Dr David CathieCathie Associates
Offshore pile design: International practice
Offshore pile design : International practice
Outline
Pile design in the offshore industry
International standards and methods
Pile resistance (capacity) methods – API, CPT
Pile driveability
Pile driving monitoring
Piled tripods for wind converters – key issues
Conclusions
Oil and gas industry has a lot of experience in developing both small and large offshore projects.
Practical solutions are available today for safe design of tripod structures
Offshore pile design : International practice
Tallest offshore piled structureBullwinkle, GOM
Bullwinkle piles:28 x 84”OD, 165m long
Bullwinkle platform:529m high412m water depth
Offshore pile design : International practice
Offshore oil & gas industry
10,000+ platforms worldwide
~99% piled jacket structures
Location Ground conditions
Gulf of MexicoOffshore BrazilWest Africa
Normally consolidated clay
Middle EastAustralia
Carbonate soils, sands, calcarenite
Far East Loose to medium dense sands, soft clays
North Sea Medium dense to very dense sands, very soft to hard clays
Offshore pile design : International practice
Pile sizes – piling hammers
Typical pile OD: 1.2 – 2.4m (1.8 – 2.4m in N.Sea)
Typical length: 40 – 100m
Pile hammers:
90-150 kJ hydraulic hammers for typical “small” piles 600kJ or more for large piles
Put in table with rated energyIHC rangeMenck MHU range is similar
Offshore pile design : International practice
Pile design in the offshore industry
Industry is risk adverse, and highly cost conscious
Consequences of structural failure leading to shutdown are very high, and unacceptable
Consequences of installation delays are very high, and unacceptable (production delayed, cost overrun)
Innovation is seen as risky and must have very high cost-benefit
Reliability of pile design is very high (no failures reported). Belief that methods are conservative to very conservative. No account taken of ageing effects.
Offshore pile design : International practice
International Standards
API RP2A – WSD 29th edition, 2000
API RP2A – LRFD, 1st edition 1993
DNV classification notes No. 30.4, 1992 (based on API 1987)
ISO 19902:2007 Fixed steel offshore structures (based on API)
API RP2A – WSD 29th edition, 2000, errata and supplement 3, 2007, provides the Commentary on CPT-based methods for pile capacity (C6.4.3c)
Offshore pile design : International practice
API pile design approach
Pile capacity/resistance methods
Main text API 93 method CPT methods in commentary in 2007 edition
Axial/lateral response
Cyclic loads
Offshore pile design : International practice
API Main text method
Shaft resistance
f = K σv’ tanδ
f <= flim
Offshore pile design : International practice
API 2007 - CPT methods
Motivation
Research programs
Key features
Database
Industry acceptance
Offshore pile design : International practice
API 2007 CPT-based pile resistance
Motivation
Improve reliability and reduce conservatism Based on more fundamental understanding of pile
behaviour Practical method capturing basic mechanics of driven
pile Direct use of CPT results in silica sand
Research Programs
Euripides (started 1995) in Eemshaven, Netherlands: dense to very dense sands
Pile load tests in Dunkirk (dense to very dense marine sands) – CLAROM site
Pile load test in Labenne (loose to medium dense sand), LCPC site.
Offshore pile design : International practice
Key features of CPT methods
Direct use of cone resistance (qc) to determine radial stress (σ’rc)
Effect of distance from pile tip
“friction fatigue” or degradation during driving as pile progresses
Unit shaft resistance based on residual soil-pile friction angle
Offshore pile design : International practice
CPT methods – database
ICP database: 20 open-ended tubular piles in sand
Length: 2m to 47m Diameter: 0.07m to 2.0m (average 0.65m) Range of Dr at tip: 57-96%
UWA database: 32 open-ended tubular piles in sand
Length: 5.3m to 79.1m Diameter: 0.36m to 2.0m (average 0.73m) Range of Dr at tip: 15-100% (average 68%) Range of Dr along shaft: 23-100% (average 74%)
Offshore pile design : International practice
CPT method – application by Shell
0
20
40
60
80
0 4 8 12 16
Penetratio
n B
elo
w S
eaflo
or [m
]
Dynamic SRD [MN]
ICP
API
0
5
10
15
20
25
30
35
0 4 8 12 16 20
Penetratio
n B
elo
w S
eaflo
or [m
]
Dynamic SRD [MN]
ICP
API
Overy (2007) The Use of ICP Design Methods for the Foundations of Nine Platforms installed in the UK North Sea, Int. Offshore Site Investigation and Geotechnics Conference
Offshore pile design : International practice
CPT Method – industry acceptance
Adopted by Shell UK in 1996 for requalification of a number of North Sea platforms (Overy, 2007)
Pile length: 26m to 87m Diameter: 0.66m to 2.13m Variable soil conditions
Adopted by API, 2007 as a “recent and more reliable method ...considered fundamentally better..”
