Study of the functional design of a floating offshore breakwater

These days, green energy is getting more and more attention in our society, and with this, the construction of offshore wind farms is gaining interest. With the development of these farms, a need for constant maintenance is created. This means a constant presence of maintenance vessels, crew boats, and equipment in the wind farm area will be necessary. In view of this, it is interesting to investigate the concept of an offshore shelter location. This location would have two main functionalities: a sheltering location for the vessels, and a logistic function. One solution to this problem could be the creation of an offshore harbour based on floating breakwaters (FB). This option is investigated in this dissertation. The starting point of this report is the determination of the hydraulic and structural boundary conditions. Hydraulic boundary conditions were obtained by analyzing time series of measured wave heights and directions, provided by IMDC. Structural boundary conditions were determined based on the new offshore support vessel presented by Offshore Wind Assistance N.V. (OWA). After these boundary conditions are defined, a preliminary design, based on previous research, is made. This preliminary design is then modeled in MILDwave software, which lead to an optimization of the FB length, and a study of different FB layouts. Since the motions of the FB need to be limited to assure safe working conditions, a motion analysis is performed using AQUA+ software. From this it will become clear that there will be a problem regarding the limitations of these motions. In view of these findings, the design of a heave FB is proposed.

Karen Merlevede

breakwater

Study of the functional design of a floating offshore

Academiejaar 2011-2012

Faculteit Ingenieurswetenschappen en Architectuur

Voorzitter: prof dr ir Julien De Rouck

Vakgroep Civiele Techniek

Master in de ingenieurswetenschappen: bouwkunde

Masterproef ingediend tot het behalen van de academische graad vanBegeleiders: ir Vicky Stratigaki, Piet Haerens (IMDC)

Promotor: prof dr ir Peter Troch

Karen Merlevede

breakwater

Study of the functional design of a floating offshore

Academiejaar 2011-2012

Faculteit Ingenieurswetenschappen en Architectuur

Voorzitter: prof dr ir Julien De Rouck

Vakgroep Civiele Techniek

Master in de ingenieurswetenschappen: bouwkunde

Masterproef ingediend tot het behalen van de academische graad vanBegeleiders: ir Vicky Stratigaki, Piet Haerens (IMDC)

Promotor: prof dr ir Peter Troch

- Lao Tzu

de Schoesitter voor de aangename babbels op IMDC en de info over moonpools ed EvertLataire wil ik bedanken om mij als bouwkundig studentje in te leiden in een stukje van demaritieme wereld Verder wil de belangrijkste mensen in mijn leven bedanken, mijn familie Inhet bijzonder mijn ouders, voor hun onvoorwaardelijke steun en omdat ze mij de kans hebbengegeven om burgerlijk te gaan studeren Mijn zus, Anne, voor het tussen-thesis-door-tripje

en de vele tips over maritieme toepassingen, Torretje, voor de uitleg over verankeringen, mijnbroer Stijn, voor de fietstelefoontjes! Ook bedankt aan de BWC, het waren aangename mid-dagen op de magnel Ann sorry als ik je gefrustreerd heb door teveel snipperdagen te nemen!Verder wil ik ook een zot leuk mannetje bedanken, Wally! Danku om altijd te luisteren naarmijn ’zottigheid’, ik ben blij dat je onder mij woont! Djanxke, bedankt om zo geduldig te zijn.Dat kan niet altijd even gemakkelijk zijn, maar voor familie heb je natuurlijk wel iets over!And last but not least, wil ik mijn partner in crime, Bo, bedanken omdat we samen altijdzulke goede mopjes en plannen maken Ooit gaan we de nacho’s terugvinden!

Copyright

The author gives permission to make this master dissertation available for consultation and

to copy parts of this master dissertation for personal use In the case of any other use, thelimitations of the copyright have to be respected, in particular with regard to the obligation

to state expressly the source when quoting results from this master dissertation

ii

of a floating offshore breakwater

byKaren Merlevede

Master dissertation submitted in order to obtain the academic degree of

Master of Civil Engineering (major Water and Transportation)

Head supervisor: Prof Dr Ir P TrochSupervisor: Ir P Haerens

Department of Civil EngineeringHead of department: Prof Dr Ir J De Rouck

Faculty of EngineeringGhent UniversityAcademic year: 2011–2012

The starting point of this report is the determination of the hydraulic and structural boundaryconditions Hydraulic boundary conditions were obtained by analyzing time series of measuredwave heights and directions, provided by IMDC Structural boundary conditions were deter-mined based on the new offshore support vessel presented by Offshore Wind Assistance N.V.(OWA)

After these boundary conditions are defined, a preliminary design, based on previous research,

is made This preliminary design is then modeled in MILDwave software, which lead to anoptimization of the FB length, and a study of different FB layouts

Since the motions of the FB need to be limited to assure safe working conditions, a motionanalysis is performed using AQUA+ software From this it will become clear that there will

be a problem regarding the limitations of these motions In view of these findings, the design

Study of the functional design of a floating offshore

breakwater

Karen MerlevedeSupervisor(s): Peter Troch, Piet Haerens

Abstract — This article discusses a theoretical approach in the design of

an offshore floating breakwater (FB) The choice of hydraulic and

struc-tural boundary conditions is discussed, after which a preliminary design is

made This design is then optimized using MILDwave software A motion

analysis is performed with AQUA+ software Finally, a design for a heave

floating breakwater is proposed.

Keywords —floating breakwater, offshore wind farms, offshore harbour,

heave floating breakwater

I I NTRODUCTION

NOWADAYS , green energy is getting more and more

atten-tion in our society The European directive 2009/28/EC [1]

states that Belgium needs to obtain 13% of the electricity

con-sumption from renewable energy sources by 2020 To

accom-plish this, the installation of offshore wind farms (OWF) is

gain-ing interest With the development of these offshore wind farms,

a need for constant maintenance is created This means a

con-stant presence of maintenance vessels, crew boats, and

equip-ment in the wind farm area will be necessary In view of this,

it is interesting to investigate the concept of an offshore shelter

location This location would have two main functionalities: a

sheltering location for the vessels, and a logistic function One

solution to this problem could be the creation of an offshore

har-bour based on floating breakwaters (FB).

II H YDRAULIC BOUNDARY CONDITIONS

Time series of registered wave heights and directions over a

period of 20 years have been provided by IMDC By

construct-ing several JAVA tools, this data was analyzed usconstruct-ing ACES

soft-ware, and afterwards presented graphically in excel The

bound-ary conditions will be determined for two cases.

• Case 1: working conditions, for which 95% workability in

normal weather conditions is intended in this design These

boundary conditions will be used for the preliminary design and

the motion analysis It is noted that case 1 circumstances also

as-sume that waves incident perpendicular to the longitudinal axis

of the FB The design wave height and period are different for

each direction However, it is seen that most waves are coming

from the SW direction, which is why the FB will be oriented

per-pendicular to this direction The design wave height and period

for case 1 circumstances will be those of the SW direction In

the analysis of possible FB layouts, the individual wave heights

and periods per direction will be taken into account.

• Case 2: a storm with a return period of 50 years, used for the

design of the mooring system.

Table I summarizs the applied hydraulic boundaries both case

1 and 2 Water level, wind and current speed are only important

K Merlevede is with the Civil Engineering Department, Ghent University

(UGent), Gent, Belgium E-mail: Karen.Merlevede@gmail.com

in the the design of the mooring system and are therefore only determined for case 2 conditions.

TABLE I: Hydraulic boundaries

Case 1 Case 2

H des 2,5 m 5,0 m

T des 9 s 10 s Waterlevel - 6,25 m TAW Wind speed - 25 m/s Current speeds - 1 m/s Return period - 50 y

III S TRUCTURAL BOUNDARY CONDITIONS

III-A Design vessel

In [2], a new Offshore Wind Assistance (OWA) support vessel

is presented This vessel will not only be used for crew transfer, but also for seabed survey, scour monitoring and cable inspec- tion, etc It has a beam over all of 10,04 m, a length over all

of 25,75 m, and a draught of 1,75 m The preliminary design will be influenced by the design vessel in length It will be as- sumed that only one OWA vessel will be mooring at the floating breakwater, and that this requires a minimum length of 50 m III-B Safe working criteria

The criteria to ensure safe working conditions are listed here.

