Long term effects of cyclic loading on suction caisson foundations in sand

Long

term

effects

of

cyclic loading

on

suction

caisson

foundations

in sand

Prof. Frits van Tol TU Delft

/

Deltares lng. Rene Thijssen Volker lnfraDesign

/

SPT )ffshore ln Cristina Lupea TU Detft

-+-".-ffi;+

O ,.: a lr .a a .(

)

:(

.

r.'-" ì .-7;-| --a,? I E Ð o

\

,J'

Figure 1 - Sketch af the cansidered structure, óMW wind turbtne with a th ree- le g jacket su bstructu re

INTROÐUCTION

Recently pubtished

statistics by

the

European

Wind

Energy Association, EWEA [2012], have shown that the investments in the offshore wind energy sector have ìncreased

from under

€500 mittion in 2001 to €4500 miLLion in 2012. Accor-ding

to

DTI [20011,30% of these investments is re[ated to foundation

costs.

ln order

to

reduce

the overat[ costs two

soLutions were identified: increasing

the turbine

capacity and/or moving

f urther f rom the shore where higher wind power

coutd be tapped in. Both these sotutions lead to an increase in the foundation size and its costs. According to Senders [2008), significant project costs during instaltation are caused by stand-by periods as consequence

of

bad weather condi-tions. Thus, decreasing

the number of

offshore

Figure 2 - Comparison of modell.ing capabiLities af the avai{abLe approaches

operations required by simptifying

the

founda-tion instattafounda-tion may reduce costs significantty.

[aboratory and/or in-situ testing is done.

Suction caisson foundations provide a simptified

instattatlon

procedure.

According

to

Señders [2008] and Byrne & HouLsby [2003) suction cais-sons

within

a mutti-footing configuration are a viab[e economicaI and environmental.l.y f riendLy

sotution

Unfortunatety,

in

the current

guide-Lines and standards Limited guidance

is

given retated

to the

assessment

of their

long term

performance throughout the 25 year design [ife time [characterised by over 108 loading cycLes). Lupea [20'1 3) gives an overview of the avaitable approaches and suggests an aLternative method to assess the tong term behaviour of suction cais-sons under cyclic loading conditions. The current articte presents onLy a summary of these appro-aches The focus is to present the resutts of the modeLl.ing carried out, as these may simptify the design process, provided that validation through

PROBLEM DESCRIPTION

The effects

of

[ong

term

verticaI cyctic loading on a suction caisson embedded in sand are to be determined in the context of :

.

Loss of stabitity

-

the reduction of subsoiI bearing capacity due to pore pressure bu itd- u p;

.

Loss of serviceabitity

-

accumutation of differentiaI setttements Ii.e. introduction of rotati on ).

It is to be noted that the governing requirements

are

retated

to

the

overatL

titting of the

wind

turbine

structure,

thus

imposing

strict

boun-daries retated

to

differentiaI sett[ement

of

the suction caissons underneath the three teg jacket structure (see figure 1 I

AVAILABLE APPRAACHES

The two most commonty used standards within It*âti!'e procFs -eror

gÊntration (trplicit +

control steFsJ, tr-¡¡ge awbtr 0f ptrmettrs with mcieæ physl ml mming, mlibration båEed m lÂborÂtor5¡ ttrts. Ittrat¡ve proffis -wor gmmation (*plicit + @ntro¡ lteps] , Large nmbtr of pilåmettr$ soi¡ d¿tÀ for ca¡ibration. Focos - steel comFgntrts ed not soi¡.se¡ected dttrie ref. steel,05t 1SS01-4 for geotechniml dæ!gnJ eñ'eclivÞ stress method¡ - all coobibutions to pore prëEur$ m$t be included cyciic load "=" qu!-static Liúitåìi ms 5Þæsæ/ Pore prssuæ

Stiffas s / Relâtive dtr ity

st¡eriBth mdy'or C¿Þacitf.