But qualified by offering 4 alternative methods Should be used only by qualified engineers
Offshore pile design : International practice
API pile design approach
Axial/lateral response
T-Z/Q-Z and P-Y standardised approach Mainly based on research in 1980’s
Cyclic loading
Axial Axial and lateral effects uncoupled Long (=flexible) piles can experience capacity degradation
in clay soils (due to strain softening) Wave loading rate effect may compensate for degradation.
Lateral Cyclic effects included by softening P-Y response near
seabed and reducing peak lateral pressures Methods proposed are guidelines only (but everyone uses
them)
Offshore pile design : International practice
Pile driveability - SRD
Soil resistance during driving (SRD)
Alm and Hamre (2001) method Database 18 installations, 1.83 – 2.74m OD, up to 90m
penetration, MHU 1000-3000, IHC S-400, S-2300 Key feature: degradation of shaft resistance as pile
passes, calibrated to database
Offshore pile design : International practice
Pile driveability – wave equation
Wave equation (SRD v Blow count)
GRL WEAP – same quake, damping soil model as used by Alm & Hamre
Blow count v depth
Pile acceptance criteria
Offshore pile design : International practice
Pile driveability prediction
SRD v Depth SRD v Blow count Blow count v Depth
0
10
20
30
40
0 50 100 150 200 250
PILE
PEN
ETRA
TIO
N B
ELO
W M
UD
LIN
E (M
ETRES
)
BLOWS PER 0.25 METRE
MHU 800S (Eff. = 80%), Best Estimate SRD
MHU 800S (Eff. = 80%), High Estimate SRD
MHU 800S (Eff. = 95%), Best Estimate SRD
MHU 800S (Eff. = 95%), High Estimate SRD
0
25
50
75
100
0 50 100 150 200 250
SOIL
RES
ISTA
NC
E TO
DRI
VIN
G (M
N)
BLOWS PER 0.25 METRE
MHU 500T (Eff. = 80%), 40m penetration
MHU 500T (Eff. = 95%), 40m penetration
MHU 500T (Eff. = 80%), 20m penetration
MHU 500T (Eff. = 95%), 20m penetration
0
10
20
30
40
0 25 50 75 100
PILE
PEN
ETRA
TIO
N B
ELO
W M
UD
LIN
E (M
ETRE
S)
SOIL RESISTANCE TO DRIVING (MN)
Best Estimate SRD
High Estimate SRD
Offshore pile design : International practice
Pile driving monitoring
Purposes
Confirming pile resistance during driving, or after set-up, SRD
Correlate SRD with calculated static pile resistance for the site.
Establish reliable pile acceptance criteria (blow count) based on a calibrated wave equation & SRD model
Monitor the stresses at pile top and to correlate with risks of tip buckling (when driving in rock)
Offshore pile design : International practice
Pile driving monitoring
Operationally, the offshore environment is extremely challenging.