• The waves on the lee side of the structure need to be ated to 1 m to ensure a sheltering environment [3] The direc- tional maximum value of C can be determined by dividing 1 m

attenu-by the directional design wave height.

• The heave motion needs to be limited to 1m, the roll motion

to 5° and the pitch motion to 1° [4].

• The wave overtopping needs to be limited to 0,01 m 3 /m/s [5].

IV P RELIMINARY D ESIGN

The preliminary design will be based on case 1 boundary conditions Two processes of energy transportation are impor- tant for the preliminary design: diffraction and transmission Diffraction considerations will lead to an optimal length, while transmission will lead to an optimal width/draught ratio IV-A Diffraction

Diffraction can be quantified using Wiegel diagrams [6] However, these are developed for semi-infinite breakwaters In this case of an offshore floating breakwater, the gap method

as described in the Shore Protection Manual [7] is applicable.

IV-B Transmission

[8], [9], [10], and [11] developed approaches to quantify the

transmssion process These approaches are applied to testcases

by [12], [13], and [14] From this it is found that the equation

by [11] provides the most accurate results The transmission

according to [11] is given by

C t = gT

2 sinh(k(d − D)) 2π 2 (W + d tanh(3, 5Dd)) cosh(kd) (1) Using this equation, and assuming an initial width of 40 m,

leads to a minimum draught of 8 m.

IV-C Overtopping

The mechanism of overtopping will determine the necessary

freeboard of the structure Using equations proposed by [15] and

the limitations for overtopping discharge leads to a minimum

freeboard of 4 m.

V MILD WAVE M ODEL

MILDwave [16] is a wave propagation model based on the

depth-integrated mild-slope equations of [17].

To model an object in the wave field, the cells are assigned a

certain absorption coefficient (S) This coefficient ranges from

zero to one; zero meaning the cell consists out of water, and

one meaning the cell is fully reflective and does not absorb any

energy The difficulty is that MILDwave does not offer a specific

input for floating objects [18] studied the layout of a farm of

floating wave energy converters (WEC) using MILDwave, and

found that the best way to model a floating object is to assign a

linearly varying S over the width of the structure This approach

is verified by applying this technique to the same testcases that

were used to determine the best applicable equation.

VI S TUDY OF THE LAYOUT USING MILD WAVE

Three types of FB layout will be modeled in MILDwave A

beam shape, an L shape, and a U shape.

VI-A Beam shaped FB

The beam shaped structure is oriented perpendicular to the

SW direction, where most waves are coming from The results

show that the length of the FB can be reduced to 150 m.

VI-B L-shaped FB

Since the fourth quadrant on the wind rose also produces

rel-atively high waves, an L-shape is the subject of this section.

Three types in particular are studied: L/150/150, L/150/100, and

L/100/100 The first number stands for the length of the side

perpendicular to the SW, while the second number is the length

of the leg perpendicular to the NW The L/150/150 layout

atten-that C is well beneath the maximum allowable value for SSE-W directions This is why the last L-shape modeled is L/100/100.

In this case, the FB is efficient for waves coming from the NWN segment However, waves coming from the first quadrant are not attenuated sufficiently This is why the U-shaped FB will

SSW-be studied in the next section.

In every L-shaped layout, problems with reflecting waves are present This is the case when waves are attacking the leeward side of the structure The reflection decreases when reducing the length of the legs The asymmetrical layout showed the most negative reflection properties Generally, the L/100/100 layout was found to be the most satisfying.

VI-C U-shaped FB The final layout modeled, is a U-shaped FB of which the par- allel sides measure 100 m, and the connecting side 155 m This layout offers sufficient attenuation for waves coming from the

SW to the NE However, for waves coming from the south, the attenuation is significantly lower that in the case of an L-shape This is because the waves are reflected inside the U-shape, am- plifying the resulting wave heights For waves coming from the SSE, SE, and ESE, the resulting wave heights are even higher than the incoming wave heights A comparison between the beam shaped FB, the U-shaped FB and the L/100/100 config- uration is shown in figure 1.

Fig 1: Comparison between the beam shaped FB, L/100/100, and U-shaped FB

This figure shows that the directions of sufficient attenuation and the directions for which C exceeds one are different for each layout It is found that the wave amplifying directions in the case

of L/100/100 are more harmful than in the case of the U-shaped

FB, because the incoming waves are smaller in the latter case Nonetheless, this reflection is to be damped as much as possible,

VII M OTION A NALYSIS

The motions of the FB need to be limited [4] states that three

motions in particular have to be studied: heave, roll, and pitch.

The period of resonance of floating bodies for these motions is

usually found somewhere between 5 s and 20 s, an interval that

also contains the design wave periods.

The motion analysis is performed using AQUA+ software, and

results in Response Amplitude Operators, or RAO’s They are

defined by [19]

Response(t) = (RAO)η(t) (2) where η(t) is the wave profile as a function of time, t The

calculations were performed for different wave incidences: 0°;

22,5°; 45°; 67,5°; and 90° The results of this analysis are listed

here.

• The maximum heave RAO amplitude equals 1,61 m/m for a

wave period of 10 s; case 1 conditions will result in a heave

motion of 4 m,

• the maximum pitch RAO amplitude equals 1,18 °/m for a

wave period of 10 s; case 1 conditions will result in a pitch

mo-tion of 2,95°,

• the maximum roll RAO amplitude equals 2,2 °/m for a wave

period of 9 s; case 1 conditions will result in a roll motion of

5,5°.

None of these motions fall within the limits proposed by [4].

A mooring line anchoring system will not be able to restrain

these motions sufficiently Reducing the motions is possible by

changing the FB layout, adding a moonpool, or adding a skirt.

Furthermore, the mooring system can be designed in such a way

that the motions can be restrained Two possible alternatives are

proposed: a tension leg mooring system, and a heave FB The

latter will be researched extensively in the next section.

VIII H EAVE F LOATING B REAKWATER

[12] performed research on this type of FB, and compared it

to a regular fixed breakwater According to his research a heave

FB will always be more efficient than the fixed type because of

the extra damping by the heave motion itself, causing additional

loss of wave energy In this section, the piles will be designed to

make sure their dimensions are realistic First the forces acting

on the FB and the piles need to be determined In both cases

these are forces due to wind, current and waves.

VIII-A Forces acting on the floating breakwater

VIII-A.1 Wind and current

Wind and current forces are calculated using the approach

de-scribed in [20] This leads to a wind force of 485 kN and a

cur-rent force of 1 231 kN The approximating points of application

are 38,25 m and 32,25 m above the sea bed, respectively.

VIII-A.2 Wave forces

Wave forces are calculated by the Froude-Krylov theory as

described in [19] However these equations are only valid

for fully submerged objects, which leads to an overestimation.

The approach by Goda [21] delivers a more realistic result,

93 949 kN, with a point of application of 34,16 m above the sea

VIII-B.2 Wave forces According to [22], wave forces on piles can be calculated us- ing the Morison equation This leads to a total wave force of

1 323 kN, with a point of application of 19,70 m above the sea bed.

VIII-C Pile design Assuming 6 piles are present in the design of the heave FB, means each pile will take on 1/6 of the total force acting on the

FB itself The total bending moment for one pile at the sea bed equals 573 839 kNm.

VIII-C.1 Wall thickness Using the approach described in [23] a wall thickness of 0,08 m can be determined.

VIII-C.2 Penetration depth

In [23] methods of Vandepitte [24] are described to determine the penetration depth Following this approach leads to a mini- mum depth of 28 m below the sea bed.

VIII-C.3 Pile length The total pile length consist out of the penetration depth, the water depth, and the extreme water level This leads to a total length of 68,25 m.

VIII-C.4 Results The design for the heave floating breakwater is shown in fig- ure 2.

IX C ONCLUSIONS AND R ECOMMENDATIONS

In this text, a design is proposed for a heave FB Although roll and pitch motions are restrained in this concept, the heave motion is not A system will need to be designed to allow safe mooring at the FB, despite these up- and downward motions Alternatively research can be done on how to restrain the heave motion completely.

Wave basin experiments are strongly advised, since the proach in this text is purely theoretical Only experimental ob- servations can map the behaviour of different layouts, wave in- cidences, etc.

ap-R EFERENCES [1] European Parliament and Council, “Directive 2009/28/ec of the european parliamant and of the council,” Official Journal of the European Union,

pp 16 – 61, April 2009.

