d.4.1 – structural and naval architecture ......gz curves verification criterion according to...

D.4.1 – STRUCTURAL AND NAVAL ARCHITECTURE DESIGN BASIS

ESTEYCO S.A. (EST)

Lead authors: Lara Cerdán (EST), Carlos Cortés (EST)

Contributors: Gregorio Torres (COBRA), Georgios Georgallis (FULGOR),

Mattias Lynch (INNOSEA)

FLOTANT -Innovative, low cost, low

weight and safe floating wind

technology optimized for deep water

wind sites, has received funding from

the European Union´s Horizon 2020

research and innovation programme

under grant agreement No.815289

Deliverable 4.1 Structural and Naval Architecture Design Basis

FLOTANT has received funding from the European Union´s Horizon 2020

research and innovation programme under grant agreement No.815289 Doc.Nº: 191025-FLT-WP4_D-4.1_V3 Date: 25/10/2019

2

The FLOTANT Project owns the copyright of this document (in accordance with the terms

described in the Consortium Agreement), which is supplied confidentially and must not be used

for any purpose other than that for which it is supplied. It must not be reproduced either wholly

or partially, copied or transmitted to any person without the authorization of PLOCAN. FLOTANT

is a Cooperation Research Project funded by the European Union´s Horizon 2020 research and

innovation programme. This document reflects only the authors’ views. The Community is not

liable for any use that may be made of the information contained therein.

[ Deliverable 4.1 – Structural and Naval Architecture design basis ]

Project Acronym: FLOTANT

Project Title: Innovative, low cost, low weight and safe floating wind technology optimized for deep

water wind sites (FLOTANT).

Project Coordinators: Octavio Llinás & Ayoze Castro – The Oceanic Platform of the Canary Islands

(PLOCAN)

Programme: H2020-LC-SC3-2018

Topic: Developing solutions to reduce the cost and increase performance of renewable

technologies

Instrument: Research & Innovation Action (RIA)

Deliverable Code: 191025-FLT-WP4_D-4.1_-v3

Due date: 311019




3

DISSEMINATION LEVEL

PU: Public X

PP: Restricted to other programme participants (including the Commission

Services)

RE: Restricted to a group specified by the consortium (including the Commission

Services)

CO: Confidential, only for members of the consortium (including the Commission

Services)

DOCUMENT HISTORY

Edit./Rev. Date Name

Prepared 25/10/19 Lara Cerdán / Carlos Cortés

Checked 31/10/19 TC and PMT

Approved 31/10/19 PMT

DOCUMENT CHANGES RECORD

Edit./Rev. Date Chapters Reason for change

ESTEYCO/01 13/09/19 Whole Document Original Version

ESTEYCO/02 23/09/19 3, 6, 9, 11, 13 Turbine information, Comments from

Cobra

ESTEYCO/03 25/10/19 3, 4, 6, 9, 13 Comments from Innosea, Bureau Veritas

and Cobra; new floater configuration.




4

DISTRIBUTION LIST

Copy no.

Company/

Organization

(country)

Name and surname

1 PLOCAN (ES) Ayoze Castro, Alejandro Romero, Octavio Llinás

2 UNEXE (UK) Lars Johanning, Philipp Thies, Giovanni Rinaldi

3 UEDIN (UK) Henry Jeffrey, Anna García, Simon Robertson

4 AIMPLAS (ES) Ferrán Martí, Blai López

5 ITA-RTWH (DE) Thomas Koehler, Dominik Granich, Oscar Bareiro

6 MARIN (NL) Erik-Jan de Ridder, Sebastien Gueydon

7 TFI (IE) Paul McEvoy

8 ESTEYCO (ES) Lara Cerdán, Javier Nieto, José Serna

9 INNOSEA (FR) Mattias Lynch, Rémy Pascal, Hélène Robic

10 INEA (SI) Igor Steiner, Aleksander Preglej, Marijan Vidmar

11 TX (UK) Sean Kelly

12 HB (UK) Ian Walters

13 FULGOR (EL) George Georgallis

14 AW (HR) Miroslav Komlenovic

15 FF (ES) Bartolomé Mas

16 COBRA (ES) Sara Muñoz, Rubén Durán, Gregorio Torres

17 BV (FR) Claire-Julie Frélaut, Jonathan Boutrot, Jonathan Huet,




5

Acknowledgements

Funding for the FLOTANT project (Grant Agreement No. 815289) was received from the EU

Commission as part of the H2020 research and Innovation Programme.

The help and support, in preparing the proposal and executing the project, of the partner

institutions is also acknowledged: Plataforma Oceánica de Canarias (ES), The University of

Exeter (UK),The University of Edinburgh (UK), AIMPLAS-Asociación de Investigación

Materiales Plásticos y Conexas (ES), Rheinisch-Westfaelische Technische Hochschule Aachen

(DE), Stichting Maritiem Research Instituut Nederland (NL), Technology From Ideas Limited

(IE), Esteyco SA (ES), Innosea (FR), Inea Informatizacija Energetika Avtomatizacija DOO (SI),

Transmission Excellence Ltd (UK), Hydro Bond Engineering Limited (UK), FULGOR S.A.,

Hellenic Cables Industry (EL), Adria Winch DOO (HR), Future Fibres (ES), Cobra Instalaciones

y Servicios S.A (ES), Bureau Veritas Marine & Offshore Registre International de Classification

de Navires et eePlateformes Offshore (FR).

Abstract

The purpose of this document is to define the Design Basis of the FLOTANT platform. It will

define the principle design parameters including the operational requirements and applicable

codes. The information provided in this document forms the basis for development of the basic

design and guidelines for the production of the functional and technical specifications that meets

the project requirements.




6

TABLE OF CONTENTS

1 PURPOSE AND SCOPE .................................................................................... 11

2 ABBREVIATIONS AND ACRONYMS ................................................................. 11

3 GENERAL DESCRIPTION OF THE FLOTANT TECHNOLOGY ................................. 12 3.1 BASIC DESCRIPTION OF THE SUBSTRUCTURE .......................................................... 12 3.2 BASIC DESCRIPTION OF THE INSTALLATION PROCESS.............................................. 13 3.3 OPERATING AND MAINTENANCE PHILOSOPHY ....................................................... 14 3.4 ACTIVE HEAVE COMPENSATION SYSTEM ................................................................ 14 3.5 ACTIVE BALLAST SYSTEM ....................................................................................... 14

4 GENERAL CRITERIA ........................................................................................ 15 4.1 REFERENCE CODES, GUIDELINES AND STANDARDS ................................................. 15 4.2 REFERENCED REPORTS ........................................................................................... 16

5 MATERIAL PROPERTIES .................................................................................. 16 5.1 CONCRETE ............................................................................................................. 16 5.2 REINFORCING STEEL .............................................................................................. 16 5.3 PRESTRESSING STRANDS ....................................................................................... 17 5.4 PRESTRESSING BOLTS ............................................................................................ 17 5.5 STRUCTURAL STEEL ............................................................................................... 17 5.6 INFLATABLE BAGS .................................................................................................. 18

6 SITE SPECIFIC DATA ........................................................................................ 18 6.1 LOCATION ............................................................................................................. 18 6.2 WIND FARM CAPACITY & LAY OUT ......................................................................... 20 6.3 DESIGN LIFE ........................................................................................................... 20 6.4 DESCRIPTION OF THE WIND TURBINE ..................................................................... 21 6.5 WIND DATA ........................................................................................................... 22 6.6 WATER DEPTH AND LEVELS .................................................................................... 33 6.7 WATER DENSITY .................................................................................................... 34 6.8 SPLASH ZONE ........................................................................................................ 34 6.9 WAVE DATA .......................................................................................................... 35 6.10 WIND-WAVE COMBINED CONDITIONS ................................................................... 42 6.11 CURRENT DATA ..................................................................................................... 47 6.12 MARINE GROWTH ................................................................................................. 53 6.13 AIR DENSITY .......................................................................................................... 53 6.14 DURABILITY AND EXPOSURE CLASSES .................................................................... 53 6.15 TEMPERATURE VARIATIONS .................................................................................. 54 6.16 ICE LOADS ............................................................................................................. 57 6.17 GEOTECHNICAL CONDITIONS ................................................................................. 59

7 FUNCTIONAL REQUIREMENTS ........................................................................ 63

8 NAVAL ARCHITECTURE ................................................................................... 63 8.1 STABILITY ANALYSIS, REQUIREMENTS AND CRITERIA .............................................. 63

9 MOTIONS ANALYSIS, REQUIREMENTS AND CRITERIA ..................................... 65 9.1 Natural periods criteria. ......................................................................................... 65




7

9.2 Motions Criteria in-place conditions....................................................................... 66 9.3 Motions Criteria in Transport and Installation Operations ...................................... 68

10 STRUCTURAL ANALYSIS .................................................................................. 68 10.1 LOAD FACTORS ...................................................................................................... 69 10.2 DESIGN LOADS DURING TRANSPORT AND INSTALLATION ....................................... 69 10.3 DESIGN LOADS DURING OPERATION ...................................................................... 70 10.4 SHIP IMPACT ......................................................................................................... 71

11 MOORING SYSTEM DESIGN ............................................................................ 71 11.1 Consequence Class ................................................................................................ 72 11.2 Limit States ........................................................................................................... 72 11.3 Design Conditions .................................................................................................. 72 11.4 Load Factors .......................................................................................................... 73 11.5 Design Criteria for ULS and ALS .............................................................................. 73 11.6 Design Criterion for FLS ......................................................................................... 74 11.7 Corrosion allowances ............................................................................................ 75 11.8 Mooring lines drag and added mass coefficients .................................................... 76

12 POWER CABLE DESIGN ................................................................................... 77 12.1 Design principles ................................................................................................... 77 12.2 Functional Requirements ....................................................................................... 77 12.3 Cable and Ancillary Components Specification ....................................................... 78 12.4 Analysis Methodology ........................................................................................... 78 12.5 Loads and Loads Effects ......................................................................................... 79 12.6 Mechanical Characteristics .................................................................................... 80 12.7 Design Resistance and Design Criteria .................................................................... 83

13 LIST OF HOLDS ............................................................................................... 84

LIST OF FIGURES

FIGURE 3.1. GENERAL VIEW OF THE FLOTANT CONCEPT ........................................................... 13

FIGURE 6.2. WEST OF BARRA RELATIVE TO SCOTLAND ............................................................... 19

FIGURE 6.3. FLOTANT IN THE SOUTH EAST OF GRAN CANARIA .................................................. 20

FIGURE 6.4. CURVES OF THE TURBINE: POWER, ROTOR SPEED, TORQUE AND BLADE PITCH ......... 21

FIGURE 6.5. 1-HOUR AVERAGED FREQUENCIES DISTRIBUTION AND WEIBULL FIT FOR WOB ......... 24

FIGURE 6.6. WIND ROSE (MEAN WIND SPEED AT 19,5 M ASL) FOR WOB .................................... 25

FIGURE 6.7. TURBULENCE INTENSITY FOR DIFFERENT WIND TURBINE CLASSES, AS DEFINED IN

IEC-61400-1 ............................................................................................................................... 26

