEAD SHEATH

The lead sheath is manufactured by extrusion. The benefits of lead sheath is the contribution to the sub-marine power cable stability on the seabed due to its high weight, impermeability for water ingression and humidity diffusion. However, lead sheath has also weak points like fatigue sensitivity and thermal cycling sensitivity initiating micro-cracks. In addition, the lead sheath’s softness causes problems during manufac-turing, transport and installation and therefore needs careful protection.

ALUMINUM SHEATH

Aluminum is manufactured by extrusion, welding or lamination. Extruded aluminum sheaths are not used anymore due to its corrosion sensitivity.

COPPER SHEATH

Copper sheaths are made of welded and corrugated copper strips. The benefits of copper sheath is its favourable fatigue and corrosion properties. It is also able to carry short-circuit currents.

STAINLESS STEEL SHEATH

The benefits of stainless steel sheath is its positive mechanical and fatigue properties, weldability, corrosion resistance and impermeability[42]. However, the drawbacks of stainless steel sheath are vulnerability to stress corrosion cracking (SCC) and thermal fatigue caused by large variations in temperature[43].

POLYMERIC SHEATH

Polymeric sheaths are only applied to MV submarine power cables as explained above. The polymeric ma-terials that are commonly used are HDPE and LDPE due to its corrosion and abrasion protection proper-ties.

After the sheath different layers of protection and bedding can be added in order to protect the armoring from the metallic sheath and to add compressibility to the cable. Compressibility is added by the bedding layers in order to keep all layers well together.

3.1.5. A

RMOR

The armor is the second last layer of the submarine power cable and is laid after the protection and bedding. The armor aims to provide structural integrity to the cable. In addition the armor protects the cable from mechanical loading and takes care of the tension stability. Types of mechanical loading that the cable has to deal with are installation tools, fishing gear and anchors. The armor consists of metal wires, commonly steel, and are wrapped around the cable with the lay length as variable. The lay length is the distance covered by the wire after one circle around the cable. Whether the armor lay-length is short or long depends on the tensional stability, bending stiffness and torsional stability that is required. Common guidelines for armor designs are[39]:

• more steel provides better protection

• harder wires provide a better protection

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3.2.AVAILABLE SUBMARINE POWER CABLES 24

• a short-lay rock armoring provides a better protection against lateral impacts at the expense of ten-sional force.

3.1.6. O

UTER SERVING

The last layer of the submarine power cable is the outer serving. This layer provides protection to the ar-mor from external stresses and corrosion during loading, laying and burying of the submarine power cable. Commonly a polymer material is used as outer serving.

3.1.7. O

PTICAL FIBRES

An optional component, but very valuable is the addition of optical fibres to the power cable. Generally, optical fibres are added for the following purposes:

• Fault detection and location

• Data transmission

• Measurements of temperature (DTS), strain (DSS), insulation (PDM), etc.

• Detection of local movements of the submarine power cable

For 3C power cables, the optical fibres are integrated between the cable cores. For 1C power cables, it is mostly preferred to lay the optical fibre and power cable separately, since the different characteristics of these two can conflict with each other.

3.2. A

VAILABLE SUBMARINE POWER CABLES

Presently, there is an enormous amount of submarine power cables available on the market in almost every shape, size, length, etc. due to the diverse applications. To get a general idea about the different available power cables, below the most important submarine power cable properties are discussed[39]:

• LV, MV, HV and EHV submarine power cables: Submarine power cables can be categorized by its

volt-age. The voltage of cables can be divided into four categories namely low voltage (LV), medium voltage (MV), high voltage (HV) and extra high voltage (EHV). In this report the emphasis is on MV (1 kV - 35 KV) and HV (35 kV - 230 kV) cables, because these voltages will be typically used by FOWFs for interar-ray and export purposes respectively.

• Alternating current (AC) and direct current (DC): The current in the submarine power cable can be

either AC or DC depending on the application. The AC is a three phase current meaning that three conductors are needed to transmit the electrical power. The DC needs two conductors to obtain a closed electrical circuit.

• Single core (1C) and three core (3C) cables: For AC, three cable cores are needed due to the three phase

transmission instead of one for DC. These three cables can be separated from each other in three 1C cables or bounded together into a 3C cable. Mostly 3C instead of three 1C cables are applied to MV and HV cables due to its higher current-carrying capacity and lower armor losses.

To provide a compact overview of all available submarine power cables,Figure 3.3shows the five most generic submarine cable types and represents the majority of installed submarine power cables. From these generic cables, number 1 and 2 inFigure 3.3are most applicable to FOWFs with the 33 kV cable for direct power transmission to shore and the 150 kV cable for larger remote wind farms. Because of the maturity of sub-marine power cables, there is a wide variety of these cables on the market available. This makes it easier for FOWF developers to implement the right submarine power cable to export the generated power to shore at the desired voltage.

Development of dynamic power cables for commercial floating wind farms