[2] the GeoSea Newsflash, “Owa fast crew transfer vessel,” Stan ers, p 11, 2011.

Messemaek-[3] J De Rouck, Zee- en Havenbouw, Universiteit Gent, 2011.

[4] Pianc, “Criteria for movements of moored ships in harbours,” Supplement

to bulletin n° 88, 1995.

Fig 2: Heave Floating Breakwater

[5] T Pullen, NWH Allsop, T Bruce, A Kortenhaus, H Sch¨uttrumpf, and

JW van der Meer, “Wave overtopping of sea defences and related

struc-tures: Assessment manual,” Die K¨uste: Archive for research and

technol-ogy on the north sea and baltic coast, 2007.

[6] R.L Wiegel, “Diffraction of waves by semi-infinite breakwaters,” Journal

of Hydraulic Div., 1962.

[7] Corps of Engineers US Army, Shore Protection Manual, Coastal

Engi-neering Research Center, 1984.

[8] E.O Macagno, “Experimental study of the effects of the passage of a wave

beneath an obstacle,” Proceedings of Acad´emie des Sciences, Paris, 1953.

[9] D.B Jones, “Transportable breakwater - a survey of concepts,” Naval

Civil Engineering Laboratory, 1971.

[10] J.J Stoker, Water waves The mathetmatical theory with applications,

In-terscience Publishers New York, 1957.

[11] H Wagner, A G¨otz, R Reinsch, and HJ Kaiser, “Schwimmende

wellen-brecher im einsatz in einem tagenbaurestsee mitteldeutschlands,”

Binnen-schifffahrt ZfB, 2011.

[12] E Tolba, Behaviour of Floating Breakwaters under Wave Action, Ph.D.

thesis, Bergische Unversit¨at, 1999.

[13] E K Koutandos and C Koutitas, “Floating breakwater response to wave

ac-tion using a boussinesq model coupled with a 2dv elliptic solver,” Journal

of Waterway, Port, Coastal and Ocean Engineering, pp 243–255, 2004.

[14] T Nakamura, N Mizutani, N Hur, and D S Kim, “A study of the layout of

floating breakwater units,” in proceedings of The International Offshore

and Polar Engineers Conference, 2003.

[15] C Franco and L Franco, “Overtopping formulas for caisson breakwaters

with nonbreaking 3d waves,” Journal of waterway, port, coastal and ocean

engineering, pp 98–108, march/april 1999.

[16] P Troch, V Stratigaky, and L Baelus, “Reference manual of mildwave,”

2011.

[17] AC Radder and MW Dingemans, “Canonical equations for almost

peri-odic, weakly nonlinear gravity waves.,” Wave motion, pp 473–485, 1985.

[18] Charlotte Beels, Optimization of the Lay-Out of a Farm of Wave Energy

Converters in the North Sea, Ph.D thesis, Ghent University, 2010.

[19] S.K Chakrabarti, Hydrodynamics of Offshore Structures, WIT Press,

1987.

[20] Pianc, “Floating breakwaters, a practical guide for design and

construc-=2

Studie van het functioneel ontwerp van een

drijvende offshore golfbreker

Karen MerlevedeSupervisor(s): Peter Troch, Piet Haerens

Abstract —Dit artikel bespreekt het theoretisch ontwerp van een offshore

drijvende golfbreker (ENG: floating breakwater (FB)) De keuze voor

hy-draulische en structurele randvoorwaarden wordt besproken, waarna een

voorontwerp gemaakt wordt Dit ontwerp wordt dan geoptimaliseerd met

behulp van MILDwave software Een bewegingsanalyse wordt uitgevoerd

met behulp van AQUA+ software Uiteindelijk wordt een finaal ontwerp

voor een heave floating breakwater voorgesteld.

Keywords — drijvende golfbreker, offshore windmolenparken, offshore

haven, heave floating breakwater

I I NLEIDING

DEZER dagen is groene energie niet meer weg te denken

uit onze maatschappij De Europese richtlijn 2009/28/EC

stelt dat Belgi¨e 13% van zijn energieconsumptie uit

hernieuw-bare bronnen moet halen tegen 2020 [1] Het is niet

verwon-derlijk dat offshore windmolenparken meer en meer interesse

opwekken De ontwikkeling van deze parken, brengt een

con-stante nood aan onderhoud met zich mee In dit opzicht kan het

interessant zijn om offshore een schuilhaven te voorzien Deze

kan meteen ook een logistieke functie hebben Een mogelijke

oplossing voor dit vraagstuk is de aanleg van een offshore

drij-vende golfbreker waaraan de schepen kunnen afmeren.

II H YDRAULISCHE RANDVOORWAARDEN

Tijdreeksen van geregistreerde golfhoogtes en -richtingen

over een periode van 20 jaar werden aangereikt door IMDC.

Deze data werd geordend door middel van verschillende tools,

geprogrammeerd in JAVA, waarna ze geanalyseerd werd in

ACES De randvoorwaarden worden bepaald voor twee

speci-fieke gevallen.

• Case 1: werkomstandigheden, waarbij 95% werkbaarheid

wordt beoogd in normale weersomstandigheden Deze

rand-voorwaarden zullen gebruikt worden bij het voorontwerp van

de FB, en de bewegingsanalyse Hierbij wordt opgemerkt dat

case 1 omstandigheden overeenkomen met het geval waarbij

golven loodrecht op de langse as van de FB invallen De

on-twerpgolfhoogte en -periode zijn verschillend voor elke richting,

maar omdat in de golfanalyse opgemerkt werd dat de meeste

golven uit de ZW richting komen, wordt de golfbreker

lood-recht op deze richting georienteerd Daarom zijn voor case 1 de

ontwerpgolfhoogte en -periode voor deze richting aangenomen.

In de analyse van de FB layout (zie verder), wordt echter

reken-ing gehouden met de ontwerpgolfhoogte en -periode voor elke

richting afzonderlijk.

• Case 2: extreme weersomstandigheden, een storm met

re-tourperiode 50 jaar, gebruikt voor het ontwerp van de

ver-ankeringen

K Merlevede, Civil Engineering Department, Ghent University (UGent),

Gent, Belgium E-mail: Karen.Merlevede@gmail.com

Tabel I vat de randvoorwaarden voor case 1 en 2 samen ter niveau, wind- en stroomsnelheid zijn enkel belangrijk in het ontwerp van de verankering en worden dan ook niet bepaald voor case 1.

Wa-TABLE I: Hydraulische randvoorwaarden

Case 1 Case 2

H des 2,5 m 5,0 m

T des 9 s 10 s Waterniveau - 6,25 m TAW Windsnelheid - 25 m/s Stroomsnelheid - 1 m/s Retourperiode - 50 j

III S TRUCTURELE RANDVOORWAARDEN

III-A Ontwerpschip

In [2], wordt een nieuw onderhoudsschip voorgesteld, twikkeld door OWA (Offshore Wind Assistance) Dit schip zal instaan voor crew transfers, zeebodem inspectie, erosie inspec- tie, kabel inspectie, enz Het heeft een LOA van 25,75 m, BOA van 10,04 m, en een diepgang van 1,75 m Het voorontwerp wordt be¨ınvloed door het ontwerpschip in die zin dat er een min- imale lengte zal vereist zijn om het schip veilig te laten afmeren Deze minimale lengte wordt hier vastgelegd op 50 m.

• het dompen moet beperkt worden tot 1 m, rollen tot 5° en stampen tot 1° [4],

• golfovertopping moet beperkt worden tot 0,01 m 3 /m/s [5].

IV V OORONTWERP

Het voorontwerp wordt gemaakt op basis van case 1 voorwaarden Twee processen van energieoverdracht zijn hier van belang; diffractie en transmissie De diffractie zal een opti- male lengte van de FB bepalen, terwijl transmissie resulteert in een optimale breedte/diepgang verhouding.

rand-Het diffractiefenomeen kan in kaart gebracht worden aan de

hand van Wiegel diagrammen [6] Omdat deze ontwikkeld

zijn voor half oneindige golfbrekers, wordt de ’gap methode’

toegepast voor offshore golfbrekers die beschreven wordt in de

Shore Protection Manual [7] Hiermee kan een minimale lengte

bepaald worden van 225 m.