üly duê to st¡tic loadins only due to rtâtic

loådl¡s D¡sp¡acffifltsy'sFe¡n Åcdmu¡âtiob Cyclic loading NotmEgdêd Soil T,lpe Pils GBS Loag þipÊplls Foudetion T'99e Hybrid Hvbrid/Constitutive Empiri cai Ëmpiriel lfodel Tlpe Sfuâiú âffimülâtiÕr Eodels

PoFe presrc build-up modcls N0n50K

DNV APr/ts()

Oth* Uttratue, R6Érch Nôffi, Ståndä'ds od/or Guidelines

Summary

0ffshore wind turbine projects have been characterised by an increase in

costs, sizes and distances from shore, EWEA {2012J This created a need of investigating the adequacy of alternative and more fìnanciatty

attrac-tive foundation types such as suction caissons. Within a mutti-footing configuration, such as a three-leg jacket structure, these foundations prove to be an advantageous sotution for increasing water

depths lt

is

due to

their

configuration that a simpì.ified design is possibLe, [eading to onty verticaI cycLic [oading conditions. This articte provides a concise

summary of the M Sc. thesis of C. Lupea and presents an approach that

may altow for the simptìfication of the [onE

term

perfonnrance

assess-ment under vertical cycLic loading of suction caissons ennbedded in

sand lt

provides a conservative theoreticaL base for the ideniification of potentiatLy damaging loads, which couLd cause significant pore pressure

buitd-up and strain accumutation. 0ne of the key conctusìons of this

research is that the foundation response is a function of both the apptied mean [oad and its cyclic amptitude for both terrsite and compresslve loading. Nonetheless, experimentaI work rnust be canried out to vatidate these results. o L 0 À =o' Soi1 H 8md nodel rith irtergrilrler atrei¡

=a1

-Enpíricel fomntes f€úr¡poletiúl

iûcre-.¡fg eccúf,uletiø r¡te - - - Iæi¡creæirg¡ccrEEl¡timrete

Îíoe

lal or cyclea

l-f

Figure 3

-

Sketch of proposed modelfor pore pressure buiLd-up (simiLar procedure for strainsJ

tion mode[ (Wichtman et aL. (201 1]). 0nce a trend

can be observed

in

pore pressure buiLd-up and

strain accu mu Lation, extra polation formulas can be used

-

see figure 3.

Loading conditions

An

important

ro[e

in

the

proposed

modeI

is

ptayed by the considered Loading conditions [see f igure 1). ln the case that was analysed

for

the

thesis, representative

for

North Sea conditions, Loading conditions were represented by

the

re-suttant forces at seabed [eve[ for the simutation of

the

25 year design

tife time of

a

óMW wind

turbine. The raw data

on

reaction forces

at

jacket

Leg- caisson interface

were

statisticatLy processed

in

order

to

determine both extreme va[ues as wet[ as the most probable operationaI

Loads.

The

extreme values

were used

in the

prede-sign

of

the

suction

caissons

for

a

three-teg jacket structure. A safety factor of 2.5 was used

in

order

to

check

the

foundation's capacity to undertake tension. The obtained caisson dimen-sions are 15 m diameter and 1ó m embedment Length, using formutas provided by API and DNV

for

undrained conditions.

A

simptified anatysis showed that the required pressures for

instatla-tion were within the [imits imposed by the water depth of 40m. As this was not the main topic of research no further analysis was carried out [i.e. buckLing anatysis).

The most probabte operational load cases were

determined

based

on the

highest number

of

occurrences

and

the

highest mean

and/or

amptitude

of

the

Load, whichever

is

governing reLated

to

pore

pressure buitd-up and

strain accumutation. Due

to

the

high safety

factors

used in the predesign, the dominant operatÌonal Loading

cases

for

the three

caissons

proved

to

be betow 10% of

the

caisson's capacity. For cLarity, the fotLowing vatues

wit[

be referring to

unfactored Loads

with

respect to the capacity of

the

15 m diameter suction caisson,

with

a'1 ó m embedment [ength.

the

offshore industry, the American Petroteum

lnstitute

(APl)

_{and Det}

Norske

Veritas

IDNVl,

provide guideLines

for the

anatysis

of

stender pipe piLes Itength (Ll over diameter ID] ratio

_'

'1

0)

and gravity based foundations IL/D.1 ]. For

suc-tion

caissons having an embedment

ratio

IL/DJ between 1

and

10, depending on

the soil

type and instaltation pressures, no specific guidance

is

given. Furthermore, these standards provide

Limited guidance

in

the

assessment

of

cyctic

Loading

on

foundations

through

empiricaI

formuLas [the API suggests the use of a

reduc-tion

factor A = 0.9

for the lateraI

resistance of

stender pites,

that

is

independent of

the

cyctic Loading characteristics;

the

DNV proposes an effective stress anatysis and

a

reduction of the

shear

stress

based

on

pore pressure buil.d-up

for gravity based foundations).

