CENTRE FOR OFFSHORE FOUNDATION SYSTEMShuniv.hongik.ac.kr/~geotech/Onlineslides/Design... · 2013-10-14 · DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS >> >> 1 OFFSHORE

DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS

susan gourvenec 2004

CENTRE FOR OFFSHORE FOUNDATION SYSTEMS

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ù conventional bearing capacity theory predicts conservative ultimate limit states under VMH loading

ù industry guidelines are based on conventional theory

ù results in over-conservative and inefficient foundation designs for offshore structures

ù offshore foundations are VERY expensive therefore the financial consequences of the conservatism is significant

>> 1 OFFSHORE FOUNDATION DESIGN

ù offshore foundations are subject to combined VMH loads

>> MOTIVATION FOR STUDY

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>> ENVIRONMENTAL LOADING


ù offshore structures are subject to considerable lateral and overturning loads from the wind, waves and currents

Frig

g Pl

atfo

rm,

Nort

h Se

a

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V

Self-weight of superstructure and

foundation system (V)

>> FOUNDATION LOADS

+

Wind & wave & current forces acting on legs (H, M)

H (often in the region of 500m)M=f(Hh)

h


ù offshore foundations are therefore required to carry general combined vertical, moment and horizontal loads (VMH)

=VMH foundation loads

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ù shallow footings with a circumferential skirt penetrating the seabed (sometimes called ‘bucket foundations’)

ù during undrained moment loading suctions are developed within the skirt providing an uplift capacity, allowing moment loads to be withstood at low vertical loads

suctions within skirt

>> SKIRTED FOOTINGS


>> CONVENTIONAL SURFACE FOOTING

uplift of footing from seabed

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gravity based structure

(up to 500m)

Trol

lA

vertically tethered tension leg platform

(up to 1500m)Sn

orre

B

>> APPLICATIONS


Trol

lOlj

e

anchored semi-submersible

(> 1500m)>>

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Troll

1410’


>> COMPARISON WITH ONSHORE

ù TrollA Platform: 470 m (1410 feet) tall

Troll A

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>> COMPARISON WITH ONSHORE


35% extra V1600% extra H500% extra M

VHM

1500 MN 29 MN

3255 MNm

2006 MN 495 MN

18850 MNm

maximum storm loads

One Shell PlazaHouston

213 m

52 m

71 m

Taywood Seltrust

168 m

183 m

foundation plan

150% bigger foundation area

A 3692 m29173 m2

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>>


>> THE PROBLEM

ù offshore design guidance is based on experience and empiricism from onshore, despite the considerable differences in the conditions

ù reflected in poor predictions of ultimate limit states for load conditions relevant to offshore foundation designs

>> AREAS FOR STUDY

ù failure of shallow foundations for conditions typically encountered offshore to quantify the degree of conservatism introduced by conventional theory (as proposed in design guidelines) and to try and identify possible practical alternative approaches

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>> 2 CONVENTIONAL BEARING CAPACITY THEORY

>> BASIC EQUATION

A area of the foundationSu undrained shear strength (uniform with depth)Nc vertical bearing capacity factor of a strip footing

= 5.14 (Prandtl, 1921) >>

Vult unit vertical bearing capacity

ù ultimate uniaxial vertical load of a strip footing resting on the surface of a homogeneous material

Vult=ASuNc

‘infinitely long’ footing

Terzaghi (1943)

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>> CORRECTION FACTOR

Vult=ASuNcKc correction factor

ù non-verticality of load i.e. inclination or eccentricityù finite foundation length

Kc = 1 – ic + sc

Source: ISO 1990 (2002)Petroleum and natural gas industries - Offshore structures Part 4 >>

load orientation factor ic = 0.5-0.5√(1-H/A’Su)

foundation shape factor sc = scv(1-2ic)B’/L

B

L

B

V

VMH

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>> LOAD ORIENTATION CORRECTIONù based on solutions for load eccentricity and load inclination

B

e

V

≡

B’

V

effective width B’ = B-2eeffective area A’ = B’ L

Meyerhof (1953)

PLUS

B

θ

B

VH

≡

Green (1954)

Vult/Asu= 0.5+0.5√(1-H/ASu) >>

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ù semi-empirical reduction factor to account for end effects


>> 3D FOUNDATION GEOMETRY CORRECTION

B

L

B

ù assume equivalent area and areal moment of inertia >> CIRCULAR FOOTINGS

BL = A = πD2

4

B3L = Ixx = πD4

12 64

D ≡ 1.128B D

≡L

B

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>> 3 FINITE ELEMENT ANALYSES (FEA)

ABAQUS (HKS 2002)

SOIL – soft clayhomogeneouslinear elastictresca plasticEu/Su=500, ν=0.49

FOOTING –concretelinear elasticE=107Eu, ν= 0.15

6D

2.5D

>> FINITE ELEMENT MESH

D

ù bonding on footing/seabed interface models uplift capacity >>

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>> 3 FINITE ELEMENT ANALYSES (FEA)

0

2

4

6

0.00 0.01 0.02 0.03 0.04

displacement

forc

e

failure load

ù ultimate limit failure loads from each analysis combines to form a continuous failure locus