IV-B Transmissie

[8], [9], [10] en [11] ontwikkelden methodes voor het

fenomeen van transmissie Door deze aanpakken toe te passen

op een aantal testcases ([12], [13] en [14]), gekozen omwille van

hun gelijkaardige hydraulische randvoorwaarden, kan bepaald

worden welke formule het meest van toepassing is in dit geval.

Er wordt besloten dat de formule door [11] het meest van

toepassing zal zijn De transmissie wordt dan beschreven door

C t = gT

2 sinh(k(d − D)) 2π 2 (W + d tanh(3, 5 D

d )) cosh(kd) (1) Aan de hand van deze formule, en een breedte van 40 m

vooropstellend, wordt een minimum diepgang berekend van

8 m.

IV-C Overtopping

Overtopping bepaalt de vrijboord van de constructie Gebruik

makend van de vergelijkingen opgesteld door [15] en de

limi-eten opgesteld door [5], wordt een minimum vrijboord

gevon-den van 4 m.

V MILD WAVE M ODEL

MILDwave [16] is een golfvoortplantingsmodel gebaseerd

op de diepte-ge¨ıntegreerde mild-slope vergelijkingen van [17].

Om een object in het golfveld te modelleren wordt er aan de

cellen een bepaalde absorptieco¨efficient (S) toegekend Deze

kan gaan van 0 tot 1, waarbij 0 staat voor een watercel en 1

voor een volledig reflectieve cel Er bestaat echter geen

een-duidige manier om drijvende objecten te modelleren in

MILD-wave [18] bestudeerde hoe een wave energy convertor (WEC)

gemodelleerd kan worden door experimentele testen te

vergeli-jken met MILDwave output Haar bevindingen tonen dat het

lineair laten vari¨eren van S de beste methode is in het geval

van WEC Deze aanpak wordt gestaafd door het modelleren van

dezelfde testcases die gebruikt werken om de ontwerpformule

voor tranmissie te bepalen.

VI S TUDIE VAN DE LAYOUT AAN DE HAND VAN

MILD WAVE

Drie mogelijkheden voor de FB layout zullen gemodelleerd

worden in MILDwave: een balkvorm, een L-vorm en een

U-vorm.

VI-A Balkvorm

Het voorontwerp wordt loodrecht op het ZW gemodelleerd,

Gezien het vierde kwadrant van de windroos ook relatief hoge golven voortbrengt, wordt een L-vorm bestudeerd In het bij- zonder worden drie types onderzocht: L/150/150, L/150/100 en L/100/100 Hierbij staat het eerste getal steeds voor de lengte van de zijde loodrecht op het ZW, en het tweede voor de zij-

de loodrecht op het NW De L/150/150 configuratie dempt ven uit het segment ZZO-N voldoende Er wordt ook opge- merkt dat de dempingsco¨effici¨ent voor het segment ZW-N vaak slechts de helft bedraagt van de maximaal toeglaten waarden Daarom wordt L/150/100 gemodelleerd Deze layout volstaat voor dezelfde richtingen als de L/150/150, met uitzondering van het noorden Het is opnieuw duidelijk dat C zich onder de max- imaal toegelaten waarde bevindt voor de richtingen ZZO-W Daarom wordt als laatste L-vorm gekozen voor L/100/100 In dit geval is de FB efficient voor golven uit het ZZO-NWN seg- ment Golven uit de noord-oostelijke richtingen worden echter niet voldoende gedempt, waardoor een U-vorm gemodelleerd zal worden in een latere fase.

gol-In elke vorm zijn problemen met reflectie zichtbaar als golven invallen op de lijzijde van de structuur Er wordt wel opgemerkt dat de reflectie afneemt als de lengte van de benen daalt in de symmetrische configuraties De asymmetrische layout zal de meest negatieve reflectie opleveren Algemeen gezien wordt de L/100/100 layout het meest bevredigend bevonden De richtin- gen waarvoor de golven voldoende gedempt worden komen min

of meer overeen met L/150/150, en de reflectie is ook lager VI-C U-vorm

Als laatste layout wordt een U-vorm bestudeerd waarvan

de evenwijdige benen 100 m meten, en de verbindende zijde

155 m Deze structuur biedt voldoende bescherming tegen ven komend uit het ZW tot het NO Echter, voor golven uit het zuiden zal de demping aanzienlijk lager zijn dan in het geval van een L-vorm Dit komt doordat de golven binnen in de U- vorm gereflecteerd worden, waardoor een onrustig golfklimaat ontstaat Voor golven komend uit het ZZO, ZO en OZO zijn de resulterende golfhoogtes groter dan de invallende Een vergeli- jking tussen deze U-vorm en de L/100/100 vorm wordt getoont

gol-in figuur 1.

doende gedempt worden en de richtingen waarvoor C groter

is dan ´e´en verschillen voor de drie configuraties Er wordt

besloten dat de invalsrichtingen waarvoor de golven versterkt

worden meer nefast zijn in het geval van L/100/100 dan voor

de U-vorm Dit is zo omdat de invallende golfhoogtes in het

laatste geval kleiner zijn Niettemin moet deze reflectie zoveel

mogelijk gedempt worden, bijvoorbeeld door het toevoegen van

absorberende inrichtingen aan de lijzijde van de structuren Er

wordt aangeraden om dit probleem van reflectie theoretisch en

experimenteel te onderzoeken.

VII B EWEGINGSANALYSE

De bewegingen van de FB moeten beperkt worden om de

vei-ligheid te waarborgen [4] stelt dat drie van de zes mogelijke

be-wegingen bestudeerd moeten worden; dompen, rollen en

stam-pen De reden hiervoor ligt in het feit dat de natuurlijke periode

voor deze drie bewegingen tussen 5 en 20 s te vinden is, m.a.w.

een interval dat ook de ontwerpperiodes omvat De beweging

van de FB wordt bestudeerd met AQUA+ software; wat

resul-teert in Response Amplitude Operators of RAO’s Deze worden

gedefineerd door [19]

Response(t) = (RAO)η(t) (2) met η(t) het golfprofiel in functie van de tijd, t De resultaten

worden hieronder opgesomd.

• De maximale RAO voor dompen bedraagt 1,61 m/m voor een

periode van 10 s; toegepast op case 1 randvoorwaarden

resul-teert dit in een dompbeweging van 4 m,

• de maximale RAO voor stampen bedraagt 1,18 °/m voor een

periode van 10 s; toegepast op case 1 randvoorwaarden

resul-teert dit in een stampbeweging van 2,95 °,

• de maximale RAO voor rollen bedraagt 2,2 °/m voor een

peri-ode van 9 s; toegepast op case 1 randvoorwaarden resulteert dit

in een rolbeweging van 5,5 °.

Geen enkele van deze bewegingen valt binnen de limieten

voorgesteld door [4] Een traditioneel verankeringssysteem met

ankerlijnen zal niet in staat zijn deze bewegingen voldoende

tegen te houden De bewegingen kunnen eventueel beperkt

wor-den door de layout van de golfbreker aan te passen, door het

to-evoegen van een moonpool, of door toto-evoegen van een skirt Dit

zijn aanpassingen aan het ontwerp van de golfbreker zelf De

be-wegingen kunnen ook tegengehouden worden door het ontwerp

van het verankeringssysteem Twee opties worden hier vermeld:

een tension leg mooring systeem, en een heave floating

break-water De laatste van deze twee wordt hierna meer in detail

besproken.

VIII H EAVE F LOATING B REAKWATER

In het onderzoek van [12] werd duidelijk dat een heave

float-ing breakwater beter presteert dan een vaste drijvende

golf-breker Dit fenomeen wordt toegeschreven aan het feit dat het

induceren en onderhouden van de heave beweging energie vergt,

waardoor er dus extra verlies aan golfenergie is Het voordeel

van een flexibele constructie gaat hier natuurlijk wel verloren In

deze paragraaf worden de palen voor dit systeem ontworpen om

te verifi¨eren of hun afmetingen realistisch zouden zijn Hierbij

is het noodzakelijk om te weten welke krachten er zullen grijpen op zowel de golfbreker als de palen zelf Deze krachten zijn het gevolg van wind, stroming en golven.

aan-VIII-A Krachten op de drijvende golfbreker VIII-A.1 Wind en stroming

Wind- en stromingskrachten worden berekend aan de hand van de methode beschreven in [20] Dit leidt tot een windkracht van 485 kN en een stromingskrachtn van 1 231 kN De aangri- jpingspunten van deze krachten bevinden zich op 38,25 m en 32,25 m respectievelijk, boven de zeebodem.