FIGURE 6.8. MONTHLY MEAN WIND SPEED AT GC ........................................................................ 28

FIGURE 6.9. MONTHLY MAXIMUM WIND SPEED AT GC .................................................................. 29

FIGURE 6.10. WIND ROSE FOR 1-HOUR MEAN SPEED AT GC........................................................ 31




8

FIGURE 6.11. WIND ROSE FOR 1-HOUR MEAN SPEED IN WINTER, SPRING, SUMMER AND AUTUMN AT

GC ............................................................................................................................................... 32

FIGURE 6.12. CORRELATION BETWEEN SIGNIFICANT WAVE HEIGHT RAW DATA AND ITS ASSOCIATED

WEIBULL DISTRIBUTION ................................................................................................................ 36

FIGURE 6.13. EXTRAPOLATION CURVE FOR PEAK PERIOD-SIGNIFICANT WAVE HEIGHT

CORRELATION ............................................................................................................................... 37

FIGURE 6.14. WEST OF BARRA WAVE ROSE (SIGNIFICANT WAVE HEIGHT) ................................... 37

FIGURE 6.15. HS WAVE ROSE AT SIMAR POINT 4038006 (1958-2019) .................................... 41

FIGURE 6.16. JOINT DISTRIBUTION BETWEEN WIND SPEED AND SIGNIFICANT WAVE HEIGHT AT GC

..................................................................................................................................................... 46

FIGURE 6.17. 50-YEAR RETURN PERIOD ENVELOPE AT GC .......................................................... 46

FIGURE 6.18. CURRENT PEAK FLOW FOR THE WEST OF BARRA REGION: CURRENT SPRING PEAK

(LEFT), CURRENT NEAP PEAK (RIGHT) FOR SITE 1 ........................................................................ 47

FIGURE 6.19. ANNUAL PROBABILITIES OF EXCEEDANCE FOR SEA ICE (LEFT) AND COLLISION WITH

ICEBERGS (RIGHT). ISO 19901-1:2005 ....................................................................................... 58

FIGURE 6.20. WEST OF BARRA SEABED GENERAL CHARACTERISTICS .......................................... 60

FIGURE 6.21. MULTIBEAM BATHYMETRY OF AN AREA IN THE VICINITY OF WEST OF BARRA .......... 61

FIGURE 6.22. GEOLOGY IN GC .................................................................................................... 62

FIGURE 8.23. GZ CURVES VERIFICATION CRITERION ACCORDING TO DNVGL-ST-N001 ............ 64

FIGURE 9.24. WIND TURBINE FRAME OF REFERENCE .................................................................. 66

FIGURE 12.25. ANALYSIS METHODOLOGY FOR DYNAMIC SUBMARINE CABLES ............................ 79

FIGURE 12.26. NUMERICAL MODEL SETUP .................................................................................. 80

FIGURE 12.27. 66KV DYNAMIC CABLE CROSS-SECTION. ............................................................ 82

LIST OF TABLES

TABLE 1 ABBREVIATIONS AND ACRONYMS .................................................................................... 11

TABLE 2 FLOTANT PLATFORM COORDINATES AT WEST OF BARRA ............................................. 19

TABLE 3 FLOTANT PLATFORM COORDINATES ............................................................................. 19

TABLE 4 TURBINE MAIN PARAMETERS .......................................................................................... 21

TABLE 5 NORMAL WIND SPEED PROFILE FOR WOB ...................................................................... 22

TABLE 6 EXTREME CONDITIONS WIND SPEED PROFILE FOR WOB (TR = 50 YEARS) ..................... 23

TABLE 7 WIND SPEED EXCEEDANCE PROBABILITY FOR WOB ....................................................... 23

TABLE 8 WEIBULL DISTRIBUTION PARAMETERS FOR WOB ............................................................ 24

TABLE 9 ANNUAL AVERAGE WIND SPEED FOR WOB ..................................................................... 25




9

TABLE 10 WIND DIRECTION DISTRIBUTION FOR WOB ................................................................... 26

TABLE 11 NORMAL WIND SPEED PROFILE FOR GC ....................................................................... 28

TABLE 12 EXTREME WIND SPEED PROFILE FOR GC (TR = 50 YEARS) .......................................... 29

TABLE 13 WIND SPEED EXCEEDANCE PROBABILITY FOR GC ........................................................ 30

TABLE 14 WATER DEPTHS (LAT) FOR BOTH SITES ....................................................................... 33

TABLE 15 WATER LEVELS FOR SITE 1 .......................................................................................... 33

TABLE 16 WATER LEVELS FOR SITE 2 .......................................................................................... 34

TABLE 17 SIGNIFICANT WAVE HEIGHT – PEAK PERIOD FREQUENCY FOR WOB ............................. 35

TABLE 18 DEFINING PARAMETERS OF THE WEIBULL DISTRIBUTION ASSOCIATED TO WOB WAVE

HEIGHT DISTRIBUTION ................................................................................................................... 36

TABLE 19 WAVE DATA FOR WOB ................................................................................................. 36

TABLE 20 WAVE DIRECTION FOR WOB ......................................................................................... 38

TABLE 21 SIGNIFICANT WAVE HEIGHT OCCURRENCE PROBABILITY DISTRIBUTION......................... 39

TABLE 22 SIGNIFICANT WAVE HEIGHT – PEAK PERIOD FREQUENCY SITE 2 ................................... 40

TABLE 23 WEIBULL PARAMETERS FOR EXTREME WAVES AT GC ................................................... 40

TABLE 24 WAVE DATA FOR GC .................................................................................................... 41

TABLE 25 WAVE DIRECTION SCATTER DIAGRAM AT GC ................................................................ 42

TABLE 26 WAVE HEIGHT OCCURRENCE DISTRIBUTION AT GC ...................................................... 42

TABLE 27 WIND- WAVE COMBINED DISTRIBUTION: HS-U10 CORRELATION FOR WOB .................. 43

TABLE 28 SECOND ORDER POLYNOMIAL EQUATION FOR WOB ..................................................... 44

TABLE 29 THIRD ORDER POLYNOMIAL EQUATION FOR WOB ......................................................... 44

TABLE 30 COMPARISON OF BOTH EQUATIONS PROPOSED FOR THE WIND-WAVE CORRELATION FOR

WOB ............................................................................................................................................ 45

TABLE 31 CURRENT INDUCED BY WIND SPEED AT SEA SURFACE .................................................. 48

TABLE 32 DEEP WATER CURRENT SPEED AT SEA SURFACE .......................................................... 49

TABLE 33 TOTAL CURRENT SPEED PROFILE ASSOCIATED TO THE 1-YEAR RETURN PERIOD

PROBABILITY................................................................................................................................. 50


PROBABILITY................................................................................................................................. 50

TABLE 35 MOST PROBABLE CURRENT DIRECTION ........................................................................ 51

TABLE 36 CURRENT INDUCED BY WIND SPEED AT SEA SURFACE AT GC ....................................... 51

TABLE 37 DEEP WATER CURRENT AT SURFACE AT GC ................................................................. 52


PROBABILITY................................................................................................................................. 53

TABLE 39 MARINE GROWTH DATA ................................................................................................ 53

TABLE 40 EXPOSURE CLASSES TO BE CONSIDERED IN THE DESIGN OF THE SUBSTRUCTURE ........ 54




10

TABLE 41 EXPOSURE CLASSES FOR EACH CONCRETE ELEMENT OF THE STRUCTURE ................... 54

TABLE 42 ISLE OF LEWIS AVERAGE MONTHLY SEAWATER TEMPERATURE. .................................... 55

TABLE 43 AIR TEMPERATURE IN WEST OF BARRA AT SEA LEVEL. SOURCE: LIFE50+ .................. 55

TABLE 44 MAX SURFACE TEMPERATURE OF WATER FOR GC (º) ................................................... 56

TABLE 45 MAX AND MIN MEAN SURFACE TEMPERATURE OF WATER FOR GC (º) ........................... 56

TABLE 46 MONTHLY MEAN (BLUE) AND MONTHLY MAX(RED) AIR TEMPERATURE (1998-2018) FOR

GC ............................................................................................................................................... 57

TABLE 47 EXTREME SNOW AND ICE ACCUMULATIONS. SOURCE OTH 2001/010 FOR WOB ......... 59

TABLE 48 SOIL PROFILE CHARACTERISTICS FOR WOB ................................................................. 61

TABLE 49 GEOTECHNICAL PARAMETERS AT GC ........................................................................... 62

TABLE 53 PARTIAL SAFETY FACTORS FOR LOADS ......................................................................... 69

TABLE 54 CRACK WIDTH LIMIT ..................................................................................................... 70

TABLE 55 LOAD FACTOR REQUIREMENT FOR DESIGN OF MOORING LINES ..................................... 73

TABLE 56 DFF FOR MOORING CHAIN ............................................................................................ 75

TABLE 57 CORROSION ALLOWANCE FOR MOORING LINES ............................................................. 76

TABLE 58 DRAG AND ADDED MASS COEFFICIENTS FOR MOORING LINES ....................................... 76

TABLE 59 MATERIAL PROPERTIES ................................................................................................ 81




11

1 PURPOSE AND SCOPE

The purpose of this document is to define the Design Basis of FLOTANT technology, a novel

hybrid concrete/plastic floater, to be implemented within FLOTANT project in 2 different sites of

the Atlantic Area (both Scotland and the Canary Islands). It will define the principle design

parameters including the operational requirements and applicable codes. The information

provided in this document forms the basis for development of the basic design and guidelines

for the production of the functional and technical specifications that meets the project

requirements.

The scope of this document is to provide guidance throughout the design of the FLOTANT

technology, but it excludes wind turbine design aspects, as well as tower internals.

2 ABBREVIATIONS AND ACRONYMS

Abbreviation/Acronym Definition

DLC Design Load Cases

DoF Degrees of Freedom

DNVGL Dot Norske Veritas Germanischer Lloyd

GM Metacentric height

PDF Probability Density Function

RNA Rotor Nacelle Assembly

WTG Wind Turbine Generator

TABLE 1 ABBREVIATIONS AND ACRONYMS




12

3 GENERAL DESCRIPTION OF THE FLOTANT TECHNOLOGY

This section is intended for general description of FLOTANT principles, components, and

technology.

3.1 BASIC DESCRIPTION OF THE SUBSTRUCTURE

The FLOTANT technology is a semisubmersible foundation for offshore wind turbines, with

concrete, steel and plastic as main materials. The basis of its design is that the inertia of the

section piercing the water surface provides a large metacentric radius, hence a large stability.

The foundation is mainly built with structural concrete instead of steel, since it is a more cost-

effective and robust material for the aggressive offshore environment, making it possible to

reduce the maintenance requirements.