The avaiLable research suggests the use of

con-stitutive modeLs with a better capacity to assess the change in soiI properties throughout each in-dividuaL cycLe. These modeLs are either based on

strain or

pore pressure accumutation. A bridge

over

the

[arge number of cycles, characteristic to offshore toading conditions, is generatty pro-vided through the use of empiricaL formu[as and

controI cyctes [Safinus et at. 201 1 ). These types

of modets shaLl. be considered as hybrids.

From

literature

search, severaI

criteria

were

identified as being significant in the assessment of this prob[em. The comparison of the avai[abte

solutions

to

the

probtem

at

hand

is

given

in

figure

2.

lt

becomes ctear

from this

figure that the research carried out covers much better the questions faced now in the offshore wind

indus-t ry.

PROPOSED MODEL

From literature

it

is

conc[uded

that

for

the probtem at hand the use of a hybrid modeI using

the

hypoptastic sand model. (Von WoLffersdorff,

199ó)

with

intergranutar

strain

INiemunis

&

Her[e,

1 997)

is to

be

preferred.

The

reasons

for this

choice reside

with the

good prediction capabiLities of the modeL and its previous usage as the base for the High CycLic Strain

-Failue Enve¡ope-Bang et al,(201Ð -Failue Enyelope-Supachawdote êt al. (2004) -Ho¡lzontal Opemtional Loa¿s

*Horizontal _E\treme Loads

z à ñ G U

î

F 2-58+05 5,0E+04 7 0E+04 6 0E+04 5 0E+04 4 0E+04 3 0E+04 2 0E+04 1.0E+04 0.0E+00 0.0E+00

1,0E+05 1.5E+05 2.0E+05 Horlzonal Capacity [kNl z E { I

r

E E E Phâse Z _F-* hptrtude [kNI

F-*.

r' Phæe 4 ÂDpübde [d,Il Fu" r (pÊñd of 1 _{rycl.) lsl} Tiûe lsl -,{ppljèd

qEi-fttic loâd (discèds¡doD of ryclic lo¡d)

. ., * - Cyc)¡cloÁd sirl (b be dtsæ6sed)

Figure 5

-

Sketch of discretised loading in faur phases

0.

h^

Figure 4 - FaiLure enveLope af the predesigned caisson

The

inftuence

of

horizontaI

toading

was investigated

by using

the

formu[as

suggested by Bang et at. [2011ì and Supachawarote et a[.

[2002']. The

latter

method is appLicabLe for ctays.

For

this

method,

the

undrained shear strength

of

sand was

determined

using

DNV

(19921

recommendations

It was conctuded

that

horizontal Loading did not yietd a significant change in the verticaI capacity

-

see figure 4. The probtem at hand was there-fore simptified to verticaLcyclic [oading onty.

The Loading conditions

were not

appl.ied

cycl.i-caLLy, but quasi-stpticatLy using

four

phases as

shown in figure 5. ln the odd numbered phases the load was instantty apptied undrained, whi[e in the even numbered phases time was altowed for consolidation for hal.f the period of one cycle The period was conservativety chosen equaI to the wave period, approximatety 7 s, even though

from

a study on

the

Loading condÌtions fottows

that the

governing cyctic loads

are

[ike[y

to

be

c[oser

to

the typicaL

wind

period of 30s. These vatues are atso c,utside the order of the opera-ting frequency of wind turbine generators.

The hypoplastic sand model with intergranular strain

The hypopLastic sand mode[

with

intergranutar

strain

is a

non-[inear

incrementaL constitutive modet. lt associates the strain rate to the stress rate, expressing aLI mechanicaI behaviouraI cha-racteristics in one sing[e tensoriaI equation The particutarity of

the

modeL

is

retated to

the

fact

that

it

does not decompose the

strain

rate into etastic or plastic parts, as one was accustomed from e[asto-pl.astic theory. The totaI

strain

is in

fact the sum of two components: an

intergranu-Lar

strain

tensor

retated to

the

deformation of interface layers at intergranutar contacts and a

component retated to the rearrangement of the

soiI sketeton

The

first

component

is

observed in reverse and neutraI toading conditions, being characteristic to hypoetastic behaviour. ln conti-nuous loading conditions both components con-tribute and the behaviour may be characterised as hypoptastic.