VIII-A.2 Golfkrachten Golfkrachten worden een eerste maal berekend aan de hand van de Froude-Krylov theorie, beschreven in [19] Deze meth- ode is echter enkel geldig voor volledig ondergedompelde ob- jecten, wat leidt tot een overschatting Daarom worden ze een tweede maal berekend, deze keer aan de hand van de meth- ode ontwikkeld door Goda [21] voor caisson golfbrekers, waar een meer realistisch drukverloop aangenomen wordt Dit leidt tot een golfkracht van 93 949 kN met een aangrijpingspunt van 34,16 m boven de zeebodem.

VIII-B Krachten op de palen Als paaldiameter wordt een waarde van 4,5 m aangenomen VIII-B.1 Wind en stroming

Wind- en stromingskrachten worden opnieuw berekend aan

de hand van de methode beschreven in [20] Dit resulteert in een windkracht van 91 kN en een stromingskracht van 263 kN VIII-B.2 Golfkrachten

Volgens [22] en [23] kunnen krachten op palen berekend den aan de hand van de Morison vergelijking Hiermee wordt een totale golfkracht van 1 323 kN berekend, met een aangri- jpingspunt van 19,70 m boven het zeebed.

wor-VIII-C Ontwerp van de palen

Er worden 6 palen gebruikt in het ontwerp, waardoor elke paal 1/6 van de krachten aangrijpend op de golfbreker zal opne- men De totale kracht op ´e´en paal wordt dan 17 489 kN, en het buigmoment 573 839 kNm.

VIII-C.1 Wanddikte Gebruik makend van de methode beschreven in [24] voor het ontwerp van monopiles, wordt een wanddikte van 0,08 m berek- end.

VIII-C.2 Insteekdiepte

In [24] wordt de methode van Vandepitte [25] gebruikt om

de insteekdiepte van monopile funderingen te bepalen neer dezelfde aanpak gevolgd wordt, wordt een minimale in- steekdiepte van 28 m gevonden.

Wan-De totale lengte van de paal bestaat uit de som van de

in-steekdiepte, de water diepte en het extreme waterniveau Dit

alles leidt tot een totale lengte van 68,25 m.

VIII-C.4 Resultaat

Het bekomen ontwerp wordt getoond in figuur 2.

Fig 2: Heave Floating Breakwater

IX C ONCLUSIE EN AANBEVELINGEN

In deze tekst werd een ontwerp voorgesteld voor een heave

floating breakwater Hoewel rol- en stampbewegingen hierdoor

vermeden worden, kan de structuur nog steeds dompen Er zal

een systeem moeten ontworpen worden om schepen, ondanks

deze beweging, tot veilig te laten afmeren aan de golfbreker Er

kan eventueel ook onderzocht worden wat het effect zou zijn

in-dien ook de dompbeweging tegengehouden wordt.

Algemeen worden golfbak testen aangeraden gezien de aanpak

in deze tekst zuiver theoretisch is Enkel experimentele

obser-vaties kunnen het gedrag van verschillende layouts e.d in kaart

brengen.

R EFERENCES [1] European Parliament and Council, “Directive 2009/28/ec of the european

parliamant and of the council,” Official Journal of the European Union,

pp 16 – 61, April 2009.

[2] the GeoSea Newsflash, “Owa fast crew transfer vessel,” Stan

Messemaek-tures: Assessment manual,” Die K¨uste: Archive for research and ogy on the north sea and baltic coast, 2007.

technol-[6] R.L Wiegel, “Diffraction of waves by semi-infinite breakwaters,” Journal

[10] J.J Stoker, Water waves The mathetmatical theory with applications, terscience Publishers New York, 1957.

In-[11] H Wagner, A G¨otz, R Reinsch, and HJ Kaiser, “Schwimmende brecher im einsatz in einem tagenbaurestsee mitteldeutschlands,” Binnen- schifffahrt ZfB, 2011.

wellen-[12] E Tolba, Behaviour of Floating Breakwaters under Wave Action, Ph.D thesis, Bergische Unversit¨at, 1999.

[13] E K Koutandos and C Koutitas, “Floating breakwater response to wave tion using a boussinesq model coupled with a 2dv elliptic solver,” Journal

ac-of Waterway, Port, Coastal and Ocean Engineering, pp 243–255, 2004 [14] T Nakamura, N Mizutani, N Hur, and D S Kim, “A study of the layout of floating breakwater units,” in proceedings of The International Offshore and Polar Engineers Conference, 2003.

[15] C Franco and L Franco, “Overtopping formulas for caisson breakwaters with nonbreaking 3d waves,” Journal of waterway, port, coastal and ocean engineering, pp 98–108, march/april 1999.

[16] P Troch, V Stratigaky, and L Baelus, “Reference manual of mildwave,” 2011.

[17] AC Radder and MW Dingemans, “Canonical equations for almost odic, weakly nonlinear gravity waves.,” Wave motion, pp 473–485, 1985 [18] Charlotte Beels, Optimization of the Lay-Out of a Farm of Wave Energy Converters in the North Sea, Ph.D thesis, Ghent University, 2010 [19] S.K Chakrabarti, Hydrodynamics of Offshore Structures, WIT Press, 1987.

peri-[20] Pianc, “Floating breakwaters, a practical guide for design and tion,” Supplement to bulletin n° 85, 1994.

construc-[21] Y Goda, Random Seas and the Design of Maritime Structures, World Scientific Publishing Company, 2000.

[22] J De Rouck, Offshore constructions, Universiteit Gent, 2011.

[23] M.C Deo, Waves and structures, Indian Institute of Technology, 2007 [24] L De Vos, Optimalisation of scour protection design for monopiles and quantification of wave run-up, Ph.D thesis, Universiteit Gent, 2008 [25] D Vandepitte, Berekeningen van constructies, Universiteit Gent, 1979.

=2

1.1 Framework of this master dissertation 1

1.2 Objectives of this master dissertation 2

1.3 Structure of this master dissertation 3

2 Literature Study 6 2.1 History of floating structures 6

2.2 Types of floating structures 8

2.3 Floating Breakwaters 10

2.3.1 Introduction 10

2.3.2 Types of floating breakwaters 11

2.3.3 Behaviour under wave action 14

2.3.4 Experimental Studies 16

2.3.5 Numerical Studies 19

2.3.6 Influence of different parameters 21

2.4 Conclusions 23

xii

3 Hydraulic boundary conditions 24

3.1 Introduction 24

3.2 Wave heights 25

3.2.1 Data processing 25

3.2.2 Design wave height 26

3.2.3 Results 26

3.3 Wave period 29

3.4 Water level 31

3.5 Wind speed 33

3.6 Current speed 33

3.7 Conclusions: boundary conditions for general design 35

4 Structural boundary conditions 36 4.1 Design vessel 36

4.2 Safe working criteria 37

4.2.1 Attenuated wave height 37

4.2.2 Motions of the FB 37

4.2.3 Overtopping 38

4.3 Conclusion 38

5 Preliminary Design 39 5.1 Introduction 39

5.2 Wave diffraction 40

5.2.1 Regular waves 41

5.2.2 Random waves 41

5.2.3 General approach 42

5.3 Wave transmission 42

5.3.1 Previous research 43

6.1 Introduction to MILDwave 53

6.2 Modeling floating objects in MILDwave 55

6.2.1 Previous research 55

6.2.2 Testcase 1: Tolba (1999) 56

6.2.3 Testcase 2: Koutandos et al (2005) 57

6.2.4 Testcase 3: Nakamura et al (2003) 57

6.2.5 Conclusion 59

7 Optimizing the preliminary design 60 7.1 Modeling the preliminary design 60

7.2 Study on the FB layout 62

7.2.1 Beam shaped layout 63

7.2.2 L shaped layout 65

7.2.3 U-shaped layout 69

7.3 Conclusions 72

8 Motion analysis 76 8.1 Introduction 76

8.2 Aqua+ 77

8.3 Response Amplitude Operators 77

8.4 Results 78

8.4.1 Pitch 78

8.4.2 Roll 78

8.4.3 Heave 79

8.5 Discussion and solutions 79

9 Heave Floating Breakwater 83 9.1 General concept 84

9.2 Forces acting on the floating breakwater 84

9.2.1 Wind and current 85

xiv

9.2.2 Waves 85

9.2.3 Conclusion 93

9.3 Forces acting on the piles 93

9.3.1 Wind and current 93

9.3.2 Waves 93

9.3.3 Conclusion 95

9.4 Pile design 96

9.4.1 Wall thickness 96

9.4.2 Penetration depth 97

9.4.3 Final pile design 99

9.5 Conclusion 99

10 Discussion and recommendations 101 10.1 Discussion 101

10.2 Recommendations 105

A Offshore Wind Farm concessions 108 B ACES output file 110 C Cumulative wave heights per direction 117 D Extreme wave heights per direction 126 E Diffraction diagrams 131 E.1 Regular waves: Wiegel (1962) 132

