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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
susan gourvenec 2004
CENTRE FOR OFFSHORE FOUNDATION SYSTEMS
CEN
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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>> ENVIRONMENTAL LOADING
>> 1 OFFSHORE FOUNDATION DESIGN
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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
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
>> 1 OFFSHORE FOUNDATION DESIGN
ù offshore foundations are therefore required to carry general combined vertical, moment and horizontal loads (VMH)
=VMH foundation loads
>>
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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
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
>> 1 OFFSHORE FOUNDATION DESIGN
>> CONVENTIONAL SURFACE FOOTING
uplift of footing from seabed
>>
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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
gravity based structure
(up to 500m)
Trol
lA
vertically tethered tension leg platform
(up to 1500m)Sn
orre
B
>> APPLICATIONS
>> 1 OFFSHORE FOUNDATION DESIGN
Trol
lOlj
e
anchored semi-submersible
(> 1500m)>>
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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
Troll
1410’
>> 1 OFFSHORE FOUNDATION DESIGN
>> COMPARISON WITH ONSHORE
ù TrollA Platform: 470 m (1410 feet) tall
Troll A
>>
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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>> COMPARISON WITH ONSHORE
>> 1 OFFSHORE FOUNDATION DESIGN
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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>>
>> 1 OFFSHORE FOUNDATION DESIGN
>> 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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>> 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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>> 2 CONVENTIONAL BEARING CAPACITY THEORY
>> 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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>> 2 CONVENTIONAL BEARING CAPACITY THEORY
>> 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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
ù semi-empirical reduction factor to account for end effects
>> 2 CONVENTIONAL BEARING CAPACITY THEORY
>> 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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>> 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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>> 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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>> 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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>> 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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>> 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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>> 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
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=Vult
V=0.9Vult
V=0.75Vult
V=0.5Vult
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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>> 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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>> 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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
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 >>
>> 5 COMPARISON – ISO vs FEA RESULTS
uplift of footing from seabed
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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
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
>> 5 COMPARISON – ISO vs FEA RESULTS
ù i.e. like comparison: no uplift
ISO
FEA
ISO: Mmax=Mult at H=0
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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
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)
>> 5 COMPARISON – ISO vs FEA RESULTS
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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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
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 >>
>> 5 COMPARISON – ISO vs FEA RESULTS
VM
H
M
H
≡
V=f(Vult)
VM
H-
M
-H
≡
V=f(Vult)
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DESIGN CONSIDERATIONS FOR OFFSHORE SHALLOW FOUNDATIONS
>> 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)
?
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