The base is divided in different cells or compartments made of plastic bags, which provide the

buoyancy in a cheaper manner than that of concrete of steel. Thus, the concrete protects the

plastic elements from the environment, without been watertight.

The following figure presents the concept during operation.




13

FIGURE 3.1. GENERAL VIEW OF THE FLOTANT CONCEPT

3.2 BASIC DESCRIPTION OF THE INSTALLATION PROCESS

The substructure is self-buoyant and self-stable, allowing for a conventional towed transport

from the port where it is assembled to the offshore wind farm.

The exact details of the installation process are yet to be decided, hence the basic step shown

here:




14

Units are fabricated in a dry dock or on top of the quay and then floated with a

semisubmersible barge, a ramp or a synchrolift.

The draft at port is limited to 12 m. At this moment, the platform does not have any

water ballast inside.

Once the platform has arrived at an area with enough depth, water is let inside the

ballast deposits to increase the draft to the desired level.

Platform towed to the wind farm (West of Barra or Southeast of Gran Canaria)

Mooring anchors connected to the hull.

The active ballast system counteracts the effect of wind, keeping the platform near

vertical.

3.3 OPERATING AND MAINTENANCE PHILOSOPHY

This section is yet to be completed, so it is included in the list of HOLDS in section 13.

3.4 ACTIVE HEAVE COMPENSATION SYSTEM


3.5 ACTIVE BALLAST SYSTEM

The floater will include a system to compensate the tilting angle generated by the wind. This

system will rely on a series of water compartments which will serve to generate a counteracting

moment. The water will be moved between these compartments via a pumping system.

The system is yet to be defined and the moment that it can provide as well, so it is included in

the list of HOLDS in section 13.




15

4 GENERAL CRITERIA

4.1 REFERENCE CODES, GUIDELINES AND STANDARDS

This section describes the reference codes and standards to be considered for the substructure

design and establishes a clear and straightforward hierarchy among them.

Level 1 – Superior standards:

DNVGL-ST-0126: Support Structures for Wind Turbines (Apr. 2016)

DNVGL-ST-C502 Offshore Concrete Structures (Feb. 2018)

DNVGL-ST-0437: Loads and site conditions for wind turbines (Nov. 2016)

EUROCODE-2 / EN-1992: Design of Concrete Structures

EUROCODE-3 / EN-1993: Design of Steel Structures

DNVGL-ST-N001: Marine operations and marine warranty (Jun. 2016)

DNVGL-ST-0119: Floating wind turbine structures (July 2018)

BV NI 572 DT: Classification and Certification of Floating Offshore Wind Turbines (January

2019)

Level 2 – General standards:

IEC 61400-1: Wind turbines – Design requirements (2005)

IEC 61400-3: Design requirements for offshore wind turbines (2009)

EUROCODE-1 / EN-1991: Actions on Structures

DNV-OS-J101: Design of Offshore Wind Turbine Structures (May. 2014)

IEC 63026 ED1: Submarine power cables with extruded insulation and their accessories for

rated voltages from 6 kV (Um = 7,2 kV) up to 60 kV (Um = 72,5 kV) - Test methods and

requirements (2020).

CIGRE TB 722: Recommendations for additional testing for submarine cables from 6 KV

(UM=7.2 KV) up to 60 KV (UM = 72.5 KV) (2018).

IEC 60228: Conductors of insulated cables (2004).

CIGRE TB 623: Recommendations for mechanical testing of submarine cables (2015).

Level 3 – Additional standards, guidelines and regulations considered:

DNVGL-RP-C205 Environmental conditions and environmental loads (Aug. 2017)

FIB Model Code for Concrete Structures (2010)

DNVGL-RP-F401: Electrical power cables in subsea applications (2017).




16

4.2 REFERENCED REPORTS

Other references are:

“Oceanographic and meteorological conditions for the design”, 2015-06-01, Iberdrola,

Project LIFE50+.

Measures from SIMAR point number 4038006, from “Puertos del Estado” (Spanish Port

Authority), from 1959 to 2019.

5 MATERIAL PROPERTIES

The reference codes to define the materials characteristics are the Eurocodes, or the

corresponding ETAs. The main structural materials used in the substructure and their main

characteristics and design parameters are shown in the following groups.

5.1 CONCRETE

Concrete is defined according to EN 1992 Table 3.1:

• Concrete C60:

Characteristic cylinder compressive strength at 28 days fck = 60 MPa

Modulus of elasticity EC = 37660 MPa

Strain at maximum strength in parabola-rectangle diagram εC0 = 2,1‰

Ultimate strain in the parabola-rectangle diagram εCU = 3,1‰

5.2 REINFORCING STEEL

Reinforcing steel is defined according to EN 1992 Section 3.2:

• Reinforcing steel B500 SD:

Characteristic yield strength fYK = 500 MPa

Modulus of elasticity ES = 200000 MPa

Ultimate strain εUD = 14%




17

5.3 PRESTRESSING STRANDS

“EN 1992 – Eurocode 2” and “prEN 10138-3” will be used as reference for the prestressing

strands in absence of a specific ETA or another equivalent certification.

• Prestressing cables Y 1860 S7 15.2 A: (EC-2 art. 3.3 and prEN 10138-3:2000)

Characteristic tensile strength fPK = 1860 MPa

Characteristic yield strength fP0,1K = 1600 MPa

Modulus of elasticity ES = 195000 MPa

Net area per strand ANET = 140 mm2

Ultimate tensile force FPK = 260 kN

Maximum prestressing load 0,8·FPK = 208 kN

Characteristic strain at maximum load εUK = 3,5%

5.4 PRESTRESSING BOLTS

Prestressing bars will be design based on their corresponding ETA, while prestressing bolts will

be designed base on “EN 1993 – Eurocode 3”.

• Prestressing bolts: (10.9, EN 14399-4)

Tensile strength Rm = 1000 MPa

Strength at 0.2% elongation Rel = 900 MPa

Modulus of elasticity ESP = 205000 MPa

5.5 STRUCTURAL STEEL

Structural steel present is defined according to “EN-1993 Design of Steel Structures”.

• Structural steel S355:

Characteristic yield strength fY = 355 MPa

Ultimate strength fu = 490 MPa

Modulus of elasticity E = 210000 MPa

Shear modulus G = 81000 MPa

Yield Strain εU ≥ 15·εY

Ultimate strain εU > 15%




18

5.6 INFLATABLE BAGS


6 SITE SPECIFIC DATA

A general description of the parameters to define all the relevant conditions in the final location

of both sites is provided in this section, including particular reference to the metocean,

geotechnical and durability conditions among others. The platform will be designed based on 2

different sites: west of Barra island in Scotland and Southeast of Gran Canaria Island in Spain.

6.1 LOCATION

6.1.1 SITE 1: WEST OF BARRA, SCOTLAND

West of Barra (WoB) selected site is located 19 km West of Barra Island, Scotland, immediately

within the 12 nm limit. This site has been identified by a previous project funded by the

European Commission under the Horizon 2020 programme, the LIFE50+, as a potential area

where tests sites for deep water floating technology could be located.

For the characterization of the oceanographic and meteorological conditions of the selected site

in West of Barra, the information provided in the public deliverable D1.1 of the above-mentioned

project LIFE50+ is used.




19

Coordinates Sexagesimal System Standard UTM System (m)

W N E N Zone

FLOTANT PLATFORM

56°53'09.60" 7°56'52.84" 564100.60 6305189.01 29 V

TABLE 2 FLOTANT PLATFORM COORDINATES AT WEST OF BARRA

FIGURE 6.2. WEST OF BARRA RELATIVE TO SCOTLAND

6.1.2 SITE 2: SOUTHEAST GRAN CANARIA, SPAIN

The second site selected for the FLOTANT project is located off the southeast coast Gran

Canaria (GC) island, in the Canary Islands, Spain. It is an area with very constant winds, but

with low extreme wind speed.

Coordinates Sexagesimal System Standard UTM System (m)

W N E N Zone

FLOTANT PLATFORM

15°19'48.00" 27°45'0.00" 467478.89 3069552.70 28 R

TABLE 3 FLOTANT PLATFORM COORDINATES




20

FIGURE 6.3. FLOTANT IN THE SOUTH EAST OF GRAN CANARIA

6.2 WIND FARM CAPACITY & LAY OUT

Each of the wind farms will include 50 turbines, totaling 600 MW each.

The actual layout will be provided in the dedicated deliverable, but as a reference, the turbines

will be arranged in rows, with a spacing between rows equivalent to 9 rotor diameters and a

spacing between turbines within a row equivalent to 7 rotor diameters.

6.3 DESIGN LIFE

The required design life for the platform and the turbine is 27 years, including 25 years of

operation, 0.5 years of installation, and 1.5 years of decommissioning.




21

6.4 DESCRIPTION OF THE WIND TURBINE

The wind turbine chosen is a generic 12 MW, adapted from the 10 MW turbine from DTU. Its

main features are shown in the following table.

WIND TURBINE CHARACTERISTICS

Output power 12 MW

Rotor diameter 195.4 m

Hub diameter 6.13 m

Hub height above sea level 119.7 m

Nacelle mass including rotor (RNA) 836 t

Distance from Tower Top to Hub Height 3.01 m

Cut-in wind speed 4.0 m/s

Rated wind speed 11.4 m/s

Cut-out wind speed 25.0 m/s

Cut-in rotor speed 3.5 rpm

Rated rotor speed 8.76 rpm

TABLE 4 TURBINE MAIN PARAMETERS

FIGURE 6.4. CURVES OF THE TURBINE: POWER, ROTOR SPEED, TORQUE AND BLADE PITCH




22

6.5 WIND DATA


As expected, given the location of this site, the wind resource is high and reliable through the

year, presenting an annual mean power density of around 1,3 kW/m2.

The main reference considered when evaluating the wind conditions of West of Barra site is the

report issued by European project LIFE50+. This document states that all the data available are

1-hour averaged wind speeds at 10 m above MSL (measurements over 31 years), so all the

numbers will be generated by extrapolating to 10-minute averaged and to other heights.

6.5.1.1 Wind Shear Profile

The best fit for the wind speed profile in normal conditions has been found to be the logarithmic

law:

The resulting 10-minute mean wind speed profile is the following (FLOTANT hub height is 119,7

m):

TABLE 5 NORMAL WIND SPEED PROFILE FOR WOB

For extreme conditions however, the best fit for the wind profile is a power law with a 0.12

exponent (α):




23

The extreme wind speed profile for a return period of 50 years would be the following:

TABLE 6 EXTREME CONDITIONS WIND SPEED PROFILE FOR WOB (TR = 50 YEARS)

Thus, the Vref, as described by IEC is 50 m/s.

6.5.1.2 Wind Speed Histogram

Following table summarizes the exceedance probability for the 1-hour averaged wind speed.