E.2 Irregular waves: Goda (2000) 133

F MILDwave testcases 134 F.1 Testcase 1: Tolba (1999) 134

F.2 Testcase 2: Koutandos et al (2005) 139

G.1 Influence of the FB length 148

G.2 Beam shaped FB 150

G.3 L/150/150 153

G.4 L/150/100 159

G.5 L/100/100 165

G.6 U shaped FB 169

xvi

C coefficient of wave attenuation Hres/Hi (-)

Cd coefficient of wave diffraction Hd/Hi (-)

Cmax maximum coefficient of wave attenuation to obtain a safe environment

Cr coefficient of wave reflection Hr/Hi (-)

Ct coefficient of wave transmission Ht/Hi (-)

FB floating breakwater

g gravitational acceleration (m/s2)

H95% wave height with 95% probablity of occurence

H1/3 significant wave height calculated from the time domain

Hd wave height after diffraction

Hi incoming wave height

Hr wave height after reflection

Ht wave height after transmission

Hm0 significant wave height calculated from the frequency domain

k dimensionless wave number (2π

L) (-)

L wave length according to Airy wave theory (m)

L0 deep water wave length gT2π2 (m)

OWA offshore wind assistance N.V

OWF offshore wind farm

q average overtopping discharge (m3/m/s)

RAO response amplitude operator

Chapter 0 List of symbols and acronyms

ρair mass density of air (1,029kg/m3)

S absorption coefficient in MILDwave (-)

TLP tension leg platform

VLFS very large floating structure

xviii

1.1 Framework of this master dissertation

Nowadays, green energy is getting more and more attention in our society The Europeandirective 2009/28/EC (Parliament and Council, 2009) states that Belgium needs to generate13% of its provision of electricity from renewable energy sources by 2020 To accomplish this,the installation of offshore wind farms (OWF) is gaining interest Actually three projectsfor the construction of OWF in the Belgian part of the North Sea are ongoing: C-Power onThorntonbank, Belwind on Bligh Bank , and Northwind on ’Bank zonder Naam’ Four othersare already planned

Thorntonbank Offshore Windfarm (C-Power) will have 55 wind turbines, located roughly 30kilometers off Zeebrugge The project started in 2007 with the construction of the first 6foundations, built using gravity base foundations These were installed in 2008 Phase 2 and

3 of the project consists of installing the remaining windmills using jacket foundations Thefirst 6 windmills have a capacity of 5 megawatt each, and are currently active The other 55windmills are under construction, and will each have a capacity of 6,15 megawatt Thorntonbank offshore wind farm will have an annual energy generation of 1 000 000 000 kWh, which

is the equivalent of the annual consumption of 600 000 inhabitants (http://www.c power.be/,2012)

The Belwind OWF has a capacity of 165 MW, and delivers energy to 330000 households/year.Located at 42 kilometers off the coast of Zeebrugge, it is the world’s most offshore locatedwind farm Its annual energy generation is estimated to be in the order of the equivalent ofthe annual energy consumption of 175 000 households

Northwind will be located on the sand bank ’Bank zonder Naam’, at 38 kilometers off thecoast, and aims to install 72 windmills with a capacity of 3 MW each

Chapter 1 Introduction

With the development of these offshore wind farms, a need for constant maintenance is ated Survey needs to be done in order to detect scour around the foundations If scour ispresent the resulting scour pits will need filling The transport cables will also need regularsurvey These surveys can be done using ROV’s or by diving inspections The foundationstructures will need to be inspected regularly as well On top of this, there will be a need, dur-ing these maintenance operations, for the supply of spare parts and containerized items (theGeoSea Newsflash, 2011), and crew transfers This means a constant presence of maintenanceships, crew boats, and equipment in the wind farm area will be necessary

cre-However, when supplies are needed, the maintenance ships have to sail to the harbour brugge or Oostende) A trip that takes at least 2 hours depending on the weather conditions,and the type of vessel In view of this it is interesting to investigate other solutions, andassess if an offshore shelter location is a valuable alternative This location would have twomain functionalities: a sheltering location for the ships, and a logistic function This way themaintenance ships can stay in position, and maintenance operations are not interrupted, whileother ships take care of the transportation of goods

(Zee-Creating an artificial island, that combines shelter for vessels doing maintenance, with otherfunctionalities, like a rescue harbour of logistic center, is one solution to answer the needs.This solution, however, is very expensive, and needs the involvement of different stake hold-ers An alternative could be the creation of a shelter harbour based on floating breakwaters.Since floating breakwaters are a common solution when working in deep water conditions, thisthesis will focus on a preliminary design for such a breakwater by performing a feasibility study

1.2 Objectives of this master dissertation

The main objective of this master dissertation is to perform a feasibility study of such a floatingstructure and propose a preliminary design, with two main focus points:

ˆ obtain sufficient wave attenuation to create safe mooring conditions

ˆ limit the motions of the structure itself to allow safe mooring of ships, and safe storage

of goods

Several parameters will be studied, such as the dimensions of the floating breakwater, thelayout, and the mooring system

This dissertation does not aim to be complete regarding the structural design of the FB It

is a starting point for further research, which, as will become clear in this report, is highlyrecommended

2

1.3 Structure of this master dissertation

Chapter 2 summarizes the results of the performed literature study A brief overview will

be given on the history of floating bodies in general, after which the focus will be put onfloating breakwaters (FB) After providing some general information about the advantages,disadvantages, types, etc the results of experimental, and numerical studies performed in thepast will be summed up From these studies it will be determined which parameters will beinvestigated further with respect to the performance of the FB

In chapter 3 and 4 the boundary conditions are determined There are two types that can

be distinguished Firstly there are the hydraulic boundaries such as design wave heights, sign wave periods, design water levels, etc Secondly there are structural boundary conditionsdepending on the design vessel, the expected wave attenuation, the motion restraints, and theovertopping limit

de-Chapter 5 concerns the preliminary design In this chapter the processes of diffraction,and transmission are treated separately Diffraction properties are viewed using diffractiondiagrams by Wiegel (1962) for regular waves, and diffraction diagrams by Goda (2000) forirregular waves Combining this with the so called gap method, as described in the ShoreProtection Manual (US Army, 1984) will give a first indication of this process Secondly thetransmission will be regarded using design guidelines of four researchers: Macagno (1953),Jones (1971), Stoker (1957), and Wagner et al (2011) To see which equation or workingmethod is more suitable for the conditions in this master dissertation, three testcases withsimilar conditions are chosen The working methods of the different authors are applied toeach of these testcases, providing the most reliable method for the preliminary design Fromthese considerations, a preliminary design is proposed which will be the starting point for therest of the studies in this report

The design will now be optimized using MILDwave In chapter 6 it is investigated how afloating object should be modeled in MILDwave software This is done by modeling the sametestcases used in chapter 5 in various ways, to see which approach provides the most consistentresults

In chapter 7 the preliminary design is optimized First the length of the FB is adjusted,after which the effect of different breakwater layouts is studied From the hydraulic bound-

Chapter 1 Introduction

subject of this chapter A simple beam shaped FB, an L-shaped FB, and finally a U-shaped FB

A very important property of the FB is of course the motion of the structure itself This iswhy in chapter 8 a motion analysis is performed using Aqua+ software that provides theresponse amplitude operators (RAO) of a simple beam shaped structure This will bring tolight that the motions of the FB are too large for safe mooring, let alone storage of goods