The

modified equation

by

Gudehus

11996J

inc[udes the inftuence of the stress leve[ (baro-tropyÌ and of density (pyknotropyl. The currently considered standard hypopl.astic sand modeI by Von Wotffersdorff has a Matsuoka-Nakai criticaI

CriticaI

state friction angl.e ["J

Granutar hardness Icontrots the shape of the Limiting void ratio curves

_-

stope) Exponent of compression law Icontrots the shape of the Lirniting void ratio curves

-c u rvatu re )

Void ratio at maximum density for p=¡ lcontrols the peak sirength) Void ratio at criticaI state for p=0 [control.s the peak and residuaI strength) Void ratio at minimum density for p=¡ (controts the initiaI stiff nessJ

Pyknotropy factor controtl.ing the peak friction angle Pyknotropy factor controtting the shear stiffness

Stiffness muttiptier for initiaI or reversed toading (stiffness increase for a 900 reversat) Stiffness mul.tiplier for neutraI Loading [stiffness increase for a 180o reversaL]

Smatl. strain stiff ness Limit (radius of etastic range, may be taken as a materiaI constantJ

Parameter adjusting stiffness reduction curve stope Iused to catibrate against cyctic test dataì

Parameter adjusting stiffness reduction curve stope (taken as materiaI constant)

n edo €c0 eio c[ f3 mR fTì1 R'.n.,. 0.

x

Tabte

I

- Hypoptastic sand modet

with intergranular

strain concept parameters

Symbol.

Description

Tabte 2 - Overview of soil characteristics

0.

o 13

t-l

CT e¿b t-t 0.4 Pr,¡utt< lkg/m3J 2072 Pi,ary lk9/m3J 1722 ky

lm/sl

2.5E-5

t-l

0.13

1.s R-"*

t-l

1.E-04

l3,

x

t-lt-l

ê¡

t-l

0.54 h lGPal 2.2

t-l

0.22 n eco

t-l

0.55 êio

t-l

0.85 ffì p

t-l

1.1 ft'ì¡

t-l

1 0.01ó-0.75 LO

20

GE1TECHN'IEK- Januari2ol4

LONG TERM EFFECTS OF CYCLIC LOAD¡NG ON SUCTION CAISSON FOUNDATIONS IN

sAND

a È

å

ñ Þ I Þ e"¡t\ t o

-Model

...TestData orB ¿rf9ol a È

å

ct

ã

Þ I Þ

e"6

_$"

-Model

...TestData 9do r lo1+orl/2 [kPq

o'F

-Model

...TestData

F

_À

å

¡

L rût c1[o/ol x 6 E

o

o

r-0E-05 1.08-04 1.08-03 1.08-02

1.0E-01

1.08+00 90 80

F70

&

å60

Iro

s

loo

È

t30

o

À20

10 0

-Model

-Test

Data

R-"

_",

B-yctê¡

[-( 0

20 40

60 80 100 t20

1+0 a t

.

Test

Data

_"Model

2

r|rn

t-¡

Figure 6 - CaLîbration of hypoplastic sand modeL parameters

stress

state

condit¡on incorporated. Any mate-riaI point within the soiI mass is described using the stress state and void ratio. Hypoptasticity

aL-lows for atI the stress hisiory to be incorporated

in the current stress, while the presence of the void ratio in the formuta makes the model more

sensitive

to

past

deformations.

According to KoLymbas [2000ì hypoptasticity has an atgebraic

formutation using

a

simpte tensoriaI

re[ation,

whitst the

hyperpl.astic modeI

is

a geometrical approach,

using pictoriaI

concepts

such

as

a

yieLd and plastic potentiaI surfaces. More

infor-mation

regarding

the

hypoptastic

sand

model can also be found in Bardet (1990] and Ling and Yang [200óì.

The hypoptastic sand modeI

with

intergranutar strain, as imptemented in Ptaxis by Maðín (2012lr,

has a totaI of 13 parameters to be catibrated, see Tabte 1. The

first

eight parameters betong to the hypopLastic sand model and are

to

be

catibra-ted

using

isotropical.Ly conso[idated undrained

triaxiaI

test

data.

The last five

parameters,

related

to

the

intergranutar [smattl strain,

are

to be calibrated using cyctic undrained test data

[cyctic undrained

direct

simpLe

shear tests

or

cyctic

undrained

triaxiaI tests)

with

varylng cyctic shear stress ratios (CSSRl.

