monopile design - agtze · pdf filemonopile design. 2. ... hettler (1981): model tests. c n y...

Addis Ababa, September 2010

Prof. Dr.-Ing. Martin AchmusInstitute of Soil Mechanics, Foundation Engineering and Waterpower Engineering

Monopile design

Monopile design Addis Abbaba, September 20102

Presentation structure:

• Design proofs required

• Calculation method (p-y)

• Consideration of large pile diameters

• Consideration of cyclic loading effects

Monopile design


Monopile foundations

• Up to now mostly monopile foundations in North and Baltic Sea• Pile diameter initially around 3m, recently 5m and more• Usual requirement: maximum permanent inclination < 0.5°• Effect of cyclic loading?• Special for offshore windmills: large diameters, large H/V ratio

Foun

datio

n

Sub

stru

ctur

eS

uper

stru

ctur

e


Projects carried out until 2008


Required design proofs for Monopiles

Bearing capacity and Serviceability under lateral (and axial) loads Consideration of cyclic effects (strength degradation / cyclic stability, accumulation of displacements)

Worst- and best case-Analyses regarding the stiffness underoperational loads (calculation of natural frequency)


DIN 1054: ks = Es/D, but admissible only for determination of bending moments and for max w < 2 cm

Design proof is obsolete if:

1) Pile is fully embedded in soil and

2) Horizontal load in LC 1 is less than 3% of vertical load and in LC 2 maximum 5% of vertical load

dph,0

Ep ≤∫=

=

Lz

z

pheykp ≤⋅=1

2

Design of horizontally loaded piles: Subgrade reaction method


p-y- method according to API RP 2A-WSD, 2000

• Non-linear load-displacement curves(p-y curves)

• p-y curves are based on field testswith up to 1m pile diameterwith up to 100 load cycles


p-y curves for sand acc. to API

1,0

p/p u

y

k z / A pu = 1.05

10

⎟⎟⎠

⎞⎜⎜⎝

⎛⋅

⋅⋅

⋅⋅= ypAzktanhpAp

uu

With:

p = Soil resistance [kN/m]

pu = maximum soil resistance [kN/m]

y = pile deflection (lateral) [m]

k = bedding modulus, dependent on ϕ´ [kN/m3]

A = Calibration factor [ - ]

static :

cyclic :

9.08.00.3 ≥⎟⎠⎞

⎜⎝⎛ −=

DzA

9.0=A(API RP-2A WSD , 2000)

Relative Density

Initi

al s

tiffn

ess


p-y curves for sand acc. to API

( ) zDCzCp 21us ⋅γ⋅⋅+⋅=

zDCp 3ud ⋅γ⋅⋅=

(API RP-2A WSD, 2000)

Max. lateral soil resistance pu [kN/m] :

with:

z = Depth below soil surface [m]

D = average pile diameter [m]

γ ´ = effective unit weight of soil [kN/m3]

C1, C2, C3 = empirical coefficients, dependent on ϕ ´ [ - ]

ϕ´ = angle of internal friction [ ° ]

the smaller value is relevant(near to surface)1)

2) (deep)

Coe

ffici

ents

Coe

ffici

ent

Angle of internal friction


p/pu y/yc

0 0

0,5 1,0

0,72 3,0

1,0 8,0

1,0 ∞

31

cu yy5,0

pp

⎥⎦

⎤⎢⎣

⎡=

1,0

p/pu

0,5

y/yc1,0 8,0

0,72

3,0

with:

p = Soil resistance [kN/m2]

pu = maximum soil resistance [kN/m2]

y = pile deflection [mm]

yc = 2,5 εc x D [mm]

εc = Strain at 0.5 σmax in undrained uniaxial compression tests

p/pu y/yc

0 0

0,5 1,0

0,72 3,0

0,72 ∞

1,0

p/p u

0,5

y/y c

0,72

static

cyclic

1,0 8,03,0 15,0

p/pu y/yc

0 0

0,5 1,0

0,72 3,0

0,72 z/zR 15,0

0,72 z/zR ∞

1,0

p/p u

0,5

y/yc1,0 8,0

0,72

3,0 15,0

0,72 zR/z

static

cyclic

Deep – if z > zR

Near to surface - z < zR

p-y curves for soft clay acc. to API

static


Max. lateral soil resistance pu [kN/m]:

With:

z = Depth below soil surface [m]

zR = Depth of the zone of reduced soil resistance (near to surface) [m]

c = undrained shear strength of undisturbed samples [kN/m2]

D = Pile diameter [m]

γ ´ = effective unit weight of the soil [kN/m3]

J = Dimensionless empirical constant between 0.25 (medium stiff clay) and 0.5 (soft clay) [ - ]

DczJzc3pu +γ+=

JcD

D6zR

+γ

=

c9pu =

(Near to surface - if z < zR )

(Deep - if z ≥ zR )

1)

2)

p-y curves for soft clay acc. to API


p-y curves for cohesive soilsO´ Neill und Gazioglu (1984): Integrated Clay Model

– no distinction between soft and stiff clay– based on 21 field tests at 11 different locations

DcNFp upult ⋅⋅⋅=

F – empirical „soil degradation“-factor

Critical pile length – pile length, from which the length has no further influence on pile behavior

for z ≤ zcrit

9

63

=

⋅+=

p

critp

NzzN

286,0

5,00,3

4

⎥⎦⎤

⎢⎣⎡

⋅=

=

DEEIL

Lz

c

ccrit

for z > zcrit

critical depth

F(Failure)Strain ε from UU- triaxial test

< 0.02 0.02 – 0.06 > 0.06Fs (static) 0.50 0.75 1.0

Fc(cyclic) 0.33 0.67 1.0Np – bearing capacity coefficient


p-y-method acc. to API underestimatesdeflectionsUnmodified application is not recommended

H-w-/H-φ-curves: Comparison API-FEM

API-Method for Monopiles ?

Displacement w in cm Rotation in °

Hor

izon

tal f

orce

in M

N

Hor

izon

tal f

orce

in M

N


Effect of large diameter

Proposal of Soerensen et al. (2010) and results of Augustesen et al. (2010)for monopiles in sand

Significant effectEstimation from numericalsimulations


Piles under cyclic horizontal loads – Test results

Alizadeh & Davisson (1970)

Hettler (1981):Model tests

NCyy

NK

NK ln11,

, +=


Cyclic LoadingOffshore guidelines (GL, DNV) demand consideration of cyclic load effects

BSH-Standard „Soil Investigations“: Cyclic laboratory tests should lead to a predictionof cyclic deformations and stability of the foundation structure.

Loading

Time


Usual requirement: Rigid clamping under design load

But: for large-diameter monopiles this leads to extreme lengths!

IGBE: Coupling of FE-simulations with cyclic triaxial tests (SDM-stiffness degradation method)


SDM method

Result (Monopile D = 7.5m, dense sand, H=15 MN, h=20m)Principle:

FE-Model

Cyclic triaxialtest device


Degradation of secant modulus under cyclic loading in the pile-soil model (schematic)

aNcp

aNcp

s

sN

EE

,

1,

1 εε =≅

( ) 21

,

1,

1

bXba

Ncp

aNcp

s

sN NEE −= ==

εε

(Huurman 1996)

sf

cycX,1

,1

σσ

=with

Degraded stiffness:

b1 and b2 are cyclic parameters to be determined in triaxial tests


Simulation of lateral pile deflection in a 1-g laboratory test using the degradation stiffness model

Simulation of plastic strain response in a cyclic triaxial test with dry sand using the degradation stiffness model

Timmerman & Wu (1969) Achmus et al. (2007)


Variation of stiffness in two pile-soil systems dependent on the number of load cycles


On the effect of rigid clamping

• Rigid clamping („vertical tangent“ or „zero toe kick“) must not alwayssecure favourable behavior under cyclic loads.

• For very large-diameter monopiles the requirement leads to too large embedded pile lengths.

Deflection in cm Number of load cycles N in 1

Dep

th b

elow

sea

bed

in m

Dim

ensi

onle

ss p

ile d

efle

ctio

ny k

,N/ y

k,1

in 1


Dependence of the pile head deflection on different loading conditions

Pile deflection lines calculated with the FE method

Investigation regarding the minimum embedded length of Monopiles


Pile deflections under cyclic loading (D = 5 m, H = 15 MN, h = 15 m)

Accumulation of horizontal pile deflections at seabed level for monopiles D = 5 m

monopile design - agtze · pdf filemonopile design. 2. ... hettler (1981): model tests. c n y...

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