TABLE 7 WIND SPEED EXCEEDANCE PROBABILITY FOR WOB




24

6.5.1.3 Weibull distribution parameters

A Weibull distribution has been fitted by the Least Square Method (LSM) to the exceedance

probability values provided in the previous section. The parameters defining this Weibull

function are given below so it is the correlation coefficient.

TABLE 8 WEIBULL DISTRIBUTION PARAMETERS FOR WOB

Figure 6.5 shows the comparison between the raw data distribution (blue) and the associated

Weibull distribution (red) from the table above, through graphic representations (on the left the

probability density function and on the right the cumulative probability) which shows the level of

accuracy obtained with the considered parameters.

FIGURE 6.5. 1-HOUR AVERAGED FREQUENCIES DISTRIBUTION AND WEIBULL FIT FOR WOB

6.5.1.4 Annual Average Wind Speed

Annual 1-hour averaged wind speed for West of Barra is 9,50 m/s at 10 m height. Table below

provides in addition to the annual averaged wind speed, the monthly averaged value, and giving

further sensitiveness in regard to its seasonal variation.




25

TABLE 9 ANNUAL AVERAGE WIND SPEED FOR WOB

6.5.1.5 Wind Rose

FIGURE 6.6. WIND ROSE (MEAN WIND SPEED AT 19,5 M ASL) FOR WOB

6.5.1.6 Scatter diagrams of 10-minunte average wind speed

The following table gathers up the mean wind speed for the different incoming wind direction

sectors. The direction, clockwise from true North, is from which the wind is blowing. Direction

measures were performed for 1-hour average direction at a height of 19,5 m (despite the mean

wind speed, that is given at 10 m height).




26

TABLE 10 WIND DIRECTION DISTRIBUTION FOR WOB

6.5.1.7 Turbulence intensity

There is no specific data for the site turbulence, so it is assigned a Class C, as described in

IEC-61400-1.

FIGURE 6.7. TURBULENCE INTENSITY FOR DIFFERENT WIND TURBINE CLASSES, AS DEFINED IN IEC-61400-1

6.5.1.8 Spectral Density

In absence of more detailed information and following DNVGL recommendations, it has been

decided to assume the Kaimal model as the most representative of wind spectral density at




27

West of Barra. The Kaimal model provides de distribution of wind energy over the different

frequencies.

6.5.1.9 Wind Gust Characteristics

No information is available at West of Barra site in regard to wind gust. Hence, reference is

made to IEC-61400-1, where it can be found mathematical models that allow characterizing

wind gust and accounting for its effects on the design load cases (DLC´s).

6.5.2 SITE 2: SOUTHEAST GRAN CANARIA

The wind data for this site has been extracted from the data provided by the SIMAR point

4038006, from the Spanish Ports Authority. This is a grid of points at which models are run to

generate wave simulations and data. This point has the following coordinates:

15°19’48.00” W 27°45’0.00” N

These simulations provide the 1-hour wind speed at 10 m above the sea level.

6.5.2.1 Wind Shear Profile

The logarithmic law has been chosen as the wind speed profile for normal conditions:

The monthly mean speed for the last 10 years is shown below, so the 1-hour mean wind speed

is set at 9.0 m/s.




28

FIGURE 6.8. MONTHLY MEAN WIND SPEED AT GC

A 1-hour mean wind speed can be extrapolated to a 10-minute mean wind speed following

DNVGL-RP-C205 section 2.3.2.11 section, providing a value of 9.83 m/s.

The resulting 10-minute mean wind speed profile is the following (FLOTANT hub height is 119,7

m):

NORMAL WIND PROFILE

Height (m) Speed (m/s)

10 9.83

20 10.48

50 11.33

100 11.98

119 12.14

TABLE 11 NORMAL WIND SPEED PROFILE FOR GC

For extreme conditions however, the best fit for the wind profile is a power law with a 0.12

exponent (α):




29

The maximum 1-hour average monthly speed for the past ten years is shown in the following

figure:

FIGURE 6.9. MONTHLY MAXIMUM WIND SPEED AT GC

The maximum 1-jour average wind speed recorded is 19.0 m/s, which is translated to 20.75 m/s

of maximum 10-minute average wind speed.

The extreme wind speed profile for a return period of 50 years would be the following:

EXTREME WIND PROFILE

Height (m) Speed (m/s)

10 20.75

20 22.55

50 25.17

100 27.35

119 27.93

TABLE 12 EXTREME WIND SPEED PROFILE FOR GC (TR = 50 YEARS)

Thus, the Vref, as described by IEC is 28 m/s.




30

6.5.2.2 Wind Speed Histogram

Following table summarizes the exceedance probability for the 1-hour averaged wind speed.

Wind speed (m/s) Frequency (%)

0.0 1.5 2.30%

1.5 3.0 7.70%

3.0 4.5 13.15%

4.5 6.0 18.90%

6.0 7.5 21.35%

7.5 9.0 18.15%

9.0 10.5 10.80%

10.5 12.0 4.80%

12.0 13.5 1.80%

13.5 15.0 0.70%

15.0 16.5 0.25%

16.5 18.0 0.05%

18.0 19.5 0.05%

TABLE 13 WIND SPEED EXCEEDANCE PROBABILITY FOR GC




31

6.5.2.3 Wind Rose

FIGURE 6.10. WIND ROSE FOR 1-HOUR MEAN SPEED AT GC




32

FIGURE 6.11. WIND ROSE FOR 1-HOUR MEAN SPEED IN WINTER, SPRING, SUMMER AND AUTUMN AT GC

6.5.2.4 Turbulence intensity

There is no specific data for the site turbulence, so it is assigned a Class C, as described in

IEC-61400-1.

6.5.2.5 Spectral Density

In absence of more detailed information and following DNVGL recommendations, it has been

decided to assume the Kaimal model as the most representative of wind spectral density at

Gran Canaria. The Kaimal model provides de distribution of wind energy over the different

frequencies.




33

6.5.2.6 Wind Gust Characteristics

No information is available at Gran Canaria site in regard to wind gust. Hence, reference is

made to IEC-61400-1, where it can be found mathematical models that allow characterizing

wind gust and accounting for its effects on the design load cases (DLC´s).

6.6 WATER DEPTH AND LEVELS

The water depths referred to LAT for both sites are:

WATER DEPTHS (m)

West of Barra 100.0

Gran Canaria 250.0

TABLE 14 WATER DEPTHS (LAT) FOR BOTH SITES


Summary of West of Barra´s water levels are given below.

WATER LEVELS FOR WEST OF BARRA, SCOTLAND

Highest Still Water Level (HSWL) 4.16

Highest Astronomical Tide (HAT) 3.16

Mean Sea Level (MSL) 2.32

Lowest Astronomical Tide (LAT) -1.48

Lowest Still Water Level (LSWL) -2.48

TABLE 15 WATER LEVELS FOR SITE 1




34


Summary of Southeast Gran Canaria water levels is given below.

WATER LEVELS FOR GRAN CANARIA

Highest Still Water Level (HSWL) 3.19

Highest Astronomical Tide (HAT) 3.11

Mean Sea Level (MSL) 1.58

Lowest Astronomical Tide (LAT) 0.00

Lowest Still Water Level (LSWL) -0.13

TABLE 16 WATER LEVELS FOR SITE 2

These values have been taken from the tide gauge in the Arinaga port.

6.7 WATER DENSITY

Seawater density is 1025 kg/m3 for both locations.

6.8 SPLASH ZONE

Definition of splash zone is in accordance with DNVGL-ST-0119 section 13.1.2:

- The upper limit of the external splash zone is the level on the floater corresponding to the

highest still water level with a recurrence period of 1 year in combination with the deepest

operational draught and increased by the crest height of a wave with height equal to the

significant wave height with a return period of 1 year or the foundation settlement, whichever is

higher.

- The lower limit of the external splash zone is the level on the floater corresponding to the

lowest still water level with a recurrence period of 1 year in combination with the shallowest

operational draught and reduced by the trough depth of a wave with height equal to the

significant wave height with a return period of 1 year.




35

6.9 WAVE DATA


The wave data for this site has been extracted from the LIFE50+ project.

6.9.1.1 Hs/Tp Scatter diagram

The following table shows the frequency distributions of significant wave height and spectral

peak period.

TABLE 17 SIGNIFICANT WAVE HEIGHT – PEAK PERIOD FREQUENCY FOR WOB

6.9.1.2 Wave height´s associated Weibull distribution

Data provided by has been statistically analysed and fitted to a Weibull curve. Parameters of

this best fit distribution function are given below as well as its correlation factor. Figure 6.12

illustrates the accuracy of Weibull distribution fit.




36

TABLE 18 DEFINING PARAMETERS OF THE WEIBULL DISTRIBUTION ASSOCIATED TO WOB WAVE HEIGHT

DISTRIBUTION

FIGURE 6.12. CORRELATION BETWEEN SIGNIFICANT WAVE HEIGHT RAW DATA AND ITS ASSOCIATED WEIBULL

DISTRIBUTION

6.9.1.3 Wave characteristic reference values (1,5,10 and 50 years return period)

Based on Weibull distribution and assuming 3-hour storms sea states, significant wave heights

associated to 50, 20, 10 and 1 year return period are provided in the following table. For each of

these values, the wave peak period has been extrapolated as the most probable value

associated to that height, in order to do so a curve fitting analysis (see below) has been

performed to allow for determining the most probable values to be associated to those wave

heights that are not contained within the available data.

Return period (years) Hs (m) Tp (s)

50 15.6 12-18

20 14.7 15.0

10 14.0 14.9

1 11.5 14.3

TABLE 19 WAVE DATA FOR WOB

Within the Life 50+ project, the wave peak period was extrapolated as the most probable value

associated to each wave height. In order to do so, a curve fitting analysis (see below) was




37

performed to allow for determining the most probable values to be associated to each wave

heights that are not contained within the available data.

FIGURE 6.13. EXTRAPOLATION CURVE FOR PEAK PERIOD-SIGNIFICANT WAVE HEIGHT CORRELATION

6.9.1.4 Wave Rose

FIGURE 6.14. WEST OF BARRA WAVE ROSE (SIGNIFICANT WAVE HEIGHT)




38

6.9.1.5 Wave direction Scatter Diagram

The following table gathers up dominant wave direction for the different incoming wave direction

sectors. The direction, clockwise from true North, is from which the waves are travelling.

TABLE 20 WAVE DIRECTION FOR WOB

6.9.1.6 Wave height occurrence distribution

The table below summarizes the occurrence probability associated to the significant wave

height for each month in the selected locations for the wind farm design in the West of Barra.

This occurrence probability is show for each month and can be used to determine the

percentage of time at which a particular wave height is not exceeded.




39

TABLE 21 SIGNIFICANT WAVE HEIGHT OCCURRENCE PROBABILITY DISTRIBUTION

6.9.1.7 Wave spectrum

No information from West of Barra is available on site to determine the most suitable wave

spectrum to characterize wave climate. However, based on its location it can be assumed, that

such oceanic regions of the Northern Europe are likely to be subjected to swell waves that have

moved out of the area in which they were generated (North of the Atlantic Ocean). Hence,

pointing to Pierson-Moskowitz as the most advisable wave spectrum model for this specific

location.