At the end of this chapter, possible solutions to this problem are proposed, one of which isdiscussed in detail in chapter 9

The difficulties disclosed in chapter 8 are addressed in chapter 9 by proposing a heave ing breakwater This type of structure is moored using vertical piles, making sure the onlypossible motion the structure can undergo is the up- and downward motion or heave In thischapter the feasibility of such a structure is investigated with regard to the pile dimensions,penetration depth, etc

float-To conclude this master dissertation, the results and concerns are summarized in chapter 10,followed by recommendations for further research

The general approach of this master dissertation is presented in figure 1.1

4

Figure 1.1: General approach

Chapter 2

Literature Study

2.1 History of floating structures

VLFS, or Very Large Floating Structures, were first introduced in the 1920s, when Edward R.Armstrong proposed a floating airport called ’Seadrome’ (figure 2.1) for transatlantic flights.His idea was to ground the airplanes in the middle of their long flight to refuel However,

in 1927 the first non-stop flight from New York to Paris took place, rendering the idea ofseadromes unnecessary (Wang and Tay, 2011)

Figure 2.1: Seadrome (Armstrong, 1929)

Further development of floating structures took place during World War II, when the USNavy Civil Engineering Corps used Armstrongs concept to build a pontoon flight deck, to be

6

positioned near Great Britain Secondly, at the end of World War II, there was a need forinstant harbours for the invasion forces on the Normandy beaches These temporary ports wereconstructed using two types of breakwaters The first type, Phoenix breakwaters, consistedout of concrete caissons that were positioned offshore and sunken down in order to create

a bottom founded breakwater The second option were the so called Bombardon floatingbreakwaters, which are presented in figure 2.2 These were floating steel structures with acruciform cross section that were anchored between the Phoenix breakwaters (Farmer, 1999).However, in 1944 a great storm caused these Bombardon FB to break loose, which lead tomassively damaged harbour infrastructure

Figure 2.2: Bombardon Floating Breakwater (Martin, 2004)

The flexibility of these floating structures lead to several studies after WO II regarding cepts, theories, and experiments with different configurations The benefits were clear: theconstructions could be built on dry land, then towed to their position where they could beeither anchored or sunk into place On top of this, it was also possible to relocate these struc-ture relatively easily

con-In 1975 the idea of VLFS was revived by Kiyonori Kikutake, a Japanese architect He designedAquapolis, a floating city, as centerpiece for the world fair in Okinawa Aquapolis stayed openuntil 1993 and was eventually towed away and dismantled in 2000 (Wang and Tay, 2011)

As the big cities grew denser and denser, there was a need for expansion of the airfields, ably outside the city In 1995 the Japanese performed a test in Tokyo bay, building a floatingrunway of 1 km length, in order to test the soundness of VLFS for use as an airport (Wang andTay, 2011) The results showed that the hydro-elastic response of the floating runway did notaffect aircraft operations, and that a floating airport could in fact be a sustainable solution.All around the world, the use of VLFS is growing in different fields From floating oil storagebases, offshore military bases and floating nuclear plants to floating stages and tennis courts.The possibilities are endless

prefer-Chapter 2 Literature Study

2.2 Types of floating structures

In general, floating bodies can be divided into two types, semi-submersible structures andpontoon structures A third option, the tension leg platform, is also discussed here because ofthe properties of the mooring system

a heavy lift vessel because of its capability to increase the draught, and thus getting quiteclose to the object that needs to be lifted (Gerwick, 2000) They’re held on location bymooring systems and dynamic positioning Semi-subs do not offer sufficient wave attenuation

to create a sheltering environment This means that if semi-sub structures are used to build

an offshore harbour, additional floating breakwaters would be required to create favourablewave conditions inside the harbour

Figure 2.3: Semi-submersible structure (Minnes, 2003)

8

Pontoon type structures

Pontoon type structures are generally used in calm waters where low wave energy is present.The structure, a rectangular hull, simply rests on the water surface, and is kept in place

by mooring systems, and/or dynamic positioning systems The main advantages of pontoonstructures are the high stability, low manufacturing costs, and easy maintenance and repair.However, they have not yet been used in open seas, where the waves are relatively high (Ali,2005) Different kind of pontoon type structures have been built Dual pontoon structuresproved to be very promising, as well as twin pontoon structures These pontoon structures arediscussed extensively in the next section A special type of floating structures are mega floats,

as shown in figure 2.4 These are large pontoon type floating structures, with at least onedimension greater than 60 m They are often protected by a breakwater Mega floats are costeffective in large water depths, environmentally friendly, and easy to construct and remove(Wang and Tay, 2011)

Figure 2.4: Mega float structure (Watanabe et al., 2004)

Tension leg structures

Tension leg platforms (TLP) are used in extremely large depths (>300 m) The mooring system

is connected to a template on the sea bed, after which the platform is partially deballasted.This results in a vertical tensile force in the wires or piles because of the buoyancy of thehull, which restricts movements in the vertical directions (heave) Horizontal movement is stillallowed, but minimal due to the restoring forces of the pretension As stated above, they aremostly used in very deep waters Which also means the TLP system is most often combinedwith semi-submersible structures (Ali, 2005) An example of a TLP structure is shown infigure 2.5

Chapter 2 Literature Study

Figure 2.5: Tension leg platform (Siddiqui and Ahmad, 2001)

ˆ the water quality is ensured which is important for marine biodiversity and ecology,

ˆ in case of ice formation they can be removed and towed to protected areas,

ˆ visual impact is minimal,

ˆ they can be rearranged into a new layout with minimum efforts

10

The main function of a floating breakwater is to attenuate waves The difficult part of signing floating breakwaters are often the connections between different modules They aresubjected to corrosion, wear, and fatigue The objective will often be to construct solid hulls

de-as long de-as possible, causing the number of connections to decrede-ase (Pianc, 1994)

Floating breakwaters can be classified by configuration or by wave attenuation mechanism.When classifying by configuration, pontoon breakwaters, mat breakwaters, A-frames, andtethered breakwaters can be distinguished (Pianc, 1994) They each have their own field ofapplication, advantages, and disadvantages that will be discussed in section 2.3.2

The classification by wave attenuation mechanism yields a distinction between reflective anddissipative structures (Pianc, 1994) In reality both processes will contribute to the attenua-tion mechanism, but one can be dominant Reflective systems ideally reflect all of the incidentwave energy An example of such a system would be the pontoon breakwater A dissipa-tive system destroys the wave energy through viscous or turbulent effects To amplify thiseffect turbulence generators have been developed, which force the wave induced flow to breakthrough slots or perforations (e.g perforated walls) The problem with these generators is thefact that their behaviour cannot yet be predicted theoretically, which means test results have

to be interpreted with care Another example of turbulence generators are open tube floatingbreakwater systems These consist out of horizontal open tubes, with randomly distributedlengths, placed under water The axis is parallel to the mean wave propagation direction Be-cause of the head losses within the tubes, and the turbulence at the shore side of the structuredue to randomness of the flow, energy is dissipated (Pianc, 1994)

Station keeping of floating breakwaters can be achieved in two ways Either by mooring lines

or by piles The use of piles eliminates sway motions, and reduces roll motions to a minimum,which leads to lower coefficients of transmission A problem with using piles are the connec-tions of the piles to the breakwater, which wear down quickly Mooring lines will always bemore suitable in deeper water However, there will also be difficulties connecting the mooringlines to the breakwaters, and, even more importantly in this case, they might allow too muchmotion of the breakwater to ensure safe working conditions (Pianc, 1994)

2.3.2 Types of floating breakwaters

Different types of floating breakwaters are discussed in this section

Chapter 2 Literature Study

Pontoon floating breakwaters

Pontoon breakwaters are the most effective type, since the overall width can be in the order

of half the wavelength, which, according to Pianc (1994), means they will attenuate the wavessufficiently They offer the best prospects for multiple use (use as a walkway, storage, etc.).Several subtypes have been developed