Figure 7 - CaLibration of intergranuLar strain concept parameters

The

catibration

procedure

requires

a

good

understanding

of

[aboratory

test

resutts,

soiI

behaviour

and

of the

inftuence

of the

modet

parameters. The soil. testing

faci[ities

in

Ptaxis

were

used

for the

catibration

of the

soil

with

respect

to

the

existing cycIic [aboratory

test

data- see figure ó and figure 7.

The values resutting from the in-situ, l'aboratory

testing

and catibration procedure

are

summa-rised in tabLe 2.

The B, parameter depends on the apptied CSSR

(see

figure

81. Therefore,

it

is

used as a bridge

between

the

[aboratory and

in-situ

conditions.

It

ensures

that

the

response

of the soil

to

the

apptied

Loading

conditions

in-situ

is

correctty extrapolated

from the

loading conditions in the [aboratory.

Extrapolation fo rmulas

The extrapolation formutas are a best

fit

based

on

the

accumulation

trend

that is

observed in

the

resutts generated by running

the

anatyses, as shown in figure 12 and figure 13.

Proble m D iscretisation

Even

though

the

probtem

is

axisymmetric

at

boundary vatue levet, the probtem was analysed

using Ptaxis 3D, due

to the

Expert Mode func-tionatity

which

atlows

the

user

to

input

aLL the

information using command [ines. This feature

attows

for

a more

rapid and

semi-automatic introduction of the [oading conditions.

Geometrica[ty,

the

probtem

was

discretised

in

two different

ways.

First

a

rectangutar soil

votume

extending

45m around the

caisson,

with

aLL

drained

boundaries

was

considered.

Secondty

only

a quarter of the

probtem was anatysed,

with

the verticaI adjacent boundaries considered undrained

-

see figure 1 0. The tatest

was chosen

in

order

to

ensure more symmetry in the automatica[[y generated mesh, as

it

was

observed

that the

non-symmetry

of the

mesh

was

inftuencing

the

resutts.

Furthermore, the

boundaries

of the soiI

volume

were

chosen at

ó.R [R

is the

radius of the suction

caisson] in order to avoid that they influence the simulation resu[ts. For the anatysis of this probLem interfa-ces were used between the caisson and the sand votume within and around it.

ln

order

to

be abte

to

observe

the

response of

the soiI

mass

to the

Loading conditions, nodes and stress points were setected both within and

,.!-o.t4

o.L2 o.10 o.08

È

(A

o

()

o.06

o.o4

o.o2 o.o0

0

500

1,ooo

1,500

2,ooo 2,500

3,ooo

Nuo

_[-l

Figure 8 - _{lnfluence of fi, on} C55,? and number of cycles required

ta reach liquefaction (N¡¡r)

outside the caisson

-

see figure 1 1

É

û

ú

o

0.10

o.09

o.08

o.o7

o.o6

o.o5

o.o4

o.o3

o.o2

o.o1

o.oo

-o.o

pr_bect

_fit

_[-] o-;o o 10 o. ¿o n ìo

----irì

a

nq- lßr hest 3st

fit

ißrt

Figure 9

-

Relatìonship between p. and C55R - curve through the values

of þ, giving fhe best fit with laboratory data

RESULTS

As

the

appIied operationa[ loads were actually

under

10%

of the

predesigned caisson's

capa-city, the resuLts showed no pore pressure

bui[d-up during the course of the 520 cycLes analysed

Isee

figure

12); having

a

symmetric

response

around the zero mean va[ue

at

different points

within the soiI mass). Pore pressure buil.d-up is

the driving factor in reducing the bearing

capa-city of sand and consequentLy endangering

sta-bitity as it was defined in the Probtem

Descripti-on. ln figure 12 it can be seen that no significant

changes occur and therefore the stabiLity of the

structure is ensured.

When evatuating the strain accumutation in time

[see

figure

13) a

trend

may be observed within

two of the points located in the soiI mass in the

caisson. The accumutation trend forthese points

represents a Iinear distribution of 1.1.10-8/cycte.

lf

this trend

is

extrapotated

[inearly

for

one

mittion

cycles,

a

tiLt

of

0.5o

of the

ful.L super-structure coutd be reached.

A

[inear

extrapolation

is

made

over one

mit-[ion cycles only, due to the fact that a signif icant

error

may be propagated. Measured [aboratory

or

in-situ

data may

provide

a

more

accurate extrapotation method.