The wave data for this site has been extracted from the data provided by the SIMAR point

4038006, from the Spanish Ports Authority.




40

6.9.2.1 Hs/Tp Scatter diagram

TABLE 22 SIGNIFICANT WAVE HEIGHT – PEAK PERIOD FREQUENCY SITE 2

6.9.2.2 Wave height´s associated Weibull distribution

There are no available data available for the extreme tendency of SIMAR points. For this

reason, the Weibull distribution will be extracted from the “Las Palmas Este” buoy, which is

located nearby. Its coordinates are:

15°23'24.00" W 28°3'0.00" N

The Weibull distribution parameters for the buoy are:

Weibull parameters

Scale coefficient (A) 0.48

Shape coefficient (k) 1.02

Location coefficient (δ) 2.02

TABLE 23 WEIBULL PARAMETERS FOR EXTREME WAVES AT GC




41

6.9.2.3 Wave characteristic reference values (1,5,10 and 50 years return period)

Return period (years) Hs (m) Tp (s)

50 5.11 12.0

20 4.69 12.0

10 4.40 10.0

1 3.35 10.0

TABLE 24 WAVE DATA FOR GC

The peak periods showed above correspond to the most probable occurrence as shown in the

scatter diagram above.

6.9.2.4 Wave Rose

FIGURE 6.15. HS WAVE ROSE AT SIMAR POINT 4038006 (1958-2019)

6.9.2.5 Wave direction Scatter Diagram




42

Wave direction (º)

0 45 90 135 180 225 270 315

Hs (m)

0.0-0.5 1.154 0.439 0.059 0.017 0.004 0.000 0.171 0.322

0.5-1.0 10.887 7.317 0.591 0.094 0.033 0.262 0.631 0.912

1.0-1.5 17.103 14.722 0.892 0.155 0.042 0.569 0.431 0.661

1.5-2.0 12.711 11.966 0.556 0.066 0.019 0.362 0.141 0.094

2.0-2.5 5.260 5.626 0.105 0.012 0.017 0.122 0.012 0.011

2.5-3.0 1.809 2.180 0.018 0.000 0.001 0.050 0.015 0.010

3.0-3.5 0.387 0.530 0.000 0.000 0.004 0.056 0.001 0.000

3.5-4.0 0.100 0.163 0.000 0.000 0.004 0.016 0.000 0.000

4.0-4.5 0.001 0.060 0.000 0.000 0.000 0.000 0.000 0.000

4.5-5.0 0.000 0.024 0.000 0.000 0.000 0.000 0.000 0.000

5.0-10 0.000 0.004 0.000 0.000 0.000 0.000 0.000 0.000

TABLE 25 WAVE DIRECTION SCATTER DIAGRAM AT GC

6.9.2.6 Wave height occurrence distribution

Hs (m) Winter Spring Summer Autumn

0.0 0.781 0.593 0.153 0.818

0.5 11.438 7.301 1.905 9.823

1.0 32.343 28.417 16.625 35.933

1.5 28.478 30.643 36.410 32.286

2.0 16.545 19.450 30.470 14.388

2.5 6.466 9.090 11.184 4.791

3.0 2.517 3.399 2.726 1.545

3.5 0.992 0.903 0.500 0.343

4.0 0.302 0.173 0.028 0.065

4.5 0.101 0.030 0.000 0.007

5.0 0.033 0.000 0.000 0.000

>5.0 0.005 0.000 0.000 0.000

TABLE 26 WAVE HEIGHT OCCURRENCE DISTRIBUTION AT GC

6.9.2.7 Wave spectrum

No information is available on site to determine the most suitable wave spectrum to characterize

wave climate. The same assumptions than those for WoB can be made, hence a Pierson-

Moskowitz as the most advisable wave spectrum model for this specific location.

6.10 WIND-WAVE COMBINED CONDITIONS




43

6.10.1 SITE 1: WEST OF BARRAS, COTLAND

Only the correlation between the mean wind speed and the significant wave height is available

for West of Barra site.

6.10.1.1 Wind-Wave climate Scattergrams

TABLE 27 WIND- WAVE COMBINED DISTRIBUTION: HS-U10 CORRELATION FOR WOB

Based on this information, it has been performed some studies to try to preview the most

probable wind speed associated to each significant wave height. To ensure the best correlation

possible with the real sea state conditions (represented by the achieved raw data), mainly two

equations have been considered:




44

Second order polynomial equation:

TABLE 28 SECOND ORDER POLYNOMIAL EQUATION FOR WOB

Third order polynomial equation:

TABLE 29 THIRD ORDER POLYNOMIAL EQUATION FOR WOB

Following table summarizes the different values checked during the selection of the most

accurate relation between the significant wave height and its associated wind speed from the

aforementioned possibilities:




45

TABLE 30 COMPARISON OF BOTH EQUATIONS PROPOSED FOR THE WIND-WAVE CORRELATION FOR WOB

It is demonstrated that the difference between the two equations is not significant inside the

range of significant wave height represented in the scatter diagram. However, when calculating

the associated wind speed to significant wave heights that are out of this aforementioned range

of values, the 3rd order polynomial equation is better adjusted to the expectable values for the

wind speed.

6.10.1.2 Wind-Wave misalignments

No met-ocean data is available about the correlation of wind direction and wave direction.


No data of the correlation between wind speed and wave height is available at the Southeast

coast of Gran Canaria. The following joint distribution corresponds to the Northeast coast of

Gran Canaria, in the PLOCAN area. The extreme values are very close together, so it is

considered to be a good reference.




46

FIGURE 6.16. JOINT DISTRIBUTION BETWEEN WIND SPEED AND SIGNIFICANT WAVE HEIGHT AT GC

The data for the graph above will be delivered via excel sheet, due to the large number of

events (almost 215000 entries). The following graph shows the envelope for the 50-year return

period.

FIGURE 6.17. 50-YEAR RETURN PERIOD ENVELOPE AT GC




47

6.11 CURRENT DATA


Surrounding Scotland seas are directly affected by oceanic circulation due to its position at the

UK Continental Shelf. The steep bathymetry of the continental slope acts as a barrier between

oceanic regions and the shelf sea systems, reducing the amount of water that can travel from

the deeper waters of the North Atlantic into the shallower waters on the continental shelf. Tidal

currents are stronger that the non-tidal in most of Scottish areas and these are better

predictable. Moreover, tidal currents are intensified in localised areas usually where the flow is

constrained by topography. This includes areas such as between Orkney and Shetland, the

Pentland Firth, off the Mull of Kintyre and Hebrides where tidal streams can be as high as 3.5-

4.5 m/s.

The non-tidal circulation on the shelf west of Scotland, (the Scottish Coastal Current) is mainly

northwards. However, this circulation is strongly affected by winds and density-driven coastal

currents and jets, which can lead to large changes in currents and even a reversal of this

general pattern for short periods.

Besides this general overview, no site-specific current data is available at West of Barra. Hence

currents at site location have been characterized based on available met-ocean numerical

model data] and making certain assumptions in regards to wind generated currents following

main recognized standards.

FIGURE 6.18. CURRENT PEAK FLOW FOR THE WEST OF BARRA REGION: CURRENT SPRING PEAK (LEFT), CURRENT NEAP PEAK (RIGHT) FOR SITE 1




48

6.11.1.1 Current induced by wind (1-/5-/10-/50-year event)

Current induced by wind has been estimated assuming that there is a direct relationship

between the 1-hour averaged wind speed at 10 m height and the current speed at surface given

by the following mathematical expression:

��(�0) = � · �1ℎ��

Where (�) coefficient will be taken as 0.03 in order to account for the worst-case scenario and

obtain a safety side current speed value.

The surface current induced by wind associated to 1 and 50-year return period, will be

determined from the 1-hour averaged wind speed with an exceedance probability of 0.05% (50-

years return period) applying the aforementioned formula.

CURRENT DATA FOR WEST OF BARRA, SCOTLAND

Current speed for a TR of 1 years (c1) 0.88 m/s


TABLE 31 CURRENT INDUCED BY WIND SPEED AT SEA SURFACE

The direction associated to these current speed values will be taken as the most probable wind

direction obtained from the scatter diagram. Therefore, wind induced current direction will be

taken as West to East direction for all cases.

6.11.1.2 Deep water current (1-/5-/10-/50-year event)

Indicative values of depth averaged currents at West of Barra location have been obtained from

HSE, «Environmental Considerations. Offshore Technology Report» 2001/010.

Values provided in the above-mentioned document, are associated to the 50-year return period

of the spring tidal current and the storm surge current components. Hence, the resulting depth

averaged current speed obtained in this clause will be calculated as the vectorial sum of each of

the terms commented above.

Under these assumptions, the 50-year return period mean spring tidal current (representative

for the biggest currents happening twice in a month) has been taken as 0.44 m/s value with

North-East direction. Analogously, the 50-year return period storm surge current component

indicative value is 0.60 m/s heading North.




49

As indicated in first paragraph, the resulting 50-year return period combined current speed is

obtained by vectorial summation of the aforementioned terms. Moreover, it has been possible to

obtain the 1-year return period current by applying correction factors given as shown in table

below.

TABLE 32 DEEP WATER CURRENT SPEED AT SEA SURFACE

6.11.1.3 Current speed profile

Since no information is available at West of Barra regarding the current speed profile, reference

is made to DNVGL-RP-C205 section 4. Based on this standard the two following mathematical

models have been used to estimate the variation of current speed with depth depending on the

type of current under consideration:

Current induced by wind

Where �0 is taken as half of the water depth at West of Barra following DNVGL

recommendations, hence �0 = 50 �.

Tidal current

Resulting current speed profiles for each of the currents defined in previous sections are given

in the following tables for the 1-year and 50-year return period currents respectively. Last

column of this table represents the vectorial summation of the aforementioned component.




50

TABLE 33 TOTAL CURRENT SPEED PROFILE ASSOCIATED TO THE 1-YEAR RETURN PERIOD PROBABILITY


6.11.1.4 Current Direction

In absence of more detailed statistical information regarding current direction, only most

probable current speed directions can be provided. Based on tidal current direction provided in

previous section and assuming that wind induced current direction will be driven by wind´s

direction, the following table provides most probable headings with respect to the North.




51

TABLE 35 MOST PROBABLE CURRENT DIRECTION


The data available for the GC site, extracted from the simulation at SIMAR point 4038006, is not

that abundant. Some of the data will be extrapolated.

