offshore pile design according to international practice

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


Tallest offshore piled structureBullwinkle, GOM

Bullwinkle piles:28 x 84”OD, 165m long

Bullwinkle platform:529m high412m water depth


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


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


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.


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)


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


API Main text method

Shaft resistance

f = K σv’ tanδ

f <= flim


API 2007 - CPT methods

Motivation

Research programs

Key features

Database

Industry acceptance


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.


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


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%)


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


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


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)


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


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


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




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



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)



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


Pile driving monitoring - underwater

Risks of mechanical damage and electrical instability require specific operational procedures

Under-water monitoring requires specific equipment


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


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


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


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 platform/tripod loading

Moment loading at mudline for a monopile is translated into axial pile loading for a tripod


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


Tripod - pile head deflections

Extract of displacement time history from 50 yr extreme event covering governing ULS peak load

Extreme deflections: Axial: 5mmLateral: 34mm


(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


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


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


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 according to international practice

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