Having

such

a

smaLl

strain

accumuLation

rate,

titting of the structure

due

to

differentiaL

settLements

is

within

the

boundaries

used

in the

offshore

wind

industry and,

therefore, serviceabitity is ensured.

An

additionaI ana[ysis

was

carried

out

to

investigate

the

foundation's behaviour

to

more

extreme conditions (storm eventsl,

based on

[oads

at

increased percentages

of

the

capa-city. Unfortunatety the modeI response in some

of

these

cases showed significant

instabil.ity

Isee

figure

15J.

ln

these

analyses

it

couLd be

observed

that the

medium dense sand initiatty

densified

within

the

caisson,

but as the

cyctic

loading continueC, [ocatised loosening

due

to

increased

pore

pressure

buiLd-up

under

the

caisson top pl.ate and centre

took

ptace. These

effects are not expected to be significant in

den-ser sand, according to Andersen (20091.

The reasons for which instabiLity and

overestima-tion of pore pressure buitd-up occurs within the

software may be related to 3 factors [Ptaxis, 2013ì:

1

Even though

the

probtem

is

symmetric

at

boundary vatue levet, the generated mesh is

not symmetric and this causes

non-symme-tric

fai[ure mechanisms;

2.

The high water depth of 40 m may have also

caused unbatanced forces

within the

soft-ware;

3.

The over-estimation

of

pore pressures was

atready expected

from the soiI

catibration

p roced u re.

Even though

for the

extreme cases no

conctu-sions coutd

be

drawn

regarding stabiLity and

servicea bitity, this modeI alLowed for the creation

of a chart that a[[ows for a safe design zone [see

figure

1ó1, within which [ong

term

performance

is ensured, by having a volumetric

strain

accu-mutation rate sma[[er or equaL to 1.1.10-8/cycte.

This

boundary

is

drawn

for

a

medium

dense

sand

Il¡

= 50%ì, considering a loading period of

approximatety 7 seconds

for the

verticaL cyc[ic

load and a [inear accumutation trend. Moreover,

the baltast weight that may be appl.ied on top of

the caisson [which provides a positive effect on

the axiaI tensionaI capacity) was not considered.

It is

important

to

notice

from this

chart,

also

found by Jardine et at. [2012], that the

foundati-on respfoundati-onse depends foundati-on both the mean load and

the amptitude of the appLied load. Moreover, it is

gænetry in operational ænditious gæmetry in *treme mnditiom

Figuur 10 - FrobLem geometry {in operationat Loading canditions futL caisson, in extreme loading condítions anly a quarter)

5500 4500 3500 2,300 1,500 500 -500 '1500 -2 500 cydð [-1

Figure 15 - Sample of pore pressure buitd-up ÌnstabiLíty as a

function of the number of cycLes for more extreme cases

accuracy

of

the

obtained

resutts.

Nonethe-[ess,

the

boundary as given

in

f

igure

1ó

provi-des

a

guidetine

for a

safe

foundation design,

though

it

might not always prove economic. This boundary is atso an indication of which cyclic Loads

need to be further investigated in order to assess

possible long

term

damages. The same figure

atso

conf

irms that

the

foundation

can

resist tensil.e loads,

but

that

capacity decreases

sig-nificantly

as

tensi[e

cyctic loading components

appear.

Taking

this

tensionaL

capacity

into account

in

the design is an important

optimiza-tion

since

in

the

design

of

suction cêissons up

to

now a[[ tensiona[ loads are normatty avoided

by

adding batlast

on the

caissons. Therefore, within certain Loading ranges this additional. weight might not be required.

The given resutts

are

the

outcome

of

the research

and

numerical analyses conducted as part of the author's M.Sc. thesis. A futt overview of aLL the considered assumptions, [imitatlons and obtained results

can

be found

in

Lupea [2013).

Nonethetess, the provided results are based

pure-[y on numericaI anatyses, thus f iel.d and/or [abora-tory testing must stil.[ be carried out for vatidation.

ACKNOWLEDGEMENTS

As

mentioned

in the

abstract,

this article

is based

on the work

carried

out for the

M.Sc.

thesis

of

C.

Lupea

as part

of

her

graduation

from

the

Geo-engineering programme

at

the TechnicaI University

of

DeLft. Great appreciation

is

given

to

dr. ir.

R.B.J.

Brinkgreve,

ir.

W.J.