6.11.2.1 Current induced by wind

This value will be obtained using the same formula than in GC:

��(�0) = � · �1ℎ��

Where (�) coefficient will be taken as 0.03 in order to account for the worst-case scenario and

obtain a safety side current speed value. The 50-year 1-hour maximum annual wind speed at

10 m from the sea level is 19.0 m/s, so the current speed induced by wind is:

CURRENT INDUCED BY WIND DATA FOR GRAN CANARIA, SPAIN


TABLE 36 CURRENT INDUCED BY WIND SPEED AT SEA SURFACE AT GC

The direction associated to these current speed values will be taken as the most probable wind

direction obtained from the scatter diagram. Therefore, wind induced current direction will be

taken as North-northeast to South-southwest direction for all cases.

6.11.2.2 Deep water current

There is no available data for the Southeast coast of Gran Canaria, so it is proposed to use the

available data for the PLOCAN area in the Northeast coast of the island. The direction of the

current is parallel to the coast, following tidal patterns, so goes NNE and SSW twice a day.




52

DEEP WATER CURRENT DATA FOR GRAN CANARIA, SPAIN


Direction of the current (º) 22.5 – 202.5

TABLE 37 DEEP WATER CURRENT AT SURFACE AT GC

6.11.2.3 Current speed profile

There is no information available on the current profile, so the same ones used above for West

of Barra are to be used here:

Current induced by wind

Where �0 is taken as half of the water depth at West of Barra following DNVGL

recommendations, hence �0 = 125 �.

Tidal current

Resulting current speed profiles for each of the currents defined in previous sections are given

in the following table 50-year return period current. Last column of this table represents the

vectorial summation of the aforementioned component. Since both components run parallel to

the coast, their values are directly added.

Depth (m) Wind

component (m/s)

Tidal component

(m/s)

Total current speed (m/s)

0.00 0.57 0.49 1.06

-10.00 0.52 0.49 1.01

-20.00 0.48 0.48 0.96

-30.00 0.43 0.48 0.91

-40.00 0.39 0.48 0.87

-50.00 0.34 0.47 0.82

-60.00 0.30 0.47 0.77

-70.00 0.25 0.47 0.72

-80.00 0.21 0.46 0.67

-90.00 0.16 0.46 0.62




53

-100.00 0.11 0.46 0.57

-110.00 0.07 0.45 0.52

-120.00 0.02 0.45 0.47

-130.00 0.00 0.44 0.44

-140.00 0.00 0.44 0.44

-150.00 0.00 0.43 0.43

-160.00 0.00 0.42 0.42

-170.00 0.00 0.42 0.42

-180.00 0.00 0.41 0.41

-190.00 0.00 0.40 0.40

-200.00 0.00 0.39 0.39

-210.00 0.00 0.38 0.38

-220.00 0.00 0.36 0.36

-230.00 0.00 0.34 0.34

-240.00 0.00 0.31 0.31

-250.00 0.00 0.00 0.00


6.12 MARINE GROWTH

According to DNVGL-ST-0437 section 2.4.11, marine growth has to be taken in account, for

both locations, following the data provided in the next table.

MARINE GROWTH THICKNESS (mm)

From 2 m above the sea level to 40 m below it 100

40 m below the sea level 50

TABLE 39 MARINE GROWTH DATA

6.13 AIR DENSITY

Air density is 1.225 kg/m3 for both locations.

6.14 DURABILITY AND EXPOSURE CLASSES




54

The exposure class to be considered in the structural and durability calculations are obtained

according to EN 1992-1-1 Table 4.1 and presented in the following paragraphs.

TABLE 40 EXPOSURE CLASSES TO BE CONSIDERED IN THE DESIGN OF THE SUBSTRUCTURE

These are the exposure classes assigned for each element:

Exposure classes

Base XC1 + XS2 + XA2

T0 level XC1 + XS3 + XA2

T1 level XC1 + XS3 + XA2

TABLE 41 EXPOSURE CLASSES FOR EACH CONCRETE ELEMENT OF THE STRUCTURE

For the steel elements, the corrosion environment will be CX/H for outdoors and C5-M/H and

C4/H for indoors, following the given classification of the ISO 9223 standard. The planned steel

tower outdoor anticorrosion protection shall follow ISO 20340 and NORSOK M501.

6.15 TEMPERATURE VARIATIONS


a) Water

Sea temperatures around Scotland are affected by local climatic conditions (heat flux with

atmosphere) and the heat transferred to the shores of Scotland by ocean currents

(advective effects). Sea surface temperatures vary with an annual cycle, lagging behind the

cycle of atmospheric temperature by around one month.

The coldest sea water temperatures are recorded in the Scottish Continental Shelf ranging

from 6ºC in winter to 14ºC in summer. Since no on-site data is available, sea-surface

temperature data has been obtained from the nearest possible location: The Isle of Lewis,

located around 120 km North East from West of Barra.




55

TABLE 42 ISLE OF LEWIS AVERAGE MONTHLY SEAWATER TEMPERATURE.

b) Air

Table below summarizes indicative values for the probable extreme maximum/minimum air

temperatures at West of Barra location as well as the lowest observed daily mean air

temperature (LODMAT). The values provided in the table below may vary in +/- 1º C.

TABLE 43 AIR TEMPERATURE IN WEST OF BARRA AT SEA LEVEL. SOURCE: LIFE50+


a) Water

Over the last 20 years, the water temperature varied from 17.4ºC in winter to 25.6ºC in

summer (extreme values).




56

TABLE 44 MAX SURFACE TEMPERATURE OF WATER FOR GC (º)

TABLE 45 MAX AND MIN MEAN SURFACE TEMPERATURE OF WATER FOR GC (º)

b) Air

Air temperature ranges from 17ºC to 30ºC over the last 20 years.




57

TABLE 46 MONTHLY MEAN (BLUE) AND MONTHLY MAX(RED) AIR TEMPERATURE (1998-2018) FOR GC

6.16 ICE LOADS


No specific information is available on site. However, in the following clause it has been

summarize relevant information that should be taken into account for this environmental

condition during the design. These values shall be taken as indicative information and are

based on conservative assumptions made from available data sources and calibrated numerical

models.

Based on the aforementioned information, sea ice and iceberg collision need not to be

considered in the design of offshore structures in the UK waters, since there is no evidence to

suggest that these events may occur. Figure below shows limit areas in the North-West Europe

region for sea ice and collision with icebergs events with an associated annual probability of

exceedance of 10-2

and 10-4

.




58

FIGURE 6.19. ANNUAL PROBABILITIES OF EXCEEDANCE FOR SEA ICE (LEFT) AND COLLISION WITH ICEBERGS

(RIGHT). ISO 19901-1:2005

Snow accumulation is more likely to occur than ice at West of Barra. Snow may settle on non-

horizontal windward-facing parts of an installation if the snow is sufficiently wet.

On vertical surfaces it is only likely to stay in position as snow for a few hours although it may

then freeze, hence remaining as ice. Snow accumulation will affect all exposed elements above

the splash zone.

Ice may form on an offshore structure through the following mechanisms: (i) freezing sea spray,

(ii) freezing fog and super-cooled cloud droplets, (iii) freezing rain and (iv) freezing old wet

snow. On a 50-year return period criterion there is no reason to believe that any of the

aforementioned mechanisms to form ice on offshore structures is of any significance at the

West of Barra site.

The following table provides indicative values for snow and ice accumulation at 57,7 º N.




59

TABLE 47 EXTREME SNOW AND ICE ACCUMULATIONS. SOURCE OTH 2001/010 FOR WOB


The Canary Islands are located in the subtropical region in the middle of the Atlantic Ocean.

There is no written register of sea ice in the area. Regarding snow it is unlikely to occur at sea

level.

6.17 GEOTECHNICAL CONDITIONS


West of Barra site lies entirely over rocky sea bottom that has been deepened by glacial

scouring action. The predominant rock type is Lewisian gneiss, which has a similar hardness to

granite.




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FIGURE 6.20. WEST OF BARRA SEABED GENERAL CHARACTERISTICS

A multibeam bathymetry is provided form a nearby area located approximately 30 km North

from West of Barra site. This information has been gathered from Joint Nature Conservation

Committee report.

As shown in the next figure, seabed is dominated by extensive areas of highly fractured

bedrock. The fractures form a regular network of gullies, some as wide as 130m with sides up to

30m in height. Although not extensively ground-truthed, the gullies appear to be infilled by

coarse sands.




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FIGURE 6.21. MULTIBEAM BATHYMETRY OF AN AREA IN THE VICINITY OF WEST OF BARRA

With all this general information gathered for the seabed characterization it is defined a

standard soil profile for de characterization of the West of Barra site seabed.

Soil Profile Characteristics

Layer Soil Type Layer Length (m) Compressive strength (MPa)

1 Rock (Basalt) 20 200

TABLE 48 SOIL PROFILE CHARACTERISTICS FOR WOB


The soil in the area is known from a report of 2009 from the company ECOS. The first meters

from the shore are pebbles, up to 15 m of depth. The next area, up to 60 m deep, has variated

granulometries of sand; and further down, we can find sand with bioclasts.




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FIGURE 6.22. GEOLOGY IN GC

Yellow corresponds to fine sand, while orange is coarse sand. The depth of the exterior line is

60 m.

These sands are very competent, with the following design parameters:

Soil Profile Characteristics

Internal friction angle 35 º

Cohesión 0 kPa

Unit weight 20 kN/m3

Deformation modulus for large strains 30 MPa

Deformation modulus for small strains 150 MPa

Poisson ratio 0.3

Shear modulus for large strains 12 MPa

Shear modulus for small strains 60 MPa

TABLE 49 GEOTECHNICAL PARAMETERS AT GC




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7 FUNCTIONAL REQUIREMENTS

The assessment of the substructure in this respect will take into account the mass, size and

requirements for the following elements:

Boat landing.

Exterior platform, on top of foundation.

Platform crane.

Power cable arrangements, including 2 exits per unit.

Switchgear, cabinets and other internal elements within the tower

8 NAVAL ARCHITECTURE

This section provides a description of the basic criteria and requirements of the naval

architecture analysis that will be carried out for the transport and installation processes

altogether with a brief description of the limiting weather conditions and main operations that are

to be performed for the installation of the substructure in its final position.

8.1 STABILITY ANALYSIS, REQUIREMENTS AND CRITERIA

This section contains the most relevant information regarding to hydrostatics and stability. All

significant design conditions must be addressed: transport and offshore installation, including

deployment at sea. Any topic/detail not included in the scope of this Design Brief must be

directly consulted in the rules and standards listed previously.

8.1.1 Wind forces and heeling curves

The wind used for overturning moment calculations should be the design wind for the

transportation, as defined in Section 4.1 of DNVGL-ST-N001: For weather-restricted operations

the design wind speed can be selected independent of statistical data.

The structure should have enough stability to withstand the wind heeling moment induced by

the action of the design wind speed, superimposed from any direction in any operating or transit

condition.

The design wind speed (Vd) for all installation stages in this project has been stablished in 15

m/s, 10-minunte average wind speed at the hub-height, which is consistent with typical limits for

cranes and workers in exposed places. Where other averaging periods or heights are needed,




64

this design value will be transformed in accordance with the applicable guidelines and wind

characteristics listed previously.