Twin pontoons or ’Catamaran-shaped pontoons’ (Pianc, 1994), as shown in figure 2.6 tribute the given mass to achieve a longer roll period This results in a more stable platformthan would be achieved with the same mass in a simple pontoon shaped floating breakwater.The corners provide additional energy loss by dissipation, and the water mass between thehulls will add damping zones, especially to the sway motion

dis-Figure 2.6: Catamaran floating breakwater

A second alternative is called the dual-pontoon type floating breakwater, and is represented

in figure 2.7, where two pontoons are connected sideways by a rigid deck Research on thistype of floating breakwater has been performed by Williams and Abdul-Azm (1997) A dualpontoon floating breakwater attenuates waves similar to a single pontoon but will also destroyenergy by turbulence between the two floating bodies

Figure 2.7: Dual pontoon floating breakwater

Mat floating breakwaters

Mat breakwater types can, for example, be built out of old tires Obviously they are a low costsolution, and easy to construct They are much less effective for use in long wave lengths thanpontoon type floating breakwaters and have a smaller design life (Farmer, 1999) A simple

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representation of a mat floating breakwater is shown in figure 2.8

Figure 2.8: Mat floating breakwater

A-frame floating breakwaters

An A-frame is a combination of vertical walls that reflect the wave energy and outriggers forstability that will also develop a large roll period This configuration is shown in figure 2.9

Figure 2.9: Aframe floating breakwater

Tethered floating breakwaters

The last type that will be discussed is the tethered floating breakwater Wave attenuation

is obtained through drag, produced during the oscillations of a field of spheres tethered toremain just below the surface (Pianc, 1994) This concept is clearified in figure 2.10

Chapter 2 Literature Study

2.3.3 Behaviour under wave action

There are three main processes that determine the behaviour of a FB: transmission, reflectionand diffraction

Wave diffraction is the process where wave energy is transferred along the crest, perpendicular

to the direction of the wave propagation, from points of greater wave height to points oflesser wave height This causes wave crests to spread into the shadow zone in the lee of thebreakwater (CEM, 2008) A graphical representation of this process is shown in figure 5.1

Figure 2.11: Diffraction process (US Army, 1984)

The transport of energy underneath, and on the sides of the structure is called transmission.Reflection is the process where energy is reflected by the structure A graphical representation

of these processes is shown in figure 2.12

Figure 2.12: Transmission process

These three processes are characterized by their respective coefficients; Ct, Cr and Cd

where Htis the transmitted wave height, Hr the reflected wave height, Hdthe diffracted waveheight, and Hi the incident wave height.

These coefficients can be combined in one overall attenuation coefficient ’C’, which is describedby

C = Hres

Hi

(2.4)

where Hres equals the resulting wave height due to the presence of the structure

Every freely floating structure has six independent degrees of motion; heave, sway, surge, yaw,pitch, and roll These motions are defined in figure 2.13

Figure 2.13: Six independent motions of a freely floating structures (Ardakani and Bridges, 2009)

For these motions, the natural frequency, which is a property inherent to the structure, can

be calculated If the wave frequency approximates the natural frequency for a certain motion,the motions of the FB will be amplified, which will render the FB to be less efficient One

of the main challenges in the design of floating breakwaters is to avoid this phenomenon ofresonance as much as possible According to Pianc (1995) the natural frequencies for sway,surge, and yaw lie in the range of 20 s to several minutes, while the frequencies of heave, pitch,and roll can be found in the range of 5 to 20 s The designer needs to have knowledge ofthe expected wave conditions in order to know which motions might cause problems He alsoneeds to understand the response of the floating body to the wave profile This behaviourbecomes even more complicated if the influence of the mooring system, and the connectionbetween modules are taken into account This is why accurate design can only be achieved by

Chapter 2 Literature Study

2.3.4 Experimental Studies

Over the years, a lot of studies were preformed to investigate the influence of several rameters on the behaviour of floating breakwaters A few of these studies are discussed hereaccording to the researched parameters

pa-Shape and dimensions

Several shapes of floating breakwaters are possible In this paragraph, the influence of theseshapes on the attenuation capacity will be discussed

Koutandos and Koutitas (2004) performed experiments in a wave flume on several models: gle fixed floating breakwaters, heave motion floating breakwaters, single fixed floating break-waters with an attached front plate (permeable and impermeable) and double fixed floatingbreakwaters They found that a single fixed floating breakwater acts more as a reflective struc-ture, while the heave motion floating breakwater was more dissipative The attached platesdid enhance the efficiency significantly, but no real difference was found between permeableand impermeable plates They concluded that a double fixed floating breakwater would be themost effective solution, but in terms of cost-effectiveness the floating breakwater with attachedplate is advised (Koutandos et al., 2005)

sin-Gesraha (2006) studied the effect of two sideboards that are very thin compared to the length

of the incident waves, and the beam of the breakwater His model is presented in figure 2.14

He made a numerical model, as well as an experimental verification, and investigated severalvariables: the exciting forces, breakwater responses, and the coefficient of transmission Hefound that adding side boards leads to larger heave motions, but other motions were lower, re-sulting in a lower coefficient of transmission He concluded that this configuration could result

in a more economical design, if the design is tuned to the incident wave frequency (Gesraha,2006) Again, the positive effect of sideboards on the attenuation capacity is shown

Dong et al (2008) examined the wave transmission coefficients of pontoon floating ters, and double pontoon floating breakwaters The results showed that the double pontoonbreakwater reduces wave energy better than the single type However, both types needed tohave a sufficiently high width in order to get a small Ct (Dong et al., 2008) The difference inattenuation capacity was not significant

breakwa-Pe˜na et al (2011) started their experiments from a reference model (model A in figure 2.15)with two side-boards (fins), and tested the influence of several variations to this basic design.The different models are presented in figure 2.15 Model B was used to determine the influence

of the width, model C for the influence of the fins and model D for the influence of the design

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Figure 2.14: Model Gesraha (2006)

Figure 2.15: Models Pe˜ na et al (2011)

When comparing model A to model B, they found that a reduction of the width by 10% has

no significant influence on the coefficient of transmission Comparing model A and C showed

an improvement of energy dissipation which obviously leads to a decrease of Ct However,they noted that the increase in dissipation is small in comparison to the potential cost of pro-longing the fins Model D showed that even with no spacing between two modules, meaningthere would be an increase of width by 50%, the energy dissipation would increase by 35%.However, increasing the spacing between two modules only leads to 5% dissipation increase,

Chapter 2 Literature Study

attenuation

Material of the Floating Breakwater

Wang and Sun (2010) first tested the influence of a porous side wall by comparing a structurewith an impermeable wall to a structure with a porous wall (porosity n 0,63) It was foundthat the transmitted wave height is higher for structures with an impermeable side wall, be-cause there is an energy accumulation in an enclosed domain In porous structures energydissipation will take place

Wang and Tay (2011) compared a porous floating breakwater to an impermeable floatingbreakwater Porous floating breakwaters are found to have a lower Cr Energy dissipation willplay a more important role for porous floating breakwater than wave reflection (Wang andTay, 2011)

Layout of the different modules

Nakamura et al (2003) investigated the effect of an inclined layout (see figure 2.16) using waveflume and wave basin experiments The layout where the units are positioned obliquely to thecenterline of the floating breakwater reduced the transmitted waves behind the breakwatermore than the conventional setting This can be seen on figure 2.16 where C, the ratio of theattenuated wave height to the incoming wave height, is shown (Nakamura et al., 2003)

Figure 2.16: Breakwater Layout Nakamura et al (2003)

Martinelli et al (2008a) also investigated the influence of different layouts with wave basinexperiments They used four different layouts, as presented in figure 2.17

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Figure 2.17: Martinelli Layout: I-shapes and J-shape (Martinelli et al., 2008a)

The 0° I-shape and the J-shape showed a similar coefficient of transmission When increasingwave obliquities, Ct decreased

The layout of the floating breakwater will clearly be an important factor As shown by mura et al (2003) the position of the structure with respect to the incoming waves will influencethe behaviour The research by Martinelli et al (2008b) indicates that an L-shaped floatingbreakwater does not give better results than an I-shaped structure

Linear Potential Theory

A first approach to study the influence of a FB is derived from wave theory If the fluid isassumed to be inviscid, incompressible, and irrotational, the fluid velocity can be described

by the velocity potential (Chakrabarti, 1987) This potential can be subdivided into differentcontributing components, and has to satisfy the Laplace equation:

Williams and Abdul-Azm (1997) used this theory to study the response of dual pontoon floating