Karreman, ir. WG. Versteijten and ir: H.J. Luger, whose guidance and support have made at[ this possibte. The authors atso

thank

a[[

the

others

Figure 1ó - lnfLuence of mean Load and ampLitude on the foundation response for medium dense sand under undrained conditions, with a verticaL loading period of approx 7s [the safe design zone is characterised by dominantly

hy-poelastic behaviour, whiLe outside of it by dominantly hypaplastic behaviour)

that have provided input during the course of this research.

REFERENCES

-

K.H. Andersen [2009ì. Bearing capacity under

cyctic loading

-

offshore, atong the coast, and on [and. the 21st Bjerrum [ecture presented in Osl.o, 23 November 2007. Canadian Geotechni-caI Journat, 4ó[5] :51 3-535.

-

J

P.

Bardet,

[19901. "Hypoptastic ModeL for

Sands." JournaI

of

Engineering

Mechanics,

116.9],, 1973-1994

-

B.W. Byrne and G.T. Houtsby [20031.

Foundati-ons

for

offshore wind

turbines.

PhilosophicaL Transactions of the RoyaI Society

4.361:2909-2930.

-

S. Bang, K. Jones, K. Kim, Y. Kim and Y. Cho

'.2011). lnctined

Loading capaclty

of

suction piLes

in

sand. 0cean Engineering, 38[7]:9'1 5

-924.

-

European

Wind

Energy

Association

IEWEA, 20131. The European offshore

wind

industry

-key trends and statistics 201 2.

-

Department of Trade and lndustry (DTl, 20011.

Monitoring

&

Evatuation

of

Btyth

0ffshore

Wind Farm.

-

G. Gudehus f 199óì. A comprehensive

constitu-tive

equation

for

granutar

materia[s. JournaI

of the

Japanese GeotechnicaI Society: Soits

a nd Fou ndatio ns 36111: 1 -12.

-

R. Jardine,

A

Puech and K.H. Andersen,

'Cy-ctic

Loading

of

Offshore Pites: PotentiaI

Ef-fects and PracticaL Design", in proceedings of

7th int.

conf. Offshore

Site

lnvestigation and Geotechnlcs, London, Sept. 201 2.

-

D. Kotymbas [2000]. Constitutive Model.l.ing of Granutar Materiats. Engineering 0n[ine

Libra-ry. Springer-Verl.ag. ISB N 97 83540669 197 .

-

H. Ling and S. Yang, [200ó1. "Unified Sand

Mo-deI Based on

the

CriticaI State and Generati-zed PLasticity." JournaI of Engineering Mecha-nics, 132[12],'l 380- 1 39 1 .

-

C. Lupea [20131. Long

term

effects

of

cyclic Loading on suction calsson foundations. M.5c. Thesis, TechnicaL University of DeLft

- D.

MaËín [20121. PLAXIS

lmplementation

of HypopLasticity. Charles U niversity of Prague.

-

A. Niemunis and L Herte

[1997].

Hypopl.astic modeI for cohesionLess soi[s with el'astlc strain

range.

Mechanics of Cohesive-FrictionaI

Ma-Ierials 2:279-299.

-

Plaxis [2013) private e-maiI communication.

-

5. Safinus, G. Sedl.acek and U. Hartwig [201 1ì.

CycLic Response

of

Granular SubsoiI

Under

a

Gravity Base Foundation

for

Offshore Wind Turbines. ln Proceedings of the 30th

lnternati-onaL Conference on Ocean, Offshore and Arctic

Engineering 0MAE, pp. 875-882.

-

M. Senders [20081. Suction Caissons

in

Sand as Tripod Foundations

for

Offshore Wind

Tur-bines.

Ph.D. Thesis, University

of

Western

Austratia-School

of Civit and Resource Engi-neen ng.

-

C. Supachawarote, M. Randol.ph and S.

Gour-venec

_[2004].

lnctined Putt-out

Capacity of

Suction Caissons.

ln

Proceedings of

the

14th

lnternationa[ Offshore and Po[ar Engineering Conference, France, pp. 500-50ó.

-

P. von Wotffersdorff (199ó1. _{A hypoptastic}

reta-tion

for

granutar materiats

with a

predefined

[imit

state surface.