The wind load in transport and installation offshore operation has two components:

Wind load on the rotor which is considered with the blades in feather situation

Wind load on the substructure

To obtain the wind heeling moments, the lever of the wind overturning force shall be taken

vertically from the center of pressure of all wind-exposed surfaces to the center of lateral

resistance of the underwater body of the unit or to the level of the mooring line attachment

points, whichever is the lower (DNVGL-ST-0119, Sec 10.2.1.4).

8.1.2 Stability Criteria in transport conditions

The floating structure shall be capable of maintaining stability during all installation conditions,

until it reaches the desired draft.

The intact design criteria are listed as follows (Section 6.2.2 of DNVGL-ST-N001):

1) The initial GM shall be positive and higher than 0.5.m (after allowing for all possible

inaccuracies in measuring it).

2) The area under the righting moment curve to the downflooding angle shall be equal to

or greater than 140% of the area under the wind heeling moment curve to the same

limiting angle.

3) The righting moment curve shall be positive over the entire range of angles from upright

to the second intercept.

FIGURE 8.23. GZ CURVES VERIFICATION CRITERION ACCORDING TO DNVGL-ST-N001




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Additionally, for towing operations the next criteria must be accomplished:

4) During towing, the static inclination in still water when subjected to 50% of required

towline pull should not exceed 2 degrees. Differential ballasting may be used to reduce

the static inclination resulting from towline pull, but by no more than 1 degree.

8.1.3 Stability Criteria in installation conditions

Once the floating platform arrives at the offshore location it is kept in position by means of

tugboats. The stability will be assessed in this situation as well. It should be noted that the

stability will be calculated in several load conditions during the ballasting sequence.

8.1.4 Damage Stability

The units remain unmanned all along the offshore installation process and are not required to

meet damage stability criteria. In accordance with in DNVGL-ST-N001 Sec 6.2.1.10, the

following additional measures shall be taken to minimize risk:

Limiting the exposure period.

Providing additional protection, such as fendering.

Minimizing vessel movements near the structure.

Dedicated procedures and experienced personnel.

9 MOTIONS ANALYSIS, REQUIREMENTS AND CRITERIA

9.1 Natural periods criteria.

To avoid large resonant effects, the FLOTANT structure and its mooring system are designed in

such a way that the resonant frequencies are shifted well outside the wave frequency range.

For a floating unit the natural periods of motions are key features and, in many ways, reflect the

design philosophy.

In order to avoid possible couplings between inclinations and heave motions it would be

recommended to shift away both natural periods. Because of the special nature of this design

there is not a specific recommendation to easily avoid coupling effects. Coupled time domain

models and tank test will clarify this uncertainty.




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9.2 Motions Criteria in-place conditions

Motion constraints are mainly imposed by wind turbine and power cable design limitations. WTG

limits are related to tilt angles and accelerations and power cable limits are mainly related to

horizontal excursions.

The convention used is shown in the next picture:

FIGURE 9.24. WIND TURBINE FRAME OF REFERENCE

9.2.1 Valid range of tilt angles.

Inclinations in both pitch and rolls DOFs are expected. It should be noted that this inclination

refers to the angle between tower axis and the vertical global axis. Therefore, the limits stated in

the lists below are applicable to WTG and the floater.

Limitations during OPERATION:

Platform yaw (10 min. maximum) < 15º (turbine to stop in case it is not corrected).

Platform yaw (10 min. standard deviation) < 3º.




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Platform tilt and/or roll (10 min. maximum) = within +/- 5º (turbine to stop in case it is not

corrected by the active ballast system).

Platform tilt (10 min. average) = within +/- 2.5º.

Platform roll (10 min. average) = within +/- 2.5º.

Platform tilt (10 min. standard deviation) < 1.5º.

Platform roll (10 min. standard deviation) < 1º.

Limitations during SURVIVAL:

+/- 5º maximum average tilt and/or roll.

+/- 7º maximum instant tilt and/or roll.

+/- 15º maximum instant tilt and/or roll in case of emergency stop (active ballast system

has to act as quickly as possible, because thrust is no longer as high).

9.2.2 Valid range of excursions.

The maximum allowed excursion is 30 m in each direction. Before that, an alarm is generated

when 15 m are reached. If reaching 30 m, the turbine is stopped.

The maximum excursions limits of the platform are yet to be confirmed, so it is included in the

list of HOLDS in section 13.

9.2.3 Valid range of heave motions

Resonant effects must be avoided and linear wave response (WF) in combination with Low

Frequency (LF) response are expected. Therefore, the amplitude of motions in heave is driven

by wave conditions and there is not a technical limit.

9.2.4 Valid range of accelerations

The maximum admissible acceleration at hub height is 0.3g in each direction.




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9.3 Motions Criteria in Transport and Installation Operations

The FLOTANT platform is unmanned during transport operations. Therefore, the limits are

imposed not by the human safety but the WTG technical limits.

The following list shows the permissible limits while towing:

- Maximum wind speed (10-minute mean): 20 m/s

- Maximum significant wave height: 3.7 m

- Towing speed: 3 knots

- Maximum nacelle acceleration: 0.06g (0.59 m/s2)

- Maximum foundation Tilt/ Roll Angle: +/- 2º.

9.3.1 Motions Response Computer Programs or Model test

Several motion analyses will be performed by mean of computer simulations. Time domain

simulations of 3h duration will be carried out following the instructions and recommendations

provided in DNVGL-ST-N001 to obtain the Most Probable Maximum Extreme (MPME). Results

will be validated with the scaled test performed in the model basin facilities.

10 STRUCTURAL ANALYSIS

This section of the document studies the environmental loads during the service life of the

turbine. It covers primary (tower, hull, mooring attachments, …) and secondary structures

(plastic bags, platforms, boat landing, …).

The structural analysis of the substructure will be performed according to reference standards

DNVGL-ST-0119 and DNVGL-ST-0126, which makes reference to EN-1992-1-1 Eurocode 2 for

the design of concrete structures and FIB Model Code 2010, which will be used primarily for the

fatigue verifications.

For the structural assessment of the structure, all the external actions (wind, waves and

currents) are considered to act in the same direction. This is a very conservative assumption,

especially for the Fatigue Limit State.




69

In the following sections, the main design loads to be used for the structural and geotechnical

verifications are presented; any additional particular design loads which may be required for the

design of the substructure will be further defined in the corresponding design reports.

10.1 LOAD FACTORS

Load factors for structural and geotechnical verifications will be obtained from section 4.3 from

DNVGL-ST-0437.

TABLE 50 PARTIAL SAFETY FACTORS FOR LOADS

10.2 DESIGN LOADS DURING TRANSPORT AND INSTALLATION

General guidelines of the loads approach during transport and installation can be summarized

as follows:

Temporary conditions to be analyzed will be towing.

Load analysis during the stages includes, but is not limited to, wind loading, sea

loading, 2nd

order & inertial forces, towlines pull, intrinsic eccentricities, etc.

The combination of the different actions, in terms of force direction, will reproduce the

most unfavorable conditions, considering the real arrangement of the substructure




70

regarding rotor plane, maximum wind force misalignment with rotor plane, maximum

soil inclination, etc.

10.3 DESIGN LOADS DURING OPERATION

This is a list of the main verifications that are to be performed in the structural analysis of the

floating platform:

Bending, shear and torsion verifications. All elements in the floating platform shall be

evaluated for the effects of both global and local bending effect and shear forces as

indicated in Eurocode 2 section 6.1, 6.2 and 6.3.

Limitation of stresses. Stress limitation in concrete elements shall be taken into account,

limiting for a value of 0.6 times the characteristics compressive strength of concrete for

the characteristic loads and 0.45 times the characteristic compressive strength of

concrete under permanent loads.

Additionally, in order to avoid permanent deformations, the strain in reinforcement shall

not exceed the strain corresponding to 0.9 fyk for characteristic extreme loads as per

DNVGL-ST-0126 [10].

Crack width limitation. Crack width requirements follow indications in DNVGL-ST-0126

section 5.8.5.4, which redirects to EN1992-1-1. FLOTANT structure uses unbonded

tendons, so according to section 7.3.1 (5) of the Eurocode 2, crack width is limited to

0.3 mm for XS2 and XS3 conditions under quasi-permanent load combination.

However, DNVGL reduces this width to 0.2 mm for members with prestressed concrete

with unbonded strands. The following table clarifies the crack width limit for each

element in the FLOTANT structure:

Element Crack width limit

Tower 0.2 mm

Lower slab 0.3 mm

TABLE 51 CRACK WIDTH LIMIT

Tightness against leakages of fluids. According to EC2 Part 3 section 7.3.1, in order to

provide Tightness Class 3, it is required to provide a minimum of 50 mm of compressed

concrete in each section for the quasi-permanent combination of actions.




71

Fatigue verification. For the fatigue verifications, the cumulative damage, calculated

according to Model Code 2010 shall be lower than 1 for all the elements in the floating

platform structure.

10.4 SHIP IMPACT

For the evaluation of the ship impact, two loads scenarios shall be considered. On the one side,

normal impact events on the boat landing by a service vessel impact and, on the other side, the

primary structure shall be designed against service vessel impacts as an abnormal event.

For the normal case, the following considerations shall be taken into account:

A maximum service vessel of a total displacement of 150 metric tons.

A service speed of 0.5 m/s.

For the abnormal case, the following considerations shall be taken into account in the design:

A maximum supply vessel of a total displacement of 2000 metric tons.

A service speed of 2.0 m/s.

The vessel shall be assumed to be drifting laterally.

Added mass coefficients for bow impact will be assumed to be equal to 1.1 while added mass

coefficient for side impact will be assumed to be 1.4 (as per Norsok N-003 section 9.3.2.3).

11 MOORING SYSTEM DESIGN

This section provides requirements for mooring lines system design.

The platform will be designed for the wind farm operating life of 25 years and their mooring

systems must be designed to be in operation for the duration of the wind farm design life without

replacement due to strength, fatigue, corrosion and abrasion.




72

11.1 Consequence Class

As per DNVGL-ST-0119 and DNVGL-ST-0126, the FLOTANT platform shall be designed to

Normal Safety Class, meaning the platform is unmanned during severe environmental loading

conditions.

For initial design purposes, as a base case, a three-line mooring system shall be assumed. The

system is considered redundant and Consequence Class 1 requirements are applicable.

The failure of a slack mooring line in the three-line system, will cause a large drift-off. It does not

necessarily imply a system without redundancy. In such case, it may be necessary to carry out

a qualification of the redundancy of the station keeping system.

To be qualified as a redundant system an ALS assessment will be performed to

demonstrate the FLOTANT is capable of withstanding loads in the damaged condition

after an accident. For this purpose, characteristic environmental loads defined as 1-year

loads can be assumed in conjunction with load factors for the ALS in the relevant safety

class.

