The document outlines international practices in offshore pile design within the oil and gas industry, emphasizing the extensive experience and safety measures for both small and large projects. It discusses various standards and methods for pile resistance, driveability, and monitoring, while also addressing innovations and reliability of designs in the context of risk aversion. The insights on tripod structures highlight key design issues, showing their advantages over monopiles, and stress the importance of adopting practical solutions based on existing industry experience.

1. Offshore pile design:
International practice
Dr David Cathie
Cathie Associates

2. Outline
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
 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
Offshore pile design : International practice

3. Tallest offshore piled structure
Bullwinkle, GOM
Bullwinkle platform: Bullwinkle piles:
529m high 28 x 84”OD, 165m long
412m water depth
Offshore pile design : International practice

4. Offshore oil & gas industry
 10,000+ platforms worldwide
 ~99% piled jacket structures
Location Ground conditions
Gulf of Mexico Normally consolidated clay
Offshore Brazil
West Africa
Middle East Carbonate soils, sands, calcarenite
Australia
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

5. 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 energy
IHC range
Menck MHU range
is similar
Offshore pile design : International practice

6. 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

7. 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

8. 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

9. API Main text method
Shaft resistance
f = K σv’ tanδ
f <= flim
Offshore pile design : International practice

10. API 2007 - CPT methods
 Motivation
 Research programs
 Key features
 Database
 Industry acceptance
Offshore pile design : International practice

11. 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

12. 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

13. 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

14. CPT method – application by Shell
Dynamic SRD [MN] Dynamic SRD [MN]
0 4 8 12 16 20 0 4 8 12 16
0 0
5 ICP
API
20
10
15
40
20
25 ICP
60 API
m
m
w
w
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B
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]
[
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f
f
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i
30
35 80
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

15. 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

16. 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

17. 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

18. 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

19. Pile driveability prediction
SOIL RESISTANCE TO DRIVING (MN) 100 BLOWS PER 0.25 METRE
MHU 500T (Eff. = 80%), 40m penetration
0 25 50 75 100 0 50 100 150 200 250
0 MHU 500T (Eff. = 95%), 40m penetration 0
MHU 800S (Eff. = 80%),
MHU 500T (Eff. = 80%), 20m penetration
Best Estimate SRD
Best Estimate SRD
75 MHU 500T (Eff. = 95%), 20m penetration MHU 800S (Eff. = 80%),
High Estimate SRD
High Estimate SRD
10 10 MHU 800S (Eff. = 95%),
Best Estimate SRD
MHU 800S (Eff. = 95%),
50 High Estimate SRD
20 20
25
M
G
O
C
N
A
V
D
R
E
S
L
T
)
(
I
30 30
M
W
M
W
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U
U
B
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B
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)
(
)
(
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0
0 50 100 150 200 250
40 BLOWS PER 0.25 METRE 40
SRD v Depth SRD v Blow count Blow count v Depth
Offshore pile design : International practice

20. 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

21. Pile driving monitoring
 Instrumentation
 Pile instrumentation consists in installing strain gauges
and accelerometers at pile top
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
Offshore pile design : International practice

22. Pile driving monitoring - underwater
Under-water monitoring requires specific equipment
Risks of mechanical damage
and electrical instability
require specific operational
procedures
Offshore pile design : International practice

23. Pile driving monitoring – signal matching
G-OCTOPUS
C126 UW PDM OLOWI PILE DRIVING ANALYZER ®
PDA OP: ACR-MHA Version 2009.098.061
PILE C1
JACKET WHT-A
BN 3/828
30/07/2009 18:23:25
80000  Signal matching (CAPWAP/TNOWAVE)
7.16 EMX 845.1 kN-m
kN m/s E2E 828.6 kN-m
CSI 229.6 MPa
F  Iteratively modifying V numerical soil model until the
a EF2 848.7 kN-m
E2F 843.1 kN-m
calculated reflective wave matches the measured waveEV2 877.5 kN-m
RMX 18448 kN
DMX 30.3 mm
DFN 14.0 mm
B w o 12
lo N. 63
8 0 .0
00 kN
LE 39.970 m
AR 2759.57 cm^2 W pM
u sd
W pCt
u p
EM 207413 MPa
51.2ms SP 77.5 kN/m3
15.60 ms WS 5123.0 m/s
Measured upward and 2 6 .7
66 EA/C 11173 kN-s/m
80000 downward waves 80000
15
LP 25.750 m
45 ms
kN kN
F12 A2 6 L/c
WD WU
- 6 67
26 .
F1: [898W] 132.7 (1)
F2: [904W] 130.2 (1)
A2: [29997] 1035 g's/v (1) Measured and
calculated
upward waves
- 0 00
80 .
51.2ms
Offshore pile design : International practice

24. Pile driving monitoring – data example
Driving Data
Capwap result
Offshore pile design : International practice

25. 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

26. 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

27. 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

28. Tripod and monopile loads
Bending moment at mudlevel during 50 year severe sea state
350000
Tripod Pile Monopile
300000
250000
200000
150000
100000
50000
0
-50000 0 20 40 60 80 100 120 140
M
m
N
o
n
b
e
k
s
-100000
r
t
]
[
-150000
-200000
time [s]
Axial (vertical) force at mudlevel during 50 year severe sea state
10000
Monopile Tripod Pile max compression Tripod max tension
5000
0
0 20 40 60 80 100 120 140
-5000
-10000
-15000
M
m
N
o
b
e
k
c
s
-20000
r
]
[
f
-25000
-30000
time [s]
Offshore pile design : International practice

29. Tripod - pile head deflections
Extract of displacement time history from 50 yr extreme event covering governing ULS
peak load
Extreme deflections: Axial: 5mm
Lateral: 34mm
Offshore pile design : International practice

30. (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

31. 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

32. Research – tripod foundations
Priority Topic Why?
1 Axial pile stiffness at working Key for accurate structural
loads dynamics
2 Lateral pile behaviour Not critical design issue for tripods
subject to cyclic loads but little is known
(fixed head, many low level
load cycles)
3 Lateral pile stiffness at 2nd order importance for structural
working loads dynamics, and for cyclic axial pile
capacity
4 Axial pile capacity under Most previous research
low level cyclic loads concentrated on higher levels of
cyclic axial load
5 Effect of ageing on pile Ageing is known to increase pile
response resistance and stiffness but the
mechanisms are not understood
Offshore pile design : International practice

33. 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
Offshore pile design : International practice