It requires specific experience and extreme precautions for data of good quality
Many attempts have resulted in failure
Instrumentation Pile instrumentation consists in installing strain
gauges and accelerometers at pile top
Offshore pile design : International practice
Pile driving monitoring - underwater
Risks of mechanical damage and electrical instability require specific operational procedures
Under-water monitoring requires specific equipment
Offshore pile design : International practice
Pile driving monitoring – signal matching
Signal matching (CAPWAP/TNOWAVE)
Iteratively modifying a numerical soil model until the calculated reflective wave matches the measured wave
15 45
-8000.0
-2666.7
2666.7
8000.0
Blow No. 1623
ms
kN
6 L/c
W up Msd
W up Cpt
Measured upward and downward
waves
Measured and calculated
upward waves
Offshore pile design : International practice
LS+P2 with D10045h set-up
LS+P2+P3 with MHU 600
47h set-upchange to MHU1000
4h set-up
LS+P1+P2+P3+P4 with MHU100037.5h set-up
LS+P2+P3+P4+P5 with MHU1000
27h set-up
Re-Strike on P5 with MHU100025h set-up
0
20
40
60
80
100
120
140
0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000
SRD (kN)
dept
h (m
)
after set-up (static capacity as per API86)SRD upper bound Stevens/PuechSRD lower bound Stevens/Puechback analysed SRDRequested CapacityCAPWAP
Pile driving monitoring – data example
Driving Data
0
20
40
60
80
100
120
140
160
0 100 200 300 400blow count (blow/m)
de
pt
(m)
0
20
40
60
80
100
120
140
160
0 200 400 600 800 1000energy (kJ)
dep
th (
m)
-
5 000
10 000
15 000
20 000
25 000
30 000
35 000
40 000
45 000
50 000
0 200 400 600 800 1000 1200
blow count bl/m
SR
D (
kN
)
Capwap result
Offshore pile design : International practice
Pile driving monitoring – accuracy and limitations
Generally within 10-15% of static tests
Best agreement in sedimentary soils (sand, clays) Agreement depends on set-up time and failure
criteria for static test Limitations
Cannot accurately differentiate between tip and shaft resistance near the base
Cannot define exact distribution of shaft resistance
Offshore pile design : International practice
Piled tripods for wind converters – key issues
Tripod foundation response very different from monopile
Structural dynamics of tripods
Insensitive to lateral stiffness Axial stiffness related to pile penetration/capacity (for
stiff piles) so natural frequencies insensitive also to detailed pile design
Cyclic axial loads much more important than cyclic lateral
Allow generously for scour – not a design problem with tripods
Need practical solutions to design foundations today!
Offshore pile design : International practice
Offshore platform/tripod loading
Moment loading at mudline for a monopile is translated into axial pile loading for a tripod
Offshore pile design : International practice
Tripod and monopile loads
-200000
-150000-100000
-50000
0
50000
100000
150000
200000
250000
300000350000
0 20 40 60 80 100 120 140
Mem
ber m
omen
ts [k
Nm
]
time [s]
Bending moment at mudlevel during 50 year severe sea state Tripod Pile Monopile
-30000
-25000
-20000
-15000
-10000
-5000
0
5000
10000
0 20 40 60 80 100 120 140
Mem
ber f
orce
s[kN
]
time [s]
Axial (vertical) force at mudlevel during 50 year severe sea state Monopile Tripod Pile max compression Tripod max tension
Offshore pile design : International practice
Tripod - pile head deflections
Extract of displacement time history from 50 yr extreme event covering governing ULS peak load
Extreme deflections: Axial: 5mmLateral: 34mm
Offshore pile design : International practice
(Geotechncial) advantages of tripods/quadripods
Not sensitive to uncertain soil parameters (operational soil modulus)
Not sensitive to scour assumptions
Lateral pile deflections are restrained by structure stiffness
Main cyclic loads transmitted as axial loading
Offshore oil & gas industry has strong preference for multiple leg structures – almost exclusively builds 3, 4 or 8 legged piled platforms for offshore operations
Other structures require specific conditions to be cost-effective
Offshore pile design : International practice
Research – tripod foundations
Must not delay design and procurement process
Solutions must be adopted today even if research to confirm or improve methods continues in parallel
Solutions are available today May be conservative but based on oil & gas
experience Use pile driving monitoring to confirm capacity
during installation Use structural monitoring to confirm
eigenfrequencies and foundation stiffness in different conditions
Not essential today but research desirable to provide improved (less conservative) methods of design i.e. reduce development costs
Offshore pile design : International practice
Research – tripod foundations
Priority
Topic Why?
1 Axial pile stiffness at working loads
Key for accurate structural dynamics
2 Lateral pile behaviour subject to cyclic loads (fixed head, many low level load cycles)
Not critical design issue for tripods but little is known
3 Lateral pile stiffness at working loads
2nd order importance for structural dynamics, and for cyclic axial pile capacity
4 Axial pile capacity under low level cyclic loads
Most previous research concentrated on higher levels of cyclic axial load
5 Effect of ageing on pile response
Ageing is known to increase pile resistance and stiffness but the mechanisms are not understood
Offshore pile design : International practice
Conclusions
International oil & gas industry has long and successful track record with piled structures
Offshore industry is conservative and risk adverse (high costs involved in all marine work)
New CPT methods of pile design have been introduced recently because of the recognition that the earlier API methods were over conservative in some circumstances (e.g. dense sand)
Cyclic loading is handled within (API) design methods for wind/wave loads for jacket or tripod structures
Tripod solutions for wave converters are very robust and insensitive to variations in foundation conditions
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