If the platform cannot be qualified as redundant then a Consequence Class 2 in the mooring

system design would be implemented.

The Consequence Class of the platform is yet to be decided, so it is included in the list of

HOLDS in section 13.

11.2 Limit States

The mooring line must be designed for the following limit states: ULS, FLS, ALS. The load

factors as a function of safety class are listed in DNVGL-OS-E301 Chapter 2, section 2,

subsection 4.2 & 4.3.

11.3 Design Conditions

Operating, Survival and Accidental design conditions are the most relevant situation to take into

account to carry out the mooring lines design.




73

The accuracy level required is Level I and therefore a Dynamic model is required. The model

shall reproduce the real dynamics of the mooring lines. The buoyancy and the drag of the lines

shall be included.

11.4 Load Factors

Requirements for load factors in the ULS and the ALS are given in the next table as a function

of safety class as reflected in DNVGL-ST-0119 section 8.2.2.6.

Load factor requirements for design of mooring lines

Limit state Load factor Safety class

Normal High

ULS ϒmean 1.30 1.50

ULS ϒdyn 1.75 2.20

ALS ϒmean 1.00 1.00

ALS ϒdyn 1.10 1.25

TABLE 52 LOAD FACTOR REQUIREMENT FOR DESIGN OF MOORING LINES

11.5 Design Criteria for ULS and ALS

The design criterion in the ULS is:

�� < ��

The design criterion in the ALS is:

�� < ��∗

For ALS purposes Td is established under an assumption of damaged mooring system in terms

of one broken mooring line.




74

When statistics of the breaking strength of a component are not available, then the

characteristic capacity of the body of the mooring line may be obtained from the minimum

breaking strength Smbs of new components as:

�� < 0.95 · ��

The design tension Td in a mooring line is the sum of two factored characteristic tension

components Tc,mean and Tc,dyn,

�� = �� · ��,�� + �� · ��,��

Where:

Tc,mean : characteristic mean tension

Tc,dyn: characteristic dynamic tension

11.6 Design Criterion for FLS

Mooring lines shall be designed against fatigue failure. The design cumulative fatigue damage

is:

�� = �� · ��

Where:

DD: design cumulative fatigue damage.

DFF: design fatigue factor.

DC: characteristic cumulative fatigue damage caused by the stress history in the

mooring line over the design life.

Requirements for the design fatigue factor DFF are given in DNVGL-ST-0119 section 8.2.5.1,

which provides the following table:




75

Consequence class DFF

1 5

2 10

TABLE 53 DFF FOR MOORING CHAIN

Predictions of fatigue life may be based on calculations of cumulative fatigue damage under the

assumption of linearly cumulative damage. The characteristic stress range history to be used for

this purpose can be based on rain-flow counting of stress cycles.

When Miner’s sum is used for prediction of linearly cumulative damage, the characteristic

cumulative damage DC is calculated as:

�� = ��,�

��,�

�

��

in which

I = number of stress range blocks in a sufficiently fine, chosen discretization of the

stress range axis.

��,� = number of stress cycles in the ith stress block, interpreted from the characteristic

long-term distribution of stress ranges, e.g. obtained by rain-flow counting.

��,� = number of cycles to failure at the stress range Δσi of the ith stress block,

interpreted from the characteristic S-N curve.

11.7 Corrosion allowances

DNVGL-ST-0119 section 13.1.3 defines the requirements for corrosion allowance for chains

(Table 13-1):




76

TABLE 54 CORROSION ALLOWANCE FOR MOORING LINES

11.8 Mooring lines drag and added mass coefficients

As per DNVGL-OS-E301 Chapter 2, section 2, subsection 2.7, the following drag coefficients

and added mass coefficients must be adopted for simulation purposes unless other information

is available:

TABLE 55 DRAG AND ADDED MASS COEFFICIENTS FOR MOORING LINES




77

12 POWER CABLE DESIGN

The dynamic submarine power cable shall be designed according to IEC 63026 (pending in

early 2020), CIGRE TB722 and IEC 60228 as far as these standards are applicable. The cable

system should take into account CIGRE TB623, DNVGL-RP-C205 and DNVGL-RP-F401

fatigue life’s safety factor and for marine growth respectively.

The special characteristic of this cable compared to similar cables in bottom fixed foundations is

the dynamic behaviour imposed by the floater. The 38/66(72.5) kV dynamic cables are

designed to interconnect floating wind turbines (inter array application).

12.1 Design principles

The power cable with end terminations (connectors, bend stiffeners) shall be designed with

respect to relevant load cases related to both installation and operation (i.e. in service).

Some of the load cases related to installation may turn out to be governing with respect to

design of the power cable and/or its terminations, conservative estimates of installation loads

should be applied at the design stage of the power cable system in order to ensure that it is

installable.

The power cable system shall be checked for the following limit states during both installation

and operation:

Ultimate Limit State (ULS)

Accidental Limit State (ALS)

Fatigue Limit State (FLS).

12.2 Functional Requirements

The power cable shall, at a minimum, be designed to meet the following general requirements:

Power transmission requirements: 12 MW. It is noted that this corresponds to the power

generated by one turbine and that inter array cables transmitting power from

subsequent turbines in a string should have a suitable cross section (e.g. for five

turbines in a string, the last cable will transmit 60 MW). In the case of the export cable

from substation, a higher voltage may be required.

Enable installation by a vessel and capable to withstand cable pull in operations in the

installation sea states defined in previous sections.




78

Operate at the specified configuration (to be defined), within the specified ambient

temperature range and the metocean conditions stated in section 6.

Enable one recovery and reinstallation, at a minimum, in order to account for

unforeseen events during the installation operation.

Ensure corrosion protection of the power cable components throughout the 25 years’

service life of the power cable.

Function as intended for the duration of its intended service life (Fatigue Assessment).

The cable supplier may consider the following in designing the power cable:

Wear and abrasion

Corrosion

Material creep

Ageing

Marine growth

12.3 Cable and Ancillary Components Specification

To accommodate the functional requirements during installation and operation, several

ancillaries are required:

Chinese finger: Pull in head

Hang off termination

Bend stiffeners

Intermediate clamps and buoys

Weak link disconnection system

12.4 Analysis Methodology

Within the scope of WP4 is to perform integrated modeling and global performance to ensure

that the power cable is validated to withstand the loadings during service life. The Analysis

Methodology followed in this project is summarized in the following figure.




79

FIGURE 12.25. ANALYSIS METHODOLOGY FOR DYNAMIC SUBMARINE CABLES

First step of the development concept is to apply the installation and environmental conditions

on an initial cable design. Next comes the numerical modeling, the results of which will be input

for the physical testing. Successful pass of these two approaches concludes to an approval of

the cable design for the specific application and validation of the numerical model. Otherwise, a

reconsideration of the cable design and/or the numerical model should be performed.

12.5 Loads and Loads Effects

Numerical modeling is essential in order to check the integrity of the power cable design for the

specific application (floater, mooring system, metocean data). A thorough presentation of the

computational flowchart followed in this work is illustrated in the following figure.




80

FIGURE 12.26. NUMERICAL MODEL SETUP

The mechanical load analysis for dynamic submarine power cable under specific installation

and environmental conditions is commonly carried out in two distinctive steps:

1. Global load analysis: The forces and motions acting on the power cable, induced

through the combined effect of the metocean environment and the aero-hydrodynamic

response of the floating structure are estimated.

2. Local analysis performed via Finite Element Analysis that seeks to determine the local

stresses’ distribution within the cross-section of the cable.

Tension and bending stress factors for various components, resulting from Local Analysis, are

then combined with the corresponding S – N curves to calculate the accumulated damage and

predict the Design Fatigue Life. In all cases, a safety factor of 10 is considered as DNVGL-RP-

F401 suggests. The design concept in terms of the fatigue life is a dynamic cable withstand of

25 years (CIGRE TB 623). If this condition is fulfilled, an approval of the cable design and

configuration is achieved.

12.6 Mechanical Characteristics

The mechanical characteristics of the dynamic cable are defined with the use of the cable

industry standard software tool UFLEX. UFLEX is a special purpose 2.5D Finite Element

Analysis program system for non-linear stress analysis of complex cable and umbilical cross-

sections developed by SINTEF. Its capabilities involve the modeling of complex cross-section

geometries, contact and friction stresses, non-linear relation of curvature and bending moment




81

and non-linear material models (https://www.sintef.no/globalassets/sintef-

ocean/pdf/uflexfactsheet_2017_des.pdf).

The Young’s modulus and Poisson’s ratio for each material is presented in the following table.

Material Young's Modulus (GPa) Poisson's ratio

Galvanized steel 200 0.3

GFRP 50 0.3

AL 68.5 0.35

CU 115 0.35

XLPE 0.5 0.4

MDPE 0.5 0.4

HDPE 0.8 0.4

Fillers 0.5 0.4

TABLE 56 MATERIAL PROPERTIES

The first step of the local analysis will involve the modeling of the entire cable cross section

(Figure 12.27). The model will take into account the multi-layered construction of the cable and

the complex interaction between these layers whilst under load. The stiffness properties,

material stresses and strains as well as load sharing amongst the components will be

determined by subjecting the cross section to different load cases.




82

FIGURE 12.27. 66KV DYNAMIC CABLE CROSS-SECTION.

On the figure above:

1) Aluminum round stranded compacted class 2 according to IEC 60228 of nominal cross-

section equal to 240 mm2, longitudinally water sealed.

2) Insulation: tree retardant XLPE super-clean.

3) Metallic screen: copper wires (CWS) or copper tape (CTS).

4) PE sheath.

5) Fillers between cores in order to give the cable a circular cross-section.

6) PE sheath.

7) GFRP braid armouring.

8) PE sheath.

9) FO cable.

The results related to the mechanical properties of the cable, calculated from Local Analysis,

which will be used in Global Analysis (ULS, ALS & FLS) are:

1) Axial, Torsional and non-linear Bending stiffness

2) Capacity Curve

3) Stress Factors




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12.7 Design Resistance and Design Criteria

The capacity curve of the cable will be calculated considering maximum copper conductor

stress of 80MPa. For the dynamic submarine power cable, concerning ULS and ALS, the

general design criterion for the components is not to extend their Yield Strength while for the

FLS is to withstand 25 years’ service life.




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13 LIST OF HOLDS

The following is a list of issues or matters that are yet to be completed or defined:

Operating and maintenance philosophy (3.2). To be completed by Cobra, Innosea,

UNEXE and/or Esteyco.

Active heave compensation system (3.4). To be completed by Adria Winch.

Active ballast system (3.5). To be completed by Cobra and Esteyco.

Inflatable bags definition and the anchoring to the concrete slab (5.6). To be completed

by Aimplas.

Maximum excursions of the platform (9.2.2). To be confirmed by Fulgor and Adria

Winch.

Consequence Class of the floaters (11.1). To be decided by Cobra.

d.4.1 – structural and naval architecture ......gz curves verification criterion according to...

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