Mechanics

of

Cohesive-FrictionaI Materlats 1l3l:251 -27 1. roo% 90% 80% 700/o 60% 50% 300/0 2oo/o 1Oof,2 oPr ú6p: -100/0 -2Aø/o -300/0 4Oo/o -s04" -600/0 -7Oo/o -aoø/ó -9Oo/o 700%

;

Ê c 0 E 0 E ts A A8putude/ c¡pactty P/d aaa

Dominæt Hjpoelastie Behaviou aDoÈinæt HJ4roplastic Behaviou

rf

,/o _{4(ro 5(}

/."'

ale

(leslgr t

zote

I

t^ r-(1-lolC

-Elcwclel

i

LONG TERM EFFEETS OF CYCLIC LOADING ON SUCTION CAISSON FOUNDATIONS IN

5AND

Figure 12 - Sa,nple of excess pare pressure bui\d-up in time at

variaus points within the soil. mass {52A cycLesi

Figure 11 - Selection af nades and stress points for nleasuretnents

-4 E-19 2 5915E+06 2 5925È+06 i-3.8-05 1 Ð,05 ,2.8-05 , - ' I -K -L-M--N-O R -S -Linear (K) -4 Ð-O5 -5-E-05 -6.8-0s

Figure 13 - Sarnpte of volumetric stra¡n accumulation in tirne at

variaus points within the soi| mass {520 cyclesl

observed that as toading within the tensi[e

regi-on occurs, due to increased amptitudes and low

mean vaLues, the undrained capacity decreases

Yet,

this

chart shows that there is

stitt

a

possi-bitity

for the

foundation

to

undertake

a

[imited

amount

of

tensiLe Loading,

thus

reducing the

totaI amount of batlast weight required. A design

comptetety

within

the

safe zone

might not

be

economic, but

it

can hel.p

the

engìneer identify

the cases that wiLL cause neither significant pore

pressure buil.d-up nor strain accumuLation.

The obtained resutts represent a

first

step into

having a better understanding of the behaviour

of

suction

caissons embedded

in

sand

under

verticaI

cyctic loading. Additionatty, Laboratory

modeI

testing

is

stitt

required

for

vaLidation.

Furthermore,

the

sensitivity

of the

foundation

response

to

change

in

retative density

of

the

sand,

compressibil.ity,

effects

of

preshearing

shouLd also be investigated

CONCLUSIANS

The prob[em anatysed during the course of this

research refers to the Long

term

performance of

suction caissons under cyctic Loading. This was

investigated

in

order

to

ctarify

the

adequacy of

suction caissons as foundations for Large (ó MWì

offshore wind turbines.

An

important aspect

that

has been discovered

and proved

with

this

investigation

is that

for

a

three leg jacket structure, suction caissons have a significant advantage in horizontaI resistance:

due to the specific Aeometry of the prob[em and

the

choice

of

Large diameter suction caissons,

the

horizontaI resistance

is

much [arger

than

the appLied horizontaI Loads. Thus, the probtem

was

reduced

to the

behaviour

of the

suction

caisson under verticaI cycLic [oading only.

Voidratio (e) [.10'3]

Endofcycle5

Endofcycle25 m,0q æ5,00 m,æ ?É,w æ,æ 7å,û m,m óñ,æ ffi,æ ffi,w ffi,00 5E,m ffi,æ %,æ s,m lE,@ 49,æ lã,æ

50

Endof 100

The mode[ proposed

to

anatyse

the

behaviour

under

verticaL cyctic Loading

is

based

on

the

hypopl.astic

sand

modeL

with

intergranular

strain

concept integrated.

lt

has proven

to

be a

good tooI

for the

prediction of the foundation's

behaviour

for

severai

hundreds

of

cyctes for specific Loading ranges. The soil. condition

un-der investigation is represented by sand, as one

of

the

most

commonLy encountered

soiI

types

within the

North Sea. The medium dense sand [lD

=

50%ì used

for

the

catibration

of the

soil mass, represents

a

conservative case,

as

the

encountered sand within the offshore soiL

inves-tigations in the North Sea is generatty dense to

very dense. CycLic Loading effects are not

expec-ted

to

be significant ìn denser sand (Andersen,

2009).

For

the

obtained

resuLts

the

overestimation

of

pore

pressure,

from

the

soiI

catibration

phase, proves

to

be prob[ematic in defining the

Figure 14 -Densif ication effects within the medium dense sano under extreme laading conditi-ons {change in vaid ratio]

@

x 7 6 2 o I -K-L-M-N-O P-Q d Ê. ¿!, 10 3 o úl 0, llir û o È, 6 û o q t( ftì 2 -2 ,8

23

GEOTECHNIEK- JANUAT:2OI4