>>

>> LOAD PATHSù combined VMH and (VM-H)

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>> 4 FAILURE LOCI

>> H & M LOADSPACE AT CONSTANT V LOAD

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1H/ASu

M/A

DS

u

VH,M

load space non-dimensionalised by footing geometry and undrained shear strength

Su

D

A=πD2/4

z

-H:M H:M >>

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>> 4 FAILURE LOCI – FINITE ELEMENT ANALYSES (FEA)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1H/ASu

M/A

DS

u

>> BASELINE CASE V=0 (idealised situation as implies weightless structure)

Mmax

ù maximum moment mobilised in conjunction with lateral load >>

Mult, i.e. at H=0

ù ultimate moment corresponds to zero lateral load

ASYMMETRIC LOCUS

ù locus is asymmetric

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>> 4 FAILURE LOCI – FINITE ELEMENT ANALYSES (FEA)

>> NON-ZERO VERTICAL LOAD

ù maximum capacity at V=0ù diminishing load capacity with increasing vertical loadù shape of locus is function of V load >>

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1H/ASu

M/A

DS

u

V=0.9Vult

V=0.75Vult

V=0.5Vult

V=0V=0.25Vult

V=Vult

ù constant intervals

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1H/ASu

M/A

DS

u

>> V=0.5Vult

ù maximum capacity at V=0.5Vult due to ‘lift-off’ at lower V

>> 4 FAILURE LOCI – CONVENTIONAL CALCULATION (ISO)

SYMMETRICALQUASI-LINEAR

LOCUS

ù symmetry reflects neglecting the difference in modes of H & M ù quasi-linearity indicates inadequacy of representation of

simultaneous load inclination and eccentricity >>

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>> 4 FAILURE LOCI– CONVENTIONAL CALCULATION (ISO)

>> CHANGES IN VERTICAL LOADù diminishing load capacity with increasing vertical load

0

0.1

0.2

0.3

0.4

0.5

0.6

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-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1H/ASu

M/A

DS

u

V=Vult

V=0.9Vult

V=0.75Vult

V=0.5Vult

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>> 4 FAILURE LOCI – CONVENTIONAL CALCULATION (ISO)

V=0.5Vult

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1H/ASu

M/A

DS

u

AND with DECREASING vertical load

V=0.25Vult

V=0.1Vult

V=0

ù zero moment capacity at zero vertical load

>> CHANGES IN VERTICAL LOADù diminishing load capacity with increasing vertical load

V=Vult

V=0.9Vult

V=0.75Vult

V=0.5Vult

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>> V=0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1H/ASu

M/A

DS

u

ISO

FEA

ZERO load capacity at zero vertical load

MAXIMUM load capacity at zero vertical load

>> 5 COMPARISON – ISO vs FEA RESULTS

OVERLOOKED LOAD CAPACITY

ù oversight of tension capacity at low vertical loads >>

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ù uplift of footing from the seabed under moment loading at low vertical loads

>> UPLIFT

ù the reduced bearing area due to uplift causes reduced capacityù vertical loads in excess of 0.5Vult are sufficient to maintain foundation

contact with the seabedù … but many offshore foundation systems would operate at working

loads less than half their ultimate vertical capacity >>


uplift of footing from seabed

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1H/ASu

M/A

DS

u

>> V=0.5Vult

ù inadequcy of superposition of VH and VM solutions (quasi-linearity)ù neglect of difference between VHM and V-HM modes (symmetry) >>

FEA: Mmax (>Mult), H>0

OVERLOOKED LOAD CAPACITYOVERLOOKED

LOAD CAPACITY


ù i.e. like comparison: no uplift

ISO

FEA

ISO: Mmax=Mult at H=0

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ù conventional theory is based on solutions for load inclination and vertical load eccentricity and breaks down under superposition

>> ECCENTRIC OR INCLINED LOADS (VM or VH)


eccentrically applied vertical load

VM

üM

V

centrally applied inclined load

V

H

üH

V>>

û

eccentrically applied inclined load

VM

H

M

H

V=f(Vult)

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ù conventional method reflects locus from VHM quadrant for negative H>> SYMMETRY: VHM vs V-HM

ù VHM is NOT physically equivalent to V-HM >>


VM

H

M

H

≡

V=f(Vult)

VM

H-

M

-H

≡

V=f(Vult)

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>> 6 SUMMARY

ù conventional bearing capacity theory inadequately represents theshape of the failure loci and underestimates the magnitude of the bearing capacity of shallow foundations under general combined VMH loading

ù superposition of solutions for inclined (VH) and eccentric (VM) loading failing to represent the response to inclined eccentric (VMH) loading in conjunction with the neglect of the differences between the H & M modes of loading (i.e. HM and –HM)

ù conservatism is exacerbated for conditions of low vertical load as the conventional bearing capacity theory does not account for tensile capacity achieved with foundation skirts

>> FOR THE CONDITIONS INVESTIGATED VM

H(i.e. undrained bearing failure of a circular

footing on a homogenous soil)

?

Documents

CENTRE FOR OFFSHORE FOUNDATION SYSTEMShuniv.hongik.ac.kr/~geotech/Onlineslides/Design... · 2013-10-14 · DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS >> >> 1 OFFSHORE